The rainforest biome (a major habitat type) can be defined as forest growing in regions with more than 200 cm (6.5 feet) of rainfall per year. Although there are temperate rainforests (such as that of British Columbia in Canada), tropical rainforests occur between the Tropic of Cancer and the Tropic of Capricorn (23.5o N and 23.5o S). They are found in regions where the average temperatures of the three warmest and the three coldest months do not differ by more than 5o C, although there may be daily variations of more than that. Rainfall is relatively evenly distributed, which allows the growth of a heavy canopy of broad-leaved evergreen trees; however, many of these regions have distinct dry and rainy seasons. There are multiple layers of vegetation from understory shrubs to trees of more than 40 meters (130 feet) in height. There are many epiphytes and palms. These forests are limited in extent by temperature and precipitation. The three major blocks of tropical rainforest are those of the Indo-Malayan region (South and Southeast Asia), Central Africa, and Central and South America (Neotropics). There are several general types of tropical rainforests:
Lowland evergreen tropical rainforest, which has no distinct dry season, and in which most trees retain their leaves throughout the year. These are the most luxuriant forests, with many very tall canopy (“emergent”) trees, sometimes more than 45 meters in height, often with huge buttress roots. Below that lies the main (or middle) stratum, from 24 to 36 meters in height, and an underlayer of smaller, shade-loving trees. Ground vegetation is often, but not always, sparse, contrary to the popular image -”white-hatted explorer hacks his way through the impenetrable jungle” – because so little light can filter through the upper leafy layers. Many plant climbers try to reach the light by attaching to the large canopy trees.
Seasonal tropical rainforest occurs in regions with a short dry period. Some of the trees in such forests are deciduous; they may lose their leaves at the same time or flower and/or fruit simultaneously (seasonality). Many of the plant genera are the same as in evergreen forests, although the species composition is different.
Tropical semievergreen forest occurs in regions where there is a relatively long dry season. The upper tree story is deciduous (a water-saving adaptation), while the lower stories are evergreen. In deciduous (monsoon) tropical rainforests, there is a lengthened dry season, and virtually all tree species are deciduous, so that the forest is leafless during the dry periods.
There are many subdivisions of these basic types of rainforests, such as montane (mountain) forests, peat forests, cloud forests, and so on.
Tropical rainforests frequently conform to the stereotypical notion that they are riotously profuse, abundant with thousands of plant species. Although some forests are composed mainly of one dominant tree genus or family, most contain hundreds of woody plant species (compared to less than 30 in most temperate forests). They are indeed the most structurally-complex and diverse of land ecosystems, with the greatest number of species. Among the earth’s ecosystems, they are rivaled in species diversity only by coral reefs.
An intact rainforest is virtually a “closed system” in which the essential nutrients, both organic and mineral, are cycled from the soil through the vegetation and back again. This allows luxuriant forests to grow in relatively hostile environments where the soils are poor and temperatures high. Therefore forest health is dependent upon decomposing organisms – bacteria and fungi – because, without the degradation of plant and animal materials, no new growth could occur. Since warm temperatures and high humidity favor decomposition, nutrients are rapidly made available for plant growth, and so luxuriant vegetation is characteristic of these forests. In general, little organic matter is lost from the forest, because it is taken up very rapidly from the soil by the vegetation.
Primary (“virgin” or “old-growth”) forests are those which have been relatively undisturbed by human activity (although they may have been altered in the past), and which contain trees of a wide range of ages. Secondary forests are forests which have undergone some major disturbance (fire, for instance, but more often human disturbance) and so most of the vegetation is of approximately the same age. These differ from primary forests in their species composition as well as in the relative youth of the trees, because a disturbance opens a gap in the forest, in which only certain species (“pioneer species”) can grow. For the most part, the species of the primary forest are not adapted to gap conditions, as are the pioneer species (see “Forest maintenance,” Section F).
Tropical rainforests cover about 6% of the earth’s land surface. Tropical America contains about 50% of the world’s tropical rainforest, approximately four million km2 of forest cover. Southeast Asia has about 2.5 million km2, and Africa, with the smallest area, about 1.8 million km2. This does not mean, however, that rainforests have lain within their present contours for many thousands of years. For example, recent evidence indicates that the Bolivian rainforest has been gradually expanding over the past three thousand years; for the previous 50,000 years it had not extended so far south because of drier conditions (Mayle, et al., 2000).
Tropical rainforests are a very ancient biome. When the earth was much warmer, 240 million years ago, the ancient “coal forests” of primitive non-vascular plants died out and the succeeding forest, mainly tropical, consisted of ferns, conifers, cycads and other lesser-known groups. Insects were the dominant animal form on land; dinosaurs appeared only later during this period. Fossils of flowering plants (angiosperms) first appear in rocks dating from one hundred and fifty million years ago. These plants were enormously successful and eventually became the dominant land vegetation, forcing the ferns and other earlier groups to a marginal status. We know that rainforests originated sometime after 100 million years ago, and that they were the dominant forest type at the time of the disappearance of the dinosaurs (65 million years ago). They represent the world’s oldest extant biome, although current tropical forests almost certainly differ in many respects from the earlier ones. However, angiosperms still form the majority of the plant species in tropical forests.
Some scientists believe, as indicated by recent fossil data from South America, that many tropical species arose as long as 14 million years ago or more. A South American monkey skull, which has been dated as 20 million years old, has anatomical similarities to Old World monkeys, and supports the hypothesis that some New World fauna originated in Africa. Also, fossil teeth of Old World rodents which resemble those of New World porcupines have been found. These animals presumably migrated to South America by floating on “rafts” of vegetation. Many other very ancient tropical mammal fossils – marsupials, herbivores, rodents, edentates – have been found in the Andes. The Isthmus of Panama arose about seven million years ago, allowing exchanges of fauna between North and South America. Fish fossils are correspondingly ancient, and have been found far from the Amazon and Orinoco Rivers, where it was previously thought that fish species in this region evolved. There are fossils of catfishes, rays, piranhas, carnivores (flesh-eaters) and piscivores (fish-eaters), even fossils of a fish which ate fruit (and which is very similar to a fish presently living in these rivers). What does this mean? These data imply that these species arose many millions of years ago, at a time prior to the uplifting of the Andes as a mountain chain, and when this area was continuous with the region which is now the Amazon basin and covered with rainforests. Most of these fish fossils are related to those of the current tropical regions (Amazon and Orinoco basins) and not to present-day fish of the region where the fossils were found.
We who live in the industrial and technological twenty-first century and who are aware of the perpetually-increasing human influence on the earth think of prehistoric man as benign, and having had a relatively slight impact on the environment. However, this may be only a myth. Studies of game consumption by ancient hunter-gatherers in western Asia indicate that humans, although highly-dispersed and at low densities, placed significant hunting and collecting pressures on local prey populations, as indicated by shifts in the proportions of different species consumed over time. When more easily available species such as shellfish and tortoises became rare (as indicated by decreasing size of prey captured as time went on), man either migrated elsewhere or consumed species more difficult to catch, such as birds and hares. As the human population grew during the Middle and Late Paleolithic periods, shortages of the more desirable (i.e., slower-moving) edible species became chronic.
The time of human arrival often corresponds to extinctions, particularly of large animals, such as the moa of New Zealand, the flightless geese of Hawaii, and giant lemurs of Madagascar. A population of approximately 160,000 moas was hunted to extinction within a few decades by Maoris settling in New Zealand (Holdaway and Jacomb, 2000; Diamond, 2000). In Australia, 28 genera and 55 species of vertebrates disappeared after human arrival on the continent. Some of these animals were gigantic – a 200-pound kangaroo and the largest known bird, Genyornis, weighing 60 pounds (Dayton, 2001). In the western hemisphere, too, most large animals, including the fabled sabre-tooth tiger and the woolly mammoth, became extinct at about the time that humans arrived on these continents. Whether these extinctions were due to human activity or to climatic change is still being debated. However, recent computer modeling suggests that the destruction of large fauna was an almost inevitable consequence of human consumption pressures, not just because of direct hunting pressures, but because of the changes in ecosystems due to removal of certain species, particularly large herbivores (Alroy, 2001)
Much of the composition of the present rainforest, too, is probably anthropogenic (caused by human activity), although we think of it as pristine. Approximately 12% of the Amazon rainforest may have been altered by humans through prey selection, seed dispersal, and plant domestication. The vegetation of many rainforest areas has also been determined by centuries of slash-and-burn (“swidden”) agriculture, leading to a forest mosaic consisting of many stages of forest succession, interspersed with areas of climax (mature) forest. Humans also altered forests through the use of fire for hunting, for fuel, and other purposes. The presence in eastern Amazonia of plants useful to humans, such as palms of the genus Astrocaryum, is thought by some to indicate that humans had originally planted these species. Therefore, some feel that the composition of many present-day forests is the result of enrichment or alteration by human agriculture and occupation (Mann, 2000). This type of evidence for human occupation is disputed by many, however. (For discussions of the presence of ancient man in rainforests, see, for example, Bray, 2000; Mann, 2000; Pope, et al., 2001; Piperno, et al., 2000; Butzer, 1999; Denevan, 1992.)
Rainforests are the most complex and species-rich environments on earth. Their great diversity is due to the humid warm climate, the many different types of habitats, the many roles which organisms can play in forests, and the amazing specializations in reproductive strategies and other mechanisms to reduce competition within the forest. In rainforests, woody plants predominate to a much greater extent than in temperate forests. Between 45% and 53% of all rainforest plants are woody. Some of these trees are so immense, one wonders if they are as old as temperate trees, some of which have been dated to 1000 years of age or more. It is very difficult to estimate the age of tropical trees, since they do not form annual growth rings, due to the absence of seasonal growth periods. Some of the large trees (such as the dipterocarps, the dominant trees of Southeast Asian rainforests) mature at approximately 50 to 60 years of age, but they may live for hundreds of years beyond maturity. Although the potential age span may be great, few trees will attain maximum age because most will die before adulthood, succumbing to competition, parasites, strangler vines, and damage from natural catastrophes such as storms. Thus, among woody rainforest trees, a few will be of a great diameter, but most will be relatively small.
Because there are so many species in a rainforest, very few individuals of any single tree species will survive to adulthood. Thus, in the case of woody trees, there are few individuals per species (typically, one or fewer per hectare), but many species are represented per unit area. (Palm tree forests are a notable exception to this.) Gentry (1988) found 58 different species in a sample of 66 trees in a tiny 0.1 hectare plot of land in the Peruvian Amazon; of the first 50 trees sampled on a one-hectare plot of land, only two were of the same species! At most, one species may comprise 15% of the individuals in a given area. This is vastly different from temperate forests, which may consist of great tracts containing only one or a few species of trees. The scarcity of individuals of a single species can be advantageous, although it exacerbates the difficulty of cross-pollination, and since it reduces the probability of disease, pest and predator transmission. However, smaller trees of the same species and rare species tend to be more clumped than would be expected if distribution were random, whereas larger-diameter trees are less aggregated (Condit, et al., 2000). Non-woody plants such as herbs, climbers and rattans are usually more densely distributed throughout the forest than are trees.
A rainforest is a cohesive ecological unit, but a dynamic one, in which the proportion of different species will vary over time and space. Indeed, the composition of a rainforest can change dramatically within a very short distance, because of differences in altitude, soil type, water distribution and so on. Tuomisto, Ruokolainen and Yli-Halla (2003) attribute the distribution characteristics of two species of plants in Amazonian forests mainly to variations in environmental factors, such as soils, and, to a lesser degree, to limitations on dispersal mechanisms. They suggest that their results may be valid for many other species and other tropical forests, as well.
The average height of the canopy trees in rainforests is 35 – 42 meters (115 – 137 feet) with a crown diameter of 13 – 22 meters (42 – 72 ft), but the tallest trees may reach 84 meters (275 feet). Trees apparently reach these heights because of competition for light; trees of the same species will be much shorter if they are kept in solitary conditions. The roots of tropical rainforest trees are superficial, since the topsoil tends to be thin, and so some of the largest trees are anchored by large horizontal roots (buttresses) which protrude above the ground. Leaves tend to be large and long, and much of the energy the plant takes in goes to making them, since a high rate of photosynthesis is required to survive competition for nutrients, light and space. These characteristics permit very high productivity of both plants and animals, and tropical rainforests produce more mass of both plants and animals than do other types of forests. Interestingly, although we regard rainforests as important sources of timber, tropical rainforest trees produce proportionally less wood and more leaves than do temperate forest trees.
Although there are many different tropical rainforests, organisms playing equivalent roles in ecosystems, appear very similar, regardless of where they are found. Corresponding patterns of rainfall, temperature, humidity, and soil type result in similar – but not identical – ecosystems in tropical forests in Asia, Africa and the Americas. These forests all have trees which are sun-loving; those which prefer shade; plants which grow on trees (lianas and creepers); and many other types. Animals of all kinds are also to be found – those which eat seeds or fruits or vegetation or other animals; those which live on the ground, or in the forest canopy, or in dead trees; those which are active in daytime or at night. Plants or animals in different forests but fulfilling equivalent functions (occupying similar niches) will not be of the same species, but will have many similarities. This is so because the conditions of their particular habitats require similar adaptations. Trees exposed to bright sunlight need to be protected against drying (dessication), for example, and so will have adaptations which permit them to survive under these conditions; those living in shade must be able to photosynthesize under restricted light conditions. Over long periods of time, then, species in equivalent habitats on different continents will adapt to their circumstances and will appear very much alike, a phenomenon known as “evolutionary convergence.” Because of the evolutionary consequences of natural selection, tropical rainforests everywhere may appear very similar.
Forests are dynamic units, which consist of individuals at all stages of their life cycles. We can think of forests as mosaics, containing a variety of patches in different phases of restoration. Forests have evolved their own system of regeneration, which is termed succession. Succession, a change in the composition of the forest over time, occurs because openings (“gaps”) continually appear in forests, and because the seeds and seedlings of different species have varied requirements for germination and growth. When an opening does appear, certain plants establish themselves in them and form a pioneer forest, only to be replaced later by other species adapted to the changing conditions, and eventually resulting in a stable forest composition – the climax forest. This process is described below.
Natural disturbances play a great role in forest succession. Trees die of old age, or are struck by lightning, or are blown over by wind, or are knocked down by other falling trees. When this occurs, gaps appear in the forest canopy, which alters the environment for all of the plants surrounding the open area, be it large or small. The frequency of and degree to which catastrophic events occur depends on the forest. In much of the Amazon, and on Borneo, there are few catastrophic events, and therefore forest regeneration is usually confined to small gaps where one or a few trees have died. In the flood (varzea) forests in Peru, there are “stripes” with different patterns of species, each of which represents a stage of succession. Because these areas are subject to annual flooding, heavy siltation, and annual changes in the river course, a climax forest is never attained – only a series of pioneer forests of varying ages. On the other hand, in Papua-New Guinea, where volcanic activity and earthquakes are fairly frequent and where there has been a long tradition of shifting cultivation, there are large areas of regenerating forest and therefore a fairly elaborate mosaic of pioneer and climax forest.
Tropical rainforests go through several stages during regeneration. A small gap, such as that caused by the death of a small tree, or the loss of a limb, will not alter much in the forest. Limbs from other trees will fill in a gap and their shade will prevent the growth of most seedlings on the forest floor, except for those which do not require much light. A somewhat larger gap changes the physical state of that area of the forest. There will be more light, heat, and wind on the forest floor where a gap forms. The forest floor of the gap will become hotter and drier than previously, although more rain will reach the ground (but it will be dried quickly by the sun). The temperature here can be as much as 10o C higher than under the canopy. Even the wave lengths of light reaching the forest floor are altered. Normally canopy trees absorb “red” (long) wavelengths and the forest floor receives only about 2% of the photosynthetically-active wavelengths (440-700 nm) in small spots or flecks. But in a gap, more of the light reaching the forest floor will be in the red wavelength range (660-770 nm). Under these circumstances, the slower-growing and shade-tolerant seedlings in the understory cannot survive, and are gradually replaced by seedlings of fast-growing and light-tolerant species (the so-called pioneer species). Pioneer species are characterized most importantly by a requirement for strong light for seed germination and seedling establishment, and also, in general, aggressiveness, tolerance to dessication, rapid growth, early reproduction, efficient seed dispersal mechanisms, and small seeds with long dormancy periods. (The fig, Ficus insipida, for instance, has a photosynthetic rate six and one-half times greater than that of any other known species, although it will probably not hold that record when other tropical pioneer species are examined. And it must live in bright sunlight [Allen, 1996].). When a gap forms and light strikes the forest floor, their seeds, which have been dormant for a long time, are able to sprout and grow rapidly to fill in the gap. At this early successional stage, the extent of seedling sprouting and survival is determined mainly by factors in the environment – competition, nutrient availability, temperature, degree of shade, etc. Then those seedlings which survive begin to influence their own environment as they grow – by producing shade, using soil organic matter, and producing new types of habitats. At the same time, epiphytes and climbing plants begin to colonize the growing young trees.
The early pioneer trees are often low and short-lived, and may be replaced by longer-lived, taller pioneer species which form a higher canopy forest. Meanwhile, the seedlings of climax species remain undeveloped in the shade of the pioneer trees, as they do not do well under gap conditions of high temperatures and high light intensities. Moreover, specimens of climax species in gaps appear to be susceptible to attack by boring insects. The pioneer species will eventually, however, be replaced by climax species since, as pioneer seedlings require light, they cannot survive and reproduce under the newly-formed (pioneer) canopy. Climax species in general have large seeds with substantial nutrient reserves (so they can wait), have shade-tolerant seedlings, are slow-growing relative to the pioneer species, and are self-perpetuating, since once the pioneer trees form a canopy, the climax species’ shade-tolerant seedlings can grow in their shade. As large pioneer trees die, conditions are conducive for the small trees of the climax species to grow rapidly and take their place. (While climax species’ seedlings are shade-requiring, older specimens actively seek light.) Once established, a climax forest can reproduce itself endlessly, since it provides shade for its seedlings, and the large trees have attained the canopy. This series of events is what Whitmore (1998) calls a “shifting mosaic steady state.” (Although pioneer and climax species are defined here as having quite different characteristics, in truth all of these species lie on a continuum and it is not always easy to define these terms.) However, decade-long studies on Nicaraguan forest after very large gaps were created by a hurricane indicated that, when very extensive gaps form in tropical forests, pioneer species do not succeed in repressing the growth of other species. This is apparently because there are relatively few seeds or seedlings of pioneer species in the forest, and so they are not able to “flood the market” nor to capture the lion’s share of available space and light, as they do when smaller gaps occur (Vandermeer, et al., 2000).
There is a very large genetic pool, and incredible diversity among the organisms in rainforests. There are species which are adapted to almost every condition which may occur during and after forest perturbation. For example, there are fast-growing species, species which require sun, those which require shade, those with rapidly-germinating seeds, others with seeds with long dormancy periods – in short, whatever happens in a forest, there will be species which can exploit the situation and thrive. However, diversity in a secondary (successional) forest tends to be lower, at least at first, than in primary forest. Palm diversity, for one, is very low in secondary forests.
How rapidly do large forest trees reach their destination in the canopy? Little is known of this topic so crucial to forest management programs and to our ability to restore degraded tropical forests. Recently, however, Clark and Clark (2001) found that most trees do not grow constantly or at maximum rates – there are too many obstacles and too much competition. Some trees even decreased in height while striving for the canopy when they were damaged or died back partially due to adverse conditions. To attain even half of their final stature might require 35 to 85 years.
Rainforests are especially vulnerable ecosystems. The seeds of the woody species are fragile, many forest plant seeds cannot tolerate sun and thus cannot disperse across cleared land, the soil is fragile and easily eroded by heavy rainfall, and many species have a very limited distribution and will be decimated by removal of even small areas of forest. Rainforest regeneration is slow, and the forest may indeed never regenerate, at least to its original condition. The ancient Cambodian capital city of Angkor was deserted in 1431, and while the forest around it has regrown, after almost 600 years it is still different from the original climax forest in this area. The forests in the areas of Guatemala where the Maya lived and which they abandoned as long as 1200 years ago are less diverse than other forests nearby; in fact, many of the tree species in these forests are those which had been used and presumably planted by the Maya.
Man-made disturbances have been much more dramatic, for the most part, than natural ones. During the El Niño of 1982-1983, there was a great drought in Southeast Asia and many fires – both man-made and natural – occurred. El Niño is a recurring climatic pattern during which warmer water from the western Pacific region moves eastward into the cooler eastern Pacific. Normally the temperature differential of the water between the western and eastern portions of the Pacific sets up trade winds which blow strongly from east to west, causing ocean currents to flow westward. In El Niño years, because warmer water pushes into cooler regions, the temperature differential is less and the trade winds weaken. Because ocean and atmosphere are so closely linked, heat and water exchanges are altered, which influences rainfall patterns in the tropics and in temperate zones as well. During the aforementioned El Niño period, three million hectares of forest were destroyed in Kalimantan (Indonesian Borneo), one third of which had been recently logged and where much flammable dead dry wood had been left. Another third was primary forest which had been desiccated and killed by the drought. Elsewhere, settlers moved in along logging roads and set fire to what remained of the forest after logging. Thus, the drying effects of El Niño were exacerbated by human activities. The huge gaps which are formed by logging and fires are not amenable to normal regeneration processes, and in many cases the resulting forest (if any) will be vastly different from the previous one. Hunting by settlers and loggers leads to the loss of animals which are the major seed dispersers and which are therefore vital in tree reproduction. In Sabah (North Borneo) much land, previously forested, is still grassland after a drought and an ensuing fire in 1915. (Whitmore, 1998.)
A mature rainforest is an association of many different types of flora and fauna, and is composed of innumerable communities which act together to form an incredibly complex entity. The communities which comprise the rainforest can vary in many ways: temperature, moisture, species present, available habitats, soil type, and topography. The number and kinds of organisms present will influence the form of the ecosystem, and, conversely, the environment in which these organisms find themselves will influence their functioning. Certain species, known as keystone species, are thought to have dominant roles in an ecosystem, although this is still somewhat controversial. The idea is that, if a keystone species is absent, the ecosystem will change dramatically or damaged irreparably. Thus it is not necessarily the number of species per se which is vital to an ecosystem, but the presence of certain essential species, such as seed-dispersing mammals or the canopy trees of rainforests.
The functioning of an ecosystem, at base, depends upon the ways in which energy is used by organisms in the system. These processes consist of the capture, transfer, and loss of energy. Rainforest organisms acquire nutrients by obtaining moisture from clouds, intercepting rainfall, and directly fixing substances, especially carbon and nitrogen, from the atmosphere. Epiphytes, which have no roots, are vital in the rainforest ecosystem, as they obtain most of their nutrients from the atmosphere and so act as a major pathway for transferring nutrients from the atmosphere to other vegetation. They do this by intercepting and storing water, concentrating nutrients in their tissues, and eventually transferring nutrients to the soil through their stems or by dying. This is extremely important since the nutrient content of tropical soils is often so low. Plants store nutrients in their tissues; in fact, most nutrients in tropical forests are contained within the organisms, not in the soil. Most nutrients in tropical rainforest soils therefore come from dead and decaying organisms – plant, animal, fungal and microbial.
• Rainforests appear chaotic, but are quite highly organized into vertical strata to a degree unknown in temperate forests. (It should be noted that some ecologists do not recognize the presence of strata in rainforests). There are the tallest trees, or “emergents,” sometimes more than 50 meters in height, which appear at irregular intervals above the almost continuous mid-layer. These largest trees,
• comprising less than 10% of forest trees, contain approximately half of the above-ground biomass of the forest. Emergent species require much light and can tolerate high temperatures and the drying effects of direct sunlight. The trees in the understory layers are very numerous and competitive and have less expansive crowns and more slender trunks than do emergents. Still closer to the forest floor are small trees and shrubs, and, on the forest floor, there are a variety of non-woody plants, seedlings, and herbs. The forest is a mosaic composed of multitudes of small groupings, based on their histories. For example, wherever a gap has formed, either by treefalls or some other mechanism, a new grouping arises, unique within the forest. The composition of these small units depends upon the size of the gap, the availability of light, the temperature and humidity, soil type, and the kinds of seedlings or seeds available. The forest structure reflects the necessities of life for plants: the need to support themselves, the requirement for sufficient light for photosynthesis, and their pattern of continuous growth. Young trees will have slender trunks and grow rapidly toward the light, allocating most of their resources to growth rather than to reproduction. Only when a tree has reached a height where it has sufficient light can it afford to expend energy on flowering and fruiting and to reduce its growth rate. At this point tall canopy trees make large crowns to support their more massive root and trunk structures, whereas in the understory there is no space in the crowd for a large crown.
Tropical forests are very rich in species, but not uniformly so. The number and kinds of species found in any given area depend heavily upon its history: the immigration, evolution and survival of species under past and current conditions. Rainfall, light, soil fertility, and the abiotic (nonbiological) environment are also determining features. Each species has a range of conditions under which it can survive and reproduce. Some can function only within a very narrow range of conditions; others have a broader tolerance. For plants, rainfall is usually the most critical variable; species richness of trees in the Neotropics can increase as much as six times as rainfall increases from one meter to four meters per year (Wright, 1996). Soil fertility is especially important for understory herbs and shrubs, and even for trees. In Borneo, for instance, the diversity of the large hardwood canopy trees (dipterocarps) is greatest when the soil is of intermediate fertility (Wright, 1996).
Little is known of the interactions between species abundance and various aspects of ecosystem functioning in tropical rainforests. However, soil nutrient levels are greater in areas where there is a greater diversity of plant species. Nutrient retention is also greater, as is the maintenance of soil processes favorable for plant growth. Soils under monoculture suffer more nutrient depletion in comparison with forested areas; in some cases this depletion is so severe as to lead to plant death (Silver, et al., 1996). A certain degree of diversity seems essential to maintain soil-plant interactions at a level which can support plant life. There are many pathways of nutrient flow in natural tropical forests, and these must be maintained for the forest to survive. Synergistic interactions among species are also thought to affect resource consumption and, thereby, ecosystem productivity. For example, Cardinale, Palmer and Collins (2002) found that increasing the diversity of aquatic arthropods boosted their feeding success (“facilitation”). A biodiverse ecosystem may also be able to resist the invasion of exotic species more readily than a less diverse one. Kennedy, et al. (2002), demonstrated this principle in experimental grassland plots. In these experiments, there were fewer invading plants and these invaders were more limited in size in biodiverse grass plots than in less diverse ones.
The role of rainforests in the global carbon cycle is complex and little known. Plants and animals contain a great deal of carbon, which they take up as carbon dioxide (CO2) during growth and photosynthesis, and which they release to the atmosphere during respiration and decomposition. Although rainforests form less than half of the total forest on earth, their leaf systems comprise approximately 70% of the world’s total leaf surface area. Rainforests have ten times more leaf area than temperate forests of comparable size and fifty times more than grasslands. It is not surprising, then, that they account for between 30% and 50% of total primary productivity (photosynthesis) in terrestrial systems, although they cover only 6% of the total land area of the earth. This means that they store more carbon (as sugars and starches) per unit area than any other type of ecosystem. Rainforests are thought to contain between 40% and 50% of the carbon in the terrestrial biomass (Phillips, et al., 1998), which has been estimated as more than 17 kilograms of carbon per square meter. The rainforests of Amazonia contain between 14 and 40 kilograms of carbon per square meter. The soils lying under rainforests also contain substantial amounts of carbon (in roots, microorganisms, soil fungi and plants), which amounts to about 27% of global soil carbon (Lodge, et al., 1996).
Not all carbon storage occurs within above-ground plant vegetation. At least 40% (and perhaps as much as two-thirds) of the carbon in tropical forests is found below ground in root systems and soil organic matter. Forests (including temperate forests) have been estimated to contain 330 gigatons (1015 tons) of carbon in the vegetation and 660 gigatons of carbon in soil organic matter (Noble and Dirzo, 1997). Amazonian soils contain from four to nine kilograms of carbon in the upper 50 centimeters of the soil layer, while pasturelands contain only about one kilogram per square meter, in contrast. Thus, tropical forests are critical elements in the carbon cycle of the planet. When forests are cleared and burned, 30 – 60% of the carbon is lost to the atmosphere; unburned vegetation decays and is lost within ten years.
The importance of rainforests in the carbon cycle depends on the extent of the forest, the amount of carbon stored per unit area (as plant body or as organic material in the soil), and the rate at which carbon is “fixed” by the plants during photosynthesis
Nitrogen is an essential nutritional element for all plants and animals. There is a large reservoir of nitrogen in the air, but it is in a chemical form which is unavailable to plants and must therefore be “fixed” into usable form as nitrates or nitrites by soil microorganisms. As much as 130 metric tons of nitrogen is fixed annually by terrestrial systems, and is then available for use by plants. Nitrogen is released into the soil and water when the plants die, or when the herbivores which have consumed the plants die, or excrete nitrogen compounds. Humans have greatly altered this cycle by their own fixation of additional nitrogen for fertilizers (more than 80 million metric tons in 1990) and by the release of nitrogen into the atmosphere by fossil fuel combustion and land conversion. Human cultivation of legumes also increases nitrogen entry into the soil. All in all, human activities add as much fixed nitrogen to the land as comes from natural sources (Vitousek, et al., 1997).
Because of the great extent of rainforests, they are a significant ingredient in the global nitrogen cycle. They fix a great deal of nitrogen by means of their huge microbial populations; nevertheless, many tropical rainforests are limited in their growth by low nitrogen levels because they lose tremendous quantities of nitrogen into the soil and, to almost as great an extent, into water as dissolved nitrogen (Perakis and Hedin, 2002).
Freshwater is an essential resource which is under increasing pressure. Dams and other diversionary activities, particularly agriculture, have diverted a huge amount of the world’s fresh water for human use. Humans now use more than 50% of the available fresh water of the earth, and this proportion is en route to increase to 70% in the next half-century. Therefore it behooves us to attend to all factors which affect the water cycle. Although the role of rainforests in the global water cycle is relatively small compared to that of the oceans, it is nevertheless extremely important. Rainforests influence the hydrologic cycle in the following ways:
1) Precipitation
Rainforests release water vapor by transpiration through leaves and evaporation (evapotranspiration, or water lost through the pores in leaves and evaporated by heat). The loss of water to the air by leaves is a critical part of the water cycle of the earth, as, by this means, the water vapor content of the air is continually replenished. More than 50% (even as much as 75% in dense rainforest) of the precipitation striking a rainforest is returned to the atmosphere by evapotranspiration and, consequently, relatively little will end up in rivers and other waters. Most of the water released by evapotranspiration to the atmosphere as water vapor will be returned to the forest as rain, so rainforests provide their own rainfall. Although forests account for only about 15%-20% of global water evaporation, approximately 65% of the rainfall over land is due to them. Lowered levels of atmospheric water vapor reduce cloud cover and rainfall, so if forest is removed, rainfall in that region will be substantially reduced. This will have dire consequences for even large rainforest reserves. If they are surrounded by deforested land, they will not be able to generate sufficient rain to support themselves, and they are doomed to perish. Evapotranspiration also has a cooling effect, as it takes energy to vaporize water from leaves, and keeps the temperature in the forest relatively constant.
2) Water regulation
The movement of water into rivers and other waterways is modulated by forest vegetation. Vegetation increases the ability of soils to retain water, preventing floods and erosion. Since a forest can intercept as much as 50% of the rainfall, it will prevent much soil loss which might otherwise occur from the impact of rain on the land surface. Water passes from rainforests into rivers and streams with much less force, reducing erosion and the threat of floods.
Overall, we do not have a great deal of quantitative data on the effects of rainforests on climate, but it is known that rainforests have an effect on heat balance, influence air currents, release a great deal of moisture into the air via evapotranspiration (which moderates temperature), and absorb some CO2 from the atmosphere (the amount is presently being hotly debated).
1) Temperature
There is currently considerable controversy over whether the earth’s average temperature is increasing or not (“global warming”), although most scientists are convinced that it is. An increase of approximately 0.6oC has occurred during this past century, and temperature ranges are decreasing (Walther, et al., 2002). Kremen, et al., (2000), estimate that global temperatures will increase between 1o and 4oC over the next century; Töpfer (2001) and Harvell, et al., (2002) suggest 1.4 to 5.8oC, predictions corroborated by Houghton, (1995), who calculated a rate of temperature increase of 0.3oC per decade, or 3oC by the year 2100. While this subject is beyond the scope of this essay, we should note that there is a natural “greenhouse” effect caused by the emission of gases of various kinds into the atmosphere – from plant and animal respiration (carbon dioxide, CO2), from animal digestion (methane, CH4), from volcanic activity, from evaporation (water vapor) and others. Secondly, human activity is increasing the amounts of many of these gases in the atmosphere. For example, carbon dioxide concentrations have increased by 30% in the past 150 years, and ozone levels have more than tripled over the past century (Percy, et al., 2002). Thirdly, the earth’s surface temperature depends upon the amount and types of these gases contained in the atmosphere and the albedo of the surfaces which can absorb heat. (Albedo is the fraction of solar radiation reflected from a surface, here mainly from plant bodies and, to a lesser extent, soil.) There is some evidence that energy emissions from the tropics have increased to a substantial degree during the past 20 years, which appears to have some relationship to cloud cover and tropical air circulation shifts. In some areas the air masses near the equator moved upwardly more strongly than usual, which decreased cloud cover and humidity in these areas. Whether or not these alterations are part of the normal climatic cycles is not known (Hartmann, 2002; Chen, Carlson and Del Genio, 2002; Wielicki, et al., 2002).
a. water vapor: Albedo depends upon the type of land cover (desert, forest, grassland, etc.). Forests absorb a higher proportion of the energy impinging on the earth’s surface than do other types of ecosystems. A forest canopy can absorb up to 93% of the solar radiation (energy) falling on it, of which less than 2% is utilized for photosynthesis. Most of this energy is absorbed in the evaporation of water from the forest vegetation. The resultant water vapor disperses heat into the atmosphere, and so heat (energy) is transferred from the surface of the earth to the atmosphere. Water vapor, as clouds, can also reflect heat energy. So it can be cooling or heating, depending upon the type and volume of cloud cover. These forest effects are not restricted to the tropics. As warm moist air moves north from tropical forested areas, heat is transferred to northern regions, and humidity and rainfall increase. Thus, tropical water balance/heat balance is linked to that of temperate areas.
b. carbon dioxide: There has been considerable controversy about whether or not rainforests act as carbon “sinks,” by net absorption of carbon dioxide from the atmosphere. This phenomenon is known as sequestration. Plants do absorb huge amounts of carbon dioxide from the atmosphere in photosynthesis, during which is converted to sugars which are oxidized or “burned” to support growth and metabolism. However, the end result of oxidation (respiration) is carbon dioxide, which is released as a waste product into the air, and a great deal of carbon dioxide enters the atmosphere in this way. Carbon dioxide is one of the “greenhouse” gases which contribute to the insulating effects of the atmosphere, and so the question of whether or not forests provide net sequestration of carbon dioxide (that is, whether forests absorb more carbon dioxide during photosynthesis than they give off during respiration) is extremely important, given current rates of deforestation and the fact that mankind is emitting huge amounts of carbon annually into the atmosphere (partly from deforestation).
The concentration of carbon dioxide in the atmosphere in the atmosphere has increased by 30% in the past 300 years, with half of that increase occurring during the past 40 years. Seven petagrams (one petagram = 1015 grams, or two billion tons) are exuded annually into the atmosphere by the combustion of fossil fuels, and 1-2 petagrams from burning of forests in the tropics (Prentice and Lloyd, 1998). Palm, et al., (1986) gives a higher estimate – between 0.4 and 4.2 petagrams of carbon emitted annually from the combustion of tropical forests, with between 0.15 and 0.43 petagrams coming from Southeast Asia alone. Schimel et al., (2001) estimate that land-use changes (mainly tropical deforestation) result in the emission of 0.6 to 2.5 gigatons of carbon to the atmosphere annually. (For comparison, United States per capita emissions are more than 5 metric tons per year, in comparison with an average of 0.6 metric tons in developing countries [Baer, et al., 2000].) Kremen, et al., (2000), state that 20% to 30% of total carbon emissions is due to tropical deforestation. Half of emitted carbon remains in the air, a quarter is absorbed by the ocean, and the remainder, approximately two petagrams, must be being taken up (sequestered) by terrestrial systems.
As much as 40% of this sequestration might be due to uptake by tropical forests (Adam, 2001). However, estimates as to the extent of uptake vary greatly. At present it is not possible to establish a figure with any exactitude because of the difficulty of obtaining accurate measurements on such a vast scale, because of the diversity of tropical forests, and because carbon absorption varies with climate, soil type, species composition, whether the forest is primary of secondary, age of the forest, etc. As an estimate, tropical forests sequester on the order of 0.7 billion tons of carbon annually (as much as 200 tons per hectare per year), approximately one-and-a half to two times as much as temperate forests (Moffat, 1997). Prentice and Lloyd (1998) calculate that Amazonia has a net sequestration rate of 0.1 Pg of carbon per year (i.e., excess uptake of carbon by plants over carbon released by deforestation). Schimel, et al., (2001) suggest that tropical forests absorb more carbon than they emit, a net “sink” of approximately 0.4 Gt per year. Pimentel, et al., (1997) give a figure of 2.5 metric tons of carbon sequestered per hectare per year for tropical forests. The large trees of the central Amazon alone have been estimated to sequester 0.2-0.3 petagrams of carbon annually (Chambers, et al., 2001). Increased net carbon dioxide sequestration by forests may be due in part to current higher growth levels. (Paradoxically, elevated growth rates occur because of increased quantities of atmospheric CO2 and the higher global temperatures due to greenhouse gases.)
Secondary forests, because of their rapid growth rates, accumulate carbon more rapidly than primary forests. However, it is difficult to calculate this rate, because many factors are involved – the age distribution of trees in the forest, for one, is very significant (Nelson, et al., 2000). Another factor in this equation seems to be that intact tropical forest trees, at least, continue to accumulate carbon in wood for as much as a century after there has been a surge in productivity (growth). The rate of storage as wood depends, however, on the size of the tree – large-diameter trees (that is, older ones) store more carbon, relatively, than smaller ones. An interesting article by Percy, et al., (2002) demonstrates that, in temperate trees such as aspens, at least, higher levels of atmospheric carbon dioxide increase tree diameter (i.e., growth). High plant diversity, such as is characteristic of tropical rainforests, also encourages CO2 sequestration (see Biodiversity Part II), although this has so far been demonstrated only in temperate forests. Thus, the high usage of atmospheric carbon by forests may allow them to buffer climate change by regulation, but their ability to absorb CO2 is limited by a number of factors: the finite nature of plant growth; the rapid reduction of tropical forest land because of deforestation; the fact that most forests are now found in regions of lesser fertility, where their growth capacity (and therefore ability to sequester carbon) is problematic (Oren, 2001), and the fact that deleterious climate changes (such as El Niño) can depress forest growth. Schimel, et al., (2001) warn that ongoing climate change as well as the maturation of secondary forests will reduce sinks, and that the terrestrial sink may vanish.
There is no question, however, that deforestation and burning of tropical forest releases huge quantities of carbon dioxide into the atmosphere (see above for quantity). It is rather ironic that humans now expect forests to absorb the carbon dioxide released when they are cut down and/or burnt!
c. methane: The atmospheric concentration of methane (CH4), another major greenhouse gas, is rising at a rate of approximately 1% per year. About 60% of methane emissions are anthropogenic (from human activities), mainly from landfills, coal mining, oil and natural gas systems, domestic ruminants (cattle, sheep, goats), animal wastes and wastewater, rice cultivation, and burning of biomass (Hogan, Hoffman, & Thompson, 1991). Methane released from wet tropical forests comprises perhaps 6-8% of total global methane emissions (Lodge, et al., 1996). Termites, which are ubiquitous in tropical forests, interestingly, account for a great deal of this methane. But most tropical methane release comes from the denitrification (breakdown of nitrogen-containing compounds) of organic material by microbes. Soils can act as “sinks” for methane, and, to some extent, offset methane emissions from termites.
d. nitrogen: The addition of nitrogen to the soil by human activities (See Section 1D) equals that added by natural sources. This includes the mobilization of more than 550 metric tons of nitrogen by land conversion, more than 80 million metric tons by fertilizer usage, and more than 20 million metric tons from the burning of fossil fuels (Vitousek, et al., 1997). The consequences are an increase in the greenhouse gas nitrous oxide (NO) as well as other reactive nitrogen compounds such as ammonia (NH3) and nitric oxide (N2O), components of acid rain and smog. In addition, the disproportionate presence of nitrogen in soils and water causes eutrophication.
(For discussions of these issues see Lawton, et al., 2001; Peñuelas and Filella, 2001; Potter, 1999; Scholes and Noble, 2001; Chambers, et al., 2001; Adam, 2001; Oren, et al., 2001; Sarmiento, 2000; Ferber, 2001; Prentice and Lloyd, 1998; Phillips, et al., 1998; Hogan, Hoffman and Thompson, 1991; Houghton, 1995; Houghton, 2000; Palm, Houghton and Melillo, 1986; von Storch and Stehr, 2000; Töpfter, 2001; Clancy, 1998; Baer, et al., 2000; Detwiler and Hall, 1988; Shaver, et al., 2000; Moffat, 1997; Vitousek, et al., 1997. There are many other articles in the literature, as well.)
e. Case in point: The Brazilian Amazon is about five million km2 in extent, of which four million km2 is forested. By 1988, 5.6% of this area had been deforested (230,000 km2), and approximately 10-15,000 km2 of primary forest are cut annually, although estimates vary considerably. Some say that as much as 30% of Brazilian forests is now gone. Carbon released as a consequence of deforestation, mainly from burning but also from the decomposition of forest vegetation after logging, provides Brazil with the credential of having the fourth greatest carbon emissions in the world (after the USA, countries of the former Soviet Union, and China) (Nelson, et al., 2000). Overall, rainforest destruction is the cause of 20%- 30% of global greenhouse gas emissions (Kremen, et al., 2000).
2) Air currents
Forests modulate air currents and reduce the turbulence of the air above them.
3) Moisture
This is related to the effects on temperature. [See Sections I, and J.]
To summarize, rainforests are of crucial importance because they recycle approximately 50% of impinging rainwater. A reduction of the forest cover will reduce this recycling activity, which in turn restricts rainfall, leads to a longer dry season, and reduces cloud cover. All of these factors will increase temperature and lower humidity, which have a negative feedback action on rainfall, reducing it yet further. These events will not only affect the forest directly – eliminating or reducing species which depend on lower temperatures and high humidity, but will alter weather conditions elsewhere on the earth (see Deforestation, Part III).
Soils are the basis for all terrestrial life, and the soil on which a forest grows is a critical determinant of forest type and vegetation. Soils are the transformers, regulators, buffers, and water and nutrient filtration systems of the forest. They act as links between the nutrient and mineral cycles and the atmosphere. They provide physical support for plants; they absorb, retain, and release water; they provide essential minerals and other chemical compounds for plant growth and maintenance; and they provide “waste disposal” services and nutrient cycling services through their microorganisms and soil fauna.
1) Soil formation
Soil is created by an immensely slow process involving the weathering of bedrock. Bedrock is dissociated by water and heat, and gradually forms particles at a rate of a few millimeters per millennium, although in the tropics weathering is more rapid than in temperate climates because of heavy rainfall and high temperatures. Because weathering is such a slow process, only minuscule amounts of minerals are generated by it. Some of these minerals remain dissolved or suspended in the moisture surrounding soil particles, and represent nutrients for future plant growth. In addition, the roots of plants fracture rocks, while soil organisms produce acids and CO2 which also help break the rocks down. While soil is being formed, the minerals associated with the particles are continuously lost by leaching as water passes over the soil or rock. Generally mineral losses are balanced by continual weathering of the rock. The minerals must be in a form which plants can absorb, but generally no more than 10% of soil minerals are, so that plant growth is often limited by their availability. As plants become established on the soil particles, and they (and animals) die and are decomposed by bacteria and fungi, their organic material mixes with the particles and accumulates near the surface to form topsoil. In this way, organic matter (humus) is added to the soil, making it able to support life. Since humus is resistant to further decomposition, it prevents compaction of soil and also plays a role in soil chemistry. Humus in soil also absorbs a great deal of water and prevents runoff.
2) Soils of tropical rainforests
Rainforests are very fragile habitats. In many places they are “wet deserts,” which grow on soils poor in nutrients. In many tropical regions, the bedrock is very old and weathered, and, consequently, depleted in minerals and nutrients. Mineral release is also inhibited by the acidic nature of many tropical soils. The soil types derived from the bedrock underlying tropical forests are mainly soils called oxisols and ultisols. (There are many kinds of tropical soils, each with its own characteristic array of minerals; see Richter and Babbar, 1991, for a detailed discussion). Oxisols have a high aluminum and iron oxide content and a low silica content. Ultisols are highly-weathered, acidic soils and are less frequently found than oxisols. These two types of soils, generally of low fertility, comprise about 43% of the soils under tropical rainforests (Hoffman and Carroll, 1995). Another 40% consists of variably fertile soils, some of which are suitable for agriculture, but many of which have low pH, poor physical structure, low phosphorus and other nutrient deficiencies, or high salt or aluminum levels. Interestingly, tropical soils can vary a great deal within a relatively small area, which leads to a variety of vegetation types because of differences in nutrient concentrations and availability, variations in the ability of the soil types to retain water, and the like. (Many other factors are also involved in determining the vegetation which grows in any particular area.)
Oxisols are acidic soils and contain considerable quantities of iron and aluminum. These minerals form insoluble compounds with phosphorus, which decreases the availability of the latter to plants. Also, under dry conditions and, particularly in soils with high iron contents and low silicate content, the oxides in oxisols form impermeable layers, known as laterite, below the surface. Thus, when the forests overlying such oxisols are cut down, the logged area becomes much drier and eroded, and this often leads to laterization. This will not happen if the surface is covered with trees and vegetation. Because laterite is impermeable, rain will run off quickly, leading to erosion and flooding. Laterization is not reversible.
Many tropical soils are acidic and depleted in weatherable minerals such as calcium, potassium and magnesium, essential for plants. Many lowland forests are limited by a lack of phosphorus, or sometimes calcium and magnesium; others, on spodosols (periodically-flooded sands) seem to be limited by low nitrogen levels. But plant growth is dependent upon the presence and interactions of many nutrients. To add to the intricacy of the situation, the presence – or limitation – of one mineral may affect the uptake and metabolism of others. For instance, the ability of leguminous trees to “fix” atmospheric nitrogen and convert it to nitrates and nitrites may be compromised by deficiencies in iron, molybdenum and/or calcium. Because there are so many types of tropical soils, and their mineral profiles are so complex, not a great deal is known about them.
Many essential elements such as calcium and potassium are easily leached out by the heavy tropical rainfall, further reducing soil nutrient levels. There are few nutrients more than 5cm (2 inches) below the surface of the soil in tropical rainforests. This poverty of soils (which is common but not universal in rainforests) has the consequence that the forest is dependent on the recycling of nutrients, most of which are contained within the vegetation and not in the soil, unlike temperate forests. Because many rainforest trees are evergreen and drop their leaves infrequently, there is relatively little “litterfall” in comparison with temperate forests. Leaves and dead plants and animals which fall on the forest floor are rapidly decomposed by fungi and bacteria, and the resulting chemical compounds are quickly reabsorbed by the living plants. Plants on tropical soils typically recycle 60% to 80% of nutrients, and in the case of calcium and phosphorus, more than 99% of these minerals appear to be recaptured from the soil by the roots of forest trees. The remainder of necessary nutrients must come from soil or from rainfall. [See also Section G5.]
How do plants retain nutrients under such stringent conditions? Many have adaptations which allow them to exploit the limited quantities of nutrients in tropical soils. Root biomass is very high where soils are infertile, so that plants can “locate” whatever nutrients might be available. Because so much energy must be invested in root systems, energy expenditures for leaves are minimized by retaining them for a considerable time, although this risks attack by pests. As an adaptation to this situation, many tropical plants form tough leaves containing noxious tannins and reinforced with woody fibers. Such leaves are unfortunately resistant to decomposition and inhibit the cycling of nutrients. Rainforest plants also produce vast quantities of leaves to capture as efficiently as possible the abundant CO2 in the air. Additionally the canopies as well as “crooks” in branches capture organic matter and provide a medium for its decomposition before it can reach the ground. Essential ingredients in nutrient acquisition and transfer are the associations tropical forest plants form with many species of flora and fauna in the soil; these organisms promote nutrient recycling and soil aeration. Forest trees grow on a mat of fungi [mycorrhizae, see above in Section G10e], which absorb nutrients, phosphorus and other minerals and transfer them to roots. The many soil fungi, bacteria and other detritivores rapidly decompose organic material on the forest floor, and these compounds become part of the soil’s nutrient supply. High-quality soil may have as much as 4000 kilograms of fungi per square meter, and 3000 kilograms of bacteria in the same volume. Trees form associations with nitrogen-fixing bacteria and fungi, which can extract gaseous nitrogen from the air and convert it to compounds usable by plants. In addition, forest soils contain large numbers of arthropods (perhaps 200,000), earthworms (which aerate the soil and increase water infiltration), nematodes, and many other organisms. Forests, by means of their roots, stabilize the soil and thereby reduce runoff into rivers and lakes, and, eventually, into the oceans. Where soils are stable, balanced nutrient relationships between freshwater bodies and land are maintained.
Since many tropical soils are already heavily weathered, they are highly vulnerable to nutrient loss and this is why many tropical soils are difficult arenas for the establishment of agriculture. Only about 20% of tropical soils are suitable for conventional agriculture, and many of these are found in alluvial plains and volcanic highlands. Disruptions of the nutrient cycle by clearing or burning (usually for agriculture or pasture) can be catastrophic for the soil, as nutrients will be rapidly lost and often the soil cannot support the same species as before, only an impoverished flora. A deforested experimental plot in Peru gave only moderate yields even with substantial inputs of fertilizer, but without them, yields dropped to zero. Even one crop depleted the soil too much for a good yield (Buol, 1995). Then, too, the organic materials contained in the nutrient-rich food crops are removed from the fields, not recycled as in a forest, and so are lost to the soil. Only the “refuse” or nonconsumable parts of the plants remain after harvesting to be decomposed and return to the soil as nutrients
Biodiversity simply means the sum of all of the variation in nature – the kind and number of species, their association into units (communities or ecosystems), or, at another level, the genes which are present in all of earth’s organisms and their arrangement, including genetic variation within species. The term includes functional diversity (such as nutrient capture and other ecological functions) as well as simply species diversity. However, most scientific studies of biodiversity have considered only species diversity and how it changes, particularly with regard to latitude. As a rule of thumb, there are many more species (often several times as many) in the tropical latitudes than there are in temperate zones. This is known as the “biodiversity gradient,” and occurs in both northern and southern hemispheres. Moving from higher latitudes to lower, one sees that biodiversity of both plants and animals (and presumably microbes and fungi) increases to a substantial degree. E.O. Wilson (1992, p. 196) illustrates this beautifully in his list of breeding bird species at various latitudes: Greenland, 56; Labrador, 81; New York State, 195; Guatemala, 469; Colombia, 1525. But this generalization must be modified because species richness in the tropics varies with longitude, altitude, soil type, topography, temperature, and rainfall, among other factors.
There are many estimates of the biological diversity (in terms of species richness) of this planet, although relatively few species have been scientifically described – about 1.5 – 1.8 million in all (mostly insects, followed by plants and vertebrates). Even considering the described species alone, we have careful studies of only 1%. And we have very little reliable biodiversity data, not even species descriptions, on other groups, such as most bacteria, fungi, non-vascular plants, and invertebrates. [In this discussion we will exclude bacteria and fungi because so little is known of their taxonomy]. There may well be ten million species (some estimate as many as 30 million) existing today. The tropics have the greatest biodiversity on the planet, and, within the tropics, the areas richest in species are the rainforests. It is estimated that tropical forests, comprising only 6% of the world’s surface area, contain one-half to three-quarters of the earth’s species of plants and animals. This is in part because the groups of organisms which contain the most species (arthropods and flowering plants) are found in high concentrations in tropical forests. These species, although numerous, tend to have smaller geographical ranges than temperate species, and there is considerable endemism (the restriction of a species to a circumscribed area or region). Europe north of the Alps has fifty species of trees; eastern North America, 171; but even a small area of tropical forest may have 100 or 200 species of trees of reasonable size (Whitmore, 1995). In Borneo, 3200 species of plants can be found in 100 hectares of rainforest. In fact, a land area of 0.5 km2 in some tropical forests contains more tree species than does the entire land mass of Europe and North America combined.
The tropical rainforests richest in species are those of Southeast Asia; the poorest, those in Africa. This may be because Africa has mainly seasonal forest with relatively low rainfall and a long history of human intervention. Here there are few palms – only about 100 species compared to 1400 in Australasia – and 403 known species of orchids, compared to more than 5000 in Malesia. Other species – epiphytes and lianas, are comparably fewer in number than in other tropical regions. Within Africa, west-central Africa n forests have the highest biodiversity Malesia (the region of Southeast Asia including Malaysia and the western part of Indonesia), which has many mountains and islands, has at least 30,000 species of plants. Within this area, Borneo and peninsular Malaysia have the greatest variety of species. Here the dominant trees are called dipterocarps, of which Borneo alone has 267 species. Indonesia has more species of flowering plants, amphibians, birds and reptiles than all of Africa. The Mekong river, which passes through Laos and Vietnam, has more than 110 species of snails, and Asia has more than 80 genera of freshwater crabs and many turtles. Sri Lanka, although it has only 750 km2 of forest (less than 5% of its original forest cover), has recently been discovered to have more than 140 species of frogs (Meegaskumbura, et al., 2002). Within the Neotropics, the upper Amazon is the richest in the number of species. Amazonia, which also has an extremely rich flora and fauna, is dominated by leguminous trees of many genera and species. Here one will find 2000 species of bromeliads (the pineapple family) and 837 species of palms. There are 1383 known species of fish in Brazil alone and 456 in Central America (as compared to 192 species in Europe). Colombia, which is not very large, has perhaps the third most diverse forest in the world. It has 1815 bird species, 142 of which are endemic; approximately 700 species of amphibia, 367 of which are found nowhere else; and between 45,000 and 51,000 plants species, one-third of which are endemic. It has 10%- 20% of the worlds orchids.
Old-growth forests have greater biodiversity than younger forests, although differences may not be very considerable under conditions of natural disturbance and recovery. Lowland forests in regions with evenly-distributed rainfall also tend to have greater diversity, while soil fertility appears to have a lesser effect than rainfall levels. In Southeast Asia, diversity declines where soils are rich in magnesium and phosphorus.
Most of the organisms in rainforests are undescribed and unknown. Even today it is quite common to read reports of new mammals found in tropical forests. Last year the black-capped dwarf marmoset, previously unknown and one of the world’s smallest monkeys, was found in the Brazilian Amazon. It is the seventh new monkey species found in Brazil during the past seven years. More undoubtedly await discovery. Horns of an unknown wild ox were found a few years ago in a market in Vietnam. Tiny newly-discovered rodents tumble into buckets sunk into the ground in Madagascar. A spiny mouse species, its only relatives 1000 miles away in the Andes, has been found in Brazil’s Amazon basin. Three new species of mouse lemurs, the world’s smallest primates, have just been discovered in Madagascar. Lawrence Heaney of the Field Museum has found 11 new species of mammals in the Philippines within the past few years; he estimates that the number of known mammal species will rise from the current 4600+ to 8000 (Morell, 1996). Most of these will be found in the tropics. And mammals are perhaps the best-known group (because of their relatively large sizes and our interest in our closer relatives). We know woefully little of other types of organisms in the rainforest, and much of what we do know is simply an artefact of availability. Species which seem to be limited in distribution (i.e., endemic) may appear to be so simply because no one has collected them elsewhere. Other species, which are in fact common, have only been recently described (such as Caryodaphnopsis fosteri, one of the commonest tree species in upper Amazonian Peru, not described until 1986). A major timber tree, “asceite caspi,” the source of most construction wood in this part of Peru, has recently been found to be a previously undescribed species of Caraipa (Gentry, 1992).
All tropical forest regions contain related organisms, for reasons of biogeography. Hundreds of millions of years ago, almost all land was in the form of one large continent, Pangaea. Plants and animals both were widely distributed across this continent, with few geographical barriers to impede their dispersal. More than 200 million years ago, this land mass began to break up into two parts: Laurasia (which would eventually disintegrate into North America, Europe, Asia, Greenland and Iceland) and Gondwanaland (later to break up to become South America, Africa, Australia, Antarctica and India). Most current tropical areas arose from Gondwanaland, and, as the evolution of flowering plants had begun before its breakup, there are many similarities among the plants in tropical forest areas. Today there are more than 300 pantropical (i.e., appearing in all tropical areas) flowering plant genera and almost 60 pantropical families. Moreover, plants at high elevations, wherever they are, resemble each other more than they do plants in the lowlands of their own area. This is what Terborgh (1992a) calls “global parallelism” and indicates the conservative persistence of groups for millions of years.
However, there are also many regional differences within tropical areas because much evolution has occurred since Gondwanaland broke up. Southeast Asia has many conifers, while there are only two species in the New World tropics and one in Africa; dipterocarps are found only in Southeast Asia, and so on (Whitmore, 1995). An interesting case in point is that of Malesia. When one part of Gondwanaland moved north, it collided with a part of southern Laurasia, and these merged to create what is now called Malesia. Since Gondwanaland and Laurasia both had unique sets of flora and fauna, western and eastern Malesia, as their “descendants,” also have very different organisms. This abrupt demarcation between types of organisms in Southeast Asia is known as “Wallace’s line” (after Alfred Russel Wallace, who explored this region in the 19th century), and lies between the island of Lombok in Indonesia and islands farther east, such as Sumba, Timor, The Moluccas, and New Guinea. Dipterocarps, the huge hardwood trees so prominent in Southeast Asian forests, are found from the Malay Peninsula west through Sri Lanka, and fossils of such species have been found in East Africa, but they are not found east of the Wallace Line. Pitcher plants (Nepenthes) are also found from Madagascar to the Malayan peninsula but not farther east. But organisms in Madagascar have relatives in India and South America, so these areas must have been linked for a considerable period of time after the initial breakup of Gondwanaland. The plants of eastern and western Malesia are less sharply demarcated than are the animals, partly because flowering plants arose before the breakup of Pangaea (mentioned above) and partly because they can disperse over long distances, even across large bodies of water. In South America, the emergence of the Andes mountains separated east and west portions of the continent, and today the coastal forests and the Amazon basin have quite different species.
1) Introduction
Why is so much of the flora and fauna of the earth concentrated in only 6% of the land surface? The answer is at least partially historical. For the first three billion years of life on earth, there was little increase in the number of species, but later diversification became explosive. Species and other taxonomic groups arose and disappeared, with an average duration for a species of probably less than 10 million years. But, during the Permian period, about 240 million years ago, between 77% and 96% of the marine fauna became extinct (Wilson, 1989; see also Gibbs, 2001).
Land organisms were also significantly affected, if not to the same extent. In the late Cretaceous period there was another massive extinction (65 million years ago) of approximately 50% of existing species, and other lesser extinction events have followed. These extinctions opened the field for the radiation and development of many new species (Raup, 1988).
2) Explanations for tropical rainforest biodiversity
As mentioned, changes in land masses over geological time have provided the impetus for the evolution of a great variety of taxonomic groups. As the original continent Pangaea split into Laurasia and Gondwana and as these land masses became sundered, organisms were separated from each other, thereafter following different evolutionary paths (but note the conservatism at higher taxonomic level in tropical forests, mentioned above). It is generally thought that one of the major mechanisms of the evolution of new species is natural selection following geographical isolation of groups within the same species. After a long period of time, the separated groups – should they remain apart – may diverge sufficiently to be considered separate species. But the number of different taxonomic groups increases disproportionately in tropical regions. Why?
a. Age of the tropical forest biome: Since tropical rainforests are thought to be the oldest biome on Earth, it is not surprising that they contain the most species. They have had the most time for their inhabitants to diversify, an idea bruited by Alfred Russel Wallace. On the other hand, certain rainforest groups appear to be relatively recent in origin. The tropical tree Inga, a widely dispersed genus, is very prevalent in the Neotropics, and an analysis of the ribosomal and chloroplast DNAs of more than 10% of Inga species indicates that this genus underwent marked speciation more recently than ten – and perhaps only three to six million years ago (Richardson, et al., 2001). The origin of the species in this genus is, therefore, quite recent, and, although the molecular techniques used have considerable uncertainties, they can distinguish between ancient (say, more than 30 million years ago) and recent events. How typical this genus is of tropical plants is not clear, although it may not be highly representative. The diversity of rainforests lies in its genera and families, as well as in species, which indicates that diversity arose quite far back in the past when these families and genera were themselves diversifying.
b. Large Area: Still another answer, also partial, is that the tropics are enormous, spreading across the waistline of the globe. Increasing size provides ample opportunity for geographic separation for groups within a species. In conjunction with this is the fact that this broad band girdling the planet has, overall, fairly constant temperature and humidity.
c. Geographical isolation due to changes in sea level, glaciation, and other factors: Some of the speciation which occurred in Amazonia and other tropical regions may have been due to warmer periods during which the sea level rose sufficiently to isolate fragments of these regions. In these cases, speciation would have been driven by geographical isolation. Of course there is little way of knowing whether or not present-day biogeographical distributions of species are correlated with earlier distributions. Evidence is scarce, as the tropical fossil record is poor. (There are very few rocks, and fossilization rates are low). We also do not know whether glaciations were accompanied by great extinctions in tropical areas. Additionally, most of the evidence we have has come from Amazonia, and what is the case there may not be true of other tropical regions. The Southeast Asian region, for instance, doesn’t demonstrate these areas of high diversity, and was probably continuously forested.
In the case of the Amazon basin, which is traversed by so many river systems, it would be natural to assume that speciation might occur when groups of individuals of the same species are separated by the larger rivers. However, DNA evidence from some species of mammals in Amazonia suggests that this did not occur, although many species appear to have formed after being isolated by the uplifting of the Andes mountains. This could account for some increase in the number of species.
In Malesia, the sea level was much lower during the ice ages, so many land masses now separated by water were linked by land bridges; these later disappeared due to climatic fluctuations. DNA evidence indicates that at least some species diversified when these land areas became isolated. The spineless hedgehog of Southeast Asia falls into distinct groupings on the mainland and on the islands; the striped rabbit of Sumatra is very different in its mitochondrial DNA from its relatives in Laos.
d. Benign character of physical environment: Greater species richness might also be related to the relatively uniform climatic and to some extent, physical, conditions over great areas in the tropics (which is not the case for most temperate areas). Here we are considering temperature, rainfall (and in some cases, soil type). The less stressful environment – warm, humid and predictable – is beneficial for the existence of organisms; more rigorous climates, as the Arctic, contain relatively few individuals and species. Because of the equitable climate, tropical forests provide relatively constant food supplies, so that organisms can specialize on one or a few food sources in the expectation that they will be widely available during the year. For instance, insects are always available in the tropical forest, so that army ants have a constant supply of food. There are no such ants in temperate forests, since in cold weather they would starve for lack of prey. However, the relationships between environment and speciation must be extremely complex, because vegetation influences climate very substantially through its effects on temperature and precipitation. Possibly fewer species are lost by natural selection in regions with a more benign climate, and in which there have been no glaciations. (It is true, though, that for most of the history of the Earth, the climate has been warmer than it is now, but those areas which are presently temperate and frigid climates do not have the diversity which we find in tropical rainforests.)
e. Heterogeneity of biological environment: Evidence suggests that biodiversity has been enhanced by the enormous heterogeneity of the internal rainforest environment. The highly varied kinds of habitats available in tropical areas (differing according to altitude, rainfall, seasonality, soil type, swampiness, etc.) have led to the evolution of a myriad of plants and animals specialized for each of them. Even areas of apparently fairly uniform forest may vary in soil type, topography, or altitude and each area has its own assemblages of plant and animal species and its own ecological webs. Often habitats to which organisms have adapted are very localized, some as small as 5 to 10 km2. Therefore it is not surprising that tropical rainforests should have high diversity, since they contain so many of these specialized habitats. Amazonia, in particular, has great habitat heterogeneity because of its many large river systems, which provide seasonally-flooded forest plains with transitional forests (varzea), palm swamps where the forest is perpetually flooded, lake margins, and, between them, terra firme forests (which can themselves be divided into those on clay soils and those on sandy soils, and the latter of which can be either dry, or waterlogged, after rainfall); limestone outcrops; cloud forests; lakes, rivers, and streams. The river margins and intermittently-flooded areas allow for a variety of stages of forest succession. There are also numerous soil types, which very strongly influence the organisms which live on them and which provide great opportunities for specialist organisms. As an example, many different families of plants in the Amazon basin can live only on a certain type of clay soil almost devoid of phosphorus. These species are endemic and, curiously, many of them have adopted very large thick leaves, although they are not closely related.
Tropical rainforests have a number of layers, or strata, which provide habitats. There are the tall emergent canopy, several mid-layers, an understory, and ground-level herbs and shrubs. Canopy trees are relatively few in number because they are so large and have huge crowns. Ground-dwelling plants are also limited in variety and, surprisingly, are not more diverse than those in temperate forests, perhaps because of the very limited light reaching the forest floor. Under these conditions, survival is difficult for ground-level plants. So most of the diversity of plants in tropical forests lies in the middle strata. The great height of the emergent trees allows much vertical space for other trees and plants, and in this way may promote diversity. Because the sun lies overhead during the entire year, there is a great deal of light available to support the plants in the lower strata, more than twice as much as is available to a temperate forest. And since plants in the tropics don’t suffer from (low) temperature stress, they can devote their energies to growth and reproduction at even very low light intensities. Light intensities in tropical forests are also very patchy and heterogeneous. Thus, plants living under the tall canopy can specialize in exploiting particular light regimes, many of which are not available in temperate forests. And with plant diversity comes animal diversity, since all of these plants provide food and shelter for animals.
Many different sources of food and types of shelter are available in tropical forests. Because of this, organisms with varied requirements (or, put another way, species with different niches – or total roles in the ecosystem) can be accommodated. Where there are many food resources – seeds, fruits, small rodents, reptiles and amphibians, myriads of insects – a highly varied set of animal, plant, bacterial, and fungal species will be there to feed on them. Some species depend upon highly specific types of food sources. Certain birds have bills suited to cracking large seeds or nuts; others, with smaller beaks, make use of small seeds. Species divide up the resources and habitats in such a way as to lessen competition and improve survival. Closely-related animals and plants have evolved slightly different life styles, so that there are species which can utilize every habitat and food source of the forest. More products, more consumers!
Thus, tropical rainforests provide many opportunities for a variety of life styles. The structural complexity of an ecosystem appears to affect the number of species found within it, and is at least part of the biodiversity picture (Nelson, et al., 1991).
f. The prevalence of specialized habitats: The fact that a great number of tropical plants which are restricted to specialized habitats (and are therefore called “habitat specialists”) has given rise to another explanation for tropical biodiversity. Many specialist plants survive only in areas of unusual habitat, which suggests that much of the speciation in the tropics (at least of plants) might have arisen through adaptation for specialized habitats. For instance, the small neotropical plant genus Phryganocydia has only three species, two of which have arisen as apparent offspring of P. corymbosa. The parent species has wind-borne seeds, but the two derivative, swamp-dwelling species have wingless seeds which are dispersed by water. Another derivative group (not yet a separate species) lives in varzea forests and has seeds with partial wings – a representative of incipient speciation. Here selection for specialized habitats is occurring.
g. Energy/productivity levels: First, there are much more energy and productivity in low latitudes and therefore much more biomass (both more individuals and more species) can exist in these regions. There is some evidence that energy levels influence biodiversity; however, the exact nature of the relationship between various forms of energy and the number of species is unclear. High mean annual temperature, primary productivity, and evapotranspiration rates are probably all involved, but we do not know whether or not the higher energy levels found in rainforests are causal factors in the generation of biodiversity. The richness of tropical rainforests in plant life is perhaps due to high levels of solar energy (Nee, 2002). (A brief discussion of issues in f and g is found in Burslem, Garwood, & Thomas, 2001.)
h. Presence of pathogens: Fairly recently it has been proposed that some of the great diversity of tree species in tropical forests might be (at least in part) the result of the activity of pathogens (van der Putten, 2000). In rainforests (and probably in some temperate forests as well), many insects and some other herbivores are adapted to survive on a single species of tree or plant. When a tree becomes infected or infested, other young trees in the vicinity of infected ones will be attacked more severely by pathogens than those farther away from the source of infestation. But if the pathogen can attack only one species of tree, trees of other species can become established near the infected individual without harm. Thus trees of any one species will not be able to survive in close proximity to each, and will become widely dispersed throughout a forest. Under these circumstances, trees of many different species will be present within a small area (unlike many temperate forests, which may consist mainly of one or a few species). Soil pathogens may play a similar role in stimulating the biodiversity of soil flora and fauna.
i. Natural disturbances: There is some evidence that natural disturbances can maintain species diversity, at least among forest plants (and, since they depend upon and are adapted to plants, animals as well). Storms and high winds are common in tropical areas, and frequently lead to considerable damage and the formation of fairly large gaps in forests. When the gap in the forest is small (as when one or a few trees fall), pioneer species will normally enter the gap and flourish, eventually being replaced by climax tree species. If the gap is larger, there may not be sufficient seeds and seedlings of pioneer species to populate the gap, and so the seedlings of other species as well can become established. Thus these gap areas will have a high diversity of plant species compared to undisturbed forest. The formation of large gaps may be essential to the maintenance of diversity in rainforests.
j. Mountains as diversity refuges: Some mountain regions contain clusters of newer species as well as older ones, which has led to the hypothesis that mountains provide stable habitats for species – older species being maintained and new ones forming. There is evidence for tropical bird speciation in mountainous areas of east Africa (greenbuls) and the Andes (spinetails). According to this scenario, mountains act as refuges because they contain many types of habitats in which species can persist by migrating to appropriate altitudes. After organisms move into varied habitats at different altitudes and were thereby separated from each other, speciation occurs.
Tropical regions are large, as well as topographically complex. As mentioned above, the complexity of the rainforest environment allows for considerable specialization of organisms, and the great size of the tropics allows geographic isolation of groups (incipient species) from each other. The stability of tropical areas, in which there are no great fluctuations of temperature or rainfall, allows survival of these separated groups, so that, over time, isolated groups could diverge, eventually becoming new species (speciation by natural selection). Small changes in climate which might provide an impetus for natural selection could be due to natural planetary perturbations, such as “Milankovich cycles,” oscillations in the earth’s orbit. (For further information, see Terborgh, 1992a).
Unfortunately, molecular evidence (as well as almost every other kind of data – taxonomic, pollen analysis) from tropical species is scarce and so it is difficult to choose among hypotheses by which one might explain the exuberant diversity characteristic of the tropical forests. Perhaps all these factors are involved.
The species which surround us now have evolved to their present states during the long history of life on earth, perhaps three billion years or more. These organisms provide services which are essential to survival for humankind and all other occupants of the planet. They maintain the cycles of organic and inorganic substances necessary for life; they regulate climate; they maintain the cycle of rainfall and evaporation, they provide and maintain the nutrients in soils; they are the transformers of the light energy of the sun into chemical energy (sugars), and they provide many other services
Tropical and subtropical areas are home to 170,000 of the approximately 250,000 known species of vascular land plants, and most of these species are in rainforests. Half of these species are in the New World, with at least 60,000 species in the Amazon, 35,000 in Africa (+ 8500 in Madagascar), and 40,000 in Southeast Asia. Forty thousand species inhabit Colombia, Peru, and Ecuador alone on only 2% of the earth’s land surface. In contrast, the entire British Isles have 1380 plant species and Europe only 11,500 (Soepadmo, 1995). In the Atlantic forest of Brazil 427 species of trees were found by Cardoso da Silva and Tabarelli, (2000), but even more – 476 tree species in a 2½ acre plot (the highest recorded number in the world) – were identified in that forest by a group from the New York Botanical Garden (Brooke, 1996). In Malaysian rainforests, there are more than 800 species of trees exist on relatively small plots (Durning, 1989; Condit, et al., 2000); in Borneo, 700 tree species were found on 10 one-hectare plots (Wilson, 1988). Madagascar has 8000 endemic species of plants (Green and Sussman, 1990), out of a total of 10,000. Non-tree species are even more diverse. In Ecuador, there are (or were) more than 10,000 plant species in the lowlands and foothills west of the Andes. These forests are now almost gone; at the Rio Palenque Science Center only one square kilometer of primary forest remains, but that tiny remnant contains 1200 plant species, of which 43 are endemic to this site (Wilson, 1992). In Malesia, there are 16 families of flowering plants; each family contains more than 500 species. The largest group is the orchids, with 6500 species (Soepadmo, 1995). Plant diversity appears greatest in lowland areas with abundant and regular rainfall, while soil fertility seems to be a less important factor.
Tropical plants are very diverse, ranging from very tiny (some orchids, aquatic plants, and saprophytes) to the enormous (Southeast Asian dipterocarps). This phenomenal diversity is due to the moist warm climate, the availability of many and diverse habitats remaining from geological history, and the ability of indigenous plants to adapt, evolve and invade new habitats (see above). Remember, however, that along with the great species diversity of tropical forest plants goes a paucity of individuals of any given species.
1) Trees
Trees are the predominant form of vegetation in rainforests. They comprise 68% of (known) plant species in central Amazonia (Gentry, 1992). Each tropical rainforest region has certain predominant genera of trees. For example, in the New World there are many trees of the Brazil nut family. In Malesia, the forests are dominated by giant dipterocarps, which provide as much as 70% of canopy tree biomass and 80% of the tallest canopy trees. Some families are strictly tropical (as is the case with the nutmegs of Southeast Asia), while others are concentrated in the tropics (such as bananas and ebonies). Although there are some genera in common among the three large rainforest regions, they share almost no species. Each larger region has local variations in composition as well, and many species are found only in a certain circumscribed area of a forest. Such species are known as endemics. The number of tree species increases proportionally with rainfall, so that dry tropical forests are much impoverished compared to wet forests, with approximately threefold fewer species. African forests contain fewer families, genera and species than Southeast Asia or the Neotropics because they are largely seasonal forests with relatively low annual rainfall.
Rainforest trees come in all sizes – very tall emergent canopy trees, medium-sized trees with their canopies in the middle layers, and small, spindly trees (if any) in the lower layers of the forest. Tropical trees have many types of crowns – with a single apical shoot or many, for example, or they may form tufts at the base (bamboos, bananas), or have a single trunk. Roots, which are for anchorage of the tree in the soil and for absorption of water and nutrients, are very important elements in rainforest ecology. Some trees have a single deep tap root; others have “sinker” roots which descend from other roots or from buttresses. Nevertheless, in rainforests, most of the trees have relatively few deep roots; most of the root mass lies in the upper 0.3 m of soil. This is because tropical soils tend to be very shallow, and because rainfall is high, so that nutrients tend not to sink into the soil but to be leached out quickly. Therefore the roots must “snatch” nutrients and water before they run off. Most tree species grow intermittently; this is especially true in the seasonal forests.
There is no real distinction between deciduous and evergreen species in tropical forests. Leaves may fall continuously in some species, others may shed all their crown leaves periodically, or may bud new leaves before the old leaves fall – and there are all intermediate stages. It is not clear what triggers leaf loss in tropical forests, but it is not always related to seasonal changes (if any).
a. Palms: Palms form a very significant proportion of tree species in most tropical forests. In Amazonia, they comprise perhaps 20% of the total number of plant genera. Inventories of palms in Peruvian terra firme forests yielded 23 species within 0.27 hectares (Kahn and de Granville, 1992). Palms seem to be most diverse in terra firme forests, but are also found in wetter forests which are waterlogged intermittently or are permanently flooded (swamp forests). These latter forests provide more difficult living conditions for trees, and there are correspondingly fewer palm species in them than in drier areas. In swampy areas, palms tend to cluster together. In the Huallaga River valley in Peru, up to 207 palm individuals have been found within a single hectare. Mangrove forests, perhaps because of their acidic and anoxic waters, contain few palm species. The same is true of high elevations, but some lowland species manage to survive there, along with species endemic to montane forests. Despite the great diversity of palm species, palm communities in a particular area tend to be dominated by one or only a few species. In Peruvian terra firme forest (see above), two species of palm, Lepidocaryum tessmani and Jessenia bataua, form almost 80% of the palm community.
Palms are not among the tallest trees, but some of them are of substantial height, up to 50 meters. However, most are less than 40 m tall and form a large part of the understory of many rainforests. Palms may be single- or multiple-stemmed, have large or small leaves, be tall, short, climbing, or even prostrate. In the latter forms, the stem “creeps” along the ground. In some species, stilt (aerial) roots are produced from the stems. Other species which live in anoxic environments such as waterlogged or flooded areas may have “pneumatophores,” complexes of rootlets which protrude from vertical roots and absorb oxygen.
Palms are productive and prolific plants, and so are extremely important in rainforest ecosystems because their fruits provide an essential food source for many animals, mammals in particular. Some palms may fruit only every year or two, while others (such as Mauritania flexuosa of the Amazon basin), may produce two to six inflorescences annually. One tree of this species produced 2190 fruits in a single inflorescence (Kahn and de Granville, 1992). A more typical palm might still produce hundreds of fruits at a time. In the Amazon, at least, palms tend to flower at the end of the dry season and fruit during the ensuing rainy season (in the Amazon, the flood season). Most palms are dioecious, that is, produce either male or female flowers, so that it is necessary to maintain large tracts of palm trees in close proximity to each other to ensure that both male- and female-flowered plants will be present.
Because palms often occur in dense tracts, unlike most other rainforest trees, their leaves are a significant source of leaf litter, which enhances the levels of organic matter (and thus, soil fertility) in palm swamps. A hectare of Mauritania flexuosa produces approximately 15.8 tons of dry litter annually, whereas a terra firme forest produces about 7.8 tons per hectare (Kahn and de Granville, 1992). Because of the large amount of organic matter decaying in the waters of these swamps, swamp soils tend to be highly acidic (histosols).
Palms also provide habitats for a number of animals, especially arthropods, and are in turn pollinated by many species of insects – beetles, bees and flies.
Palms are an important component of secondary forests because some have rapidly-germinating seeds and seedlings which are tolerant of sun. Other species appear only when a canopy is established. Many palms are found in deforested areas because, being useful plants, they are often retained when timber trees are removed, and because some of them are resistant to burning. Thus, palms may comprise an unusually high proportion of secondary forest trees.
b. Figs (Ficus sp.): Figs are among the most important plants in rainforests. There are many species (450 in Malesia alone), and as there are always some species in fruit, they provide a major food source for many fruit-eating mammals and birds. Figs can be huge or dwarf; there are figs which are lianas, shrubs, and stranglers. Figs are interesting in that the trees are dioecious, as are most palms. Each fig species is pollinated by a particular species of wasp.
Many fig species are “stranglers,” a life style they share with a varied group of other climbing plants. These species begin life as seeds which have been dropped on the branches of trees, from which the sprouts send roots down to the ground. When the roots reach the soil, the fig plant enlarges in diameter and may send a network of branches around the trunk of its host, which becomes increasingly constricted as its unwanted guest grows in diameter. In this way the fig may eventually kill the tree, which earns it the title “strangler fig.”
c. Dipterocarps: Dipterocarps are generally very large trees which are the dominant vegetation form in forests from Sri Lanka and India across through the Philippines. In Peninsular Malaysia, 30% of the trees with a diameter of 30 cm or more were found to be dipterocarps (Manokaran, 2002). These trees are the major emergent and canopy trees and represent a large proportion of the plant biomass in Southeast Asian primary forests; they are only a very small proportion of secondary forests.
2) Other plants
There are many types of plants other than trees in rainforests. Approximately one-sixth of tropical plant species are epiphytes (plants which are not rooted in the soil), and up to 50% are shrubs and herbs.
a. Lianas: Lianas are woody vine-like plants which can grow quite thick (15 cm in diameter and 70 m in length is common) and which “climb” trees to reach the light, holding on by a variety of devices such as winding twigs, roots, thorns, tendrils, and hooks. They often reach the canopy and may have large crowns. Little is known of the function of lianas in the forest (other than ensuring their own survival), but they do provide protection for animals and stabilize trees against wind and other natural forces, perhaps also regulating the microclimate at the same time. There are many species of lianas, and they constitute approximately 8% of rainforest species. Rattans are examples of lianas with economic importance; so much so that in some areas (Southeast Asia) they have been collected so heavily that they are endangered.
b. Epiphytes: Epiphytes are non-woody plants that have no contact with soil, but grow entirely on trees, which they use as conduits to sunlight. Among them are orchids, some ferns, and bromeliads. They may or may not be partially or entirely parasitic. They provide many habitats for plants and animals – homes for ants, for example. They often have interesting adaptations. Those which climb into the upper canopy have structures which enable them to survive in an environment with high temperatures and low humidity. The leaves of such epiphytes may have thick cuticles and leathery leaves, and form a variety of types of water storage organs (such as leaf bases formed into tiny “water tanks” or nutrient traps). These plants are much more common than one might think; in western Ecuador, epiphytes constitute up to 25% of the plant species in wet forests (Gentry, 1992).
c. Bamboos: Bamboos are hollow-stemmed woody grasses, which may be quite tall, up to 30 meters in height. Bamboos are unusual for tropical rainforest plants because they occur in clumps, with closely-packed trunks and a thick subcanopy. Bamboos flower synchronously after growing asexually for many decades, and this event is followed by a die-off of the stems.
d. Epiphylls: These are lesser-known forest denizens, and include many “primitive” types of plants such as liverworts, mosses, and lichens.
e. Hemi-parasites: This group consists of plants which are partially parasitic, but which also provide some of their own nutrients by photosynthesis. Among these is the mistletoe family, of which there are more than 1100 species in the tropics.
f. Parasites: Some plants are complete parasites, like the spectacular Rafflesia which is a huge plant found on Borneo. Rafflesia and some other parasitic plants have a foul smell, which attracts flies, their pollinators.
g. Herbs: These ground-dwelling and sometimes inconspicuous plants are highly diverse in tropical forests. Gentry (1992) reports that herbs and shrubs constitute as much as 50% of plant diversity in Ecuadorian forests, and almost as many in Southeast Asian forests. Among forest plants which are herbaceous are bananas, gingers, and taro.
3) Flowering and fruiting
Many rainforest trees flower, although the popular perception of these forests is one of unbroken masses of greenery. The fruits of these trees provide essential foods for many forest animals. Mass flowering and fruiting is characteristic of many groups of tropical trees, especially the dipterocarps of Southeast Asia. Flowering generally does not occur annually, as in many temperate species, but at two- to ten-year intervals, with several species flowering more or less simultaneously.
Mass flowering can occur over small or very large areas, and during the flowering period, a great number of seeds are produced and many fruits set. This may be advantageous to the trees, since if many seeds are produced, the likelihood that some will survive and germinate is greatly enhanced. Although most of the seedlings will die, victims of competition for light, some are certain to survive and grow. The massive production of seeds is very important in forest ecology, since seeds are very nutritious and desirable foods, particularly for wild pigs and other mammals, major seed predators. What triggers these flowering events? The cues are not well known, but it is necessary for the tree to be emergent and have its crown in the light prior to the initiation of reproductive activity. A slight drop in temperature may provide such a cue (Ashton, Givnish and Appanah, 1988).
Some other trees and smaller plants produce flowers and fruits continuously and so are extremely important food sources for forest animals. Figs (Ficus) are among the most extensively distributed of these, and are found in both neotropical and Southeast Asian forests.
4) Endemism
Many, if not most, plant species in tropical rainforests are irregularly distributed. If a species is found uniquely in a restricted area, it is called endemic. Endemism is greatest in isolated or unusual habitats – forest ridges, isolated valleys, and islands. In fact, areas of high endemism can be considered as “islands” of specialized species and are particularly important for conservation. Since they contain unique species, many will become extinct if these areas are logged or damaged. Thus, they are “extinction hotspots.” Endemism is very common in tropical rainforests, perhaps for historical reasons, and because there are so many localized habitats available.
5) Habitat specificity
Many tropical plant species are specialists, which is a bit surprising in these areas which appear so abundant, even profligate, in those factors which support life: water, light, warmth. But many tropical plants have adapted to quite specific local conditions. The result is that certain areas have a greater variety of plants than others; among these are coastal Brazil and Ecuador and parts of the Amazon; in Asia, northern Borneo, peninsular Malaysia and New Caledonia. These regions also have a great many endemic species, as many as 20% of the plants. These regions can be considered “evolutionary hotspots”; they are especially significant in that, if rainforests are removed from them, many more species will be lost than if the deforestation occurred elsewhere. Because of their great biodiversity, they are vital as genetic resources for evolution. (See Gentry, 1992, for a review of the above two topics.)
Tropical rainforests are just as rich in animal life as in plants, although the animal biomass is lower than that of plants. Although only a fraction of the total number of species in these forests has been discovered and classified, we know that there are at least twice as many species of mammals and birds in tropical forests as in temperate forests. Assuming that this relationship also holds for insects and invertebrate animals, the tropics contain at least three million species of animals, two-thirds of the world’s total. All of these species evolved in forest habitats and so they are closely adapted to the unique conditions within these forests. Only a few small groups of organisms (conifers, aphids, salamanders) are more numerous in temperate regions than in tropical areas.
Each of the three major tropical forest areas has its unique set of species. Most leaf-eating species live in Africa and Asia, while America has most of the frugivores (fruit-eaters) and insectivores (insect-eaters). More bird and bat species live in the American tropics than in either Southeast Asia or Africa, and the Amazon basin alone contains half the known species of freshwater fish. Three hundred species of mammals are also found there, an enormous number. Borneo, within Malesia, has more than 200 described mammal species, 350 species of birds, 200 species of reptiles and 80 species of amphibia (and probably many more which have not as yet been described), as well as half of the known species of fish (freshwater and saltwater) (Payne, 1995). Many rainforest mammals are arboreal (45% of non flying and non gliding mammals in Borneo, compared to 15% in temperate forests in the eastern United States), and most are nocturnal. Although the vast majority of animal species are insects, we have few clues as to the actual number of insect species in tropical rainforests – certainly the true number will be in the millions. Because of the great diversity of animals, there are relatively few individuals of any particular species, so that members of any one species have to maximize all avenues of reproduction and communication.
Since there are so many tropical species, one can ask how each can survive the intense competition in the rainforest. As is true of plants, animals are “compartmentalized” in their spaces and life styles within the forest, each species existing in its own niche. Animals may specialize in time or in space; they may be nocturnal or diurnal; they may live in the canopy or on the ground, on water or vegetation or on other animals; they may differ in feeding preferences, reproductive patterns, and so on. Scientists conducting a study of neotropical bats on Barro Colorado Island, Panama (Whitmore, 1998), discovered nine bat “guilds,” which were defined by feeding preferences, mainly with regard to the size of their insect prey. In Gabon, Africa, five species of nocturnal lorises segregate themselves by both food preferences and living space. Some live in the canopy (one insect-eating species, one frugivorous one, one insectivorous one) and two species live in the undergrowth (one insectivorous, the other frugivorous). Thus they never have to compete directly for space or food supply. In Ecuador, 74 species of frogs and toads have been found in one area of rainforest, most of which differed in reproductive modes. Some species lay their eggs on water, others on vegetation, in cavities in trees, in depressions in the soil, in foam nests, or on the backs of the females. Yet others have staggered reproductive periods, so that only a few species breed simultaneously, thus avoiding competition among the young for food, space and water (Whitmore, 1998).
a. Pollination: Many animals are essential in the reproductive processes of forest plants. Bats are known to be pollinators of more than 300 plant species (many of which are economically important as timber, fuel, fiber, medicine, or dyes). In Southeast Asia, bats pollinate popular forest fruits such as durian, banana and mango (Payne, 1995). Nectar-feeding birds and insects such as beetles, bees, and wasps are even more important than bats in tropical forests. In Panama, birds pollinate between 40% and 80% of forest plants (Karr, 1994).
Some plants share a pollinator species with other plant species, but as the plants of the different species don’t flower simultaneously, competition for the pollinators’ attentions is avoided (and potential hybridization between different species is inhibited). For example, Costa Rican Heliconia species which share the same bird pollinator produce flowers at different times of the year. Some species of trees are pollinated by only one or a few species of thrips, which will not pollinate other species of trees. Plants also have special means to ensure cross-pollination. Individuals of Oroxylum species in Malesia produce nectar at night, but only in tiny bursts, so as to encourage its bat pollinators to visit a number of plants. Figs are pollinated by fig wasps, the females of which become trapped in the fruit forming from the flower they pollinated. These wasps can travel over great distances – more than 14 kilometers – to pollinate trees.
The animal pollinators and the flowering parts of the plants are well-adapted to each other. Flowers pollinated by birds are bright and have watery nectar; bat-pollinated flowers open at night and have viscous nectar, while those pollinated by bees open in daytime and often have bright colors and footholds. Some flowers have potent odors to attract beetles and flies.
b. Seed dispersal: The seeds of many forest plants are distributed by animals. Most of these plants produce fruits which are desirable as food. The fruits of the many fig species (genus Ficus), for instance, are a major source of food for a number of animals, which through their movements, distribute the seeds throughout the forest. Among these animals are bats, monkeys, tree shrews, squirrels, and larger animals such as deer, elephants, wild cattle, and civet cats, all of which consume the fruits and discard the seeds. The seeds are then left to germinate if they are in favorable conditions. In some cases, seeds must pass through the gut of an animal before they can germinate. The gut enzymes break down the tough seed coat so germination can occur. Rodents eat seeds, but often bury some of them for future consumption, and some of the buried seeds may eventually sprout.
The seeds of more than 26 species of trees are dispersed by fish. On Borneo alone, 13 species of fish feed on fruits and are major seed dispersers. Frugivorous birds are also crucial for seed dispersal of a variety of plants. The Sulawesi red-knobbed hornbill, Aceros cassidix, is an effective seed disperser for many of the plant species which it consumes. The male may bring as many as 265 fruits of a single species to its nest during a visit, and seeds of 33 different species of fruits are used to feed the young. The birds can carry seeds as far as one kilometer to the nesting site. Small seeds are passed through the baby birds’ guts; larger seeds are regurgitated in the vicinity of the nest by the young. Discarded and excreted seeds germinate under the nest, where the forest becomes enriched with the plant species preferred by these birds. Interestingly, though, there are few plants of the genus Ficus underneath hornbill nests, although Ficus fruits form more than 70% of the birds’ diet during the breeding season (Kinnaird, 1998).
Animals may also be essential for the dispersal of spores of mycorrhizal fungi. The feces of some Australian rainforest mammals contain the spores of many species of these fungi. Some of these mammals probably ingest the spores while they are foraging for food, and disperse them accidentally. The very high concentration of sporocarps of certain species of fungi in the guts of certain rodents (rat kangaroos [Potoridae], bandicoots [Peramelidae] and others) suggest that these animals seek out the fungi as part of their diets. The scat of the white-tailed rat (Uromas caudimaculatus), for example, contains the spores of many different fungi (Reddell, Spain, and Hopkins, 1997).
c. Forest maintenance: Vertebrates undoubtedly play a role in forest maintenance. Many primates and ungulates eat leaves and shoots, and their selection of food sources may alter the balance among different species of plants. When these animals eat certain species of plants and avoid others, the ones which are consumed will be suppressed, while those not eaten will gain a competitive advantage. Orangutans feed on the shoots and soft stems of plants such as climbing bamboo, a plant which, by invading forest gaps, can hinder the regeneration of forest trees. Therefore a healthy orangutan population will prevent forest gaps from being overwhelmed by bamboos and other non-tree species, and leave room for tree regeneration and succession. Pigs, in their search for food, turn over the soil, and rodents burrow and damage vegetation, but these soil disturbances enhance seed germination. Even when animals burrow in holes in trees, their waste products may provide nutrients for the trees. In a few cases, animals serve as dinner for plants.
d. Forest “webs”: A web is an association of different species. Animals are important parts of tropical forest webs, which can be highly intricate and involve many species – plants, animals, microorganisms. [A discussion of webs is too complicated to enter into here; consult any general biology or ecology text.] One example of a complex web comprises the Passifloraceae family of the Neotropics and the animals associated with it. This plant group consists of approximately 500 species of small trees and climbers, which are fed upon by butterflies, beetles, bugs and moths. One small area may contain ten to fifteen different Passifloracea species, each with its specific associated pollinators and predators (ten to fifteen separate webs). Why don’t these webs compete? In this case, competition is inhibited by several mechanisms. The time of flowering of each species of plant is staggered, and each is pollinated by an animal (bee, bird, bat, or moth), which is “monogamous” to that plant but which may feed on different Passifloracea species at other times of the year. Thus the animals provide an essential link between and among plant webs and are critical for their functioning. As another example of interlocking webs, the various species of euglossine bees of the Americas pollinate 30 to 50 different plant species; any one species of bee may pollinate as many as 12 plant species. Thus, these insects interconnect many otherwise separate webs involving these plants. Humans have not always recognized the critical nature of such webs. Brazil nut trees, indigenous to South American rainforests, require the agouti, a small rodent, which is a seed predator, and which opens and disperses Brazil nut seeds throughout the forest. These trees also require (among other things), bees as pollinators. Brazil nut plantations fail because the pollinating bees need alternative sources of nectar when Brazil nut trees are not in flower. These sources are present in an intact forest but not in a monoculture plantation.
e. Maintenance of plant diversity: It has long been posited that herbivores might affect the distribution of plant species within rainforests by eating the seedlings of abundant plant species and allowing the seedlings of scarcer species to grow. This has recently been demonstrated by comparing two forests in Mexico, one of which had an intact fauna, the second of which had lost almost half of its large mammals to hunting or to the pet trade. The latter forest, Los Tuxtlas, had a dense understory of seedlings of only a few species, while Montes Azules, the former, had a sparser understory but one with great diversity of plant species. Also, when large mammals were excluded from experimental plots in Montes Azules forest, there was a decline in the number of plant species represented in the understory (Kaiser, 2001b).
Much has been written lately about the catastrophic loss of biodiversity, (species extinction), which has been occurring during the twentieth century. Current extinction rates are estimated as between one thousand and ten thousand times the “natural” rate which existed before humans appeared (Pimentel, et al., 1997, among others). The rate may be even higher, since we know so little about how many species exist on Earth. Unlike past massive extinction events, this one can be traced directly to human activities, which replace natural ecosystems with fewer, species-poor systems, such as pastures, agricultural plots, and tree farms. Human responsibility in species extinction is not only recent (See above, Part I, Section D1). When the Polynesians migrated across the Pacific Ocean several thousand years ago, they initiated the disappearance of 2000 species of birds. When Europeans arrived in Hawaii, they extinguished 10% of the native plants, and more are going. Of the 135 bird species now present in Hawaii, only eleven still have populations large enough to ensure that they will survive (at least in the immediate future); twelve others are rare, and at least twelve more are endangered.
Not knowing how many species exist, by a factor of 10 or more (but, at conservative estimates, at least 3.4-10 million), we cannot estimate how many have already become extinct or are poised on the knife edge of destruction. Very roughly, we can say that 50% of the world’s (recent) species have become extinct or are threatened with extinction. It may be that as many as 10% of them have already disappeared. Kremen, et al., (2000) estimate that 14,000 to 40,000 species are lost annually from tropical forests alone. More than 20,000 species have been listed by organizations as “at risk of extinction” (and these are only conspicuous species). Some estimate that 25% of present species will have vanished by 2010, a rate of extinction comparable to that of the great extinctions of the Cretaceous period. This represents a rate one thousand times the normal one.
Over the past century, approximately 20 mammalian species have disappeared; to recover this number of species would take 200 centuries. Many groups of organisms have lost between 5% and 20% of their species as a consequence of human activities. And we are not at an end. Pimm and Raven (2000) estimate that 12% of all plants and 11% of all bird species, for example, are endangered and will probably become extinct within the medium range. Vitousek (1997) corroborates the 11% value for birds. As natural systems become increasingly fragmented, species will vanish with accelerating frequency.
The greatest loss of biodiversity is occurring in tropical rainforests and coral reefs, because they have such high species diversity and have been subjected to such heavy exploitation pressures. Rainforests have now been eliminated from 50% of the area on which they formerly existed. The coastal forest of Ecuador, the Atlantic forest of Brazil, the forests of Madagascar, many West African forests – all have been virtually destroyed in the past 40 years. If tropical forests continue to be cut at current rates, by the 22nd century all of the rainforests outside of reserves will have been cut or seriously disturbed. Because of this, tropical rainforests everywhere are heading for massive extinctions. The extinction problem is magnified in rainforests, not only because of their generally high levels of biodiversity, but also because many rainforest species are very specialized and have narrow ranges (a high degree of endemism). Therefore, removing even a relatively small portion of forest may destroy many species, and can multiply the extinction rate. The clearing of one mountain ridge-top in Peru led to the disappearance of 90 endemic plant species, for example. Of course, species have always become extinct, over time. Some species in the past have existed for only one or two million years; others have lasted more than ten million years. These extinctions have been due to changes in habitat, climate, and other alterations in environment to which the organisms could not adapt. However, during recent history the extinction rate has escalated greatly. Today, most of the loss of species is due to habitat destruction, abetted by overexploitation – hunting, fishing, and gathering.
Thus, the future for tropical forests is one of increasing fragmentation, extreme reduction in size, and extraordinary extinctions of flora and fauna. Very little of the remaining forest lies within the approximately 5000 reserves which have been set up by national or state governments (4% of land area in Africa, 2% in Latin America, and 6% in Asia). Even in most reserves, protection is only nominal. Often the governments in countries with large areas of tropical forest have no funds to expend upon conservation or protection of reserves and parks. Additionally even those whose job it is to protect reserves are vulnerable to bribery (often because of low salaries) or to importuning or threats from loggers, hunters and collectors. Those areas at most risk for biodiversity loss are in South Asia, The Philippines, the Caribbean, western Ecuador, tropical Andes and Madagascar.
“…the evolutionary impoverishment of the impending extinction spasm, plus the numbers of species involved and the telescoped time scale of the phenomenon, may result in the greatest single setback to life’s abundance and diversity since the first flickerings of life almost 4 billion years ago.” (Myers,
Some years ago Norman Myers coined the term “biodiversity hotspots” to indicate areas of the globe which contain a high degree of biodiversity, and which should get high priority for conservation (Myers, 1988b). A similar concept is Russell Mittermeier’s “major tropical wilderness areas, of which he identified three: the Upper Amazon, the Congo Basin and New Guinea/Melanesia (Mittermeier, 1998). He also uses the term “megadiversity countries” for nations with extremely high biodiversity (Mittermeier, 1988; Mittermeier, et al., 1998; Mittermeier, Myers & Mittermeier, 2000). Recently, Olson, et al., (2001), suggest a finer-tuned method of evaluating terrestrial biodiversity by utilizing a concept based on biogeography, taxonomy, and ecology. These new units they call “ecoregions.” By this means they have subdivided the globe into 14 biomes, eight biogeographic zones, and 867 ecoregions. Under this system, the tropics contain 463 ecoregions. These scientists hope that the finer resolution attained by this method will assist in identification, comparison, and conservation of the most biodiverse regions and those with high endemism. (See also Gentry, 1992.)
Myers’ hotspots now consist of only 10% of their former area. They also contain only about 12% of their former primary vegetation, in comparison with 50% for tropical forests overall (Pimm and Raven, 2000). Nevertheless, they still contain 44% of vascular plant species and 35% of terrestrial vertebrate species (Hardner and Rice, 2002). Unfortunately for their continued existence as areas of great biodiversity, some of them also have high (human) population densities. In 1995, the population density in Myers’ designated 25 hotspots averaged 73 people per km2 (Cincotta, Wisnewski & Engelman, 2000). These areas also contain 44% of known vascular plant species (133,149) and 35% (9,645)of known vertebrate species (Myers, et al., 2000), as well as highly-depleted habitats. Between 1995 and 2000, nineteen of these areas had population growth rates higher than the global average. In 1995, 75 million people lived in the three major tropical wilderness areas indicated by Mittermeier, about eight people per km2. But the population growth rate in these areas is 3.1% per year, more than twice the global average (Cincotta, Wisnewski & Engelmann, 2000). It seems likely that the hotspots will lose 40% of their species if current rates of habitat loss continue, even if for only another decade.
Human population growth has emerged as the driving force behind tropical forest loss and species extinction. Although generalized population growth rates mask many features of population increase, such as non-uniform distribution, size of initial population (larger populations will add more individuals than smaller ones even if the growth rate is the same), and affluence (and therefore ability to consume goods), high growth rates in tropical areas are good indicators of trouble for forests. Many of the areas where human population density is the highest are also those which are biodiversity “hotspots,” areas of great biodiversity. Another factor is urbanization. Urban areas can disturb forests far distant from regions of population concentration – over a radius of 100 km or more. Just as pertinent, affluent people in temperate regions create a demand for tropical products, a demand which provides great impetus for destructive activities in tropical forests. War and civil disturbances (as in the Republic of Congo) are also highly destructive, as people flee from conflicts into the forests, and those seeking food sources strip the forests of edible plants and animals. The rapid growth of human numbers in tropical areas represents trouble for rainforests.
Land-use change is the major factor in biodiversity loss; somewhat less significant are climate change, alterations in nitrogen cycles, the introduction of exotic species into ecosystems, and disruptions of the carbon cycle by alterations in atmospheric carbon dioxide levels.
“The world we want is considerably different from the one we are creating.” (Houghton, 1995)
“Our findings show one compelling reason why this [current development practices not providing human benefits as they should] is the case: our relentless conversion and degradation of remaining natural habitats is eroding overall human welfare for short-term private gain.” (Balmford, et al., 2002).
What should be the goal of conservation in the 21st century? One can think of many, but perhaps the most critical are to maintain vital ecosystem “services” (ecological processes which support life on earth), to preserve a considerable degree of genetic diversity, both of variety of species and within species, and to use the resources of nature sustainably. We must plan our conservation efforts with the following points in mind and so choose the areas for maximum conservation effort with care: (i) the less damage, the greater the number of species the forest can sustain (ii) the more species, the greater the value of the forest for conservation purposes (iii) the larger the original area of the forest, the greater the number of species (iv) the more diverse the topography and soil, the greater the number of species (v) each different topographical area will probably contain endemic species (vi) forests with valuable species (forest products) need protection (vii) in conserving species, a viable population of each species is necessary (viii) a sufficient area for each species is essential (ix) all types of forest must be preserved (lowland, montane, peatswamp, dry, floods, etc.) (Jacobs, 1988).
Tropical rainforests, undisturbed, contain as much diversity as they can under present global conditions. We must remember that any change, therefore, will lead to impoverishment of diversity. All of the above criteria must be accompanied, first of all, by a reduction in human population growth rates, and the sustainable use of our global “life-support systems” – food, water, air, energy, “sink” capacities. Our emphases always have been on human life, but without valuing the nonhuman aspects of our environment – including the other living species – it will be impossible to make improvements in, or, indeed, keep stable, the “human condition.” We must realize that our current development patterns are impossible to sustain and that our highly resilient life-support systems will eventually reach their limits (and some may already have done so). An improvement in human lives and a reduction in poverty must come from the redistribution of goods and services, qualitative rather than quantitative development, population stabilization, and community action. As Goodland (1995) stated, “The growth debate emphasizes the scale of the growing economic subsystem relative to the finite ecosystem.” We must realize that the earth’s ecosystems are, indeed, finite.
• A. Means of conserving tropical rainforests
o 1) Drastic reduction of human population growth
o 2) Improvement of land use
o 3) Improvement of forest management
o 4) Institution of changes in public policies toward forests
o 5) Protection of forest land
o 6) Improvement of agricultural methods and productivity
o 7) Modification of economic and legal systems
o 8) Reduction of social and economic imbalances
o 9) Reduction of anthropogenic effects on forests
o 10) Utilization of indigenous species for resources
o 11) Establishment of national centers for the conservation of threatened and endangered species
o 12) Increase in basic research on tropical rainforests
o 13) Regarding tropical rainforest preservation as an asset in economic calculations
o 14) Institution of economic measures favorable to rainforest preservation
o 15) Reformation of trade policies
o 16) Reduction of poverty, both urban and rural
o 17) “Community-based” conservation
o 18) Promotion of the rights of indigenous peoples
o 19) Increasing international pressures
o 20) Improving environmental education
o 21) Reduction of waste
o 22) Reduction of demand
o 23) Market reform
o 24) Care of secondary forests
o 25) Thinking on a large scale
Orang Yang Untung dalam logging operation…
• THE WINNERS – The big-time “winning interests” in a tropical nation which is cutting down its rainforests are those few groups who obtain large gains from forest removal. These include ranchers in Latin America, loggers in Southeast Asia (producing for the global or domestic market), and – as always – politicians. People needing agricultural land are the main “winners” in Africa, due to that continent’s rapid rate of population growth. The small number of individuals who gain from deforestation gain immensely and protect their interests via concessions from governments, (even if these activities result in a loss to the country or state). Other winners are consumers from the industrialized world, consumers who by deforestation can buy less expensive goods (foods, fibers, and wood for construction materials and furniture).
• THE LOSERS – All denizens of tropical countries lose (except the few mentioned above). Many indigenous people and poor people lose their land and livelihoods when large companies move in to profit from forests. Even people from temperate regions lose, because they must forego the many values of the forests, such as ecosystem services, climate stabilization, and scientific and aesthetic values.
• One comment on the present use of resources in Southeast Asia will suffice for all other tropical areas as well: “The remarkable feature of the present phase is the manner in which this resource frontier has become the property of whole nations, developed for gain under an exploiting and modernizing ethos in an era that, in a longer historical context, looks like one of frenzy.” (Brookfield, et al., 1993
Usia 40
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