Note: The following units are used in this document:   
1 hectare (ha) = 10,000 m²  =  2.47 acres
1 km² = 100 hectares (ha)
1 metric ton = 2000 kilograms
1 gigaton = 109 metric tons
1 petagram (pg) = 1015 grams (gm)

Unless otherwise stated, all units used in this document are metric.

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 (23.5^o N and 23.5^o 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 5^o 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.

1) Prehistoric humans and extinctions

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).

2) Prehistoric human alterations of rainforests

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.)

3) Examples of rainforests seriously altered by human activities

a. Polynesia: The great stone statues of Easter Island are familiar to most of us, but we know very little about this society, which collapsed sometime after 1500 A.D. Deforestation of the island, which had been covered with palms and other trees, was complete by this time and all forest species had become extinct. The island is now barren and treeless. Deforestation led to soil erosion, and, therefore, poor agricultural capacity, and the lack of trees also meant that no boats could be built for fishing. In other regions of Polynesia, forest destruction degraded land so that it became unproductive. This has occurred in New Zealand, Hawaii, the Cook and Society Islands, and elsewhere (Diamond, 1986).

b. Mexico: Central America is famous for the ruins of its splendid civilizations, but is less well known as the point of origin of agriculture in the New World. Pollen of presumably cultivated maize (Zea) has been found in lowland tropical areas (later the home of the Olmec civilization) and is associated with clearings (indicated by charcoal deposits and grass pollens). Such pollens have been dated to 5100 B.C. Pollen from about 5000 B.C. appears to be from domesticated plants. Other species, such as manioc, sunflowers, and cotton, were apparently domesticated at a later time (Pope, et al., 2001). New World agriculture may have begun within its tropical forests.

For many years, people burned forests to provide land for agriculture. This made them more open and altered their species composition to favor species tolerant of fire or species which were able to reproduce quickly under the conditions existing after burning. Because of human activities, grasslands and cultivated fields replaced forests in many areas. Agricultural methods varied over the centuries. Sometimes forests were stripped, probably by slash-and-burn agriculture; at other times practices were more benign (agroforestry; terracing and raised-field methods). During the tenure of the Maya, there were several episodes of rapid population decline, at least partially attributable to deforestation. Around 800 A.D. the classical Mayan civilization collapsed, and population levels decreased substantially. Around 1250, the population again began to increase, and agriculture and the demand for fuel wood and housing intensified. At the same time, soil erosion increased as the inevitable consequence of deforestation, particularly on hillsides. Erosion reached enormous levels (as much as 85 tons of soil per acre annually). By the time the Spanish arrived, the Mayan civilization had already been weakened, at least partially because of environmental factors, and fell easily to the conquistadors (Abrams, 1996; Stevens, 1993).

c. South America: Some anthropologists believe that the flooded forest (varzea) regions of the Amazon basin were the sites of relatively large societies of indigenous peoples. Recently the remains of what might have been transportation canals, agricultural fields, mounds and other constructions have been found in the Beni region (now savanna) of Bolivia, and in Colombian savannas bordering the Caribbean, as well as smaller areas in Ecuador, Peru, Guyana, Surinam, Venezuela, and Central America (Mann, 2000; Bray, 2000). This has led some people to believe that there were dense populations and elaborate cultures in these regions hundreds of years ago, but that they were abandoned between 300 and 600 years ago. Thus, the interspersion of savanna and tropical forest now present would have been at least in part anthropogenic. The argument is that only after humans had made the area favorable for forests - by mulching, enriching the soil, depositing wastes - could the current fragmented forest-savanna environment have arisen. If this hypothesis were validated, it could have important consequences for our future treatment of the Amazon basin. It has led to the argument that there is no necessary incompatibility between human use and biodiversity in the tropics. Others believe that the fragile ecology of the region and its nutrient-poor, acidic soils could not have supported extensive agriculture or large populations, and that the heavy rains and flooding which occur in this area could account for many of the "artefacts," such as large mounds. The contemporary cultures of indigenous peoples, according to this view, represent the best accommodation to the environmental limits of Amazonia.

d. Central America: At La Selva in Costa Rica, remnants of prehistoric maize cultivation have been found - archeological structures, ceramics and lithic objects, which date from approximately 1000 B.C. - 500 A.D. Charcoal, which indicates burning, has also been found in the soils of this forest, and swamp sediments contain maize pollen associated with charcoal which is approximately 700 years old. In Panama, five to-seven thousand-year-old starch grains of manioc, yams, maize and arrowroot have been found on milling stones (Piperno, et al., 2000).

4) European discovery of tropical rainforests

When Alexander the Great conquered the Punjab in India in the fourth century B.C., western man first became conscious of rainforests. For hundreds of years Europe relied on the information brought home by Alexander’s soldiers; this information was codified in the same century by Theophrastus in his Enquiry into plants. Not much was added to this unreliable source until Europeans began to explore beyond their continent in the sixteenth and seventeenth centuries. During the course of these explorations many plants and animals were sent back to Europe, where people struggled to put them into taxonomic order. The inherent bias of these samples led to many misconceptions. Linnaeus, the great Swedish taxonomist who established the modern taxonomic system, for example, believed that tropical regions contained relatively few species and were relatively uniform. Only in the late 18th and in the 19th century, with the voyages of such men as Alexander von Humboldt, Charles Darwin, and Alfred Russel Wallace, were rainforests recognized as the amazingly complex biomes they are in fact. Since then, scientists have endeavored to collect, categorize, and describe the organisms, ecology, and soils of rainforests, and there have been even more assiduous efforts by commercial interests to exploit rainforests and their products - mainly timber, but also wood for fuel (charcoal), rubber, medicinal plants, tree resins, fruits, fibers, and food products.

Approximately one-third of the surface of the earth (3.54 billion hectares) is presently covered with forest. Of this area, about 150 million hectares are tree plantations, and 500 million additional hectares are actively managed by humans (Noble and Dirzo, 1997). Still more forest has felt the impact of human activities such as collecting, hunting, and setting fires.

1) Size and age of the vegetation

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.

2) Distribution of trees in the forest

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.

3) General features of tropical plants

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.

4) Analogous nature of tropical species

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.

5) The three major tropical rainforests

a. Africa: Approximately 9% of Africa’s land mass is covered with rainforests, but on this continent, the forest is almost entirely seasonal and in comparison with the Neotropics and Malesia, rainfall is relatively low, and the trees are fairly low in stature. Much of Africa had been deforested relatively recently, and has become savanna. There are four forest zones - the Guinea forest of recent origin on the west coast, the Nigerian forest, the Cameroon-Gabon forest, and the Congo forest, which is by far the largest. These forests are all species-poor in comparison to those in Asia and the Americas, and many large pantropical families (such as the palms, lianas and epiphytes) are poorly represented. There are many important timber trees but few other economically-valuable species. The African forests are relatively impoverished because they lie in areas which are somewhat dry, with stressful alternating dry and wet seasons. These forests have always been subject to intense human pressure; probably they had all been cut down in the past and so are, in actuality, secondary forests.

b. Indo-Malayan region (South and Southeast Asia): Southeast Asia is highly mountainous and insular, with an intensely humid and hot climate. The forests had not been subjected to much human pressure in the past, since the human population had been relatively low until recently. There are two main areas - the western portion, consisting of Borneo, Malaya, and Indonesia (together forming Malesia), along with southern Thailand, Burma, Cambodia, Vietnam and Laos, and the east, extending from New Guinea to the Solomon Islands and Northeastern Australia. During the glacial period, the sea level was low and much of this area was a continuous land mass. The islands and mainland of western Asia contain a variety of types of forest - lowland, highland, swamp, and peat, with rich volcanic soils in some places (Java, for instance) and infertile soils in areas away from mountain chains and alluvial plains. The lowland forests of Malesia are dominated by dipterocarps, huge canopy trees with great timber potential, which has made these the most threatened forests in the world. Of the sixteen dipterocarp genera, ten are found in Malesia, and 386 of the 550 dipterocarp species are Malesian. In contrast there is but one genus in Africa. In keeping with this, the flora of this area is the richest of any rainforests, with 25,000 -30,000 species, two-thirds of which are found only in lowland forest. There are many useful plants, such as fruit trees, spice trees, and palms.

c. Neotropics: The Neotropical forests are those of Central and South America, with Amazonia being the richest region. As many as 60% of the trees in Amazonia are leguminous (nitrogen-fixing), and there are numerous species of bromeliads (the pineapple/orchid group), epiphytes, and palms. Here, too, are found many useful plants - fruits, medicinal plants, cocoa, vanilla, rubber. There are many varieties of forest here, too, and because of the extensive tributary system associated with the Amazon River, there are both flood (varzea) and highland (terra firme) forests. The varzea forests are seasonally flooded. The climate is generally hot and humid, although there are seasonally drier areas as well.

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 10^o 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.)

1) Introduction

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.

2) Rainforest structure

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.

3) Forest niches

Tropical rainforests vary because of differences in latitude, altitude, soils, or water supply. Because of their immense diversity, they provide many different niches in which organisms can live. A niche consists an organism’s total role and interactions with its ecosystem and environment. Generally speaking, niches will be related to vertical position in the forest, i.e., above the canopy, in the top canopy, in the understory, in the lower part of the forest, or on the ground. Of course, there are horizontal niches as well, and there are many other niches in rivers and lakes, in flooded, cloud, and montane forests. The important point here is not to categorize all possible niches and habitats within forests, but to recognize their immense number and diversity. This is an opportunity for an explosion of species, and so rainforests are generally the most species-rich ecosystems on earth, rivaled only by coral reefs. The forest has many microclimates - places with a variety of temperatures, shady and sunny areas, moister and drier areas, and so on. The trees provide habitats for climbers, epiphytes, vines, insects, birds and arboreal animals. The forest growth cycle provides yet more habitats - differing sizes of gaps in the forest, variations in the forest canopy cover, differing fruiting cycles, and on and on. Another important feature of rainforests is that, since large trees of any one species are not usually close neighbors, any organisms whose habitat includes that tree species must of necessity have a large range. In contrast, trees in the understory are much closer together, and form a continuous layer of foliage which provides habitat for many arboreal animals (and plants). The much greater diversity of animals in rainforests in comparison with temperate forests consists mainly of arboreal animals - particularly insects.

Because of these almost innumerable variations the forest is a mosaic of varied groupings of species, both plant and animal. Most of these species never move from one level of the forest to another, each layer having its own particular characteristics. Many rainforest species are specialists; that is, they can live in only one of the myriads of niches or habitats available in a forest. Often, the organism is so specialized that it lives only in a single area or in patches of unusual habitat.

4) Productivity

Productivity is one of the primary features of any ecosystem. Most of the matter and energy within forests are contained in the "biomass" - the matter of the organisms which comprise the forest. Green plants, which photosynthesize and convert solar energy into chemical energy (i.e., carbohydrates) are known as primary producers (autotrophs), and their photosynthetic capacity, primary production. Secondary producers (heterotrophs) eat the primary producers and convert some of this energy into their own biomass. Decomposers are organisms which decompose organic matter and regulate nutrient cycling within the forest ecosystem.

Net primary productivity (NPP) is the energy converted, for the most part by green plants, from solar energy to chemical energy (primarily in the form of sugars) minus the energy lost through these converters by their respiration (i.e., their life processes). Net primary productivity is affected by temperature, by the availability of water, carbon dioxide (CO₂), and nutrients, and by the efficiency of conversion of light energy to the chemical energy of carbohydrates. Since in the tropics where rainforests are found, water, light, and high temperatures are readily available, and there is a dense concentration of green plants at all levels from the lower stories to the canopy, it is no wonder that these forests have very high levels of productivity.

Net primary productivity appears to be greater in forests containing a variety of plant species than in a homogeneous system such as an agricultural monoculture. This is understandable if we consider that organisms sharing similar habitats also share certain characteristics. For example, if they are emergent canopy trees, the tallest trees in the forest, they will necessarily be light and heat tolerant, able to reduce moisture loss through their leaves, and so forth. Each species of canopy tree will, however, have evolved its own specific mechanisms to deal with these difficult conditions and to photosynthesize efficiently under them. For instance, many canopy trees have thick or waxy leaves to prevent water loss. Trees in the understory or lower canopy must live in partial shade, and at higher humidity and lower temperatures than canopy trees, and they have evolved features which allow them to cope with these conditions. Ground-level vegetation is in almost continual shade and very high humidity, and must be adapted to these conditions. Different species are also adapted to utilize resources in varied ways. Some may be more productive in acidic soils, others in neutral soils; others may flourish at low or high levels of some nutrient or water. Thus, where environmental conditions are detrimental to one species, individuals of other species, with different requirements or tolerances, may be able to survive. Having many species in an ecosystem is therefore a mechanism for fully exploiting the resources available in an area and for buffering the system against unstable conditions or catastrophes.

5) Nutrient cycling

Most tropical forests live "on the edge." The input of nutrients into a tropical rainforest is generally lower than the demand, so the plants must recycle a high percentage of their nutrients in order to survive. This occurs through the decomposition of dead leaves, plants, and animals by soil microbes and the uptake by plants of chemicals released during decomposition. Some additional nutrients may be derived from leaves leached by rainfall; others may drift in from smoke or aerosols. But the rainforest is to a great extent a closed system. It modifies its own supply of resources and thus growing conditions through its influences on the soil and on nutrient cycling. The cycling of nutrients in rainforests is a very important conservation mechanism, as, since nutrients are rapidly taken up from the soil into plants, nutrient leaching by high rainfall is minimized. High biodiversity is beneficial, because different species of plants have varied chemical compositions and nutrient-cycling patterns, and so are able to exploit many opportunities in the environment.

Of course the forest must have ample supplies of oxygen, hydrogen and carbon (supplied from the atmosphere or water or decomposing organisms; normally these are not limiting because of the recycling of these elements). Carbon and nitrogen enter the soil when plants die or shed leaves, and these elements augment the fertility and water-holding capacity of the soil. Calcium, phosphorus, potassium, magnesium and selenium are also essential minerals which are scarce in certain soils, and may become limiting elements (i.e., compounds or elements which are essential to plants, and, when scarce, restrict growth) for rainforest growth and productivity. Micronutrients such as iron, boron, manganese, copper, zinc, molybdenum and chlorine are necessary in tiny quantities, and may also act as limiting elements. In some soils of the tropics (certain oxysols and ultisols), minerals such as calcium, potassium and magnesium, which, since they are derived from the weathering of rock, have been exhausted by leaching during centuries of heavy rainfall, and may be limiting factors. In other tropical soils, either phosphorus or nitrogen may be an important limiting factor. Phosphorus is often a limiting factor in lowland forests, and nitrogen in montane forests with shallow soils. Forests on nitrogen-impoverished ultisols in lowland Amazonia are often filled with leguminous trees, which have nitrogen-fixing microbes associated with their roots. Plant growth is also regulated by a complex of interactions among nutrients. One element may limit the cycling or accumulation of other elements, as, for example, nitrogen accumulation may be limited by the low availability of water or other nutrients. Little is known of these interactions.

Even forests on poor soils may grow very well. The soils on which Brazilian forests grow are generally relatively infertile, but the leaves of the trees accumulate high concentrations of nutrients. Nevertheless, Brazilian forest litterfalls are just as nutrient-rich as those in forests at La Selva, Costa Rica, where soils have much higher nutrient levels (Proctor, 1995).

The roots of most forest plants lie fairly shallow, within a foot of the surface. Because of this, any disruption to the soil surface will have serious consequences for root structure. In rainforests on deeper soils, as in river valleys, many nutrients seep into the soil and nutrient cycles are mainly closed systems, with few nutrients entering from outside. In forests which lie on shallow soils, such as hillsides, there may be some nutrient input from the decomposition and weathering of rock.

6) Species richness

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.

7) Age of vegetation

We know that individual trees belonging to certain temperate species can be very old (cf. sequoias and bristlecone pines), but we know much less about the trees of tropical rainforests. These trees cannot be dated by examining annual rings, as many temperate trees can, since rings do not form or are irregular in the absence of regular annual climatic cycles. By means of carbon dating (dating based on the degree of decay of radioactive carbon in the wood), some rainforest trees have been estimated to be more than 1,400 years old (Chambers, et al., 1998). Although plants grow rapidly in the rainforest environment, because of competition, most will die, and only a few will grow into huge "emergents" (trees which are so tall that they emerge from the canopy). Estimates of time required for an emergent tree to grow from a seedling to 100 cm in diameter range from 90 to 600 years (Chambers, et al., 1998). Some species of trees may grow rapidly when emerging through the canopy and then grow slowly to maximum size. Other species may grow slowly and regularly throughout their life span (confounding the conventional idea of rapidly growing tropical species). Other species may have more complex growth patterns, but we do not know much about them.

8) Forest microclimates

Because the vegetation in rainforests is frequently so tall and dense, a variety of microclimates are available to forest organisms.

a. Light: Canopy trees are exposed to an extremely high light regime. They are not protected at all from the rays of the sun, which are very intense because of the low latitude. In addition, the light regime is fairly constant throughout the year. Below the canopy there is considerably less light, as mid- and low-level vegetation is screened from the sun by the crowns of the canopy trees. Here only plants which are somewhat shade-tolerant will be able to survive. This vegetation is always attempting to reach the light, sometimes by climbing up trees or other plants. Only about 1% of available light reaches the forest floor and, consequently, ground vegetation is limited to shade-hardy species. This is why the ground is not heavily vegetated in many tropical forests.

b. Moisture and vapor pressure: Vapor pressure is another important element in forest microclimates. Since 80% of rainfall reaches the forest floor, moisture available to roots is probably not a limiting factor for growth. However, the vapor pressure (the amount of water vapor in the air), which is produced by the evaporation of rainfall and from transpiration (which is the water released during metabolic processes in the plant), depends upon the degree of air saturation, wind, and air temperature, all of which vary from the canopy to the forest floor. Generally vapor pressure decreases from lower to upper strata of the forest. Vapor pressure is much more variable in the canopy (because of high evaporation rates) than in lower, more protected layers of the forest, and at the forest floor, water vapor flux (variation) is only 25% of that in the canopy. This is partially responsible for so-called "microclimate" levels of vegetation growing on the forest trees. Some plants, for example certain epiphytes in the canopy, can tolerate diurnal (daily) changes in water vapor levels; others, living at lower levels, need a relatively constant degree of saturation.

c. Temperature: A third important factor in forest ecology is temperature. Within the lower strata of the forest the temperature will be lower than in the canopy by 7-10^o C. Temperature affects the rate of chemical reactions, the oxidation of humus in the soil and other processes. High temperatures will also increase the dessication (drying) rate. In the soil the temperature varies little, and is rarely lower than 23^o C. Just above the surface the temperature varies by only 5^o C or so (falling between 22^o C - 27^o C, generally), and therefore activities in the soil and on the forest floor are not usually interrupted by temperature variations.

The interactions among all of these factors, and their influence on important physiological activities, (particularly photosynthesis) determine how and where plants grow.

How are emergent trees able to flourish so luxuriantly under the difficult conditions in the canopy? There is a plethora of mechanisms, such as varying photosynthetic and CO₂ exchange rates in the leaves, numerous water-retention devices. A fig tree, Ficus insipida, has the highest known photosynthetic rate, which enables it to capitalize on the high light intensities available in forest gaps and thereby to fill these gaps rapidly. Canopy trees, which live in a highly variable environment, may produce different types of leaves: during the wet and dry seasons. Some produce thick leaves during the dry season, which, although water-retentive, are inefficient at capturing sunlight under the cloudy conditions which prevail during the rainy season, and are therefore replaced by thinner leaves at that time. A canopy tree, Anacardium excelsum, produces whorls of leaves enclosing dead air space. When water vapor is released from the leaves during photosynthesis, this dead air space retains much of it, moisturizing the surrounding leaves and preventing desiccation. Total leaf area is also variable. Rainforest trees probably maintain maximal leaf area for only about one-third of the year, usually prior to flowering. Leaf area may vary by 50%- 300% during the wet season.

9) Symbiotic interactions

Symbiotic interactions are those in which two or more species interact very closely. These associations may be beneficial or negative to one or the other party, or neutral. There are innumerable instances of symbiotic interactions in rainforests, of which a few are mentioned here.

a. Ants and epiphytes: In the Neotropics some ant species collect the seeds of epiphytes and plant them in ant "gardens" which they fertilize with feces that they collect. The plants in turn provide starch and sugar secretions for the ants. The swollen tubers of some epiphytes (Rubiaceae, for example) provide housing chambers for other ants, while the ants in turn provide excrement and humus for the nourishment of the plant.

b. Ants and Macaranga trees: In Southeast Asian forests, Macaranga trees provide shelter for ants and entice them with starch grains, while the ants repel insect predators and cut off encroaching climbing plants. The small caterpillars of the Lycaenid butterfly Arhopala are tolerated in small numbers because they produce a sugary solution when they are touched by the ants, and so they eat tree leaves in safety.

c. Azteca ants and Cecropia trees: Most trees of the genus Cecropia are associated with ants, which live in their hollow stems and feed on glycogen-rich compounds exuded from organs at the bases of the leaf petioles. The most common ants found are leaf-cutter ants of the genus Azteca, which protect the trees against encroaching vines and against the invasion of other leaf-cutter ants (such as of the genus Atta). Cecropia trees which are home to ants are attacked less frequently than others, even if their leaves are more palatable than other species of Cecropia.

d. Leaf-cutter (attine) ants and fungi: For a long time we have known that certain species of ants have a "partnership" with fungi. One of the most complex associations of this type exists between attine ants and their fungi, associations which apparently have evolved over 50 million years (Currie, 2003). Ant species specialize in particular groups of fungi. Some species of Attini ants cut large numbers of leaves, carrying them long distances to chambers in their underground nests, which may extend over a considerable area and contain more than one thousand chambers. (Other ant species utilize instead vegetation, flowers, insect remains, or discarded matter such as dead grass.) The leaf-cutter ants chew the leaves into pulp and scrape away the leaves’ waxy coating, which is a defense against fungi. The leaves are then used to provide food for a leucocoprinid fungus, which the ants prune and fertilize, and which compliantly produces structures ("gongylidia") which the ants eat. The fungus is grown from a small pellet which the queen ant carries from her mother’s nest to a new nest. The fungi normally contain insecticides as a defense mechanism, but when in the garden, they degrade these toxic compounds, removing them from the fungal tissue eaten by the ants. Recently, it has been realized that the ant-fungal association is even more complex. Yet another parasitic fungus of the genus Escovopsis inhabits fungus gardens and is apparently accidentally carried to the nest by hitching a ride on the ants’ bodies. When the garden is healthy, these parasitic fungi are restrained by an antibiotic exuded from bacilli of the genus Streptomyces, members of which live on certain surfaces of the ants’ bodies. But when the garden is stressed, or if the ants are removed, the Escovopsis fungi explode in numbers and overwhelm the fungal garden. Then the ant population will decline due to lack of food, at least until another garden can be established. It appears that still other compounds produced by the ants may act to inhibit the growth of alien bacteria and fungi which might invade the garden, although the exact roles of these secretions are not yet known (Ariniello, 1999; Currie, 1999, 2003).

e. Plants and butterflies: Certain Passifloraceae plants have odd relationships with Heliconiine butterflies. The butterflies lay their eggs on the tips of the plant shoots which the caterpillars like to eat. When there are no eggs on the shoots, the plant produces yellow nectaries which mimic eggs, or other structures (stipules) which look like young caterpillars. Thus the "occupied" leaves are ignored by butterflies and the plant is spared. However, some butterflies will probe to see whether or not eggs are actually present and thereby circumvent the plants’ defenses.

f. Figs and wasps: Very common are highly specific relationships between a pollinator species and a plant, such as those between figs and their wasp pollinators. Figs are dioecious, that is, they have separate male and female plants. Each species of fig has its own species of pollinating wasp, both sexes of which develop within "gall (male) flowers," from which they escape and then mate inside a fig fruit. The male dies, and the female wasp leaves the fruit, picking up pollen from the male flower within the fig. She then flies to another tree which has young figs, and enters a fruit. If the fig is female, and contains female flowers, pollen on her body will fertilize them; seeds will subsequently form. Or, the fig may, rather, be "male" and have sterile gall flowers, in which she lays an egg. The wasp grub developing from this egg consumes the ovary of the gall flower and develops into an adult wasp. And so the cycle repeats itself.

g. Termites and pitcher plants: Flesh-eating is common among animals, rare among plants. Among the best-known of the carnivorous plants are the pitcher plants, which drown insects and other very small prey in their pitchers. The pitcher, which contains nectar or some sweet substance, generally has a slippery edge, so that insects attracted to the plant’s nectar will slip into the pitcher and not be able to climb out. The Southeast Asian pitcher plant, Nepenthes albomarginata, like some animals, has a distinct preference for what it eats - it likes termites. Thousands of termites, mainly of the genus Hospitalitermes, can be found in the pitchers of this plant in Brunei. The termite lives on fungi and algae, and also likes the rim hairs of pitchers, which attraction leads to the insects’ demise. The lead termites of a column return to the column after they have contacted the hairs (trichromes) of the pitcher, and their message entices the follower termites to the pitcher (Merbach, et al., 2002). In this case the termites, at least the ones which escape, get food, and so do the pitcher plants, although the advantage definitely seems to lie with the plant!

10) Roles of fungi and other microbes

In tropical forests all organisms are dependent to some extent on bacteria and fungi. Some animals such as wood and leaf-eating insects depend on symbiotic gut microbes to digest cellulose in their food supply, while other insects utilize fungi directly as a food source.

a. nutrient cycling:

i) Decomposition: Without microbes, organic matter on the forest floor and in the soil would never decompose. The rate at which these microorganisms decompose dead material is directly responsible for the availability of nutrients for plants. As the humidity and temperatures in rainforests are high, conditions are ideal for rapid microbial decomposition. However, the rates of decomposition will differ according to which microorganisms are present, the character of the organic matter, the physical and chemical environment of the soil and so forth. Apparently microbial and fungal populations are quite sensitive to fluctuations in soil moisture and other disturbances.

ii) Nitrogen fixation: Nitrogen fixation is an essential function of microbes in forests. Without bacteria which are capable of converting gaseous nitrogen into nitrates and nitrites which plants can utilize, rainforest soils would rapidly become depleted of this essential mineral in usable form. Many million tons of nitrogen are converted annually and added to the soil by these organisms. In the many tropical soils which are nutrient-poor, only nitrogen-fixing bacteria allow plants to survive.

b. Tree dispersion and other ecological effects: Pathogenic microbes play a role in preventing "clumping" of trees or plants of a particular species and ensuring their wide dispersal throughout the forest. When plants of one species live close together, they are subject to attacks by pathogenic agents, while if they are more widely dispersed, transmission of disease agents is more difficult. In this way, the presence of microbial and fungal pathogens play a role in structuring the composition of tropical forests, by ensuring that most individuals of a given (tree) species will be fairly widely dispersed (van der Putten, 2000). This has implications for monocrops, such as oil palm, soy beans, and rubber, which are being raised on converted rainforest land in many tropical areas. For example, the large gaps in the forests made to create oil palm forests in Malaysia and Indonesia have contributed to the spread of the root-rot fungus Ganoderma, and another root-rot fungus, Phytophthora cinnamon, spread widely after logging in Australia. Similarly, cutting in Eucalyptus forests in Australia has led to a great increase in severity of outbreaks of this pathogen (Lodge, et al., 1996; Gilbert and Hubbell, 1996). These outbreaks have many ancillary effects, including alterations in forest structure, changes in animal populations (including endangerment of rare species), and decreases in tree density.

c. Food sources: Microbes provide food for many small organisms in forests and also as agents which allow the digestion of otherwise indigestible food sources in the guts of many animals. Fungi are important food sources for some invertebrates such, as ants and their fungus gardens (see above) and beetles. On the island of Sulawesi (Celebes), 40% of the beetles feed on fungi (Lodge, et al., 1996).

d. Regulators of population size: Pathogenic microorganisms have important effects on the population size of any organisms which they infect. For instance, defoliation of green plants is restricted by the attacks of pathogens on insect predators of those plants.

e. Mycorrhizae: Many fungi are present in rainforest soils, and some form close associations with tree roots. These presumably symbiotic associations are known as mycorrhizae. Certainly the associations, in which the fungal hyphae penetrate or ensheathe the root, are very close, although they may not all be beneficial to the plant. Up to 90% of all tree roots are involved in these associations. The fungi colonize roots through the spread of the hyphae or the dispersion of spores. The exact role of mycorrhizae in forest ecology is not clear. Some believe that they are involved in nutrient capture and that much carbon and other minerals (nitrogen and phosphorus, especially) are transferred from the soil to the roots by mycorrhizal associations. In turn, the plant passes manufactured carbon compounds to the fungi, and since the mycorrhizae are themselves eaten by soil organisms, carbon is transferred rapidly from the host tree to the soil ecosystem. Mycorrhizae apparently facilitate the uptake of water by roots and increase the resistance of roots to pathogens. All in all, mycorrhizal associations appear to play key roles in growth, nutrient cycling and primary productivity in tropical rainforests. They also appear to have some control over the structure of the plant community and the course of succession. Where forest is disturbed, plants which do not form mycorrhizal associations will predominate; later, as the fungi invade the area, there will be a succession of plants which tolerate and, later, require these associations. This scenario is complicated by the fact that fungi also undergo succession, and that these changes may play a role in the successional dynamics of plant species (Bever, et al., 2001).

Mycorrhizal fungi also act as social agents, as they interconnect trees through their hyphae. This may mean that trees can transfer carbon among themselves via the fungal mat, so that trees in the shade (and thus less able to photosynthesize) are "subsidized" by well-illuminated trees. It is possible that young trees in shady environments are enabled to survive by this mechanism, at least until they can extend their branches into the canopy. It has been speculated that forests are less competitive than they appear, particularly if the mycorrhizae act to reduce competition for nutrients by equitable distribution (Read, 1977). In one experiment, tree seedlings were found to transfer carbon between species bidirectionally (Simard, et al., 1997). However, Proctor, (1995) warns that little is known about the role of mycorrhizae in tropical forests, and therefore one must be cautious about assessing the roles of these fungi in these ecosystems.

Little is known about the ecology of mycorrhizae, but they appear to have a narrow range of tolerance; some can colonize more than one species of tree; others apparently cannot. Mycorrhizae don’t seem to reform easily in disturbed or logged environments. In one experiment, seedling roots became infected with fungus only when they were in contact with living mycorrhizal-root associations during the early stages of their growth (Lodge, et al., 1996). We don’t even know if all of these associations are essential or beneficial to the tree. However, if these associations are important, as they appear to be, disturbances of forests by logging may contribute to further forest destruction by disrupting them.

11) Reproduction in rainforest flora

As noted above, individuals of any one species tend to be fairly widely dispersed within the forest. Because of this, plants must disperse their pollen over relatively great distances. Large forest trees may be fertilized by pollen from trees more than 140 away (and very frequently, even farther), even though there may be numerous individuals of that species in the intervening area (Chase, et al., 1996). Thus the "breeding population" of a species can be dispersed over a very large area, and may consist of more than 750 individuals, in the case of figs (Nason, Herre & Hamrick, 1998). This has implications for forest preservation planning. More information on reproduction may be found in sections on flowering (see II E3) and animal roles in pollination and seed dispersal (see II F1).

12) Plant-soil interactions

Through the weathering of rocks to soil, essential minerals such as phosphorus, calcium and magnesium are released by lichens, algae, fungi and bacteria. Nutrients are also added to the soil by other inputs from the abiotic environment, as well as by the decomposition of fallen leaves ("litter fall") and plant and animal bodies. These nutrients can then be taken up from the soil by plant roots. The amount of nutrient uptake by plants is dependent upon the quantity of root hairs in the soil. Since tropical soils are typically shallow, many of the root hairs are close to the surface, and are particularly numerous in nutrient-poor soils. Microbes are vital to plant-soil interactions since they affect the availability of nutrients from the soil. (See G10, Role of fungi; also G5, Nutrient cycling).

13) Rainforest stability and disturbance

No ecosystem is stable, and rainforests are no exceptions to this rule. Rainforests are subject to many types of natural disturbances which may further affect population sizes and proportions: fire, flooding, storms, winds, alterations in rainfall, and, in some places, volcanic or other seismic activities. Even where there is no apparent variation in conditions, the populations of many species will fluctuate over time. This is true for plants as well as animals. In Panamanian research plots, the proportion of shrubs in plant populations varies from approximately 10% to 40%, depending upon conditions such as pest levels and the presence of pathogens (Wright, 1996). Insect population booms and busts are well known, and lizard populations in tropical forests appear highly variable. Nevertheless, in undisturbed forests, although there is continual flux in the abundance of any one species, the overall composition of the forest tends to remain virtually the same. In Peninsular Malaysia, several two-hectare plots of rainforest maintained their species compositions for many years with relatively little variation. Over the period of the studies (24-38 years), the researchers found a low percentage of local extinctions (17% or less) or immigration (17% or less), mainly of rare species. For the most part, species which were abundant as adults were also abundant as juveniles. Thus, the structure of the forest is expected to remain similar over time (Manokaran, 2002).

a. Fire: Catastrophic fire is a relatively rare event in most rainforests which are devoid of humans (in contrast to temperate forests). In Amazonia, for example, natural major fires occur in any given area approximately every 440-1550 years. This is so because, where there is a closed canopy forest, fires are inhibited by the humidity, which often reaches 65% and more. Nevertheless, charcoal deposits (indicating previous fires) which predate human occupation have been located in the Amazon forest.

The probability of fires occurring in rainforests is greatly increased by the opening of the canopy by logging. This decreases the relative humidity and increases the ambient temperature. Logging activities also leave behind "slash," the remnants of trees and underbrush, which are dry and much more susceptible to burning than intact forest. A 50% reduction of the canopy cover can increase the average temperature of an area of forest by 10^o C. and may reduce the humidity by 35% or more. While a primary forest will burn under conditions of drought (and even small clearings can reduce humidity sufficiently to facilitate burning), a selectively-logged forest can catch fire after less than a week without rain, and a secondary forest, after eight to ten rainless days. In a forest which has burned, fallen branches and other combustible materials cover the ground, and highly flammable weeds and grasses invade the open areas. All of these add to the combustible fuel in the forest; thus, previously-burned forests are more susceptible to recurring fires at frequent intervals than are intact forests (Cochrane, et al., 1999).

Forest trees are able to survive and resprout following low-intensity fires. The probability of severe tree damage is much greater after repetitive fire, and, with repeated burning, most trees will be killed. Even large trees will be destroyed (up to 98%, compared to 45% of large trees in the initial fire). If fires occur more frequently than every 90 years, many trees will be lost; if more often than every 20 years, the burned area may become deforested entirely. Currently, in some parts of the Amazon, fires recur within less than five years. These secondary fires often occur in forests adjacent to land being burned for agricultural purposes, whereupon the area may become entirely treeless and reduced to scrub or grassland, probably irreversibly (Cochrane, et al., 1999).

b. Wind: In some regions there are severe storms with high winds. The largest and tallest trees generally have the strongest root systems and are able to resist wind activity, while smaller trees may be blown over and killed. Usually the forest composition will not be much altered, since the downed trees and undergrowth will resprout, or be replaced by natural succession (See Section F).

c. Flooding: Flooding is a natural event in some lowland rainforests, as in the varzea forests of the Amazon Basin, where many riparian (riverside) trees and plants spend months partially or wholly submerged during the rainy season. These plants are adapted to the flood cycle and are undamaged by long periods of submersion. Many do not even lose their leaves. Unusual flooding (often caused by human activity), however, can destroy the forest along riverbanks, leading to succession.

d. Geological activity: Long ago, glaciation altered the mix of plants and animals in forested areas by changing the physical environment in which they lived. Today, earthquakes and volcanic activity occur in a number of tropical areas, and can destroy or disrupt considerable areas of forest. Some will eventually become reforested; if the damaged area is very large, perhaps not.

e. Invasions by "exotic" (non-native) organisms: A very serious problem in many forests is the invasion by exotic species introduced mainly by human activities. Neotropical forests have been invaded by Africanized bees, and the fruit fly Drosophila malerkotliana from India has also been introduced into many of these forests. Tropical forests seem to have little resistance to such invasions, and the newly-introduced species frequently supplant or threaten native species, as they have no local enemies to control their populations.

14) Roles of pathogens

Pathogens of plants are extremely important but little-known elements of tropical ecosystems. These pathogens include fungi, bacteria, protozoa, viruses, and worms (nematodes, in particular). They play a variety of roles - regulating populations, restricting plant distribution, reducing/increasing diversity, creating gaps in the canopy, regulating the reproduction and growth of the host, and affecting the availability of food/shelter for animals. In so doing pathogens are instrumental in normal community functioning (as endemic pathogens), but when they undergo population surges – and become epidemic – they can have dramatic and sometimes catastrophic effects on the forest. This occurs when the pathogen takes advantage of susceptibility in the host plant and a conducive environment; such conditions are usually brought about by human activities or extreme climatic conditions. Humans remove natural controls on plant pathogens by disturbing the natural ecosystems during the establishment of agricultural systems, by monocropping, with pollution, road construction, and the introduction of foreign species, which can act as hosts for pathogens, and by the introduction of foreign pathogens. The last-named have been the cause of the most extreme disease episodes (Gilbert and Hubbell, 1996).

When individuals of a single species live close together, pathogens can easily move from one to another, and mortality is high. Light-dependent tea seedlings in the Amazon grouped in open areas have a high mortality rate due to the "witches’ broom" fungus, so that only a few trees survive to become adults. The survivors are widely spaced, and so, by means of a pathogen, the establishment of a monoculture is prevented (Lodge, et al., 1996). Pathogens can also create canopy gaps (by killing certain individuals in an area) and promote successional events and diversity. Interestingly, Percy et al. (2002) found that trees responded to elevated levels of CO₂ and ozone (two greenhouse gases) by increasing production of waxes, which provide leaves with protection from pathogens, as well as other chemical protective agents.

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 (Vit