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.

A. What do we mean by biodiversity?

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.

B. How much biodiversity is found in tropical rainforests?

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.

D. Why is there so much biodiversity in tropical rainforests?

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 (see below).

[More information will be added to this section in the near future. Here are only a few examples of rainforest plants.]

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).

1) Roles of animals in tropical rainforests

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).

2) Distribution (population size in different areas)

Animals, like plants, are limited in the location and size of their populations by many factors. Competition, predation, availability of water, food and habitat - all will affect where animals are found. Perhaps the most important of these factors is food. Animals will be prevalent where there are adequate sources of nutrition. In tropical rainforests, foods are sporadically distributed throughout the forest, since few species are clustered. Nor is animal prey. Therefore, some animals, such as deer, pigs, and primates, must migrate to satisfy their nutritional needs. Many animals depend on a variety of fruits, which are produced at different times of year in the forest. Some plant species, such as figs, produce fruit all year, and provide a major source of food. When prime food supplies decline, animals may switch to less desirable alternative foods. For instance, when fruits are scarce, some primates will shift to eating palm nuts (capuchin monkeys, with strong jaws), or strangler fig fruits and insects (squirrel monkeys). Some animals, like the tamarins of South America, find a major food resource in plant nectars, and will follow the flowering of their preferred plant species.

Little is known about other factors which regulate animal distribution. Some animals are prevalent on mineral-rich soils. In Africa, many herbivores are found where soils are rich in sodium, phosphorus and magnesium. The locations of rhinoceros and elephant populations may be determined (at least partially) by the availability of these minerals. In Southeast Asian forests, where leaves are often low in sodium, animals are abundant near mineral-rich salt licks. The proboscis monkey of Southeast Asia lives in forests which grow on soils with a high mineral content. Perhaps only plants growing on this type of soil provide adequate minerals and food for their breeding and survival.

Different types of habitats are required by various kinds of animals. Some animals, such as many birds, require undisturbed primary forest. Other animals like tapirs do well in disturbed forests and secondary growth. Orangutans, found only on Borneo, require forest cover, but can survive even in secondary forest. They prefer lowland swampy areas, and are more rare in hardwood (dipterocarp) forests.

3) Kinds of animals - a tiny sample of the astounding biodiversity in tropical rainforests

[Additions will be made to this section in the future.]

a. Invertebrates:

i) Arthropods: Insects are by far the most numerous arthropods, and, indeed, animals, by a huge margin, both in terms of number of species and in numbers of individuals. Erwin and Scott (1980), in a famous study of insects in a Panamanian rainforest, found so many species of insects in a single canopy tree that Erwin (1982) later estimated that there are 30 million species of insects in the world. This estimate is not accepted by everyone, but it is clear that there are many more species of insects (not to mention other organisms) on earth than we have identified, or even suspected, until recently. The majority of the species which Erwin found - 83% of the beetle species, for instance - are indigenous to a particular tree species, a particular area, and in some cases, perhaps, even to an individual tree. Of 1080 beetle species in four different types of lowland rainforest in Brazil, he determined that only 1% of the species were found in all four. This is perhaps not too unexpected, since the plots were fairly far apart, but even in two plots only 50 meters apart, less than 9% of the species were found in both (Erwin, 1988). This is a remarkable degree of endemism.

a. Butterflies: Butterflies are conspicuous residents of tropical forests and there are many species of them. In Brazil, 800 species have been identified within an area of approximately 3 km², and probably there are more than 1500. In comparison, in all of eastern North America there are 440 species; in Europe and North Africa, there are 380 species.

b. Ants: In Peru, a single tree was found to contain 43 species of ants, from 26 genera. This is approximately equal to the total ant fauna of the entire British Isles (Wilson, 1988). The common army ant, Eciton burchelli, of the Neotropics has the curious role of keeping populations of other ant species under control. Ants of this species penetrate the forest in columns of 15 meters width and one or two meters depth, and move up to 200 meters per day. As the column passes along the forest floor, it attacks other ant colonies, as well as other insects which it encounters en route. Besides keeping other species of insects in hand (or in mouth) this species is an essential ingredient in the Neotropical rainforest because it provides food for many birds by "flushing" out insects which flee in advance of the ant column. Some of these birds, among them several species of antbirds (Hylophylax naevioides, the spotted antbird; Gymnopithys leucaspis, the bicolored antbird; and Phaenostictus mcleannani, the ocellated antbird), are obligate (i.e., compulsory) army ant followers. Other birds snatch insects wherever they can, but are roused by army ant columns to follow them. Such are the barred forest falcon (Micrastur ruficollis), the plain brown woodcreeper (Dendrocincla fuliginosa) and the bright-rumped attila (Attila spadiceus). Thus these ants are considered keystone species in these forests, as so many species depend upon them for food and population regulation. Moreover, they are indicators of forest fragmentation. When a forest is cut into fragments, they lose their army ant colonies and, in addition, their avian followers. Such birds are the first to disappear from forest fragments, but will reappear when forest is allowed to regenerate around such fragments (Matlock, 2001).

c. Beetles: Erwin (see above) estimated that most of the insect species he found, perhaps 40%, are beetles. This may be an overestimate, yet further surveys of living canopy beetles yielded many new species; 70 species of beetles have been found in the tendrils of canopy lianas alone. New species continue to be found at undiminishing rates in canopy surveys.

d. Termites: All tropical areas are replete with termites. As mentioned above, termites in their great profusion play a role in global warming by their release of methane and CO₂. These are social insects and live in large nests - underground, hanging in trees, or in large earthen mounds. They have worker, soldier and queen castes. Termites consume wood, and are able to do so because they have protozoans in their guts which themselves are hosts for symbiotic spirochetes (bacteria) which produce cellulase, an enzyme which can dissociate the cellulose in plant cell walls. Many termite nests are composed of digested wood and fecal material which acts as a glue to hold the nest together. Termites carry litter from the forest floor to their nests, and so the nests contain considerable quantities of nutrients. These nests are frequently abandoned, and form patches of high nutrient content. In this way termites may affect nutrient cycling and soil quality. Many tropical woods are extremely hard (that is, the walls of the woody cells are heavily armored), and some scientists speculate that they have evolved in this way in response to the assaults of termites. Some woods also contain toxic compounds which aid in repelling termites.

b. Vertebrates:

i) Fish: Many species of fish inhabit the waterways of tropical rainforests. The Amazon River and its tributaries, because of their size and extent, contain the greatest known diversity of freshwater vertebrates, mainly fish, perhaps 2000 species, of which about 90% are endemic. Some of these we know from the tropical fish trade, while others are known only to local inhabitants, still others are a major food, as well as income, source. Much of the commercial fishing in tropical countries is done in freshwater, rather than marine, environments.

Tropical freshwater ecosystems are closely intertwined with the surrounding forest. Nutrients such as phosphorus and nitrogen normally enter the waterways as rain leaches them from forest soils. They are essential elements for aquatic algae, which provide food for plankton and insects, which are eaten by small fish, which in turn are eaten by larger fish. Changes in land use disrupt this chain by altering the amount of nutrients in the water, and so on up the chain. Surprisingly, fruits and seeds are also extremely important to aquatic systems in some rainforests. There are at least 200 species of fruit and seed-eating fish in the Amazon region, an adaptation unique to this area. Generally, environmental destruction in the Amazon has been the major factor in the decline in fish populations. Removal of forests, even in areas far removed from a fish’s habitat, can have dire consequences.

a..The tambaqui (Colossoma macropomum) is a huge fish which lives in river channels of the Amazon flood plains. A large specimen reaches more than one meter in length, and weighs as much as 25 kilograms. As these are slow-growing animals, they require seven years to attain this size (Salati, et al., 1993). The young are carried into the plains during the rainy season and shelter under "floating meadows" - floating vegetation which forms in sunny patches where the trees do not overhang the water. These fish live on zooplankton, algae and grass seeds when young and as adults seek fruits, nuts and seeds (particularly seeds of the rubber tree) which have fallen into the water. They locate food by scent. At the end of the flood season, the young go into lakes in the floodplain, and the adults migrate to the river channel, where they live on the fat reserves they have accumulated during the flood season. The tambaqui is an important food species, and tasty. This has led to their overexploitation by commercial fishing operations, which set nets across river channels or use gillnets and seines in the lakes to catch juveniles. Now, a fishing boat must travel two or three thousand kilometers to find large tambaqui; the average size of a tambaqui in the market in Manaus, Brazil, is rarely above the legal limit of 55 cm, although an adult tambaqui can be more than one meter in length (Salata, et al., 1993).

ii) Amphibians: Amphibians are in trouble. Everywhere on the globe, amphibians are declining in numbers. In most cases, the cause is probably loss of habitat and unsustainable use. Other factors are pollution and the introduction of competitive or predatory exotic species. In some cases, disease appears to be a culprit (see below, Section G 5e), but the overall picture is little understood for tropical forest species. (For amphibian and reptile declines, see Gibbons, et al., 2000.)

a. Frogs: Frogs are obliged to live near water, since their skins must be kept moist in order for respiration to occur across the epidermis. Most lay bunches of eggs in shallow water; these eggs hatch into tadpoles, which later metamorphose into juvenile frogs. However, many frogs have developed very exotic reproductive mechanisms, and may lay their eggs in foam nests, in holes in trees, in the water-filled cavities of bromeliad leaves; some give birth to live young. [See a link elsewhere on this website for the frogs of Peru.]

i) Poison-dart frogs (Dendrobatidae): Among the most interesting of the tropical frogs are the poison-dart frogs, which produce potent toxins in their skin which can be used for making poison for arrow tips. These frogs are highly-colored, often with brilliant stripes along their backs - red, yellow, blue, green or orange. These frogs are terrestrial, and lay their eggs on land. After the eggs hatch, the female will carry the tadpoles to water, where she feeds them with her own unfertilized eggs - apparently in response to a behavioral cue from the tadpoles!

b. Toads: Toads are terrestrial and have tough, impervious skins, unlike frogs. They are not usually poisonous, but a few, such as the giant marine toad of the Americas, are similar to the poison-dart frogs in that they secrete toxins in their skins.

iii) Reptiles:

a. Lizards: There are many types of lizards in tropical forests, where they play a variety of roles, from vegetarian to carnivore.

i) Iguanas: Iguanas are common in neotropical forests and are among the largest reptiles in the Western Hemisphere. When adult, they can attain a length of six feet or more. Although vegetarians, they look ferocious because of their dorsal spines, and can cause serious injury to a predator with a lash of their tails or a swipe of their long claws. They eat fruits and leaves and are found near water.

ii) Tegu lizards: These are fairly large lizards of the Neotropics. They can reach 4½ feet in length. They are carnivorous, eating small animals and eggs. Some are partially aquatic, like the iguanas.

iii) Geckos: Geckos are ubiquitous creatures of all tropical forests. They live entirely on insects, and are highly territorial. They warn invaders in their space by thumping their dewlaps against the surface to which they are clinging. They also make a surprisingly loud call resembling "GECK-O, GECK-O," hence their name. Their toes have minute protuberances which act as tiny suction cups, enabling them to cling to vertical surfaces and ceilings. The large Tokay gecko often has beautiful blue and yellow spots on its body, and has a formidable call, which varies from one area to another. One, in southern Thailand, has a call which sounds like "FOOT-BALL, FOOT-BALL," and begins calling each evening at exactly the same time - 6:20 p.m.

b. Snakes: Snakes, poisonous or not, are very compatible with tropical forest ecosystems and there are many hundreds of species in the tropics. In the whole of California there are 30 species of snakes, while a few square miles of Peruvian rainforest may contain 75. Moreover, tropical snakes are much more diverse in anatomy, life style and habitat than are temperate snakes. Nothing is known of the conservation status of most tropical snakes, but many species of snakes in temperate climates are endangered. As with other animals, the fragmentation and destruction or degradation of habitat is the major factor in their population decline. For some snakes, the introduction of invasive species into their areas has been devastating. The introduction of the mongoose into Caribbean islands decimated snake populations; rats had a similar effect on boa populations in Mauritius. Many snakes are killed for their skins, for food, because they are thought to impinge negatively on human activities, or because of fear; they are also captured for the pet trade. Snakes, because of their diets, compete with other predator species, mainly raptors and carnivorous mammals, in the many habitats in which they live.

i) Constrictors: Constrictors are the largest of snakes and kill their prey by coiling around the prey animal in a way that rapidly interferes with breathing and blood circulation. They are tree-dwelling and eat all kinds of animals - rodents, other reptiles, and birds. Big constrictors can consume remarkably large prey, even some which is more than half their weight. They may use hunting or ambush tactics. The anacondas in the Neotropics, the pythons in Southeast Asia, Africa, and Australia, and the widely-distributed boa constrictors are the best known of these snakes.

Boa constrictors range widely in a variety of habitats, give birth to live young (viviparity), are mainly terrestrial, and everywhere feed only on vertebrates such as iguanas and other lizards, rodents, and, occasionally, birds. They can wait for prey for days by lying coiled in auspicious places, such as near rodent burrows. It is reported that very large constrictors can eat adult humans (a reticulated python in Indonesia, for one; [Greene, 1997]). Madagascar has boas closely-related to South American boa constrictors, although they are relatively small (less than 2 meters long). They are rainforest snakes and live in the vicinity of water. The Calabar burrowing boa of West African forests, unlike other boas, lays eggs, and may use its tail to protect its head from the parents of the baby mice which it seeks for its prey. Some anacondas are aquatic, and may be quite large; the heaviest known snakes belong to this group.

Pythons generally have teeth on their upper jaw just below the snout, lay eggs (oviparity) and, like boas, are found in a variety of habitats, from the arid parts of Australia to tropical rainforests. The green tree python is specialized for living in the tropical rainforest canopy, but more commonly pythons are terrestrial.

ii) Vipers, adders and pitvipers are found throughout the tropics, except Australia, and are characterized by triangular heads. They have extremely diverse modes of life, habitats, and types of prey. Some are oviparous, others are viviparous; some (pitvipers) remain with their young, others do not. These snakes are highly poisonous (some more than others) and include the fer-de-lance and bushmaster of the Neotropics and the green viper of Southeast Asia. Although many vipers live in rainforests, only one, the Eyelash pitviper of Honduras is on the CITES list of endangered species. Vipers are mostly terrestrial, although some are arboreal. Generally they have a fairly sedentary way of life, and ambush their prey. They kill their prey with potent toxins and, like other snakes, can ingest large prey because of their flexible jaws. They typically bite their prey, release it, and find it by following its scent. Its sense of smell is so keen it can discriminate its poisoned victim from other animals.

Pitvipers are distinguished by pits, which sense infrared radiation, above their eyes. Blind vipers can sense a mouse from some distance away because of these organs. It seems that these organs are not primarily for prey detection, but may have some defensive function. Most vipers also have a tail spine which can be vibrated against vegetation to make a noise as a defensive mechanism. (Note that the rattlesnake is a viper.) Tropical specimens, such as the Malayan pitviper, can be quite large, and reach lengths of 1.5 meters.

iii) The elapid snakes include the coral snakes (American), the cobras (Asian), the mambas (African) and other poisonous snakes which have large, immovable fangs in the front of their mouths and often some smaller teeth behind. Their venoms are neurotoxins, which affect the nervous system of the prey. Death of the victim comes by paralysis of the diaphragm muscles, leading to suffocation. Most elapids are oviparous, but a few are viviparous.

The most famous elapids are the cobras, which are middle-sized to large snakes. They have characteristic expanding "hoods" which make them appear fearsome. The famous "spitting" cobras of Asia and Africa have fangs modified to eject venom toward the eyes of a perceived adversary. They eat vertebrates, other snakes and even fish.

iv) Nonpoisonous, nonconstrictor snakes: There are many varieties of innocuous (to humans) snakes, although they are all carnivores. Among these are the small tree snakes, such as the paradise tree snake of Southeast Asia, a small thin green snake which is entirely arboreal.

c. Crocodilians: Crocodiles, alligators and caimans are all warm-weather reptiles, and many types are found in the rivers of tropical rainforests. The caiman is a small alligator of South America, while crocodiles are found in Africa and Asia. They spend most of their time in the water, since their food supply consists of fish, as well as mammals, birds and reptiles which they catch near or in the water. They are egg-laying reptiles and make their nests in river banks.

Reptiles, following the pathway of amphibians, are now undergoing severe population declines all over the globe. The magnitude of this phenomenon is difficult to determine, but it is substantial. Habitat loss is the most likely factor in a great deal of this decline, with the introduction of exotic species (usually predators, but in some cases other reptilian species which compete for space and food), pollution (fertilizers, herbicides and pesticides, and other contaminants), diseases and parasitism, global warming and unsustainable use (for the pet trade, food, skins and biological research) are all implicated (Gibbons, et al., 2000).

iv) Birds: Approximately 2500 species of birds live in tropical forests. In New World rainforests, a single square kilometer may contain several hundred bird species. The great variety of food and habitats available permits birds to specialize in food sources, type of shelter, and nesting sites, so that many species can live in a small area. Thus there are many species of thrushes, for example, each specializing in a different diet without competing with the others. Many birds are frugivores (fruit-eaters); others are insectivores (insect eaters); still others eat nectar. Some eat insects from leaf, twig, and trunk surfaces; still others search in bark for insects; woodpeckers burrow into the wood for their prey. Hummingbirds live almost exclusively on nectar from plants; in doing so many of these nectar-loving birds transfer pollen from one plant to another. Because of the relatively equable climate, tropical forests provide food and shelter all year around, and so birds need not spend most of their time searching for food sources. This frees them for elaborate mating rituals and displays. Birds are extremely important forest denizens, since they provide many services such as pollination and insect control.

a. Selected birds of tropical forests [See also elsewhere on this website for birds of Peru.]

i) Bird of paradise: Birds of paradise, which live only in New Guinea, have been objects of human interest for many years because of their remarkable adaptations for reproduction. Males are brightly-colored and have elaborate, sometimes elongated, tail feathers which they display for the females in complex mating rituals. The male must first find a suitable display area, and may fight with other males for one. Only a few males occupy such a site at any one time. The blue bird of paradise of the highlands of eastern New Guinea is an extremist - he hangs upside down from a branch and sways back and forth, while displaying his bright iridescent blue plumage and long tail feathers. At the same time he makes a loud whirring noise to attract females, which observe his display from the periphery. The blue bird of paradise’s gorgeous feathers have led not only to its survival - by encouraging mating! - but to its decline. The desire for bird plumes for hats in developed countries during the last century and a continuing demand for feathers for headdresses by local inhabitants of New Guinea have caused its near extermination. During the early 20^th century, eighty thousand skins per year were exported from New Guinea by European settlers. As more and more forest is converted for agriculture, this species has become quite rare, as it requires intact forest as its habitat. Some other species of birds of paradise can adapt more readily to disturbed and cultivated areas and so are less vulnerable to extinction pressures. The use of modern firearms has accelerated the efficiency of hunting of a population already decimated by the loss of habitat.

ii) Bowerbirds: The colorful bowerbirds, which live in New Guinea and Australia, construct remarkable display grounds known as "bowers" out of sticks and grasses. They decorate the bowers with ornaments such as snail shells, fruits, flowers and - close to human habitation - the detritus of human civilization: buttons, bottle tops, shotgun cartridges. The female is courted by the male in his bower, where he displays his magnificent plumage, imitates bird calls and other forest sounds, and may offer the female some fruit.

iii) Kagu: The flightless kagu of New Caledonia has been isolated for eons by continental (or, in this case, "island") drift. Ten million years ago New Caledonia was part of Australia, but it subsequently broke off, isolating all of its species. Many of these species are endemic. The peculiar kagus live and eat on the forest floor, and consume snails (which they crack with their bills), earthworms and other invertebrates. The kagu is a bird of the dense forest and was once common, but the population has been decimated by hunting and the loss of its forest habitat by logging, conversion for agriculture, and nickel mining. In addition, species introduced by humans have damaged the forest floor habitat of the kagu. Pigs disturb the soil; deer eat low-lying vegetation which shelters them; dogs kill them; and rats attack them. They are also in demand by zoos and for pets.

iv) Hornbills (Family Bucerotidae): Hornbills are large and spectacular birds of Southeast Asian primary forests. These birds live in flocks and forage high in the canopy. They have a distinctive bill with a huge "casque" or horny growth on the upper surface, which is hollow and acts as a resonating chamber when the bird calls. The bill also acts as a tool for obtaining fruit, for digging into trees for insects, for catching the small mammals and birds which are part of its diet, and for excavating nest holes in trees. The female lays her single egg in such a hole, subsequently incarcerating herself within it by constructing a wall of mud and saliva. She leaves a small slit through which her mate can feed her for the three months or more required for the egg to hatch and for the chick to become large enough to leave the nest. She also throws her fecal pellets out the slit! During this period, she loses all her feathers. The chick remains with its parents for several years, during which time the adults do not reproduce again.

v) Quetzal (Pharomachrus pavoninus): The quetzal is a bird of the Central American cloud forest at altitudes above 1500 meters. These forests are cool and very humid, which suits the conspicuous quetzal, with its brilliant red-and-green coloring and a three-foot-long tail. It is such a spectacular bird that its image became a very important one to the Maya and Aztecs, to the extent that it was incorporated into the plumed serpent deity Quetzalcoatl. Quetzals perch up high in the canopy, where they eat fruits and insects. However, they nest lower down in dead tree stumps, where they lay two eggs. Unfortunately these birds are now quite rare in accessible forests, where they are still hunted for feathers and for export as pets.

vi) Hoatzin (Opisthocomus hoazini): The hoatzin is a bizarre bird of the Amazon rainforest and was long thought to be related to Archaeopteryx, the reptile-like primitive bird, and therefore more closely related to dinosaurs than other birds. This association was made since the chicks are provided with two functional claws on the wing (as are found in Archaeopteryx fossils). They use them to pull themselves back to their nests after they have swum away from danger. These claws disappear at the time of the molt to the adult plumage. But it now appears that the hoatzin is instead related to the cuckoo and is not at all a "primitive" bird. It lives in trees along rivers and lakes, and is never found far from water. It is a poor flyer because of its diet, which consists almost entirely of the leaves of riparian (riverside) plants and some fruits - at least 52 different species! This necessitates a huge crop, almost 10% of the bird’s body weight, for microbial breakdown of these large quantities of greenery. Since the crop is very bulky, there is less room for flight apparatus and so the hoatzin remains, for the most part, sedentary. Two or three oblong pinkish-colored spotted eggs are laid and incubated by breeding pairs, while nonbreeding individuals act as assistants.

vii) Hummingbirds (Family Trochilidae): Mention should also be made of the magnificent hummingbirds so characteristic of neotropical rainforests, many of which delight us in northern regions during warm weather. This is a very large group in the Neotropics; Colombia has 143 species; Brazil, 90. These are tiny birds, almost the smallest of vertebrates, some of which may weigh less than two grams. Even the largest weigh only 18 grams or so. Hummingbirds have color vision, and are attracted to their food sources by the bright colors of flowers, such as the red and yellow flowers of epiphytes and bromeliads. They detect ultraviolet light reflections from flowers which use this device to captivate pollinators. They have very long bills for "sipping" nectar from flowers, and their bills are adapted to the shape and depth of the types of flowers which provide the preferred food for that hummingbird species. In fact, the hummingbird bill acts as a pumping mechanism for the rapid ingestion of nectar. It cannot waste time, since it is hovering while drinking. Hummingbirds may be able to consume as much as eight times their weight daily, and they must have a high sugar intake (although they may eat some small insects as well) because of their very high metabolic rate. Nevertheless, hummingbirds prefer nectar of a relatively low sugar concentration (around 20%) which prevents competition with bees, which pollinate and obtain nourishment from plants which produce nectar with high sugar concentrations (up to 80%). Because of this behavior, hummingbirds must visit thousands of flowers daily, making them important pollinating animals in rainforests, especially of bromeliads. Hummingbirds are able to hover above flowers because they have an extremely high frequency wing beat (up to 80 strokes per second, although 30 to 50 is more usual), especially when they are trying to escape. They can fly up, down, backwards, forwards, and sideways or even on their backs. Hummingbirds lay two relatively large eggs, which hatch in about two weeks.

viii) Raptors: Mention should be made of the many hawks, kites, eagles, vultures, and other carnivorous birds which abound in the tropics. As an example we can describe the white-bellied fish eagle (Haliaeetus leucogaster), which ranges from the Indian subcontinent to Southeast Asia, southern China, New Guinea and Australia. This bird is a medium-sized eagle, grey and white as an adult, with a black-and-white tail. It lives along coastal areas, although it may fly far inland in search of prey, and usually breeds in large trees (preferably more than 30 meters) on islands. It makes large nests about 1.5 meters in diameter, and, as with other eagles, the nest is reused many times. Two eggs are laid. This eagle feeds only on fish, which it catches by "gaffing" with its talons. It eats its prey "on the wing."

v) Mammals:

a. Primates: Primates are found largely in tropical areas (90% of the approximately 200 species); few are temperate. (Perhaps this accounts for the eternal human seeking for warmth and sunshine!). Brazil and Madagascar together house 40% of all primate species; when one adds Zaire and Indonesia this figure rises to 75%. Brazil alone has 53 species (27% of the total number), of which many are endangered or almost extinct. Of these 53, thirteen are found nowhere else. Madagascar too possesses many (29) species of primates, all of which are lemurs, and most of which are unique. Forty per cent of lemur species are endangered (Mittermeier, 1988).

i) Tamarins: The black-chested mustached tamarin (Saguinus mystax) is a tiny monkey of the Peruvian and Brazilian Amazon. It is arboreal and lives on fruit and insects in mature rainforests, and seems unable to adapt to secondary forest. This monkey has black or dark brown fur all over, except around the nose and mouth, where short tufts of white hair form the "mustaches" of its name. The four species of lion tamarins are found exclusively in the almost-vanished Atlantic rainforest of Brazil and are therefore highly endangered. Amazingly, the black-faced lion tamarin, which has golden hair surrounding a black face, was not discovered until 1990. It lives on fruits and insects. This monkey is monogamous (unlike most primates), be