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