The question, "Why are there so many species in the tropics?" is one that has intrigued scientists for almost a century. Scores of theories have been put forth to account for the increase in diversity in almost all taxa along a gradient of decreasing latitude. Pianka (1966) reviewed the major theories of the time and lumped them into six categories: the time theory; the theory of climatic stability; the theory of spatial heterogeneity; the competition hypothesis; the predation hypothesis; and the productivity hypothesis.
The time theory assumes that all communities tend to diversify in time, and that older communities therefore have more species than younger ones. Temperate regions are considered to be impoverished due to recent glaciations and other disturbances. However, Deevy (1949) argued that only in cases where barriers to dispersal are pronounced can the ecological time theory be of importance in determining species diversity. Where there are no barriers, species can spread rapidly.
A hypothesis that has been popular is that there are more species in the tropics because there are more ecological niches. Fischer (1960), in his review of the concept, explained as follows: "A given environment provides a variety of possible ways for organisms to make a living, and the organisms themselves greatly multiply the number of these ecologic niches, in which properly adapted species can prosper and procreate." Because the lowland tropics have been least affected by climatic fluctuations in geological history, there has been more time for species to exploit all the available niches.
The theory of climatic stability, similar to the niche theory, hypothesizes that because of the relative constancy of resources, regions with stable climates allow the evolution of finer specializations and adaptations than do areas with more erratic climatic regimes (Klopfer 1959). This results in
"smaller niches" and more species occupying the unit habitat space. However, it is not clear whether climates in the tropics actually are more stable when stability is defined as deviation from an average. A cyclic weather pattern also can be defined as stable if the cycle is regular. Even if stability is defined as deviation from a physical average, stability can be low in the tropics when it is defined as deviation from an average that a species can tolerate.
The theory of spatial heterogeneity assumes that there is a general increase in environmental complexity as one proceeds towards the tropics. The more heterogeneous and complex the physical environment, the more complex and diverse the plant and animal communities supported by that environment tend to be. However, it has been difficult to show that the tropical environment, on the scale in which diversity is usually measured (1 ha or less), is any more complex than the environment at higher latitudes. For a regional scale, it can be argued that there are more habitats in the tropics than at high latitudes (Simpson 1964). For example, Costa Rica has a whole range of habitats from low-altitude tropical to middle-altitude temperate to high-altitude boreal habitats, whereas regions of higher latitude progressively lose some of these habitats. Janzen (1967) explained the reason for this by pointing out that it is the seasonality at high latitudes that causes reduction of habitats on mountains. Species there are better adapted to fluctuating temperatures and can migrate more easily up and down slope. On tropical mountains, species do not have to adapt to seasonal change, and therefore their range is often restricted to a particular narrow band of temperatures that occur at a particular elevation. Temperature barriers along an elevational gradient are therefore greater in the tropics, and mountainsides can be partitioned into more niches, each with its own complement of species.
However, the question of micro-spatial heterogeneity in the tropics vs. higher latitudes remains unanswered. How is micro-spatial heterogeneity defined? If it is defined as the number of species that occupy a space, then the theory becomes circular. There are more species in the tropics because tropical ecosystems have more niches. However, the number of niches in an ecosystem is determined by the number of species in that ecosystem.
The competition hypothesis is based on the idea that competition is the most important factor of evolution in the tropics, whereas natural selection at higher latitudes is controlled mainly by physical factors such as drought and cold (Dobzhansky 1950). Such catastrophic mortality factors are said to be rare in the tropics and thus competition for resources becomes keener and niches become smaller, resulting in a greater opportunity for new species to evolve. However, there is little evidence that catastrophic events, especially droughts, are any less common in the tropics than at higher latitudes.
The predation hypothesis contradicts the competition hypothesis. It claims that there are more predators and/or parasites in the tropics and that these hold down individual prey populations enough to lower the level of competi tion between and among them. The lowered level of competition then allows the addition and co-existence of new intermediate prey type, which in turn support new predators in the system. However, the predation hypothesis does not explain why there are more predators and/or parasites in the tropics to begin with. If there are more predators because there are more prey species, but there are more prey species because there are more predators, then the argument is circular.
The productivity hypothesis states that greater production results in greater diversity (Connell and Orias 1964). The idea is that in regions where productivity of plant species is high, more food is available for herbivores. Species that would not survive in areas of low productivity can survive in the tropics because there is an excess of available energy. The hypothesis rests on the observation that there is a correlation between high primary productivity and high diversity in many ecosystems. However, correlation does not necessarily prove cause and effect. A third unexamined factor such as rich soil could result in both high productivity and high diversity.
According to Rosenzweig (1995), Terborgh (1973) "cut the Gordian knot" of interwoven explanations for high species diversity in the tropics. The tropics, he noted, are richer than any other place because they are more extensive than any other place. He noted that the land area of the northern and southern tropics is roughly double that of any other zone. With so much more territory to explore, there is much more opportunity to harbor species. However, temperate and tropical regions of comparable size differ in diversity (MacArthur 1969). For example, there are thousands of square miles of land in North America, Europe, and Asia that do not have diversity comparable to the tropical region.
Recently, Hubbell (2001) has criticized the niche assembly rules to explain patterns of biodiversity. He proposed that dispersal assemblies, that is, groups of species that have become assembled purely on the basis of which seeds happened to reach a particular location have equal or greater importance than differences in microhabitat (niche) in the observed pattern of species distribution. The idea is that all species at the same trophic level (for example, trees) are ecologically equivalent and that dispersal of seeds from parent trees can account for observed patterns of diversity. However, Condit et al. (2002) found that the dispersal theory alone cannot account for species distributions except in small uniform areas that have been colonized by early and mid-successional species that do not require particular environmental conditions, as do mature forest species.
Local disturbance has been suggested by Connell (1978) as the explanation for high diversity in tropical forests. His hypothesis, called the "intermediate disturbance hypothesis", postulates that maximum diversity occurs in ecosystems that are subjected to intermediate regimes of disturbance. Molino and Sabatier (2001) tested the hypothesis in French Guiana and found that diver sity was higher in areas with light-intensity disturbances such as tree falls and selective cuts. However, to say that these disturbances are the cause of greater diversity in the tropics is to ignore the element of scale (Willis and Whittaker 2002). Most of the species that invade disturbed areas in the American tropics are pioneer species. On small-scale plots (20 X 20 m) such pioneers will increase the diversity if there has been a disturbance such as a tree fall within these plots. However, these species are extremely common and widespread. Therefore, they will increase the diversity of a plot where disturbance occurs, but they will not increase the number of species occurring at the landscape level.
Intermediate levels of disturbance will not increase the diversity of local endemic species in a mature forest because these species have adapted over the millennia to the conditions of a mature forest. Intermediate disturbance will increase diversity due to immigration of common pioneer species. Diversity can be high in a disturbed area following the invasion of pioneer species in a mature or "climax" community (Whittaker 1975). Conservationists are generally more concerned with preserving species endemic to mature communities. Pioneer species are rarely considered endangered species. Logging, shifting cultivation, and other anthropogenic disturbances ensure the survival of pioneer species, often considered to be weeds.
In the end, there is little agreement on the reasons for high diversity in the tropics. Perhaps the Eurocentric perspective of scientists caused them to look at the problem from the wrong viewpoint. Perhaps more progress could have been made on the diversity question had there been a scientist from the tropics who, while traveling in Europe or North America, would have asked, "Why are there so few species at high latitudes?" Then it would be clear from the evidence in Section 2.1 that diversity tends to decrease with increases in stress. Diversity is highest where stress is least, that is, where temperatures are optimum year round, rainfall is adequate, and nutrients are plentiful and well balanced.
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