Community structure in ecology is defined by species richness and population abundance, characteristics that some scientists have argued are produced by unrelated, chance processes. But new research by a team of scientists in Germany and the United States has taken chance out of the biodiversity equation. After uncovering similarities in the community structure of tropical forests in three regions of the world, the scientists concluded that species richness and abundance must be governed by related, deterministic processes, including predation and disease.

The latest research, published in the journal Science, is not the first to challenge the notion that chance processes are the major drivers of evolutionary diversification, an idea otherwise known as neutral ecological theory, which was introduced in 2001 by American ecologist Stephen Hubbell. This controversial theory describes speciation and extinction processes as stochastic, or nondeterministic, such that each species progresses and evolves in random fashion in space and time. Thus, under neutral theory, differences between similar species within a community have no bearing on whether or not each species is successful.

Neutral theory is based on what Hubbell described as functional equivalence, the hypothesis that species of similar position within a community experience the same birth, death, dispersal, and speciation rates. Beyond this “symmetry” in vital traits, Hubbell argued, species of the same trophic level may differ in any of a number of ways. Furthermore, as long as symmetry is maintained, complex processes, including competition, may exist in neutral theory. However, important ecological processes, such as predation and disease, are asymmetrical, involving dissimilar species from different trophic levels, and therefore fall beyond the scope of neutral theory.

Neutral theory predicts that evolutionary diversification and population abundance are unrelated across independently evolved ecological communities in different parts of the world. However, as the authors of the Science study found, the number of tree species and the number of individuals in tree families were comparable for forest plots in Africa, tropical America (the Neotropics), and Southeast Asia. Between regions, such as between forest plots in Ecuador and Malaysia, correlations were strongest among higher taxonomic groups, namely families and orders, and were weakest among genera and species. In contrast, within regions, such as among forest plots in the Neotropics alone, there were strong correlations in the number of species in genera. The researchers suspect that the latter phenomenon may be due to common ancestry and migration of plants within regions, resulting in the homogenization of floras and similarities in lower level taxonomic structure.

Comparisons with the fossil record revealed that the representation of families and the number of species within families have been conserved within regions over tens of millions of years. Thus, similarities in family-level species richness have persisted in different regions despite independent speciation and extinction processes. The findings suggest that, while nature can be unpredictable, deterministic processes strongly influence tree diversity and abundance at regional levels in tropical forests. These insights could prove valuable to forest conservation and efforts to protect rainforest biodiversity.

While it seems probable that random processes work in tandem with deterministic processes, the scientists also suggest that the abundance and diversity of plant species in tropical forests could be influenced by the Janzen-Connell effect. According to this hypothesis, predation on seeds and seedlings by insects, herbivores, and pathogens creates openings between different species of plants, thereby providing opportunity for colonization by other species.
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Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.