One of the most perplexing issues in ecology is the coexistence of large numbers of species within communities across a range of spatial scales. The rainforests of South America and southeast Asia, for instance, are home to a high diversity of plant species, where over 200 species can be found in 1 ha of forest. On coral reefs, species richness is high at both large spatial scales (e.g., over 1500 species of fish and 400 corals on the Great Barrier Reef) and at the scale of small patches of reefs (e.g., over 200 species of fish on a reef only 50 m wide). The coexistence of large numbers of species is particularly puzzling given that many coexisting species seem to perform the same ecological roles within plant and animal assemblages. If so many species are doing the same thing, how can they all coexist in a world where resources are often limited and there is competition for these resources? To fully appreciate how such large numbers of species can coexist, we need to have a thorough understanding of the dynamics of their populations and, in particular, of how individuals within a species' population interact with each other, and with populations of other species. Indeed, the coexistence of individuals within a population is intriguing, given that interactions within species can in fact be stronger than interactions among the populations of different species. The ecologist George E. Hutchinson posed the Paradox of the Plankton in 1961, questioning how so many species of marine plankton could coexist in a relatively homogenous medium with limited food, seemingly in violation of Gause's rules of competitive exclusion. The research that Hutchinson stimulated over the next 45 years has given us a broad understanding of how organisms can coexist.
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