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III.2 Competition, Neutrality, and Community Organization Jérôme Chave OUTLINE 1. Introduction 2. Species abundance 3. Species co-occurrence 4. Body size 5. Evolution and competition 6. Reconciling historical and mechanistic interpretations Competition has long been thought to play a foremost role in the organization of ecological communities, and this has been a core concept in the building of niche theory. However , many observed patterns in nature are difficult to reconcile with the predictions of niche theory, and they reflect the historical nature of community assembly. The neutral theory of biodiversity has recently been developed to provide an alternative interpretation of patterns in community organization in the absence of competitive difference among coexisting species. GLOSSARY Hutchinsonian ratio. Body size ratio of the larger species over the smaller species in a pair of species; niche theory predicts that co-occurring species should have larger body size ratio than expected by chance neutrality. Assumption of equivalence in individuals’ prospects of reproduction or of death, irrespective of the species they belong to phylogenetic overdispersion and clustering. The tendency of species to be on average more or less (respectively ) evolutionarily related in a sample than in the larger species pool species abundance distribution F(n). Number of species with exactly n individuals in a sample 1. INTRODUCTION Ecological communities are complex assemblages of organisms shaped by environmental constraints and interacting through a variety of ecological processes but also reflecting historical contingencies. Understanding these processes has been a central goal of animal and plant ecology for almost a century. Early on, researchers put forward the idea that communities were tightly organized associations of species, with sharp boundaries, and that they were amenable to classification, just as species can be classified taxonomically . This idea may be related to the famous work of the Russian experimentalist Georgyi Gause. He and others developed research projects on simple species assemblages easily amenable to experiments, such as yeast and paramecia (Gause, 1934). Because these systems included only a few species, typically two, and controlled environmental conditions, it was possible to find the theoretical conditions under which species may coexist stably in association. Nicely, these experiments could be reframed into the mathematical theory developed by the Italian mathematician Vito Volterra (1931). The major finding of both theory and experiments was that, of two species with identical ecological requirements and competing for limited resources in a stable environment, one will eventually exclude the other. This result later developed into a fundamental principle, called the principle of competitive exclusion. The view that ecological communities should be the result of tight species association did not remain uncriticized over these years. In 1926, the plant ecologist and taxonomist Henry Gleason, for instance, suggested that species do not, as a rule, live their lives in tight associations, a view that was echoed in 1935 by one of the most prominent British ecologists of the first half of the twentieth century, Sir Arthur Tansley. Some authors even suggested the extreme view that species interactions play no role in community organization (Andrewartha and Birch, 1954). This longstanding debate in ecology was admirably synthesized by Robert MacArthur (1972). Although he acknowledged the role of history in shaping species assemblages, he noted ‘‘unravelling the history of a phenomenon has always appealed to some people and describing the machinery of the phenomenon to others.’’ He further suggested that the ecologist tends to be ‘‘machinery oriented,’’ whereas the biogeographer tends to be ‘‘history oriented.’’ To MacArthur, useful patterns of species diversity are shaped by repeatable phenomena, not by chance events. An important issue here is: How do experimental findings generalize to natural communities and to more than a couple of species? This question was most clearly addressed in 1959 by the American limnologist G. Evelyn Hutchinson, one of the most influential ecologists of the twentieth century. Central to the understanding of species coexistence is the concept of the ecological niche of a species, the biological equipment of a species in relation to competition and to environmental conditions. For animals, niches may be defined in terms of the food of a species, as developed early on by Charles Elton. For plants, an operational definition of the niche is more problematical, even though much research has been devoted to quantifying plant lifehistory strategies. Although the concept of the niche was developed historically earlier, it was Hutchinson and his students, including MacArthur, who proposed an operational definition of this concept to most ecologists (for...


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