Ecological Systems in Hierarchical Perspecitive: Breaks in Community Structure and Other Consequences

Explanation of the patterns of species abundances is important because it may help in understanding mechanisms structuring communities. A general conceptual model is proposed and examined as an alternative to previous propositions, which focused too narrowly on specific mechanisms. According to this model, viewing the structure of an environment as a nested hierarchy of habitat units provides a general mechanism sufficient to account for empirically established regularities in species abundances. Various biotic and abiotic factors can be considered as specific mechanisms sorting biological components into respective levels and compartments of the hierarchical structure of the environment. Because "sizes" of these compartments vary in a predictable way as a function of their position in the hierarchy, so should the abundances. The model may provide a conceptual framework that allows evaluation of the relative contributions of competition, predation, and other biological interactions. The operational and highly simplified version of the model uses spatial or temporal measures of ecological ranges of species to facilitate testing of the general model. The operational model makes three qualitative and quanti- tative predictions: (1) although the species display a continuous gradation of properties, the nested hierarchy of habitat units should lead to clustering of species at distinct levels; (2) generalists should be relatively more successful than specialists; (3) relative abundances of species should be predictable from their position in the hierarchical structure. An analysis of eight communities, including flatworms, aquatic insects, foraminiferans, rodents, and birds, supports these predictions. The clusters, or "breaks" in community structure, appear to be a new and possibly general property. Moreover, quantitative predictions of relative abundances for these communities are in surprisingly good agreement with the actual abundances. A hierarchical structure of the environment appears promising in accounting for some poorly explained community-level phenomena, such as correlation between the species range and abundance, and differences between abundance structures of communities in rigorous and less severe environments. The model is compatible with commonly ob- served as well as irregular patterns of distribution of species abundances, with high local abundance of some species, with differentiation of extinction probabilities, and with scale dependence of ecological phenomena.

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