Size Bimodality in Monospecific Populations: A Critical Review of Potential Mechanisms

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Abstract
Changes in size distributions, including changes that produce bimodality, result from the interaction of several characteristics of the individuals composing the population with factors that influence those characteristics. The critical factors can be classified as (1) the initial distribution of individual sizes; (2) the distribution of growth rates among the individuals; (3) the size and time dependence of the growth rate of each individual; and (4) mortality that may affect each size class differently. Each of these factors may be influenced by an individual''s genetic makeup, as well as by abiotic and biotic environmental effects. We classify the biological mechanisms that may affect these factors in a two-way table as either "inherent" (i.e., genetic) or "imposed" and as either "noninteractive" or "interactive", depending on whether they require interactions among the individual organisms in order to be expressed. We review examples of each of these mechanisms from the population biology of fish and terrestrial plants. In many cases the mathematical models that have been developed independently to explain bimodality in these two groups of organisms show remarkable convergence and strengthen our generalizations about mechanisms affecting size distributions. The class of imposed, interactive mechanisms, which includes competition, provides many of the examples of bimodality that we find most interesting ecologically. However, these mechanisms are probably the least likely of all four classes to produce the bimodalities found in nature. Imposed, noninteractive mechanisms resulting from the effect of temporal and spatial heterogeneity on all four of the factors influencing size distributions are far more likely than interactive mechanisms to explain size bimodalities in nature.