A measure of fitness which is an explicit function of feeding efficiency and vulnerability to predation is introduced. Feeding efficiency is a function of feeding and respiration rates and the nature of the food supply. Defining fitness as a function of body size reveals several potential explanations for the evolution of life—histories and for mechanisms of competition in zooplankton populations. Utilizing availability data for Daphnia pulex and evaluating fitness for several temperatures, food—size distributions, and predatory regimes gives results which are very consistent with what is known of the ecology and demography of this species. A large variability in fitness is shown to exist between size classes, very small and very large individuals being particularly prone to food limitation. Thus, for a specified environment, there is an optimal body size at which an organism maximizes its contribution to the persistence of the population in terms of survival and/or reproduction. I suggest that natural selection should lead to maximum reproductive effort at this size at the expense of growth, and present several examples which support this idea. As alternative to the size—efficiency hypothesis, I suggest several mechanisms whereby small species may outcomplete or coexist with larger ones.

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