A Theory of Trophic Strategies: The Evolution of Facultative Specialists

An adaptation of Vandermeer's (1972) revision of niche theory based upon a generalization of the familiar logistic model describes optimal resource utilization patterns for populations at all stages of growth. These are focused into three trophic strategies, two of which are expected to evolve where the relationship between population size and resource abundance (or the rates of resource utilization and provision) is constant and one where this relationship is variable. When population size is constantly less than or equal to that which could be sustained by available resources, obligate specialization and generalization, respectively, are the optimal strategies. When this relationship varies, facultative strategists that could specialize when their population sizes were less than those that could be sustained by available resources and generalize when they were limited by resources would be at a selective advantage. Since such variation characterizes most environments and obligate strategists are limited to constant ones, most consumers should be facultative. Observations of trophic behavior (Murdoch and Oaten 1975) are consistent with this theory, although its evaluation awaits feeding experiments in which the abundance of predators relative to their prey, as well as the frequencies of alternatives, are varied. The results of Cook and Miller (1977) seem to corroborate this theory, but the potential importance of facultative strategists in shaping and maintaining the structure of communities (Glasser 1979, 1982) suggests that it warrants further attention.