Optimal Foraging for Complementary Resources

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Abstract
Foraging for complementary resources is widespread in nature. The conventional approaches to optimal foraging are inadequate to describe this phenomenon, since they are predicated on perfect substitution among alternative foods in terms of energy yield per unit time. A more qualitative modeling approach derived from economics is proposed to take explicitly into account the degree of resource substitutability for predators requiring balanced diets. The model yields hypotheses that differ markedly from those derived under the assumptions of perfect resource substitution. Prey species cannot be ranked (ordinally or cardinally) in terms of their value to predators, since the benefits derived from consumption of 1 prey type depend on the quantities of complementary resources consumed. Prey selectivity may be affected by the relative abundance of any potential food source. When food resources are complementary predator fitness may be greater on mixed diets than on single species diets. A laboratory protozoan system was manipulated to test aspects of this model of optimal foraging. The predator Stentor coeruleus was fed on pairs of prey species drawn from Euglena gracilis, Chlamydomonas reinhardti and Tetrahymena pyriformis. Stentor preferences for prey were partial rather than absolute, reflecting its needs for a balanced diet. The degree of food selectivity for 1 prey type was dependent on its relative abundance and the relative abundance of alternative prey species. Stentor reproduced better when feeding on mixed diets than on single prey species diets in those cases in which it selected an optimal food bundle, while it reproduced equally well on mixed or single prey species feedings in those cases in which Stentor were indifferent to choices among alternative prey.