Most animals exploit a narrower range of food items than they are capable of consuming. In an effort to understand the constraints that may determine diet width, ecologists have organized their evaluations of food selection into a body of theory called optimal foraging theory (Schoener 1986; Begon et al. 1996; Perry and Pianka 1997). Two distinct approaches have developed to address this issue. The first considers that an animal selects among various food or prey items that are distributed in some fashion (e.g., clumped) throughout a generally suitable habitat. The second approach examines how animals discriminate among various patches of habitat that vary in productivity and suitability (Morrison et al. 1992) and can be viewed as an evaluation of habitat selection.
Early efforts to understand diet width relied on evaluating potential food items in terms of cost (search and handling time) and benefit (energy) (MacArthur and Pianka 1966; Charnov 1976). According to the basic assumptions of foraging theory, an animal should have a diet that maximizes energy intake and minimizes time to obtain nourishment (Schoener 1971). As such, food or prey items are ranked by profitability and added to the diet as long as there is an increase in net energy intake. The optimal diet model provides several useful predictions. If handling times (the time needed to pursue, capture, and consume) are typically short, the consumer should be a generalist (use a wide range of foods or prey). On the other hand, if handling times are long, the consumer should specialize on the most profitable foods. Consider prey selection by wolves (Canis lupus) that are usually in close proximity to large ungulates, such as moose. The time and energy required to capture a moose may be considerable. As a result, wolves may specialize on the most profitable or vulnerable segments of the population (juveniles and older animals in poor condition). Optimal foraging theory also predicts that a consumer should have a broader diet in an unproductive environment or during lean periods than in a productive environment or periods of food abundance (Gray 1987).
Although optimal foraging theory has provided an important platform for understanding consumer—prey relationships, the successful application of this theory to understanding diets of free-ranging vertebrates have been limited (Perry and Pianka 1997). The predictions of this theory are based on a series of assumptions that may not be justified (Pierce and Ollason 1987; Perry and Pianka 1997). The first is that the foraging behavior exhibited by present-day animals was favored by natural selection and continues to enhance the fitness of animals; the second is that high fitness is achieved by a high rate of net energy intake (Begon et al. 1996). Numerous field investigations in the last decade have revealed that diet selection is probably the consequence of fairly complex interactions of external, internal, and phylogenetic factors (figure 5.4).
External factors may include prey availability, risk of predation (Lima and Dill 1990) and social interactions (e.g., competition) (Perry and Pianka 1997). Internal factors include animal condition or hunger (McNamara and Houston 1984), learned experiences, age, sex and reproductive state, macro- and micronutrient requirements, and concentration of toxins or distasteful compounds. Phylogenetic factors include morphological constraints (e.g., mouth shape), sensory limitations, and physiological limitations. With such a complex array factors now known to affect foraging decisions, hindsight is quite clear: General models will probably fall short in contributing to our understanding of foraging patterns. However, recent innovations (e.g., using phylogenetic comparative methods) and continued use of manipulative experiments will undoubtedly advance our ability to identify parameters that are influential in complex environments.
C = physical condition P = prédation risk F = food quality I = competition
Figure 5.4 Internal and external factors affecting foraging decisions by a lagomorph (drawing courtesy of D. F. Smith).
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