The last three chapters discussed how regulatory and economic principles apply to experiments on feeding, the allocation of time among various activities, and choice between different ways of getting reward. But a critical test of any approach is how well it can deal with the way animals behave in nature. It is hard to devise a fair test, of course, because in nature, nothing is controlled and observations are rarely comprehensive. Nevertheless, applications of simple optimality principles to problems of diet selection and foraging in natural or close-to-natural conditions have been reasonably successful.

Diet selection and functional response

The study of foraging in natural environments is organized around a few simple quantitative arguments, some discovered independently by several workers. The principle (discussed later in the chapter) that an optimal diet from a set of nutritionally equivalent prey differing in profitability is just the top N-most-profitable types was independently proposed by at least nine different people. Some other principles are borrowed from economic theory. The basic issues are essentially the same as those discussed under the heading of “optimal behavior” in Chapters 8 and 9: how animals choose either between equivalent foods (i.e., perfect substitutes) or between imperfect substitutes. There are also close parallels between many natural situations and some of the reinforcement schedules described earlier and similar processes seem to be involved.

The starting point for theoretical/optimality analysis is the net rate of energy intake, and the beginning assumption is that animals act so as to maximize it. This assumption has fairly straightforward consequences for choice of diet, allocation of time to patches, search for camouflaged (cryptic) prey, and the spatial pattern of search in environments with different food distributions. I will deal with each of these cases in turn.

Functional response

How does the amount a predator takes of a particular prey type depend on its density? The answer to this question is of great interest to ecologists because of its bearing on the stability of predator–prey systems. For example, if individual prey are at greater risk when prey density is high than when it is low (negative feedback), predation will have a stabilizing effect on prey populations; but if risk is less at high densities than at low densities (positive feedback), the stage is set for explosive increases and decreases in prey population size.