Natural selection and population density-feedback II. The evolution of functional response curves

The nature of the functional response may be qualitatively understood as follows. Sigmoid responses to one food type may arise, in the presence of alternate foods, as a result of optimal feeding and foraging behavior. Sigmoid curves resulting from this cause I term class A curves. The same curve may also arise in the absence of alternate foods as a result of learning, individual variations in the level of food density at which predators begin feeding, or training effects. The latter I have termed class B curves. At very high food densities, a drop in food intake per predator might occur because of the tendency for predators to take easily found and captured items first and to become more selective when food is very common. Such “dome-shaped” curves have been found in the laboratory but should be rare in nature. Computer simulation of a three trophic-level system, using the phenotypic selection model of Emlen, indicates that natural selection acting on prey should encourage sigmoidality in the predator's class B functional response, at least in disturbed environments. The opposite force arises from selection acting on predators. However, given the magnitudes of growth efficiencies (see Eq. (8), it appears that at least for terrestrial vertebrates, selection on prey species is more important than selection on predators for determining functional responses. Accordingly, prey-predator systems occupying highly variable environments are expected to show more marked type III (class B) curves than systems in more stable areas. Finally, the role of functional response for prey-predator stability is discussed. Class A (alternate food) responses may result in population control for prey in multiple prey systems. Peterman and Pikitch have modeled systems in which type III functional response by predators, in systems where predation varies independently of prey, may lead to double equilibria. This picture is clouded, however, when predator populations are interactive with their food, though double equilibria are still possible (J. M. Emlen, 1984, “Population Biology: The Coevolution of Population Dynamics and Behavior,” Macmillan, New York, in press).