The response of predators to a choice of prey is influenced by prey frequency, density and visibility, and the type of background against which the predator is searching. These factors affect the likelihood that frequency-dependent predation will maintain polymorphisms. Experiments have been carried out in which human subjects searched a computer monitor screen for 'prey' objects (Ws and commas) against a background of randomly placed blocks of Ms and full stops (points). The frequency and density of the prey were varied, as well as their chance of occurring against the background element on which they are most cryptic. The display moved, so that only part could be seen at a time and the background frequency was also varied between trials. Under the conditions of most of the trials, rare objects were found more than proportionally (selection was anti-apostatic). Niche-choice by prey reduced this effect, as did a reduction in density. The relative visibility of the two prey objects changed with background frequency, as would be expected, but so did the frequency dependence. The data suggest that at one end of the range of background frequency the anti-apostatic effect changes to a pro-apostatic one which would maintain polymorphism. This result indicates that background variability may itself be an important factor in determining whether predators behave in such a way as to develop visible polymorphisms in prey species.Keywords: density, distribution patterns, environmental heterogeneity, frequency-dependent selection, heterogeneity, selection.
IntroductionThe response of predators to a choice of prey has been investigated from the point of view of optimization of predator effort (Greenwood, 1984), the handling of complex signals by the predator (e.g. Bond, 1983) and the ecogenetics of polymorphic prey species (Clarke, 1969;Murdoch & Oaten, 1975). It has been shown by a number of different groups that predators as diverse as birds and molluscs often take proportionally fewer of the rare forms of prey and more of the common forms than are available to them, thus exerting apostatic selection on the prey population (Clarke, 1962). This response may also be density-dependent. The response of predators to a choice of polymorphic prey (reviewed by ) is part of the more general question of how predators handle the problem of searching for prey with a variety of colour, pattern and degree of crypsis against complex backgrounds (reviewed by Endler, 1988). In birds, an apostatic (or pro-apostatic, Greenwood, 1984) response at relatively low densities has been shown to turn into an antiapostatic response as the density increases (Cook & Miller, 1977;Horsley et al., 1979; Allen & Anderson, 67 1984), so that rare types are taken proportionally more frequently, while in mammals the response has been demonstrated to be anti-apostatic (Greenwood et al., 1 984a, b). In addition to density, another factor which may affect the response is the degree of heterogeneity of the background from which the prey are taken. In land mol...