FREQUENCY-DEPENDENT SELECTION WITH BACKGROUND HETEROGENEITY

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.