Analysing Experiments on Frequency-Dependent Selection by Predators

Abstract

(1) Predators sometimes eat disproportionately more of the more abundant forms of their prey, thus promoting diversity in the prey population. (2) Such selection may be investigated by exposing a series of prey populations with different relative frequencies of the various types to a series of predators. (3) We use a simple model to describe the outcome of such a series of trials, relating the relative frequencies of the various prey types eaten to the relative frequencies available. Frequency-independent and frequency-dependent components of selection are separated in the model. (4) Previous criticism of the model does not appear to be valid. An alternative model due to Manly seems a priori unrealistic and potentially misleading. (5) We have investigated the fit of our model to all the available data. In most cases it is good. Manly's model is also a good fit to most of the data, except at extreme prey frequencies. (6) Experiments of the type considered need to be carefully designed if the results from different experiments are to be comparable. (7) They are more useful than most of the other types of experiment and observation that have been used for investigating frequency-dependent selection. (8) Appendix 1 describes two techniques for fitting our model to the data from experiments with two prey types. (9) Appendix 2 describes an approximate technique for fitting our model to the data from experiments with more than two prey types and illustrates it with a worked example.