Predation on polymorphic prey : the effects and behavioural bases of frequency-dependent and frequency-independent selection

Tucker, Graham Michael (1988) Predation on polymorphic prey : the effects and behavioural bases of frequency-dependent and frequency-independent selection. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Apostatic selection (frequency-dependent selection by predators) has been demonstrated in several studies and theoreticaa work indicates that it has the potential to maintain polymorphism. Although apostatic selection has been proposed as a process that may help maintain colour polymorphisms in molluscs such as Cepaea and Littorina there is little evidence to support this. The underlying behavioural basis of apostatic selection is also poorly understood. The experiments described in this thesis attempted to establish: (1) whether predators exert frequency-independent visual selection on visually polymorphic molluscs and which factors affect the degree and direction of selection, (2) whether predators could exert apostatic selection on artificial populations of Cepaea and Littorina, (3) the behavioural mechanism underlying apostatic selection. A two year field study of snail predation by song thrushes (Turdus pilomelos) revealed selection on the colour morphs of Cepaea. During the winter yellow five-banded morphs were at a selective advantage and pink unbanded at a disadvantage. Yellow five-banded morphs were also at a selective advantage in the summer as were yellow unbanded morphs, whilst morphs with fused bands were at a disadvantage. No significant changes in selection were observed in either predation period. An experiment was carried out to test whether captive song thrushes would feed in an apostatic manner on an artificial dimorphic population of Cepaea. Although the results gave some indication that selection was frequency-dependent they were not conclusive, because of the small number of replicates. Another experiment tested whether shore crabs (Carcinus maenas) exert frequency-independent visual and/or apostatic selection on an artificial dimorphic population of Lottirina mariae. The results indicated that there was no visual or apostatic selection. Some potential behavioural mechanisms causing apostatic selection were investigated using humans as `predators'. In the first series of experiments `populations' of dimorphic `prey' (at one of seven frequencies) were presented on the screen of a computer colour monitor to subjects who were asked to use a light pen to remove each prey they detected. Each subject either had the prey presented against a matching background or against a background that made them conspicuous. The results showed that apostatic selection occurred, but only when the prey were inconspicuous. The results also showed that the apostatic selection was caused by some aspect of the subjects' searching behaviour. Further experiments revealed that the overall degree of selection decreased with increasing density, although the individual effects on the frequency-dependent and frequency-independent components could not be established. A second series of computer trials explicitly tested whether humans formed search images or modified their search rate when searching for cryptic prey images. In these experiments cryptic prey were presented singly amongst a series of blank screens and the subjects only had to detect whether a prey item was present. The results showed that the subjects did modify their search rate and this improved their ability to detect the prey. However, apostatic selection occurred when the prey were presented in unequal frequencies and this suggests that search images were formed.