Melanism and Selective Predation in three Species of Moths

Abstract

In large-scale mark/release/recapture experiments with the polymorphic moth Biston betularia (L.), Kettlewell (1955a, 1956) convincingly demonstrated that birds were taking a disproportionate number of the form which was most conspicuous on the predominant backgrounds at different sites. Differential elimination of the morphs of B. betularia also occurred in experiments in which dead, frozen moths were exposed to predation on trees (Clarke & Sheppard 1966; Bishop 1972; Lees & Creed 1975). Bishop (1972) and Lees & Creed (1975) both found situations in which the melanic form, carbonaria, was present at a high frequency, even though it was at a cryptic disadvantage, relative to the typical form. This is consistent with Ford's (1937, 1975) suggestions that melanic polymorphism in moth species which rely on crypsis depends upon the balance between the selective advantages of both the non-visual and visual attributes controlled by the melanic genes. The results of Bishop (1972) and of Lees & Creed (1975) emphasize the need to estimate selective predation at a number of sites in order to examine the relative importance of non-visual and visual components of selection for different species. Lees, Creed & Duckett (1973) and Bishop et al. (1975) compared melanic frequencies with epiphyte cover (in addition to other variables) at a number of sites. Bishop et al. (1975) have emphasized the difficulties in deducing, from such analyses, whether non-visual or visual selective factors are more likely to be influencing melanic frequency. This can only be done by making more direct estimates of the selective predation of the morphs at different sites. Although predation experiments and mark/release experiments have been carried out with B. betularia at a number of sites, the time and resources required limits the number of sites that can be investigated during the flight period of the moth. However, Kettlewell (1955a, 1956) showed that the conspicuousness of the melanic and typical morphs of B. betularia to humans and to natural predators (birds) were related. Further confirmation of this was obtained by Lees & Creed (1975). The field experiments, described in this paper, were carried out to compare the consistency of different methods of estimating selective predation of the morphs. The work was undertaken in connection with surveys of the relative crypsis of the melanic and typical morphs of three species in southern Britain.