Functional response and area-restricted search in a predator: seasonal exploitation of anurans by the European polecat, Mustela putorius

A study of the feeding habits and movements of 11 radiotracked polecats Mus tela putorius in western France revealed that seasonal predation upon agile frogs, Rana dalmatina, was directly influenced by prey abundance and distr ibution. Although dietary structure showed the importance of mammalian prey (71.5%), polecats exploited nocturnal, terrestrial anurans in spring (31.6 %). The periodic activity of anurans at spawning sites led both to a maximu m density in spring and to a patchy distribution. The monthly variations in anuran dietary occurrences were associated with changes in frog availabili ty. The functional response of polecats to frog density was sigmoidal share d (type 3 response). Frog consumption rate increased more slowly than prey density but frogs were actively removed at higher density. It is therefore suggested that frog populations were moderately affected by the predator an d this density dependent effect tends to stabilise anuran populations. Pred ation upon anurans was also correlated with a prey dispersion index as reve aled by a polynomial regression. Polecats concentrated their predation on s pawning congregations of the breeding adult frogs. Movements were smallest in spring and polecats changed their track length by increasing the differe nce between a succession of small movements and of longer journeys towards profitable sites. Changes in movements correlated with the anuran dispersio n index and the response was sigmoidal (polynomial regression) revealing an area-restricted search. This response may be regarded as an 'aggregative r esponse' according to the first part of the definition of Begon et al. (199 6). Functional and area-restricted search responses to the frog abundance a nd dispersion constitute an original example of predator-prey coexistence s trategies among vertebrates. I suggest that such predation could be favoure d by the individualistic habits of the mustelid.