Abstract

We studied a vertebrate predator-mammalian prey system over a two-year period to determine whether predators displayed numerical and (or) functional responses to fluctuations in prey resources. Relative abundances and diets of owls (Athene cunicularia, Bubo virginianus, and Tyto albo), and of foxes (Pseudalopex culpaeus) were determined monthly over a two-year period, simultaneously with assessments of mammalian abundances (six species of Rodentia and one of Marsupialia) at a semiarid site in north-central Chile. Mammal abundances reached maximum levels during the autumn of the two years, and declined during the respective springs, with summer and winter showing intermediate abundances. Except for P. culpaeus, the remaining three predators failed to demonstrate numerical responses to the changes in abundance of local small mammals. Except for B. virginianus, the remaining three predators did not increase their diet breadths consistently in response to reductions of mammal abundance. All predators showed strong prey preferences for some mammalian species, regardless of their abundance in the field, and thus failed to display functional responses. specifically prey switching. Comparison with similar studies suggest that numerical and functional responses are uncoupled components of predation on small mammals, occurring in all possible combinations. We speculate that only when both responses are displayed simultaneously (a necessary but not sufficient condition), predators may effectively affect mammal abundance.