THE COMPONENTS OF PREDATION ON SMALL MAMMALS IN SEMIARID CHILE - PRELIMINARY-RESULTS

We studied a vertebrate predator-mammalian prey system over a two-year period to determine whether predators displayed numerical and (or) fu nctional responses to fluctuations in prey resources. Relative abundan ces and diets of owls (Athene cunicularia, Bubo virginianus, and Tyto albo), and of foxes (Pseudalopex culpaeus) were determined monthly ove r a two-year period, simultaneously with assessments of mammalian abun dances (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. Exce pt for P. culpaeus, the remaining three predators failed to demonstrat e numerical responses to the changes in abundance of local small mamma ls. Except for B. virginianus, the remaining three predators did not i ncrease their diet breadths consistently in response to reductions of mammal abundance. All predators showed strong prey preferences for som e mammalian species, regardless of their abundance in the field, and t hus failed to display functional responses. specifically prey switchin g. Comparison with similar studies suggest that numerical and function al responses are uncoupled components of predation on small mammals, o ccurring in all possible combinations. We speculate that only when bot h responses are displayed simultaneously (a necessary but not sufficie nt condition), predators may effectively affect mammal abundance.