Body Size, Prey Size, and Community Structure in Insectivorous Mammals

Abstract

This study investigates the relationship between body size and prey size in three two—species communities of insectivorous mammals in Australia and England. In capacity all species maximized their net energy intake by feeding preferentially on large prey, but the field in larger species of insectivore in each community ate prey that were 25—29% longer than those eaten by smaller species. Direct field observations showed that the larger species of insectivores dug extensively in the topsoil and subsurface litter to obtain prey, whereas the smaller species foraged on the litter surface or in trees. The larger species also exploited denser and structurally more complex microhabitats than their smaller relatives. These differences in foraging behavior allowed their larger insectivores in each community to exploit prey that were 7—19% longer than those encountered by the smaller species. Controlled removal of the larger species of insectivore from two communities produced marked changes in the microhabitat use and prey size selection of the remaining species. In the first community, a small eutherian shrew (Sorex minutus) ate more long prey and tended to move into tall grass when a larger congener, S. araneus, was removed. In the second community a small dasyurid marsupial (Antechinus stuartii) also ate more long prey and decreased its use of trees when the larger A. swainsonii was removed. These results provide no evidence that body size directly influences prey size selection in insectivorous mammals. However, body size differences may influence prey size selection indirectly by conferring an asymmetric competitive advantage on larger insectivores, thus allowing them to actively exclude smaller species from microhabitats containing larger prey. Community diversity of insectivorous mammals should therefore depend on the availability of different foraging microhabitats, and not on the distribution of prey sizes.