Insectivore

Abstract: Summary 1 Riparian zones serve several ecological functions for bats. They provide a source of prey and likely provide favourable structural habitats and shelter from predators. Many studies have shown that bats use the space above streams, ponds or riparian vegetation as feeding habitat. These studies, however, have never distinguished between the effects of habitat structure and prey availability on the foraging activities of bats. Such effects can only be distinguished by an experimental approach. We predicted that bat activity along a stream is influenced by the number of emerged aquatic insects. 2 We evaluated the response of terrestrial consumers, insectivorous bats, to changes in the abundance of emergent aquatic insects by conducting a manipulative field experiment. In a deciduous riparian forest in Japan, aquatic insect flux from the stream to the riparian zone was controlled with an insect-proof cover over a 1·2 km stream reach. 3 We estimated the abundance of emergent aquatic and flying terrestrial arthropods near the treatment and control reaches using Malaise traps. The foraging activity of bats was evaluated in both treatment and control reaches using ultrasonic detectors. 4 The insect-proof cover effectively reduced the flux of emergent aquatic insects to the riparian zone adjacent to the treatment reach. Adjacent to the control reach, adult aquatic insect biomass was highest in spring, and then decreased gradually. Terrestrial insect biomass increased gradually during the summer at both treatment and control reaches. 5 Foraging activity of bats was correlated with insect abundance. In spring, foraging activity of bats at the control reach was significantly greater than at the treatment reach, and increased at both sites with increasing terrestrial insect abundance. 6 Our result suggests that the flux of aquatic insects emerging from streams is one of the most important factors affecting the distribution of riparian-foraging bats. As is the case with other riparian consumers, resource subsidies from streams can directly enhance the performance or population density of riparian-dependent bats. To conserve and manage bat populations, it is important to protect not only forest ecosystems, but also adjacent aquatic systems such as streams.

Abstract: Exclosure experiments have demonstrated the effects of bird predation on arthropods. In a Mexican coffee plantation, we excluded foliage-gleaning bird and bat predators from coffee plants. Effects of bats and birds were additive. In the dry season, birds reduced arthropods in coffee plants by 30%; birds and bats together reduced arthropods by 46%. In the wet season, bats reduced arthropods by 84%, whereas birds reduced them by only 58%. We conclude that previous "bird" exclosure experiments may have systematically underestimated the effects of bats.

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 captivity all species maximized their net energy intake by feeding preferentially on large prey, but in the field the larger species of insectivore in each community ate prey that were 25-29% longer than those eaten by the 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 the 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 minutes) 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 large prey. Community diversity of insectivorous mammals should therefore depend on the avail- ability of different foraging microhabitats, and not on the distribution of prey sizes.

Abstract: We analyzed the patterns of habitat use by insectivorous bats in Mexico City, one of the largest and most populated cities of the world. We tested the hypotheses that richer patches of food, expected in more vegetated areas, have higher bat activity levels, and that fast-flying species benefit most from urbanization. We compared activity of insectivorous species and relative abundance of insects in 5 habitats (large parks, small parks, illuminated open areas, residential areas, and natural forest). Sampling of bat activity and insects was conducted every 2 weeks in 12 sites per habitat during summer 2002. Measures of bat activity were based on 3,600 oneminute sequences of sound that were recorded and analyzed. The average number of taxa per site was significantly higher in the natural forest than in urban habitats, but overall bat activity was significantly higher in large parks and illuminated open areas than in small parks, residential areas and natural forest. Vespertilionid bats (Eptesicus fuscus, Myotis, and an unidentified species), along with Eumops perotis, occurred almost exclusively in extensive green areas (large parks or natural forest). The molossid Nyctinomops macrotis made the broadest use of the urban‐natural mosaic, whereas Tadarida brasiliensis used urban sites (illuminated areas and large parks) more intensively. Insect abundance was higher in large parks and natural forest, and it was significantly correlated with overall bat activity and with the number of taxa recorded per site. The observed patterns of habitat use and foraging can be explained by considering the flight and echolocation performance of species. Although some species successfully exploited highly urbanized sites, large areas with vegetation are needed to maintain the most diverse insectivorous bat fauna in Mexico City.