Background/Question/Methods Rivers and riparian areas are recognized as important foraging habitat for insectivorous bats. This is assumed to result from high aggregations of emerging aquatic insects, which provide an important food resource to foraging bats. However, riparian vegetation may influence bat foraging activity as well. Patterns in bat foraging activity within riverine landscapes are spatially dynamic. We used natural variation in aquatic insect abundance, extent and density of riparian vegetation, and flow permanence along the San Pedro River, Arizona, to examine how these factors influence spatial patterns in bat distributions. We acoustically measured bat foraging activity at 16 sites located along a ~100 km reach of this desert river during the summer of 2007. Additionally, we measured insect abundance at each of these sites using sticky traps set at 0-m and 25-m from the river.
Results/Conclusions We found significantly higher bat activity above the river at sites with low density riparian vegetation (0-100 no. trees/ha) in comparison to sites with high riparian vegetation density (>500 no. trees/ha; t-test: n_high = 4, n_low = 4, t = -2.517, df = 6, P = 0.045). Insect data collected along the San Pedro River showed similar aquatic insect abundance directly above the river at sites with low and high density vegetation, but more insects penetrating into the terrestrial landscape along reaches with low vegetation density. These results suggest that river reaches lined with dense riparian vegetation may support lower bat foraging activity because dense vegetation may hinder flight maneuverability and echolocation ability. Alternatively, by promoting increased lateral penetration of aquatic insects across the river-riparian transition, sparse vegetation may encourage a similar lateral expansion and thus, support higher overall bat activity over the river channel next to sparse riparian forests.