River and riparian areas support high bat foraging activity, presumably as a result of high aquatic insect availability; however, structural features of the landscape (river channel confinement and riparian vegetation structure) may also influence bat foraging activity. To test the hypothesis that landscape structure indirectly affects bat foraging activity by controlling the location of insect aggregations, we measured bat activity and insect availability along the South Fork Eel River in northern California, USA. The South Fork Eel River is characterized by alternating sections of canyons (confined reaches) and open floodplains consisting of rock bars with cobble to boulder size substrate (unconfined reaches). Some of the rock bars are lined with riparian vegetation (Alnus glutinosa) along the river edge, while others are not. Bat foraging activity was measured using Anabat detectors at 21 sites classified as confined, unconfined with riparian vegetation, and unconfined without riparian vegetation, May – August 2006. Insect availability was measured using emergence traps and sticky traps at 9 of the above sites three times throughout the summer. Mean bat activity ranged from 278 – 1912 calls/night and was negatively related to channel confinement (linear regression, p = 0.03). Mean bat activity did not significantly differ between unconfined sites with riparian vegetation (287 calls/night) and those without (312 calls/night). Bat activity decreased exponentially with distance from the river tracking declines in insect abundance. These patterns suggest that channel geomorphology influences the location of bat foraging activity by limiting the extent of lateral penetration of aquatic insects into the terrestrial landscape.
