Influence of river drying and insect availability on bat activity along the San Pedro River, Arizona (USA)

We evaluated the effects of flow reduction and drying on prey availability and bat activity along a desert river in southeastern Arizona. We sampled bat activity and insect availability during the dry season at perennial and intermittent sites along the San Pedro River. Intermittent sites included both temporally flowing and dry conditions during the study period. Bat activity significantly declined between May and June sampling periods but was not related to whether sites had perennial or intermittent flow. Declines in bat activity corresponded to reductions in insect availability, but only at perennial sites. Bats tracked aquatic insect availability at perennial sites but not at intermittent sites, where insects appear to actively aggregate above localized wet portions of the intermittent reaches. Finally, both bat and insect availability declined to nearly undetected levels when the river dried at 2 of 16 sites in despite increases in terrestrial insect availability. Our results indicate that intermittency affects bat activity indirectly via its effects on prey availability. Seasonal river drying appears to have complex effects on foraging decisions by bats, initially causing imperfect tracking by consumers of localized concentrations of resources, but later resulting in disappearance of both insects and bats after complete drying.