Cooperative Fish and Wildlife Research Units Program: all
Education, Research and Technical Assistance for Managing Our Natural Resources

Approximately 70% of bats are insectivores and are commonly observed foraging over rivers and streams, presumably feeding on emergent aquatic insects. In this study, we collaborated with recreational river runners and other citizen scientists in Grand Canyon, Arizona, USA to record bat activity and sample riparian insects for one hour at dusk from April through October in 2017-2020. Citizen scientists collected 1,428 paired samples on 611 sampling nights at 410 sampling sites throughout a 470 km segment of river. Light traps collected a total of 71 insect taxa and acoustic monitors detected 19 bat species. We hypothesized that bat activity would be positively related to insect catch rates. Additionally, we predicted that bat activity in the riparian zone would be unrelated to terrestrial insect abundance. We fit Bayesian regression models to test these hypotheses as well as other competing hypotheses regarding the potential effects of quantify the relation between and environmental variables including time-of-year, time-of-day, distance downstream from Glen Canyon Dam, channel width, riparian vegetation density, air temperature, and lunar phase. We found that bat activity was positively related to the abundance of aquatic flies (Diptera), which outcompeted other prey categories in our models. We also found that activity of small Myotis (Myotis californicus and M. yumanensis) was higher later in the evening and that canyon bats (Parastrellus hesperus), conversely, were more active earlier in the evening. Activity of canyon bats also varied seasonally, with peak activity in August. Our results highlight the importance of aquatic flies as prey for bats along a large, regulated river corridor and demonstrate the power of citizen science as a tool for ecosystem monitoring.