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Due to habitat degradation, climate-induced drought, and the spread of white nose syndrome, 15% of bat species are threatened by extinction and numerous species are threatened or of conservation concern. Grand Canyon National Park is a hotspot for bat diversity – 22 species have been documented, more than any other national park unit. The Colorado River ecosystem is important habitat for bats because it provides drinking water and invertebrate prey, and the riparian vegetation and canyon walls provide habitat and structure for migration corridors. 

In a recent study, federal researchers collaborated with recreational river runners to investigate the relationship between bat activity and the availability of prey emerging from the Colorado River. During each night of their expeditions, river runners monitored bat activity using acoustic detectors and quantified insect abundance using light traps. In total, these community scientists collected 1,428 paired bug and bat samples throughout a 470 km-long segment of the river between 2017-2020. Light traps collected 71 insect taxa and acoustic monitors detected 19 bat species.

The team of researchers hypothesized that bat activity would be positively related to aquatic insect abundance and unrelated to terrestrial insect abundance. They tested these hypotheses with models using the insect and bat data collected by river runners. They also used models to evaluate other factors that can influence bat activity including time of year, time of night, distance downstream from Glen Canyon Dam, channel width, riparian vegetation density, air temperature, and lunar phase.

They found that bat activity was strongly and positively related to the availability of aquatic flies (Diptera). This was true when looking at all bat species combined, as well as in models specific to the most common bats encountered along the Colorado River in Grand Canyon: Canyon bat (Parastrellus hesperus), California myotis (Myotis californicus) and Yuma myotis (Myotis yumanensis). 

These results demonstrate that bat activity in Grand Canyon is primarily influenced by the availability of aquatic insect prey. Therefore, river management and dam operations that promote the production and diversity of aquatic insect assemblages (e.g., Bug Flows) will likely also benefit bat species that forage in the riparian corridor. This study also highlights the power of community science for monitoring large ecosystems such as the Grand Canyon.

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Read the paper:  

Metcalfe, A.N., Fritzinger, C.A., Weller, T.J., Dodrill, M.J., Muehlbauer, J.D., Yackulic, C.B., Holton, P.B., Szydlo, C.M., Durning, L.E., Sankey, J.B., and Kennedy, T.A., 2023, Insectivorous bat foraging tracks the availability of aquatic flies (Diptera): The Journal of Wildlife Management, e22414, https://doi.org/10.1002/jwmg.22414.

Associated data: 

Metcalfe, A.N., Kennedy, T.A., and Fritzinger, C.A., 2023, Bat activity and insect abundance data along the Colorado River in Grand Canyon, AZ: U.S. Geological Survey data release, https://doi.org/10.5066/P9R3HLVZ.

Safety Note: Bats and Rabies

Bats are known to carry rabies, which is a zoonotic disease that can be transmitted to humans and is fatal if untreated. Bats can spread rabies to humans through their saliva, via bites or scratches. It is important to never touch or handle bats, especially if they appear sick or are exhibiting unusual behavior such as being active during the day. Anyone who has had direct contact with a bat should seek medical consult, as rabies can only be confirmed through lab testing.