A case study of the spatial relationship between bat occurrence and artificial light pollution along a bike trail in Portage County, Ohio
Light pollution is the result of misdirected artificial illumination from human activities; its occurrence coincides with the nocturnal foraging activity of bat populations and should be studied as a qualifying characteristic of suitable habitat. In the presence of artificial illumination, bats have been shown to display unnatural behaviors which, in addition to bat population declines, potentially limit the ecological benefits of stable bat populations. Therefore, the spatial patterns of bat activity should be studied to indicate abnormal bat behavior resulting from the presence of artificial illumination.
The city of Kent in Ohio experienced substantial human population growth from 1950 to 1970 and is still experiencing positive growth trends. Because artificial light follows the patterns of human settlement, Kent’s human population trend indicates that light pollution is a permanent threat for local wildlife. For this study, a bike trail owned by Kent State University was chosen as it transects a forested habitat near the campus. Along the bike trail, bat echolocation calls were measured with an ultrasonic detector during 18 nights in the summer of 2014. Bat passes were averaged by recording time to determine bat pass frequencies for each recording night. Light intensity conditions were also measured using a multimeter and photo-transistor. Spatial relationships between bat and light data were analyzed using GIS tools.
Over the study period, 933 bat calls were counted in a total of 21.87 hours of recording. When mapped spatially along the interpolated light data, the results display a bat preference for medium and high light intensities and an avoidance of dark areas across an illumination gradient. Given the probable species composition (Lasiurus cinereus, Lasiurus borealis, and Eptesicus fuscus) at the field site, it is likely these bats are increasing prey intake by foraging in brighter patches throughout the property. Although seemingly beneficial, adaptations of these bat species to exploit artificial light conditions may prove harmful to ecosystem balance by altering energy flow and creating competition.