How far from a light source can bat activity change?

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Artificial light at night threatens nocturnal wildlife by fragmenting the nighttime landscape. It affects insect-eating bats’ commuting and foraging, but responses differ by species, and few studies have examined how far these effects reach from a light source. The researchers investigated how bat flight activity changed with distance to estimate the impact range for each species.

Using acoustic detectors, they compared the activity of two species with contrasting body shapes and flight behavior: the little bent-wing bat (Miniopterus australis) and the white-striped free-tailed bat (Austronomus australis). They compared 10 artificially illuminated transects with paired control transects. Detectors were placed at 0, 10, 25 and 50 m, starting at an LED-lit pathway on treatment transects and in a dark area on control transects.

Little bent-wing bat activity was reduced even 50 m from the light sources, where illuminance was low at approximately 0.14 lx. This suggests the impact on this species extends beyond 50 m. White-striped free-tailed bats were more active on illuminated transects at 10 and 25 m, but not at 0 or 50 m. The researchers interpreted this as activity being unaffected at very bright and low illuminances, with a possible benefit at intermediate illuminances.

The researchers concluded that single light sources can have far-reaching effects and alter habitat use by light-sensitive insect-eating bats. They recommended minimizing artificial light at night in urban bushlands to help keep them viable as habitat for these species. Further research is needed to determine how far from artificial light little bent-wing bat activity returns to normal.