| Townsend's big-eared bat is a widely-distributed species that is decreasing in abundance throughout its range. It is listed as a Sensitive Species by the U.S. Forest Service and Bureau of Land Management and a species of special conservation status in most states where it occurs. To better conserve roosts for this species, I reviewed 34 peer-reviewed journal articles, technical reports and other grey literature on roost characteristics. Bats use natural and anthropogenic structures for day, night, maternity, and swarming or transitional roosts. Temperature is a likely driving factor in roost selection but this species is able to mediate temperatures behaviorally by roosting in clusters or individually. By selecting complex roost structures that provide a range of temperatures, bats can select locations within roosts with ideal temperatures. Human activities can affect this species, although recent studies have reported mixed results. I suggest that high levels of human activity can affect this species, but it can tolerate low levels. Future studies should limit research-related activities at maternity roosts to minimize disturbances to bats. I studied how roost selection affected potential exposure of bats to radon gas, a naturally occurring gas, in 59 uranium mines in summer and 37 in winter. The Environmental Protection Agency recommends an exposure level of <4pCi/L for a residence. Overall radon concentrations in mines were lower in winter (mean +/- SE: 197 +/- 90 pCi/L) than in summer (619 +/- 31 pCi/L). However, I observed higher radon concentrations at winter bat roosts (338 +/- 30 pCi/L) than summer (112 +/- 73 pCi/L). Bats spend more time in mines during winter, therefore they are exposed to higher radon concentrations for longer durations. Because bats are in torpor or hibernation during winter, thus decreasing respiration rates, I estimated their overall absorbed dose of radon was lower in winter (7 mGy) than in summer (139 mGy). Use of mines by bats was lower in summer months (4 bats) compared to winter (34 bats), therefore their exposure in summer may have little influence on their annual absorbed dose. Radon is found in high concentrations in natural caves (2 to 1,625 pCi/L), suggesting that cave-obligate bat species may have evolved in the presence of radon. Changes in seasonal radon concentrations were explained by air movement driven by temperature changes outside mines. Additional research should focus on effects of high (e.g., >100 pCi/L) radon concentrations on bats. Without additional information indicating radon has deleterious effects on bats, I recommend that land managers continue the current protocol of gating mines to exclude humans while allowing use by bats. |
