Date Published: October 31, 2018
Publisher: Public Library of Science
Author(s): Theodore J. Weller, Thomas J. Rodhouse, Daniel J. Neubaum, Patricia C. Ormsbee, Rita D. Dixon, Diana L. Popp, Jason A. Williams, Scott D. Osborn, Bruce W. Rogers, Laura O. Beard, Angela M. McIntire, Kimberly A. Hersey, Abigail Tobin, Nichole L. Bjornlie, Jennifer Foote, Dan A. Bachen, Bryce A. Maxell, Michael L. Morrison, Shawn C. Thomas, George V. Oliver, Kirk W. Navo, Michelle L. Baker.
Efforts to conserve bats in the western United States have long been impeded by a lack of information on their winter whereabouts, particularly bats in the genus Myotis. The recent arrival of white-nose syndrome in western North America has increased the urgency to characterize winter roost habitats in this region. We compiled 4,549 winter bat survey records from 2,888 unique structures across 11 western states. Myotis bats were reported from 18.5% of structures with 95% of aggregations composed of ≤10 individuals. Only 11 structures contained ≥100 Myotis individuals and 6 contained ≥500 individuals. Townsend’s big-eared bat (Corynorhinus townsendii) were reported from 38% of structures, with 72% of aggregations composed of ≤10 individuals. Aggregations of ≥100 Townsend’s big-eared bats were observed at 41 different caves or mines across 9 states. We used zero-inflated negative binomial regression to explore biogeographic patterns of winter roost counts. Myotis counts were greater in caves than mines, in more recent years, and in more easterly longitudes, northerly latitudes, higher elevations, and in areas with higher surface temperatures and lower precipitation. Townsend’s big-eared bat counts were greater in caves, during more recent years, and in more westerly longitudes. Karst topography was associated with higher Townsend’s big-eared bat counts but did not appear to influence Myotis counts. We found stable or slightly-increasing trends over time in counts for both Myotis and Townsend’s big-eared bats from 82 hibernacula surveyed ≥5 winters since 1990. Highly-dispersed winter roosting of Myotis in the western USA complicates efforts to monitor population trends and impacts of disease. However, our results reveal opportunities to monitor winter population status of Townsend’s big-eared bats across this region.
Effective conservation relies on knowledge of the whereabouts and ecological needs of animals during critical phases of their annual cycle. Most species of bats in temperate areas spend half or more of each year in hibernation, which allows them to avoid seasonal prey shortages, limit mortality due to concealment and inactivity, and reduce somatic degradation . Hence, availability of winter roosts has long been understood to contribute to patterns of bat community diversity . Hibernation has been an evolutionarily successful strategy for temperate-zone bats, allowing them to persist in cold regions with net-positive population growth rates despite low fecundity and other “slow” life history traits relative to other small mammals [1, 3]. The central importance of hibernation to temperate-zone bat biogeography underscores the need to understand bat overwintering ecology . However, our ability to do so is hampered in many regions by a fundamental lack of information about where bat hibernacula occur .
We compiled results of 4,549 winter surveys for bats from 2,888 unique structures across 11 states that were reported between the winters of 1916 and 2017 (Table 1). Across all states we categorized 78% of surveys as counts and 22% as presence surveys. Among the surveyed structures for which type was reported, 97% occurred at either caves (39%) or mines (58%); however the relative composition varied among states (Table 1). Mines represented >90% of structures surveyed in Colorado, Nevada and Utah whereas caves represented >83% of structures surveyed in Arizona, Idaho, and New Mexico. Bridges (n = 51, 41 of which were in Oregon) were the only other structure type that represented >1% of the total structures surveyed.
Efforts to conserve bats in the western United States have long been impeded by a lack of knowledge of the winter whereabouts of most species. Our results suggest that this has not been for lack of survey effort, at least in caves and mines. We found there has been considerable effort, particularly in the recent past, to survey winter hibernacula of bats in western states. Our database revealed that during 1995–2016 an average of at least 177 surveys per year were conducted across the region and during 2012–2016 the annual average was 369 surveys per year. Only 3 of the 20 hibernacula (15%) that contained ≥30 Myotis, but 30 of the 92 COTO hibernacula (33%) had first records in our database after 2011. This suggests that that recent increased survey effort has generally been effective for locating large COTO hibernacula but less effective for Myotis hibernacula. Survey effort has been relatively well-dispersed geographically and, at least for caves, has focused on areas of karst and volcanic habitat. Encouragingly, most states have been tracking negative surveys, at least in recent years, as opposed to only those surveys where bats had been detected (Fig 1). The inclusion of non-detection records greatly facilitated our biogeographic trend modeling. Structures with more bats tended to be surveyed over a greater number of years. This trend is consistent with patterns observed in northeastern USA and Europe  and represents a frugal approach to monitoring for conservation.