Cave Hibernaculum Surveys of a Townsend's Big-Eared Bat (Corynorhinus townsendii) Colony at Mount St. Helens, Washington

Abstract

The Mount St. Helens (MSH) colony of Townsend’s Big-eared Bats (Corynorhinus townsendii) uses several caves in the Cave Basalt lava flow near Mount St. Helens, Washington State, USA, including hibernacula and a maternity roost. Federal, state, and private land managers currently consider the MSH colony of Townsend’s Big-eared Bats to be 1 group, based upon apparent high-fidelity use of the known maternity roost and hibernacula, and the unique landscape of the Cave Basalt lava flow, which contains a large number of roost sites in a small area. The Cave Basalt is a clearly-definable flow of pahoehoe lava approximately 9 mi in area, containing numerous lava tubes. Though lava fields and tubes exist in other regions of the Pacific Northwest, the Cave Basalt is relatively unique for its high abundance of lava tube caves in a small area. The lava flow has been radiocarbon dated to approximately 1900 BP (Greeley and Hyde 1972), and forest ecology on the flow differs greatly from the surrounding coniferforested landscape common to western Washington State (Halliday 1962; Mullineaux and Crandell 1981; Nieland and Nieland 1994). The MSH colony of Townsend’s Big-eared Bats has been a focus of substantial conservation efforts. Wildlife biologists and personnel from multiple agencies (USDA Forest Service, Washington Department of Natural Resources, Washington Department of Fish and Wildlife) began implementing conservation measures for the MSH colony in the late 1990s, using public outreach to educate recreational cavers, implementing seasonal closures and signage at known hibernacula, and installing bat-friendly exclosure gates at multiple entrances of the known maternity roost. A parcel of land containing the main entrance of the maternity roost was purchased in 1999 from private timberland ownership by The Nature Conservancy (TNC) of Washington State (WDFW 2002). Previous efforts to assess the size of the MSH colony consisted of intermittent exit counts at the maternity roost, recorded by TNC staff, graduate students from the Vancouver branch campus of Washington State University (Green 2003), and ND Reynolds (NDR) as an ecological consultant to TNC (Reynolds 2005, 2006). Although synthesis of these efforts revealed 25 y of persistent use of the maternity roost, surveys were not systematic. A systematic, repeatable survey that could provide an index to population size was needed. In 2003, JM Wainwright proposed a program of hibernaculum surveys. A shortlist of 6 caves was identified from over 64 caves known from the Cave Basalt (per Nieland and Nieland 1994), which were likely, based on expert knowledge and detailed agency reports (Senger and Crawford 1984), to harbor the majority of bats during winter. To protect these caves, we do not report true cave names and or location information. We present data using 6 pseudonyms: ‘‘Cave A’’ through ‘‘Cave F’’. Survey methods consisted of entering hibernacula during winter and counting Townsend’s Big-eared Bats. We are acutely aware that Townsend’s Big-eared Bats are sensitive to human disturbance (BC Wildlife 1998; WBWG 2005), and waking from torpor in midwinter is metabolically expensive (Thomas and others 1990), so we developed rapid-reconnaissance survey methods (Kunz 2003; Tuttle 2003) to minimize disturbance as much as possible. We surveyed only every other year, and each cave was surveyed only once during survey years. GENERAL NOTES