Arizona has an estimated 50,000 open abandoned hardrock mines, 22,000 of these occur on BLM managed lands (Committee on Energy and Natural Resources, U.S. Senate 2008). These abandoned mines are distributed across BLM lands and create a public hazard (fig. 1). Nearly 70 percent of the abandoned mines in the western United States show signs of bat use (Tuttle and Taylor 1994). To eliminate the hazards these open mines present to the public, and to mitigate the loss of historical bat roosts (caves) these mines are being closed at an accelerating rate. The National Environmental Policy Act (NEPA) requires that any agency receiving federal funds must evaluate the effects of its proposed actions on a variety of environmental levels, including impacts to wildlife. Given that nearly 70 percent of the abandoned mines in the western United States show sign of bat use, biological surveys for bats are critical (Tuttle and Taylor 1994). These surveys play a key role in management decisions that seek to preserve and manage the habitat of bat species, which may be negatively affected by mine closure activities. The United States Fish and Wildlife Service (USFWS) strongly suggests that species listed as of concern at the state level should be managed as having endangered status. Seven former C2 species in Arizona and one federally listed species utilize mines during some stage of their life (Adams 2003).There are thousands of inactive mines on private, state and federal lands throughout Arizona and the majority have not been evaluated for bat use. With increasing concerns about the status of bat populations in Arizona and the limited information regarding the utilization of mines and caves, research with a focus on these species is imperative.
Figure 1: Example abandoned mine opening that is both a danger to the public and bat habitat.
Surveying abandoned mines for bat activity comprises two primary techniques: namely internal and external mine surveys. Internal mine surveys involve examining the internal workings of abandoned mines during seasons of bat use. Internal mine surveys are used to detect bat use in a mine during the reproductive and active periods of bat use: guano, insect parts, urine stains and the physical presence of bats all indicate bat use. Internal mine surveys provide an estimation of past and present bat use.
External surveys involve observers monitoring mine openings with visual, acoustic, motion detection and/or netting equipment. These surveys usually consist of a single visit to a mine during which bat activity is recorded as the number of bats entering or exiting a mine. Entrance and exit counts are often used to estimate the number of bats using a mine (Ludlow et al. 2000, Richter et al.1993). However, these exit and entrance counts can vary widely between nights (Hayes 1997). External surveys involve the use of night vision goggles, infrared cameras and acoustic bat detectors to record bat activity. External surveys rarely detect seasonal bat activity, such as migratory or hibernation activity, unless multiple visits are made to each mine (Altenbach 1995). Temperature and percent relative humidity of maternity and hibernation roosts are critical to bats. Townsend’s big-eared bat, (Corynorhinus townsendii) a former C2 species and a state sensitive species commonly roosts in mines. Generally these bats use mines with ambient temperatures around 15° Celsius as night roosts (Pearson et al. 1952, Twente 1955, Humphrey and Kunz 1976). Maternity roosts and day roosts exhibit temperatures of 15° Celsius or above. Townsend’s big-eared bat hibernation roosts have temperatures between zero° and 11° Celsius (Pearson et al. 1952, Twente 1955, Humphrey and Kunz 1976, Pierson et al. 1991). Studies have shown maternity roosts are found in specific microclimates within the mines.
Researchers found Yuma myotis, (Myotis yumanensis), maternity colonies located in mines possessing a relative humidity greater than 90% (Bettes 1997). Data collected on mines with and without maternity colonies suggests that maternity roost habitats are selected based on high humidity levels during the lactation periods (Bettes 1997). It has been suggested that high humidity levels decrease the amount of water loss by bats through respiration (Bettes 1997). In addition, structural features resulting from the mining activity (i.e. bald head raises, domes, stopes, and rooms) may capture the metabolic heat of the clustered bats in maternity colonies speeding the development of newborn bats (Bettes 1997). Thus internal mine features are associated with temperature and relative humidity, and play a critical role in roost selections. Detailed surveys of mines are necessary to determine bat use, type of use and microclimatic conditions of bat roosts. Due to the advantages of a definitive determination of presence and absence, internal surveys of abandoned mines are conducted whenever possible.
This approach requires continued assessment of potential hazards when conducting internal or underground surveys. Airflow, surface and ambient temperatures, and relative humidity is recorded at the entrance, working face, locations of any bat sign, and the entire length of the adit using a digital radiometer. Temperatures are collected by focusing the digital thermometer on the back above bat sign. Temperatures obtained reflect the microclimate (substrate temperature) at ceiling heights or mine features where bats may be found. Relative humidity and air temperature is recorded with a digital sling psychrometer. Daily and seasonal fluctuations in temperature and relative humidity can occur in the internal microclimate of the mine thus an insulation index is used to assess microclimate. Mine characteristics such as the geological nature of substrate, presence of crevices and fissures, and location and volume of stopes, raises, and winzes is also recorded. An internal survey conducted by an experienced bat biologist also trained and experienced in abandoned mine entry is the most efficient approach to mine bio-assessment. As of March 2010 220 mine sites have been surveyed for bat activity. Fifteen percent of these mines serve as critical bat habitat and are recommend for bat compatible closures. The remaining mines provide little of no bat habitat and will be closed permanently.
Figure 2: Cave Myotis bat (Myotis velifer) observed during internal mine surveys.
These abandoned mine surveys will enable both future management of bat habitat and mitigate the dangers these mines pose to the public. Abandoned mines are the primary roosting structures for a variety of bat species in the state and the conservation of those mines that provide bat habitat will aid in the long-term management and conservation of Arizona’s bat fauna.
For more information contact:
Joel Diamond , Arizona Game and Fish Department
5000 W. Carefree Highway Phoenix, AZ 85086-5000
Phone: (520) 742-1911 Email: firstname.lastname@example.org
Shawn F. Lowery , Arizona Game and Fish Department
5000 W. Carefree Highway Phoenix, AZ 85086-5000
(520) 742-1911 Email: email@example.com