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Status of Populations of the Endangered Gray Bat in the Western Portion of its Range
D. Blake Sasse, Richard L. Clawson, Michael J. Harvey, and Steve L. Hensley

Southeastern Naturalist, Volume 6, Number 1 (2007): 165–172

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2007 SOUTHEASTERN NATURALIST 6(1):165–172 Status of Populations of the Endangered Gray Bat in the Western Portion of its Range D. Blake Sasse1,*, Richard L. Clawson2, Michael J. Harvey3, and Steve L. Hensley4 Abstract - Forty-eight Myotis grisescens (gray bat) maternity colonies in Missouri, Arkansas, and Oklahoma were monitored from 1978 to 2002 as recommended in the US Fish and Wildlife Service recovery plan for this endangered species. Seventynine percent of colonies were stable or increasing across 3 subpopulations in this portion of the species’ range, and 9 of 14 (64%) actions required by the recovery plan in this region were entirely or partially completed. This study indicates that the dramatic decline in gray bat populations that led to its listing as endangered in 1976 may have halted and that gray bat populations appear to be recovering in the western portion of its range. Introduction The endangered Myotis grisescens Howell (gray bat) inhabits caves in the southeastern United States, where they concentrate in a small number of winter hibernacula. In the spring, they disperse to numerous caves, where they form colonies that are usually sexually segregated. Summer colonies are often large, consisting of tens of thousands of bats, with some caves harboring over 100,000 individuals. This tendency to form large aggregations made the gray bat vulnerable to both intentional and unintentional human disturbance, and the species suffered precipitous population declines before its protection under the Endangered Species Act in 1976 (Brady et al. 1982, Tuttle 1979). Factors that exacerbated this decline include impoundment of waterways, cave commercialization, natural flooding, and pesticides (Clawson 1991, Tuttle 1979). After the gray bat was listed as endangered, a recovery plan was developed by an interagency recovery team of experts (Brady et al. 1982). The plan includes goals that have to be met to down-list the species from endangered to threatened status as well as criteria that would have to be met for full recovery and removal from the list. The recovery plan includes needed conservation actions, such as purchase or physical protection of important caves, and assigns priority levels to caves used by gray bats based on the significance of the cave to the recovery of the species as determined by the best professional judgment of the recovery team. Down-listing the 1Arkansas Game and Fish Commission, # 2 Natural Resources Drive, Little Rock, AR 72205. 2Missouri Department of Conservation, 1110 South College Avenue, Columbia, MO 65201. 3Tennessee Technological University, Department of Biology, Box 5063, Cookeville, TN 38505. 4US Fish and Wildlife Service, 222 South Houston, Suite A, Tulsa, OK 74127. *Corresponding author - 166 Southeastern Naturalist Vol. 6, No. 1 gray bat from endangered to threatened status under the Endangered Species Act requires that 90% of the most important hibernacula be protected and that populations at 75% of the most important maternity colonies, designated priority 1, be documented as stable or increasing over a period of 5 years (Brady et al. 1982). This study is the first attempt to determine trends in gray bat populations in the western portion of their range over a 25-year period and to assess progress in completing gray bat recovery plan management objectives. Methods In the 1960s and 1970s, several studies examined movement patterns of gray bats in the western portion of their range, comprising Missouri, Arkansas, Oklahoma, and Kansas (Elder and Gunier 1978, Grigsby 1980, Harvey et al. 1981, LaVal and LaVal 1980, Myers 1964). Based on recovery locations of bats banded during these studies, sub-populations were described consisting of multiple caves containing maternity colonies and their associated winter hibernacula. A complicated picture of migratory patterns emerged from these studies that indicated there were 3 subpopulations in this region that are generally independent of each other, with only occasional movements of individuals from one subpopulation to another (Fig. 1). From 1978–2002, we and other cooperators collected population data from 48 maternity sites, including 6 identified as priority 1 in the gray bat Figure 1. Sub-populations of Myotis grisescens (gray bat) in the western portion of its range. 2007 D.B. Sasse, R.L. Clawson, M.J. Harvey, and S.L. Hensley 167 recovery plan. Seven known gray bat maternity sites west of the Mississippi River were not included in this study. Inadequate sample sizes, lack of consistency in use of survey techniques at some caves, or lack of quantitative estimates precluded the use of data from Bat Cave (Miller County, MO), Beck Cave (Hickory County, MO), Grandpa Chipley Cave (Camden County, MO), Saltpeter Cave (Stone County, MO), Tumbling Creek Cave (Taney County, MO), Turner Mill Spring Cave (Oregon County, MO), and Pittsburg, KS (Robertson 2003). Three of the excluded caves (Beck Cave, Saltpeter Cave, and Tumbling Creek Cave) were identified as priority-1 sites in the gray bat recovery plan (Brady et al. 1982). Population estimates of maternity colonies were made using different techniques in each state, but have generally remained consistent at each cave, so estimates at each site among years could be compared. This variation in survey methodology precludes calculation of the total population in this region or within subpopulations, but does not affect analysis of population trends at each site. Estimates at caves in Oklahoma were obtained by entering caves each winter and examining guano deposited by maternity colonies the previous summer. At each guano pile, a measuring tape was used to determine length of the pile, and the width was measured at 1-m intervals along the axis of the pile. This information, used to estimate area of the guano pile, was multiplied by 1828 bats/m2 (average roosting density) to estimate colony size (Tuttle 1975). In Missouri, population estimates were based on measurement of the largest fresh guano pile in each maternity cave. The length and average width of piles were measured and multiplied by 0.8, because piles are not entirely rectangular in shape, and multiplied by 1828 bats/m2 to estimate colony size (Tuttle 1975). In Arkansas, several methods were used, depending on suitability at each site. At some maternity caves, visual counts were made of bats exiting maternity caves at dusk, while at others, estimates of the ceiling area covered by the colonies were made during the day and multiplied by 1828 bats/m2 to estimate population size (Tuttle 1975). Population estimates for all sites were tested to determine trends with the Mann-Kendall test (Hollander and Wolfe 1973, Thompson et al. 1998). This non-parametric statistical method is based upon annual positive or negative changes in population and is useful when assumptions required for analysis using regression techniques cannot be met. The Mann-Kendall test is appropriate for use in analyzing population trends in cave dwelling bats since survey techniques vary considerably in accuracy. However, this test can only determine if the population is generally increasing or decreasing and does not take into account the magnitude of change (Thompson et al. 1998). Results From 1978–2002, populations of gray bats at 21 of 48 (44%) maternity caves showed a significantly increasing trend, 17 (35%) had no discernable trend (were considered stable), and 10 (21%) were decreasing (Table 1). In the 168 Southeastern Naturalist Vol. 6, No. 1 northern subpopulation, 87% of maternity colonies had either stable or increasing populations, while in the eastern subpopulation, 80% of maternity colonies demonstrated the same pattern. In the southern region, which contained the largest number of maternity colonies, populations were either stable Table 1. Population trends of gray bats at maternity caves in the western portion of their range from 1978–2002. Number of Probability PopulationA Cave name Data range surveys Trend valueB Eastern subpopulation (Missouri) Bat (Oregon County) 1978–2001 8 Increasing 0.002 9800 Bat (Dent County) C 1981–2002 10 Increasing < 0.001 40,800 Bat #2 1978–2001 10 Increasing < 0.001 46,250 Bat #3 1979–2001 10 Increasing < 0.001 17,150 Coalbank 1978–2001 8 Increasing < 0.031 8200 Cooks 1978–2000 7 No Trend 0.068–0.119 11,500 Dead Man 1981–2001 6 No Trend 0.360–0.500 8150 Roaring SpringsC 1981–2001 8 Decreasing 0.016 0 Saloon 1978–2002 8 Increasing 0.001 9250 Thrasher’s Ford 1981–2001 7 Decreasing 0.001–0.005 6550 Northern subpopulation (Missouri) Adkins 1978–2002 10 Increasing 0.001–0.002 26,925 Bat (Texas County) 1981–1999 6 Decreasing < 0.001 0 Blackwell 1978–2001 10 Increasing < 0.001 9000 Brown #1 1981–2001 9 No Trend 0.238 39,200 Devil’s Icebox 1987–2002 8 Increasing < 0.001 10,900 Estes 1978–2002 7 No Trend 0.191–0.281 1825 Great Spirit/IncaC 1978–2002 13 Decreasing 0.021 10,900 Holton 1978–2002 7 No Trend 0.068–0.119 2600 Lower Burnt Mill 1978–2002 6 No Trend 0.235–0.360 6525 Mary Lawson 1978–2001 12 Increasing < 0.001 34,300 Mauss-MolesC 1978–2001 10 Increasing < 0.001 76,700 Piquet 1978–1989 4 Increasing < 0.042 18,500 River 1981–2001 8 Increasing < 0.054 9800 Rocheport 1988–2001 6 Increasing < 0.001 32,650 Smittle 1981–1999 8 No Trend 0.360 33,650 Southern subpopulation Oklahoma Adair 1981–2001 10 No Trend 0.054–0.078 6964 Anticline 1978–2001 10 Decreasing 0.023–0.036 7878 Boy Scout 1982–2001 9 Increasing < 0.001 12,500 Charley Owl 1981–2000 10 No Trend 0.108–0.146 12,100 Dressler 1981–2001 9 No Trend 0.238 4500 Spavinaw Creek 1978–2002 12 Increasing < 0.001 32,355 Twin 1981–2001 12 Increasing 0.002 22,500 Missouri Bat (Ozark County) 1978–1994 7 No Trend 0.386–0.500 27,200 Blacksmith 1981–1997 6 Increasing 0.001–0.008 8200 Coolbrook 1978–1994 6 No Trend 0.235–0.360 5350 Maze 1978–1994 4 No Trend 0.167 8150 No Name 1980–1997 8 No Trend 0.089 1700 Rantz 1978–1994 5 No Trend 0.592 14,275 Stillhouse 1981–1997 5 Increasing < 0.008 10,200 2007 D.B. Sasse, R.L. Clawson, M.J. Harvey, and S.L. Hensley 169 or increasing at 18 of 23 (78%) sites. Adequate information was available to analyze population trends at 6 of 9 (66%) maternity colonies identified as priority-1 sites in the gray bat recovery plan, and of these, 4 (66%) had increasing and 2 (33%) had decreasing population trends (Table 1). There were 14 recovery actions listed in the gray bat recovery plan that relate to the western portion of the range and of these, 6 (43%) were complete, 3 (21%) were partially achieved, and 5 (36%) had not yet been performed Table 1, continued. Number of Probability PopulationA Cave name Data range surveys Trend valueB Arkansas Big Creek 1980–2002 23 Decreasing < 0.025 3068 BoneC 1979–2002 24 Increasing 0.015 136,000 Cave River 1979–2002 20 Decreasing < 0.001 50 Cave Springs 1979–2002 23 Decreasing < 0.001 2030 Horsethief 1986–2002 12 Increasing < 0.001 6800 LoganC 1979–2002 24 No trend 0.054 18,370 Newark Storm Drain 1988–2002 15 No trend 0.190–0.218 3172 Old Joe 1978–2002 25 Decreasing 0.002 6120 Summer 1982–2002 21 Decreasing 0.028 4080 AMost recent population estimate within this study period. BMann–Kendall test. CPriority-1 sites in the gray bat recovery plan. Table 2. Priority actions necessary to prevent extinction of Myotis grisescens (gray bat) in the western portion of its range (Brady et al. 1982). Action Status Maternity Caves Acquire Saltpetre Cave (MO) Remains private Acquire Maze Cave (MO) Remains private Acquire Bat Cave (MO) Remains private Protect Tumbling Creek Cave (MO) Bat-friendly gate installed Acquire Bone Cave (AR) Remains private, but has been fenced under a management agreement with between the owner and the Arkansas Game and Fish Commission Protect Beck Cave (MO) Fenced Protect Roaring Springs Cave (MO) Was leased to Missouri Department of Conservation but the lease has expired Acquire Logan Cave (AR) Purchased by US Fish and Wildlife Service Protect Mauss/Moles Cave (MO) Remains unprotected but is owned by Missouri Department of Conservation Acquire Inca (Great Spirit Cave) (MO) Purchased by Missouri Department of Conservation Hibernacula Protect Bonanza Cave (AR) Bat-friendly gate installed by US Forest Service Acquire Coffin Cave (MO) Purchased by Missouri Department of Conservation and fenced Protect Chimney Cave (MO) Closure sign posted at entrance Protect Marvel Cave (MO) Commercialized and abandoned by bats 170 Southeastern Naturalist Vol. 6, No. 1 (Table 2). Of the actions involving hibernacula, 2 (50%) were complete, 1 (25%) was partially completed, and the other is unlikely to ever be accomplished due to changes made to the cave during commercial development. Discussion The population decline that led to listing of the gray bat as endangered appears to have been halted and populations may be increasing. This study found that in the western range 79% of maternity colonies were stable or increasing, which is supported by several short-term evaluations in the region (Grigsby et al. 1993, Harvey 1996, Martin et al. 2000). Grigsby et al. (1993) observed stable or increasing populations at 4 gray bat maternity caves in Oklahoma between 1981 and 1991. In another Oklahoma study, populations at 4 gray bat maternity caves had remained the same or increased between 1981 and 1999 after internal cave gates were installed (Martin et al. 2000). Populations of gray bats in two protected Arkansas hibernacula have steadily increased from < 100 to > 250,000 individuals in recent years (Harvey 1996; M.J. Harvey, pers. observ.). Unfortunately, populations from these hibernacula have not been linked to specific summer colonies. Harvey (1996) estimated that 220,000 gray bats were found in 19 maternity and bachelor caves in Arkansas and that the population was relatively stable or increasing in the state. Colony-size estimates and trends in eastern populations of the gray bat have not been fully assessed, though studies in several states also show increasing population trends. In Kentucky, summer maternity populations increased slightly from 1989 to 2001, while 2 summer colonies in Alabama doubled in size from the 1980s to the early 1990s (Thomas and Best 2000, Wethington 2001). Ellison et al. (2003) attempted a species-wide assessment of trends in gray bat summer cave populations and found that of 76 maternity colonies, 3 (4%) were increasing, 66 (87%) had no detectable trends, and 7 (9%) had decreasing trends. However, despite these increases, this study, and that of Ellison et al. (2003), revealed several weaknesses in criteria for down-listing in the 1982 recovery plan that need to be addressed prior to any decision to change the status of this species from endangered to threatened. Given the large number of maternity sites used by gray bats, it is unlikely that all sites can be surveyed often enough in any given 5-year period to show a stable or increasing population trend as required for down-listing to threatened status. Over the 25 years of this study, 35 of 48 (73%) caves were surveyed 4– 10 times, with an average time between population counts of 3–6 years. The database used by Ellison et al. (2003) was incomplete and suffered from lower sample sizes for many of the same colonies included in this study. In situations where it is possible to survey a colony on an annual basis, a 5-year period may still not allow adequate time to examine trends, due to impreciseness inherent in methods used to survey bat colonies of this size (Tuttle 2003). 2007 D.B. Sasse, R.L. Clawson, M.J. Harvey, and S.L. Hensley 171 Recovery plan priority levels for hibernacula and maternity caves were assigned based on the “biological significance, location, vulnerability, and consensus of opinion of a variety of respondents to the cave survey” (Brady et al. 1982). These indefinite criteria make it difficult to determine how conservation efforts should be directed towards sites discovered since publication of the recovery plan or towards sites whose population has significantly changed over time (Harvey 1996). Specific priority-level criteria should be incorporated in future plan revisions, or a process should be developed for reassessment of priority levels on a regular basis to address these concerns. With completion of most recovery tasks in the western range, this portion of the recovery plan should be significantly updated to reflect actions that remain unfinished. When the recovery plan is revised, analyses should be performed with modern statistical techniques to determine appropriate survey frequencies and time intervals that support population-trend analysis. These analyses should take into account difficulties in determining population size and trends in cave dwelling bat species (McCracken 2003, Tuttle 2003). The recovery plan should also focus on identifying gray bat subpopulations and encourage coordination and synchrony of surveys of subpopulations in multiple states, allowing interim assessments of the status of subpopulations. The lack of such coordination in the past makes it impossible to utilize existing monitoring data to develop population estimates for this region over the study period. Acknowledgments We thank the many bat researchers who provided data and conducted or participated in surveys over the years including Jerry Choate, Ron Redman, Bill Puckette, Keith Martin, and Joyce Barker. Portions of this study were financed by the US Fish and Wildlife Service through the Section 6 Endangered Species Recovery Program, the Arkansas Game and Fish Commission, the US Forest Service, and the National Park Service. Literature Cited Brady, J., T. Kunz, M.D. Tuttle, and D. Wilson. 1982. Gray bat recovery plan. US Fish and Wildlife Service, Denver, CO. 28 pp. Clawson, R.L. 1991. Pesticide contamination of endangered gray bats and their prey in Boone, Franklin, and Camden counties, Missouri. Transactions of the Missouri Academy of Science 25:13–19. Elder, W.H., and W.J. Gunier. 1978. Sex ratios and seasonal movements of gray bats (Myotis grisescens) in southwestern Missouri and adjacent states. American Midland Naturalist 99:463–472. Ellison, L.E., T.J. O’Shea, M.A. Bogan, A.L. Everette, and D.M. Schneider. 2003. Existing data on colonies of bats in the United States: Summary and analysis of the US Geological Survey’s Bat Population Database. Pp. 127–237, In T.J. O’Shea and M.A. Bogan (Eds.). Monitoring Trends in Bat Populations in the United States and Territories: Problems and Prospects. US Geological Survey, Biological Resources Discipline, Fort Collins, CO. Information and Technology Report USGS/BRD/ITR—2003-0003. 274 pp. 172 Southeastern Naturalist Vol. 6, No. 1 Grigsby, E.M. 1980. The gray bat, Myotis grisescens, in the southwest portion of the Ozark plateau: Movement patterns, maternity colonies, hibernacula, and philopatry. Ph.D. Dissertation. University of Oklahoma, Norman, OK. 96 pp. Grigsby, E.M., W.L. Puckette, and K.W. Martin. 1993. Comparative numbers of gray bats (Myotis grisescens) at four maternity caves in northeastern Oklahoma in 1981 and 1991. Proceedings of the Oklahoma Academy of Science 73:35–37. Harvey, M.J. 1996. Status and management of endangered bats in Arkansas. Proceedings of the Annual Conference of the Southeastern Association of Fish and Wildlife Agencies 50:246–253. Harvey, M.J., J.J. Cassidy, and G.G. O’Hagan. 1981. Endangered bats of Arkansas: Distribution, status, ecology, and management. Report to the Arkansas Game and Fish Commission, US Forest Service and the National Park Service. Memphis State University, Memphis, TN. 137 pp. Hollander, M., and D.A. Wolfe. 1973. Nonparametric Statistical Methods. Wiley, New York, NY. 503 pp. LaVal, R.K., and M.L. LaVal. 1980. Ecological studies and management of Missouri bats, with emphasis on cave-dwelling species. Terrestrial Series #8, Missouri Department of Conservation, Jefferson City, MO. 53 pp. Martin, K.W., W.L. Puckette, S.L. Hensley, and D.M. Leslie, Jr. 2000. Internal cave gating as a means of protecting cave-dwelling bat populations in eastern Oklahoma. Proceedings of the Oklahoma Academy of Science 80:133–137. McCracken, G.F. 2003. Estimates of population sizes in summer colonies of Brazilian free-tailed bats. Pp. 21– 30, In T.J. O’Shea and M.A. Bogan (Eds.). Monitoring Trends in Bat Populations in the United States and Territories: Problems and Prospects. US Geological Survey, Biological Resources Discipline, Fort Collins, CO. Information and Technology Report USGS/BRD/ITR—2003-0003. 274 pp. Myers, R.F. 1964. Biology of three species of myotine bats in the Ozark plateau. Ph.D. Dissertation. University of Missouri, Columbia, MO. 210 pp. Robertson, S.M. 2003. Population dynamics and foraging behavior of gray bats in Pittsburg, Kansas. M.Sc. Thesis. Southwest Missouri State University, Springfield, MO. 57 pp. Thomas, D.P., and T.L. Best. 2000. Radiotelemetric assessment of movement patterns of the gray bat (Myotis grisescens) at Guntersville Reservoir, Alabama. Pp. 27–39, In B.R. Chapman and J. Laerm (Eds.). Fourth Colloquium on Conservation of Mammals in the Southeastern United States. Occasional Papers of the North Carolina Museum of Natural Science and the North Carolina Biological Survey, No. 12. Raleigh, NC. 92 pp. Thompson, W.L., G.C. White, and C. Gowan. 1998. Monitoring Vertebrate Populations. Academic Press, San Diego, CA. 365 pp. Tuttle, M.D. 1975. Population ecology of the gray bat (Myotis grisescens): Factors influencing early growth and development. University of Kansas Occasional Papers of the Museum of Natural History 36:1–24. Tuttle, M.D. 1979. Status, causes of decline, and management of endangered gray bats. Journal of Wildlife Management 43:1–17. Tuttle, M.D. 2003. Estimating population sizes of hibernating bats in caves and mines. Pp. 31–39, In T.J. O’Shea and M.A. Bogan (Eds.). Monitoring Trends in Bat Populations in the United States and Territories: Problems and Prospects. US Geological Survey, Biological Resources Discipline, Fort Collins, CO. Information and Technology Report USGS/BRD/ITR—2003-0003. 274 pp. Wethington, T.A. 2001. Status and management of endangered bats in Kentucky. Proceedings of the Annual Conference of the Southeastern Association of Fish and Wildlife Agencies 55:389–395.