Population size and recovery criteria of the threatened Lake Erie watersnake: Integrating multiple methods of population estimation

摘要

The Lake Erie watersnake, Nerodia sipedon insularurn, occurs only in the island region of western Lake Erie, an area less than 40 km in diameter. Restricted geographic distribution and declining population size resulted in this snake's classification as Threatened in the U.S. and Endangered in Ontario and Ohio. A combination of mark-recapture methods, capture rate information, and interpolation were used to estimate the current U.S. population size of Lake Erie watersnakes. A total of 121 point estimates were generated using both 'closed' population (Lincoln-Petersen, Schumacher's) and 'open' population (Jolly-Seber, Bailey's triple-catch) methods to analyze data collected from 1980-2004. Paired t-tests, comparing estimates obtained using alternative methods, were consistently non-significant. Although standard errors and confidence intervals of individual estimates were often large, standard errors of mean estimates, obtained by averaging across methods and sets of consecutive years, were markedly smaller, averaging 14% (range = 5-25%). These analyses demonstrate the utility of mark-recapture methods even in cases where sample size and recapture rates are low, as may often be true for threatened and endangered species. Another 60 estimates were obtained by applying the Lincoln-Petersen method to samples collected in consecutive years. As expected if recruitment occurs between samples, these estimates were significantly larger than those obtained using other methods. By comparing these recruitment-biased estimates to unbiased estimates, annual adult recruitment (and by extension, adult survivorship) was estimated to be 0.63. At 11 study sites for which recent (2000-2004) mark-recapture estimates were available, population density (adults/km) was highly correlated with capture rate. The regression relationship between density and capture rate was thus used to estimate watersnake population size at 19 additional sites for which only capture rate was known. Interpolations of watersnake population size were made at another 29 sites, thus encompassing the entire U.S. range of the Lake Erie watersnake. Interpolations were based on habitat suitability, land-use practices, the observed presence of watersnakes, and density at sites for which mark-recapture or capture rate estimates were available. Lake Erie watersnake densities exceeded those of many other snakes. At 30 sites for which recent estimates were available, median density = 141 adults/km of shoreline (range = 11-1107 adults/km). Mark-recapture estimates encompassing 15.8 km and capture rate estimates encompassing 19.6 km of shoreline on the U.S. islands totaled more than 6500 adult watersnakes. Including interpolated numbers at 28.3 km of uncensused shoreline to encompass the entire U.S. distribution of the Lake Erie watersnake brought this total to nearly 7700 adults. This exceeds 5555 adults, the number specified in the Population Persistence criterion of the Lake Erie Watersnake Recovery Plan. Estimates also exceeded island-specific criteria for Kelleys, South Bass, and Middle Bass Island. On North Bass Island, estimated population size (385 adults) fell short of the island-specific recovery criterion (410 adults) unless interpolated numbers at uncensused sites were included (total = 443 adults). Other criteria for delisting the Lake Erie watersnake include Habitat Protection and Management and Reduction of Human-induced Mortality. Recent land acquisitions, development of watersnake-friendly land management plans, and outreach efforts seeking to foster public appreciation and minimize watersnake-human conflicts have contributed to achievement of these goals. These actions, together with this snake's capacity for rapid population growth, make recovery and eventual delisting of the Lake Erie watersnake a real possibility.