TRACY, C/R; NUSSEAR, K/E; MEDICA, P/A; MARLOW, R/M; CORN, P/S; University of Nevada, Reno; University of Nevada, Reno; U.S. Fish and Wildlife Service, Nevada; University of Nevada, Reno; Aldo Leopold Wilderness Research Institute, Missoula: Estimates of population size in the desert tortoise may be impossible or impractical

Distance sampling is currently the standard method for estimating population density of the federally listed desert tortoise. However, this method requires much in order to calculate population densities accurately. It requires an accurate estimate of the probability that tortoises are available to be seen (Go). It requires accurate estimates of the observability of tortoises (Pa) that are available to be seen. Each of these parameters depends upon other variables including daily and seasonal influences on tortoise behavior and activity, observer differences in different habitats, local and regional differences in physiognomy and vegetation, annual differences in rainfall and food resources, and more. Small inaccuracies in these parameters caused by different levels of the determining variables make the accuracy of density estimates from distance sampling questionable, and sensitivity analyses suggest that the cost to mitigate these inaccuracies are likely prohibitive. This creates a dilemma that the population size of a listed species may, in some cases, not be determinable, and this creates new challenges requiring solid, scientific bases for determining the efficacy of conservation measures without employing the currency of population size or density to assess the effectiveness of those conservation measures.