Demographic Processes Influencing Population Viability of the Iowa Pleistocene Snail (Discus Macclintocki)

Abstract

Understanding the importance of variable local population abundance and the limited potential for dispersal and genetic exchange is crucial for the conservation of many species with limited geographic distribution and specialized habitat requirements. Because of rareity, it is often difficult to study the relative importance of variation in recruitment and survival and their net effect on population growth. We designed a survey of natural populations of endangered Iowa Pleistocene snails (Discus macclintocki) using the robust mark-recapture design to estimate population size and vital rates. A dense population remained stationary throughout the 6-y study whereas vital rates fluctuated substantially in two much smaller populations. In the smaller populations rates of growth varied from sharply increasing to sharply decreasing among years, and changes in estimated recruitment were the primary vital rate influencing these fluctuations. Snails were highly sedentary and sampling at random locations showed that the populations were highly subdivided within a site. Fluctuations in demographic rates and patchy distribution may provide the basis for substantially different rates of genetic change within and among sites. Although in the short-term, fluctuations in recruitment of these snails may influence local dynamics most substantially, long-term threats of habitat loss or climatic change will likely affect survival of adults and persistence of the populations.