Academic journal article The American Midland Naturalist

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

Academic journal article The American Midland Naturalist

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

Article excerpt

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 shortterm, 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.

INTRODUCTION

The Iowa Pleistocene snail (Discus macclintocki F.C. Baker, Discidae) is a state and federally endangered land snail that occurs on a specialized habitat type termed an algific talus slope in the karst topography region of northeastern Iowa and adjacent Wisconsin, Minnesota and Illinois. The region, often referred to as the "drifdess area," escaped the last glacial advances leaving the Paleozoic-age bedrock subject to erosion (Prior, 1991). Algific slopes, usually north facing, occur where air circulates over underground ice producing a constant stream of cold moist air through vents on to the slope. These vents are typically covered with a loose talus layer and thin plant and litter cover. Many rare plant and animal species that are considered glacial relicts persist only on these small areas of suitable habitat (U.S. Fish and Wildlife Service, 1984). Sites generally vary in size from 0.2-4 ha, although some sites are more extensive. Many sites are protected through ownership or easement by conservation organizations including the U.S. Fish and Wildlife Service, Iowa Department of Natural Resources and The Nature Conservancy, but some sites are on private land. Currently 24 of 37 sites in Iowa and Illinois where the Iowa Pleistocene snail occurs are protected by some form of conservation protection. Frest (1987) conducted extensive study of the species during the 1980s and his reports formed the basis for the species recovery plan (U.S. Fish and Wildlife Service, 1984) that guides current conservation.

The Iowa Pleistocene snail is a classic example of an endangered species with a limited geographic range, very specific habitat requirements, limited potential for dispersal and genetic exchange and variable local population abundance. Although there has been substantial interest in population and evolutionary ecology of terrestrial snails, there have been relatively few detailed studies of density, dispersal and other demographic vital rates (Williamson et al, 1977; Cowie, 1984; Baur, 1986; Sherley et al, 1998; Backeljau et al, 2001).

In general, conservation biologists agree that there is a practical need for understanding the interaction of demographic and genetic factors in extinction processes (Lehmkuhl, 1984; Lande and Barrowclough, 1987; Nunney and Campbell, 1993; Young and Clarke, 2000). Although in a theoretical sense there is some controversy about the relative importance of genetic factors and demography in determining the minimum viable sizes of wild populations (Lande, 1988; Spielman et al, 2004), in practical terms demographic fluctuations will often be of immediate concern. …

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