Density and Adult Size in Natural Populations of a Southern Appalachian Low-density Land Snail, Mesodon normalis (Pilsbry)

Abstract

-There are two principal hypotheses explaining variation in adult body size in land snails: physical environmental factors, and the effects of other individuals in the population. We examined these hypotheses in natural populations of the southern Appalachian land snail, Mesodon normalis. Repeated sampling of the populations in 8-10 different sites showed that adult shell size (data available from the 10 sites), adult capture-recapture density (data available from eight of the 10 sites) and adult abundance from relative search data (data from the 10 sites) were stable over 4 yr. However, these parameters varied significantly among sites. Density estimated from capture-recapture data varied less, year to year and site to site, than estimates of abundance from relative search data, although both showed comparable relative differences among the study sites. No significant correlations of either shell diameter or density with the six measured environmental factors (elevation, aspect, slope, cation exchange capacity, soil calcium and soil pH) across sites were found. However, adult shell size was significantly negatively associated (r2 = 0.78) with population density, suggesting that intraspecific competition may be occurring in this naturally low density gastropod species that inhabits forests. This result corresponds to that found for some of the more high-density European land snails, but was unexpected in a comparatively low-density species.