Experimental Effects of Density and Food on Growth and Mortality of the Southern Appalachian Land Gastropod, Mesodon normalis (Pilsbry)

Abstract

-Hypotheses explaining variation in adult body size in land snails include abiotic environmental factors and the effects population density have on growth rates, body sizes and mortality and fecundity. We examined some of the predictions of these hypotheses in an experimental laboratory investigation using the southern Appalachian land snail, Mesodon normalis which occurs at comparatively low densities. Young reared at lower densities grew faster and became larger after 1 yr than did snails reared at higher densities. In field samples, snails which grew faster in their 1st yr become the larger adults. In the laboratory, survivorship was density-dependent, and smaller snails were more likely to die younger than larger ones. Growth rate and body size were significantly positively correlated to per capita food and negatively to density. Density-dependent food limitation was present only during the first few months of growth. For snails becoming adults, survival, final shell diameter, time to adulthood and growth rate were independent of food level. Possible interference competitive mechanisms, including mucus effects and aggressiveness, were indirectly tested by comparing the responses of the largest snails with the replicate mean, and by examining the inequality of sizes among individuals among treatments. The results suggested that the interference mechanism was a factor (perhaps mucus) acting equally on all individuals. This study indicates that Mesodon normalis is probably not food-limited, and that size-dependent juvenile growth rate and survivorship, by responding directly to density and indirectly through the effect of density on body size, is an important regulatory component of population size.