EXPERIMENTAL EFFECTS OF DENSITY AND FOOD ON GROWTH AND MORTALITY OF THE SOUTHERN APPALACHIAN LAND GASTROPOD, MESODON NORMALIS (PILSBRY)

Hypotheses explaining variation in adult body size in land snails incl ude abiotic environmental factors and the effects population density h ave on growth rates, body sizes and mortality and fecundity. We examin ed some of the predictions of these hypotheses in an experimental labo ratory investigation using the southern Appalachian land snail, Mesodo n normalis which occurs at comparatively low densities. Young reared a t lower densities grew faster and became larger after 1 yr than did sn ails reared at higher densities. In field samples, snails which grew f aster in their Ist yr become the larger adults. In the laboratory, sur vivorship was density-dependent, and smaller snails were more likely t o die younger than larger ones. Growth rate and body size were signifi cantly positively correlated to per capita food and negatively to dens ity. Density-dependent food limitation was present only during the fir st few months of growth. For snails becoming adults, survival: final s hell diameter, time to adulthood and growth rate were independent of f ood level. Possible interference competitive mechanisms, including muc us effects and aggressiveness, were indirectly tested by comparing the responses of the largest snails with the replicate mean, and by exami ning the inequality of sizes among individuals among treatments. The r esults suggested that the interference mechanism was a factor (perhaps mucus) acting equally on all individuals. This study indicates that M esodon normalis is probably not food-limited, and that size-dependent juvenile growth rate and survivorship, by responding directly to densi ty and indirectly through tile effect of density on body size, is an i mportant regulatory component of population size.