Some aspects of the ecology of the land snail, Helicella virgata, in South Australia

Abstract

H. virgata was introduced to South Australia from Europe and has since become remarkably numerous. Its natural history is described, including breeding, growth, and food. There is normally one generation per year, the eggs mostly hatching in autumn and the young growing rapidly until spring. Marking and recapture were used to determine growth curves in the field. Snails are only active when the ground is moist and the humidity high. Even in winter dormancy is frequent, and in summer it is prolonged. H. virgata is microphagous, feeding mainly on the topsoil and surface litter, avoiding places from which these are removed. It was shown experimentally that the numbers of young produced, their rate of growth, and the length of life of adults all decreased with increasing density. This was explained in terms of increasing starvation. A natural population was studied for nearly 2 years. In places the density of this population was high and there was evidence of a consequent shortage of food, which affected young and adults differently. "Effective fecundity" (p. 509) was inversely proportional to the density of the population. There were marked aggregations, apparently reflecting the distribution of food. However, the degree of "patchiness" (Lloyd 1967) remained fairly constant with time. Over most of its range H. virgata is relatively scarce and the numbers in these places are less likely to press upon the food resources. The effects of cultivation and trampling, lack of calcium in the soil, and other factors must explain this scarcity. Weather can certainly be important; the drought of 1965-66 reduced all populations severely.