Population density affecting adult shell size of snail Cepaea nemoralis L.

Abstract

THOMAS et al.1,2 have questioned the ability of molluscs to limit their population size by self inhibition at high densities. They suggest that plant metabolites may have caused the reduction in growth and fecundity of the aquatic snail Biomphalaria glabrata Say ascribed to crowding in previous studies3,4. Field evidence for density-dependent regulation is limited. Yom-Tov5 found that the fecundity of the desert snail Trochoidea seetzeni Pfeiffer was adversely affected by population density; he considered that either self-inhibition or nutritional differences were responsible. While studying the population dynamics and energetics of the land snail Cepaea nemoralis L., we have found evidence for density-dependent effects that cannot readily be explained by resource limitation. In samples of C. nemoralis collected in 1968 to study shell polymorphism (M. A. Palles-Clark, unpublished) a negative correlation was noticed between adult shell diameter and sample size (r =−0.60, P <0.001). Snails were collected from chalk grassland between Beacon Hill and Round Down, West Sussex. A more detailed study carried out in the same area during the summer of 1973 confirmed this effect. The density of adult C. nemoralis was measured by mark-recapture at nine 20 × 20 m sites, situated 100–600 m apart. Each site was sampled in May, June and August. As there were insufficient recaptures to use multiple recapture methods for all sites, adult population densities were estimated for the second sampling occasion using the Lincoln index.