Ecology of Some Experimental Populations of the Landsnail Cepaea Nemoralis (L.). Ii. Production and Survival of Eggs and Juveniles

Abstract

Oviposition in Cepaea nemoralis occurs mainly from the end of May until the beginning of August, but there is a second peak, probably small, in egg production in October. Most of the eggs produced in late autumn die, but some survive during the winter and produce juveniles. During the summer the number of clutches produced per snail was 2.4 and mean clutch size was 72 eggs. It is suggested that some substance, possibly digestible protein and carbohydrates in the food is limiting clutch size, that the seasonal decrease in clutch size is caused by a decreasing concentration of that substance in the food and that larger snails produce larger clutches because they eat more. A comparison is made between a population which was rapidly increasing in size and one that declined. In the former egg production per snail was more than twice that in the latter and egg survival 25% better so that production of juveniles was 3 times larger. In the former some 25% of the snails that became adult did so in their first year while in the latter nu such fast growers have been observed. However, survival of juveniles in their first year in the expanding population was 20 times better. This suggests that survival of small juveniles is the key factor in the dynamics of populations of Cepaea. It is suggested that drought as well as predation by shrews, toads and frogs are the major factors affecting juvenile survival. It is argued that there may be some density governing processes acting but that their influence in population size is almost negligible.