Persistence and extinction in some Cepaea populations

Eight artificial populations of Cepaea nemoralis kept in enclosures excluding visual predators, built up to peak numbers and then crashed in periods of between 10 and 20 years. Fluctuations in numbers were caused by variation in recruitment, not in adult survival. One population survived through a very low population size (14 individuals or less), and shows no significant change in morph frequencies. The very slight fluctuation in morph frequency has a probabilty of chance occurrence of 7%. Studies of 250 years or more are probably needed to distinguish systematic effects on gene frequency from random variation. The lack of any major alteration in morph frequencies, however, suggests that there has been no genetic revolution in this population.