Population dynamics of Arianta arbustorum and Cepaea hortensis in western Norway

Abstract

Natural population of Arianta arbustorum and Cepaea hortensis in western Norway were studied for two years (1975 - 1977) using quadrat sampling. A searching efficiency test was performed to obtain more realistic density estimates of juveniles. Most juveniles of both species take three years to become adult. The main egg laying period was June-August. The clutch size varied considerably. However, there were no differences between years. Mean clutch size was 48.1 (A. arbustorum) and 37.9 (C. hortensis). The clutch size of A. arbustorum decreased significantly during the period June - September 1975, but no such trend was found in 1976. Mean adult density of A. arbustorum declined from 4.9 to 0.8 m^-2 between 1975 and 1977. Mean adult density of C. hortensis was 4.2 (1975), 4.3 (1976) and 3.3 (1977) m^-2. The size of juvenile cohorts varied. The 1975 cohort of A. arbustorum was three times as great as the 1976 cohort. In C. hortesis the 1975 cohort was half of the 1976 cohort. First year survival (October to October) of the 1975 cohort was 31 % (C. hortensis) and 15 % A. arbustorum). Annual production due to growth and reproduction was: 14.4 (1975) and 12.2 (1976) kcal. m^-2 for A. arbustorum 5.2 (1975) and 4.7 (1976) kcal. m^-2 for C. hortensis.