POPULATION-DYNAMICS OF THE SNAIL THEBA-PISANA (HELICIDAE) IN THE SANDDUNES OF NORTHERN ISRAEL

The population dynamics of the land snail Theba pisana (Helicidae) was studied for two successive years in the sand dunes of northern Israel and under laboratory conditions. The field population in autumn consi sted of two size classes: large snails of more than 10 mm in diameter and small snails of less than 5 mm in diameter. After the first rain, the large snails that emerged from aestivation laid eggs and died. Aft er 3-4 weeks, anew cohort hatched and grew intensively throughout wint er to the end of spring. However, a group of small snails ceased growi ng and entered an early aestivation. These growth-inhibited individual s retained immature size. Their proportion in the population is estima ted at 5-10% throughout the year. The population in the laboratory sho wed a similar growth pattern and size distribution as that in the fiel d. The inhibited-growth group in the laboratory was 40% of the populat ion. We conclude that Theba pisana in Israel developed a strategy of m ixed annual and biennial life cycles where part of the population is g rowth-inhibited in the first year and is retained as a reserve to ensu re the continuity of the population in the face of changing biotic and abiotic regimes in a typical Mediterraenean climatic zone.