POPULATION DYNAMICS OF THE SNAIL THEBA PISANA (HELICIDAE) IN THE SAND DUNES OF NORTHERN ISRAEL

ABSTRACT

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 consisted 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. After 3–4 weeks, a new cohort hatched and grew intensively throughout winter to the end of spring. However, a group of small snails ceased growing and entered an early aestivation. These growth-inhibited individuals retained immature size. Their proportion in the population is estimated at 5–10% throughout the year. The population in the laboratory showed a similar growth pattern and size distribution as that in the field. The inhibited-growth group in the laboratory was 40% of the population. We conclude that Theba pisana in Israel developed a strategy of mixed annual and biennial life cycles where part of the population is growth-inhibited in the first year and is retained as a reserve to ensure the continuity of the population in the face of changing biotic and abiotic regimes in a typical Mediterraenean climatic zone.