Persistence and extinction of local populations of the garden snail Helix aspersa in unfavorable environments.

In southern California, previously disturbed but currently uncultivated habitats are unfavorable environments for the introduced snail Helix aspersa. In these habitats, snails were often distributed in small, local populations only a few meters apart. Migration between neighboring populations was minimal because the snails have strong homing tendencies. Local micro-environmental differences produced differences in the demographic properties of 4 adjacent populations during studies spanning 5 years.The major unfavorable feature of these habitats was 6-8 months annual drought which caused slow and intermittent population growth. Snails nested successfully only 3 times in 5 years, but juveniles from only 2 of these cohorts reached sexual maturity. Most growth was restricted to the spring and snails were up to 4 years old when they matured, except for one population where additional water during dry weather enabled snails to mature in 6-8 months.At least 74% of deaths were caused by small mammals, and one population went extinct when adverse physical conditions prevented recruitment during a period of heavy mortality. Adverse physical conditions alone did not lead to population declines or extinctions.