Life History, Population Dynamics and Polymorphism of Theba pisana (Mollusca: Helicidae) in Australia

Abstract

(1) The land snail Thebapisana (Muller) was sampled for 3 years in a permanent pasture and adjacent roadside vegetation at Mt. Benson, South Australia. The breeding season lasted from late summer to spring. Two distinct cohorts of offspring were produced; the earlier reproduced when 1 year old, the later probably when 2 years old. (2) Samples from other sites throughout the species' range in south-eastern and southwestern Australia suggested that mixes of annual and biennial life cycles were common. However, at some northern, hot, dry sites, only annual life cycles were evident. (3) Snails were most abundant in the pasture at Mt. Benson in spring, especially near its edge, and rare in summer. The reverse was the case for the roadside vegetation. Dispersal to and from roadside aestivation sites helps to explain this pattern. Snails were common well into the pasture where suitable aestivation sites were available. Snails died in the pasture in summer if confined without access to such sites. (4) Snails were aggregated within the pasture at all times of the year, not only in summer. Small snails (< 12 mm diameter) were more restricted in microhabitat than large snails. Possible reasons for this are discussed. (5) There was no evidence of a geographic dine in the shell-banding polymorphism of T. pisana in Australia. Unbanded snails were generally most common. Slightly higher percentages of banded snails were found in sheltered habitats than in exposed habitats at Mt. Benson but there was no evidence of differences in seasonal mortality or dispersal of the different morphs. (6) Strategic baiting along fencelines to prevent invasions from adjacent areas of high snail density and heavy grazing, or slashing of vegetation, to reduce aestivation sites are suggested as ways of reducing the pest status of T. pisana in pastures.