EFFECTS OF CONTRASTING HABITATS AND WEATHER ON ABUNDANCE OF THE LAND SNAIL THEBA PISANA (O. F. MULLER, 1774): A 34-YEAR STUDY

The effects of climate on abundance depend on local environments. A predicted interaction was tested in the land snail Theba pisana, in contrasting habitats of dense Acacia thickets and adjacent areas of low open vegetation, in introduced populations in Western Australia. The snails aestivate on vegetation over summer and mortality is higher in the Open habitat than in the Acacia. Growth experiments in winter showed that this advantage of the Acacia habitat in summer is countered by more rapid growth and higher survival of juvenile snails in the Open habitat. Thirty-four successive annual censuses of adult and subadult T. pisana were analysed, to test (1) the prediction that harsher summers would reduce abundance in the Open habitat, with less impact in the sheltered Acacia, and (2) the relative importance of summer (survival during aestivation) and winter (reproduction and growth) for abundance. As predicted, abundance in the Open habitat was lower following hotter, drier and sunnier summers. More surprising, abundance in the Acacia habitat showed the reverse association. Correlations with individual weather variables indicate that, while summer temperature and sunshine are important in the Open habitat, low summer rainfall, rather than high temperatures, is associated with higher abundance in the Acacia habitat. Winter conditions do not predict subsequent abundance of adults in either habitat, indicating the greater importance of summer mortality in determining abundance, but in different ways in the two habitats. This contrast is an example of the importance of considering specific habitats in the search for effects of changing climate on abundance.