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