| Summary: | Locations in southern Iberia were sampled to assess patterns in the distributions and diversities of land molluscs and to determine the effects of habitat structure on diversity at differing scales. In Part One of the study 91 sites, each 1km , were sampled. Species with higher abundances were present at more sites than those with lower abundances. Low levels of similarity were found between the land mollusc communities. The monthly mean of sunshine hours per day during the wet period and the annual mean absolute humidity provided the best explanation of the species data. The results suggest that the heterogeneous habitats from the region allow a large number of mollusc species to co-exist in varied communities possibly controlled by historical and regional factors including climate and geology. In Part Two of the study 60 sites, each 1600m2, were sampled. Twenty sites were located in each of three habitat types (sand, steppe and garigue), and for each habitat type ten sites were sampled in both the wet period and dry period. Diversities and habitat structure were assessed at 1m , 5m and 20m . There were differences in the number of species, abundances and diversities between habitats, and between the wet and dry periods at each habitat. Increasing heterogeneity and complexity increased the number of species, but not their abundances. Components of habitat structure affected the number of species and abundances independently of the total complexity. Relationships between heterogeneity, complexity, number of species and species diversity were scale-dependent with most of the statistically significant relationships between these occurring at lm2. Increases in abundances and number of species with increasing area support the passive sampling model and the area per se hypothesis.
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