Land Snail Communities in Mediterranean Upland Grasslands

It is difficult to define precisely determinant environmental factors that explain land snail distribution and abundance, and most variables are often interrelated. In addition, studies have generally examined only a limited number of environmental variables, yielding a partial and local habitat description. Recognised determinant factors are: climate, vegetation and soil. Competitive interactions, historical factors and spatial structure are rarely included, although they could generate biogeographic patterns on a local or regional scale. The goal of the present work is to evaluate land snail habitat complexity by examining a wide range of environmental variables. Land snails from open environments were studied in a Provence, range, the Grand Luberon, characterised by low variations in pH and calcium content. Grasslands on the ridges are maintained by sheep grazing. A stratified sampling was chosen according to altitude and vegetation structure. Sites were sampled throughout the range. Different vegetation structure types were analysed, from low grasslands to shrublands. Thirty-eight environmental variables were noted for each site. They described: (1) topography and climate, (2) habitat structure, (3) flora, and (4) spatial structure of snail communities. The main analytical method used was the partial Canonical Correspondence Analysis which yielded a partition of species variation into several independent components. The four environmental variable groups were used separately. We obtained a partition of ecological variation which emphasised the predominance of habitat structure over floristic composition and the important role played by environmentally independent spatial variables. Nevertheless, it remains difficult to discriminate between the influence of climatic factors s.s., historic events and/or biotic factors within the altitudinal gradient.