La ricchezza specifica dei Vertebrati in Sicilia

Abstract

The ‘Atlas of Sicilian Vertebrates’ recorded the distribution of Vertebrates during the 1993-2006 on a UTM grid with cells of 10 x 10 km. Excluding the Chiropterans and all the vertebrates living on the surrounding small islands, 193 species (7 Amphibians, 18 Reptiles, 147 Birds, 21 Mammals) resulted to be present in Sicily. Preliminary investigations found a statistically significant correlation among species richness of each class in the UTMs, which means for example that the richest cells in Amphibians were also the richest in Reptiles, Birds, etc. This correlation persisted when the area-effect was removed by excluding all peripheral cells that included coastland smaller than 100 km2. There is a low probability that such correlation was an artefact due to the differential sampling efforts across Sicily because censuses were conducted by a high number of collaborators who acquired a thorough territorial knowledge during this study. We thus hypothesised that some macro-ecological variables could be responsible for the distribution of species richness in the territory. We considered 61 variables belonging to 13 climatic, physiographic and land use data-sets. They were preliminarily treated to rule out auto-correlation and collinearity. This procedure allowed us to extract 25 independent predictors that we regressed on species richness per each Class and on the total Vertebrate richness in the UTM cells by using the GRM method and a forward stepwise selection of variables. Most variation of total vertebrate richness in Sicily was explained by a limited set of 5 variables: 2 Corine Land Covers (forests and semi-natural areas; wetland and water corpses) and 3 climatic (annual potential evapo-transpiration range, annual mean temperature excursion and De Martonne’s aridity index). Bird species richness was explained by the whole set of 5 variables, whereas a more restricted set explained the distribution of Amphibian (forests and semi-natural areas; wetland and water corpses; annual potential evapo-transpiration range), Reptile (forests and semi-natural areas; annual potential evapotranspiration range) and Mammal (annual potential evapo-transpiration range; annual mean temperature excursion) species richness. The UTM cells with highest values of species richness were characterized by an annual potential evapo-transpiration range 1.5 times larger than in the poorest cells, and by a threefold annual mean temperature excursion. The extension of forests, semi-natural areas, wetlands and water corpses were, on average, twofold than those in the poorest areas. More species, generally Birds, live in driest areas as shown by the inverse relationship between the De Martonne’s aridity index and species richness. Apparently, in the area equivalent to a UTM cell of 100 km2, the wider is the range of climatic variation, the larger becomes the environmental heterogeneity. This, in turn, provides more suitable habitats and potential niches for species.