Biodiversity and ecology of Tuscan malacocoenoses with particular reference to the effects of human impact

For the most part, there is no doubt that human impact on biodiversity has been negative: habitat loss and degradation, pollution, overexploitation, and introduction of invasive species had played significant roles in biodiversity decline. However, human impact on Earth is not always so detrimental and its effects are not the same for all taxa. The aim of this thesis was to assess the anthropogenic impact on one of the most endangered and often unnoticed groups - terrestrial molluscs - firstly focusing on natural and then urban context. In Chapter I, effects of forest age as a surrogate for forest continuity on land snail richness and composition in Tuscan evergreen and deciduous woods were assessed. Chapter II explored the role of dispersal and local environment in urban land snail assemblages comparing different human-made habitat types in three cities in central Italy (Grosseto, Siena and Arezzo). Finally the link between man and urban environment has been further elaborated in Chapter III, where a fully synanthropic land snail species (Papillifera papillaris) has been used to assess athmosferic pollution from human activities, especially vehicular traffic, in the urban area of Siena. Concerning results obtained, Chapter I revealed that successional age as surrogate for forest continuity parameter seemed not to be a key factor in controlling land snail species richness and abundances but habitat structure more than forest age was the factor mostly involved in affecting communities; furthermore, no difference was found between evergreen and deciduous oak litter in terms of land snail richness, abundance and diversity. Focusing on urban environment, Chapter II demonstrated that cities can show relatively high land snail richness with values comparable to natural areas. After having accounted and later removed spatial structures influence, urban gradient effect turned out to be the principal component structuring urban land snail assemblages. This finding proved a likely human-mediated transport, which enhances dispersal abilities of organisms normally considered scarcely mobile. However, no establishment there would be in the absence of suitable microhabitat conditions. In particular, “vegetation cover” and “distance from the city centre” were the environmental variables that explained most of the variation in species composition. In Chapter III, results elected Papillifera papillaris a promising species to monitor the metal distribution and bioavailability in urban environments and to evaluate the possible transfer of pollutants to higher levels in food chains. Mollusc soft tissues showed very low concentrations of lithophile elements, and much higher concentrations of anthropogenic metals such as Cd, Cu, Pb, and Zn while excreta and traditional biomonitors such as mosses were the matrices most involved in lithophile contamination. Surprisingly, Papillifera papillaris tissues showed extreme Mn values. These concentations, for their magnitude, have no comparison with those reported in literature for other species of terrestrial gastropods. Excreta showed a high concentration of Pb, demonstrating the scarce bioavailability and absorption of this element and probably its limited impact on urban food chains.