THE ROLE OF EXOTIC SPECIES AND ENVIRONMENTAL STRESSORS IN THE DECLINE OF ITALIAN FRESHWATER FISH

The importance of biodiversity for ecosystem functioning and resilience is widely acknowledged. Nevertheless, biodiversity constantly declines worldwide. Among different ecosystems, freshwaters appear more vulnerable to biodiversity loss due to several anthropic disturbances, to cite some, habitat degradation, pollution and overexploitation of water resources, introduction of exotic species, overfishing and climate change. The scope and magnitude of these pressures has been investigated in many studies but some uncertainties still remain on the interplay between different pressures and especially on the effects of exotic species. This thesis investigates the decline of native fish community in Italian rivers and canals, analyzing the effects and the interactions of environmental pressures and exotic fish species. Official data from provincial and regional fish inventories were collected and digitized in homogenous datasets which, at following steps, extended from the whole Emilia-Romagna Region, to the Padano-Veneto District, and to the whole Italian territory, from North to South, including main islands, for a total of 4126 fish sampling sites along the Italian watercourses. Data analysis is organized in three main parts: 1)the assessment of the fish communities composition and the identification of the most important factors of disturbance, 2) the analysis of alpha and beta diversity of native and exotic components of the fish communities, and 3) the effects of exotic fish species on functional diversity of fish communities. The results have highlighted that 1) exotic species are widespread and in many cases the invasion degree was severe. Environmental features generally resulted less detrimental to the native fish community than exotic species. This trend reached an apparent paradox in the highly altered Oglio River, where the two dams, unpassable to fish, present in the middle course, acted as a defense, rather than an obstacle, for the surviving native community, as prevent exotic species upstream migration from the highly invaded Po River. Other evidences have further supported this observation, with exotic species pushing the native ones on the edge of local extinction in their optimal native range and displaced some residual populations on the edge of their natural altitudinal distribution. Similar results were found also in term of diversity 2) with alpha and beta native diversity higher in the upland rivers, whereas exotic species resulted dominate large rivers and canals in the lowlands. The results suggest also a homogenization process in fish communities due to few exotic species, widespread and dominant in most of lowland waterways. Inside invasion dynamic, the results 3) suggest that not all generalist traits are equally important for a successful invasion. Temperature and low oxygen tolerant species, with some eurytopic traits resulted more advantaged than native ones, and dominant in the exotic communities. On the scale of the whole Italy, a high invasion degree corresponded to lower functional diversity. Furthermore exotic species showed a lower functional diversity than native species and had more negative effects than translocated species on the diversity of fish communities. Overall, results of this thesis have also provided an updated picture on the state of the fish communities in Italy, calling for urgent need of conservation and management actions