INTERACTIVE EFFECTS OF HABITAT MODIFICATION AND INVASIVE ALIEN SPECIES ON AMPHIBIAN BIODIVERSITY

The biodiversity of our planet is facing a severe crisis. Habitat alteration, climate change, the spread of alien species, and overexploitation are major drivers of biodiversity loss. These global change drivers often act jointly or even synergistically on population declines. To identify priorities for conservation, it is thus essential to compare the relative impact of different threats on biodiversity. Amphibians are the vertebrates showing both the largest proportion of threatened species and the highest rate of decline. Out of the ~6,800 amphibian species assessed by the IUCN, more than 4,000 species are threatened by at least one stressor, and more than 2,000 by at least two stressors. Habitat modification (alteration and/or destruction) and alien species are the two factors threatening the largest number of amphibian species. For this reason, this thesis investigates the role of habitat modification and invasive alien species in determining the temporal dynamics of amphibian populations. The role of these stressors is examined at different spatial scales and jointly with other factors such as microhabitat, landscape, or climatic variables. The first study (Chapter 2) is a continental-scale analysis, which evaluates the combined effects of alien species, habitat availability, habitat changes, and climate change in determining population trends of European amphibian (and some reptile) species over the last 45 years. The results showed that populations declined more often in areas with a high number of alien species and where climate change has caused a loss of climatic suitability. The effect of habitat availability was not relevant when averaged across species; however, when excluding the two commonest species, habitat loss was the main correlate of negative population trends for the remaining species. Furthermore, I observed a strong interaction between habitat availability and the richness of alien species, which indicated that the negative impact of alien species was particularly strong for populations living in landscapes with less suitable habitat Then, the thesis focuses on a specific system: the invasion of the red swamp crayfish Procambarus clarkii in Italy, and its effect on the population dynamics of amphibians. The second study (Chapter 3) presents the updated distribution of the red swamp crayfish in Italy. Thirty years after its first record in Italy in 1989, the red swamp crayfish has invaded 80% of Italian provinces. The invasion is particularly extensive in central and northern Italy, where this invasive crayfish is present in 100% and 96% of provinces, respectively. Given the strong negative effects that the red swamp crayfish can exert on native amphibians, its widespread distribution raises concern about the future status of conservation of the Italian batrachofauna. Chapter 4 analyses site occupancy data of newts in northern Italy, in an area suffering the introduction of the invasive crayfish, in order to investigate the role of microhabitat, landscape change, and invasive species in determining the decline of two newt species through time. Initial occupancy of newts was negatively affected by landscape alteration (i.e. urban and agricultural cover) and by the presence of fish. Both species suffered a strong decline, with a net loss of site occupancy of 25%‒36% along the study period. After the crayfish invasion, the main drivers of population dynamics sharply shifted, and occupancy changes were not determined by landscape or microhabitat alterations, as the strongest predictor of local extinctions was the colonization of wetlands by invasive crayfish. The last study (Chapter 5) focuses on the effect of habitat characteristics and invasive crayfish in determining the temporal dynamics of the amphibian community in a large number of wetlands in northern Italy. Since connectivity among wetlands is fundamental for the dynamics of spatially structured populations, we considered two measures of connectivity acting on each focal wetland: incidence of the focal species, and incidence of invasive crayfish. Survival and colonization of local populations were jointly determined by factors acting at different scales. Survival probability was positively related to the area and the permanence of wetlands, while it showed a negative relationship with the occurrence of fish. Colonization probability was highest in semi-permanent wetlands and in sites with a high incidence of the focal species in nearby sites. Furthermore, the incidence of invasive crayfish emerged as one of the strongest drivers of colonization probability. In this thesis, I showed that invasive alien species are a major driver of the decline of European amphibians. The negative effect of invasive alien species acted differently across species and spatial scales. While the site-level impact was evident only for a subset of species, the landscape-level effect of invasive crayfish was ubiquitous across the whole amphibian community. Habitat modification showed a less evident effect, but invasive alien species and habitat modification can strongly interact: the negative effect of alien species is exacerbated in highly modified landscapes. It is thus essential to directly prevent and manage the spread of alien species, while simultaneously preserving natural landscapes. This can help in mitigating the negative effects of invasives, even when habitat alteration is not the main threatening factor.