Small mammals in a changing world: distributional, demographic and behavioural responses to environmental heterogeneity with implications for host-parasite-pathogen relationships

The demography of species and their functional role in the environment are governed by the interplay between individual internal state, external abiotic and biotic conditions, as well as by interspecific interactions, including host-parasite relationships. Generally, these interactions in wild systems have been investigated by long-term studies, and the different components were so far rarely evaluated ensemble. In this dissertation, I implemented a multi-factorial niche-based approach to predict small mammals’ proximate responses to environmental, climatic and anthropic factors, and their implications for host-parasite-disease spatio-temporal patterns. For this purpose, live-trapping targeting the small mammal community was performed across a wide latitudinal (Norway and Italy) and altitudinal (from 500 to 2500 m a.s.l.) gradient. Through field experimental designs based on manipulation of anthropogenic food availability, and longitudinal transects across heterogeneous habitats, I assessed the drivers of small mammal community composition and demography, and the cascading effects on the parasitic load and the circulation of common and emergent pathogens. I found that small mammal survival depended on intrinsic seasonal cycles and was enhanced by food availability only where harsh climate conditions occurred. Conversely, population size was mainly determined by climate constraints and food availability. When opportunistic, dominant rodent and subordinate vole species were sympatric, they showed opposite demographic trends in presence of anthropogenic food, with the subordinate species decreasing both survival and population size, likely pointing at exploitative and interference competition (Paper I). When the spatial components of these patterns were explicitly accounted for, I found that rodents also decreased their spatial range in presence of anthropogenic food. As the local density of the most opportunistic species increased and evenness decreased, their tick burden was amplified at high tick environmental density, especially in heavier individuals, and diluted at lower tick availability. To sum up, the overlap and aggregation of primary and secondary hosts at feeding sites, and more in general in anthropic, fragmented landscapes, likely enhances the completion of the tick life-cycle, inducing tick-burden amplification in a simplified community of hosts (Paper II). Ticks could also thrive in milder climatic conditions due to climate change, as observed along the Alpine altitudinal gradient. Similarly, the composition and altitudinal distribution of the host community can be affected by climate-related abiotic conditions. In accordance with this expectation, I detected the occurrence of generalist species (e.g. bank vole) also at high altitudes, so resulting sympatric with harsh climate specialists, such as snow and field voles. This upward distributional shift of generalist species, and vectors alongside, were also accompanied by circulation of common rodent- and vector-borne pathogens that seemed to show altitudinal segregation, an aspect that needs further investigation (Paper III). Among these, some protozoans, such as Hepatozoon spp. (Paper IV), that may hold a strong epidemiological role along the food web. In conclusion, this dissertation elucidated some complex ecological relationships that involve small mammals in sensitive ecosystems. At macro- and micro-scale, we experimentally demonstrated the cascading consequences of climate and anthropic disturbances on small mammal communities and populations, and their implications for the health of humans and ecosystems.