Alessandro Culicchi: Ecological and evolutionary effects of anthropogenic change in freshwater fish communities

Abstract

Environmental change is ongoing, driven by human activities that have profoundly modified the modern world. Natural ecosystems are directly affected by these anthropogenic activities. This thesis explores the effects of anthropogenic change on natural ecosystems. Being this an extremely broad topic, we narrow down our research to some key areas of investigation, without aiming to be exhaustive. We also explore related themes. In Paper I, we review the effects of anthropogenic change on the process of speciation. We show that human intervention can alter geography or selective regimes. In both cases, this can either lead to an increase or decrease in reproductive isolation between species. In the following papers, we focus on the study case of two sister species with similar morphology and ecological niches, the common bream (Abramis brama) and the white bream (Blicca bjoerkna). In Paper II, we demonstrate that changes in abiotic and biotic conditions in lakes can disrupt the delicate co-occurrence and abundance patterns between the two species. In particular, increases in turbidity, predation and competition pose a threat to the local survival of white bream. In Paper III, we examine the effects of abiotic and biotic conditions on the phenotype of our species of interest. For this purpose, we focus on the average individual size and its relative difference between the species as a phenotypic proxy. We show that the relative difference in size is quite insensitive to external perturbations. However, we find that the presence of pike in the lake increases the similarity in the size between the study species. This can potentially threaten niche specialisation and species cohesion. Throughout Paper II and Paper III, we investigate the temporal trends of environmental and biotic factors over the last few decades, finding an increase in temperature, turbidity and the proportion of perch, along with a decrease in the proportions of pike and roach. In Paper IV, we conduct a genomic study based on whole-genome resequencing, which reveals that the morphological approach is inadequate for the taxonomical identification of our study species. Genomic data also show that hybridisation occurs, but it does not pose an immediate threat to species cohesion in the study area. Intra-specific genetic variation is low, and it suggests a pattern of isolation-by-distance. Overall, this thesis highlights that an analysis of anthropogenic influences on natural systems should be approached from different angles, to provide a comprehensive overview of such a complex phenomenon.