Final Report Summary - INVABIOECOF (BIODIVERSITY AND ECOSYSTEM FUNCTIONING: individual-based modelling to understand and predict the consequences of biological invasions) By directly impacting biodiversity, global changes caused by the intensification of human activities can affect the relationship between biological diversity and ecosystem functioning. Quantifying how communities and ecosystems respond to these changes is crucial from both a theoretical and applied perspectives. Freshwater ecosystems provide countless services to humans, and support a rich biodiversity but they are among the most imperilled worldwide. Biological invasions of non-native species such as fish are a major driver of changes in freshwater ecosystems. Invasive fish are increasingly recognized as a significant contributor to extinction threat and biodiversity loss in fresh waters, and this is because fish invasions can display strong ecological and evolutionary impacts. For researchers, managers and policy makers interested in freshwater biodiversity, understanding, quantifying and predicting the potential impacts of invasive species is of upmost importance. Based on three innovative approach utilising state-of-the-art techniques (stable isotope ecology, functional ecology and individual-based ecology), the overall aim of the project was to determine how non-random alteration of biodiversity by perturbations may affect ecosystem functioning, using invasive fish as a non-random perturbation, to fill these knowledge gaps and support the management actions of decision makers.Our investigations revealed that the introduction of non-native species modified the trophic structure of recipient communities through an increased variability in the mean trophic position driven by the introduction of species with extreme trophic positions (herbivorous and predatory species). Analyses of stable isotope values of freshwater fish communities at the global scale demonstrated that communities containing non-native species had a different isotopic structure than communities without non-native species, although these differ between freshwater ecosystem types (i.e. lentic vs. lotic) and the characteristics (e.g. trophic position, body-size) of non-native species. Stable isotope analyses were then used to determine whether food web structure was related to ecosystem function (primary production and litter decomposition) in a set of local lakes. We found that stable isotope metrics use to characterise food web structure were sensitive to environmental pressures. We also observed that invasive fish introductions increased the level of trophic similarity within food webs. At a large geographical scale, we observed that functional and trophic diversity in communities increased as taxonomic diversity increased but the similarity between functional and trophic diversity was extremely low, demonstrating that additional site-specific factors influence trophic structure of communities. Locally, we demonstrated the importance of human activities and fishery management practices in shaping community structure and notably non-native species abundance. We also demonstrated that biological invasions strongly modified functional diversity patterns of communities since native and non-native species significantly differed in term of functional traits and niches and coexist through niche partitioning. Another locally important invader, crayfish, was demonstrated to strongly modify ecosystem functioning through a fivefold increased of litter decomposition at high invasive crayfish densities, leading to an up to 2 month earlier depletion of the leaf litter stock, strongly affecting ecosystem functioning.At the individual level, high levels intraspecific variability in functional traits and stables isotope values were observed, indicating that different life stages within an invasive species should be considered as functionally distinct entities. In addition, management activities were found to affect the patterns of intraspecific variability in wild populations (growth rate, diet composition). Trophic specialisation was then demonstrated to modify some ecosystem functions. Behavioural differences within invasive populations could be high. For instance, we demonstrated the existence of a new foraging behaviour in non-native predators consisting of capturing birds on land through intentional beaching. Differences in behavioural composition of invasive populations was demonstrated to modify prey community and ecosystem functioning, demonstrating the ecological importance of intraspecific variability in biological invasions and its importance in modeling approaches. Overall, this project has deepened our understanding on the ecological impacts of invasive fish species, providing some new knowledge to serve as decision support for freshwater management.By explicitly demonstrating the importance of intraspecific variability in biological invasion and the trophic and functional consequences of biological invasions, this project has deepened our understanding on the ecological impacts of biological; invasions, providing some new knowledge to serve as decision support for freshwater management. Importantly, the project has allowed the fellow to develop his academic career, providing him with a unique opportunity to develop his research questions, notably by obtaining additional funding for national and international projects with the project acting through leverage effect. The fellow has developed his own research group and the project was an excellent source of experience, knowledge and research outputs that allowed him to reach higher academic level and to develop independent thinking and leadership. The fellow has reinforced existing and established new national and international scientific collaborations, broadening the findings on the impacts of invasive fish species impact recipient communities and ecosystems.