Mechanisms that favour or prevent the success of alien fishes in fresh waters: Investigating interactions with co-occurring stressors

The World’s rich freshwater biodiversity has been severely compromised by non-native fish introductions. As a result, biological invasions by fish species have been increasingly studied over recent decades, with research focusing on both the ecological mechanisms behind these processes and fish impacts on biodiversity and associated services. However, invasion processes are extremely intricate and thus complex to investigate, and this complexity is increasing in a rapidly changing world. Understanding the potential synergistic effects of co-occurring stressors—particularly those related to climate change—on the establishment and impact of non-native species has been identified as a key priority for advancing invasion science. Field investigations often face challenges in isolating the influence of individual variables on invasion processes, thereby constraining the accuracy and depth of the findings. This emphasises the importance of manipulative experiments under controlled conditions, which not only allow for specific parameters to be controlled but also provide a framework for predicting changes in invasive capacity in response to the expected future changes. The general goal of the research work collected in the present thesis was to investigate the ecological mechanisms driving the success of alien fish in freshwater environments, particularly in response to the recent concurrent alterations that have affected these ecosystems globally. The roles of structural habitat complexity and increases in freshwater temperature in shaping fish invasiveness and feeding behaviour were assessed. The effects of fish biological sex and predator/prey body size on these dynamics were also investigated. A combination of field studies and manipulative experiments was conducted for this purpose. The results highlighted several key points. 1) Habitat complexity, primarily determined by aquatic vegetation cover, mitigates the negative impacts of invasive predators by leading to reduced consumption of their most profitable prey (fish prey) and increasing intraspecific predation (cannibalism), thus modifying the trophic-functional roles of fish based on their body size. 2) Rising temperatures increase prey consumption by invasive fish across various trophic levels and reduce the potential competitive capacity of native competitors. 3) Temperature affects prey size selection, with a significant interactive effect of fish sex and warming. This suggests that, while preventing fish introductions remains critical, habitat preservation and restoration may help mitigate the impacts of established invasive populations. Additionally, considering variations in sex ratios and fish responses to rising temperatures is essential to accurately identify the role of invasive species in invaded communities. Integrated management strategies that account for the synergistic effects of fish invasions and co-occurring stressors are crucial when planning actions aimed at preserving freshwater biodiversity