Biological invasions are a key driver of global environmental change and biodiversity loss at regional and global scales. Although invasive fishes are widely recognized as a major threat to aquatic biodiversity, there is a limited understanding of the mechanisms driving their ecological impacts. Many invasive fishes are functionally distinct (i.e. life history, morphology, behaviour) from native fishes, and may generate new direct and indirect biotic interactions in invaded ecosystems. This may dramatically change the architecture of food webs, leading to trophic dispersion of food webs through increased food chain length. There is an increasing need to quantify the ecological impacts of invasive species using a food web approach once this is essential to demonstrate the multiple effects associated with new biotic interactions in invaded communities.

The current proposal aims to evaluate how invasive fishes alter the structure of freshwater food webs and how environmental change strengthens these effects by modifying the interactions among species. Specifically, we will focus in Mediterranean streams, and will use the Lower Guadiana Drainage as a case-study. First, we will evaluate the long-term feeding relationships between museum-archived and contemporary fish specimens based on stable isotope analyses over a 30-years period. Second, we will quantify changes in food web structure in contemporary assemblages along gradients of increasingly invaded fish communities and environmental change, using stable isotopes, in order to evaluate the extent of trophic niche contraction and expansion for native and invasive species. Third, we will analyze intraspecific variation in functional traits for native and invasive species based on morphological analysis to identify the functional attributes of fish that drive impacts on invaded communities. Finally, our major findings will reach an international audience through publications in peer-reviewed journals, conference presentations and organization, and will be disseminated in dedicated websites to the general public and stakeholders.

To date, most studies on biological invasions in freshwater ecosystems were primarily descriptive and only reported putative ecological impacts. In particular, there is a lack of knowledge of invasive fish impacts on the food webs in Mediterranean streams. Our project will use an integrative approach, combining long-term and contemporary patterns, in an effort to understand how invasive fishes impact freshwater food webs. This will be the first study to use stable isotope analyses to quantify the impacts of invasive fishes in the Iberian Peninsula. The results from this research will provide new insights in food web ecology and invasion biology in Mediterranean streams, that might be useful in a range of dynamic and highly fragmented riverscapes.