Habitat alterations and the introduction of non-native species have many ecological impacts, including the loss of biodiversity and a deterioration of ecosystem functioning. The effects of these combined stressors on the community trophic web and functional niche are, however, not completely clear. Here, we investigated how artificial ecosystems (i.e. reservoirs) and non-native species may influence the trophic and functional niche space of freshwater fish communities. To do so, we used carbon and nitrogen stable isotope and abundance data to compute a set of isotopic, trait, and functional metrics for 13 fish communities sampled from 12 distinct ecosystems in Türkiye. We show that in reservoirs, fish were more similar in their trophic niche compared to lakes, where the trophic niche was more variable, due to higher habitat complexity. However, there were no differences in the trait and functional metrics between the two ecosystem types, suggesting a higher prey diversity than assumed in reservoirs. We also found that the number of non-native species did not affect the trophic niche space, nor the trait or functional space occupied by the fish community. This indicates that non-native species tended to overlap their trophic niche with native species, while occupying empty functional niches in the recipient community functional space. Similarly, the proportion of non-native species did not affect any trophic, trait, or functional metric, suggesting that changes in community composition were not reflected in changes in the community niche space. Moreover, we found that trait richness, but not functional richness, was positively related to the isotopic niche width and diversity, indicating that a wider occupied trait niche space corresponded with a wider occupied trophic niche and lesser interspecific similarity. Our findings underscore the complexity of ecological relationships within freshwater ecosystems and highlight the need for comprehensive management strategies to mitigate the impacts of human activities and biological invasions.

Department of Biology Faculty of Arts and Sciences Eskişehir Osmangazi University Eskişehir Türkiye

Department of Life and Environmental Sciences Bournemouth University Poole Dorset United Kingdom; Department of Ecology and Vertebrate Zoology Faculty of Biology and Environmental Protection University of Lodz Lodz Poland; Department of Basic Sciences Faculty of Fisheries Muğla Sıtkı Koçman University Muğla Türkiye

Faculty of Fisheries and Protection of Waters South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses University of South Bohemia in České Budějovice 389 25 Vodňany Czech Republic

Faculty of Fisheries and Protection of Waters South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses University of South Bohemia in České Budějovice 389 25 Vodňany Czech Republic; Department of River Ecology and Conservation Senckenberg Research Institute and Natural History Museum Frankfurt 63571 Gelnhausen Germany; CAMB Center for Applied Mathematics and Bioinformatics Gulf University for Science and Technology Kuwait

Faculty of Fisheries and Protection of Waters South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses University of South Bohemia in České Budějovice 389 25 Vodňany Czech Republic; Université de Rennes CNRS ECOBIO [ ] 35000 Rennes France

Faculty of Fisheries Ege University Bornova 35100 İzmir Türkiye; Department of Life and Environmental Sciences Bournemouth University Poole Dorset United Kingdom

Université de Rennes CNRS ECOBIO [ ] 35000 Rennes France

Vocational School of Health Services Eskişehir Osmangazi University Eskişehir Türkiye

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