Trophic interactions shift along the invasion gradient of an invasive freshwater fish

Biological invasions are among the leading drivers of freshwater biodiversity loss, causing wide-ranging detrimental impacts on recipient ecosystems. Understanding invasion success and the ecological impacts of non-native species requires evaluating populations across different invasion stages, as impacts may remain localized or propagate throughout the recipient ecosystem. The aim of this study was to assess trophic interactions and their ecological implications between invasive pumpkinseed Lepomis gibbosus and three native fish species, Squalius fellowesii , Petroleuciscus ninae , and Oxynoemacheilus germenicus , across a continuous invasion gradient in a temperate stream basin. This was achieved by examining spatial and seasonal variation in trophic position, isotopic niche width, and trophic niche overlap among populations spanning the invasion core, intermediate, and invasion-front regions. Lepomis gibbosus trophic position did not vary significantly along the invasion gradient, whereas isotopic niche width increased significantly with distance from the invasion core and also varied seasonally, being significantly larger in summer, autumn, and winter than in spring. Trophic niche overlap between L. gibbosus and all three native fish species increased significantly with distance from the invasion core. Community isotopic structure, represented by nitrogen range and mean nearest neighbour distance, also changed significantly, with both metrics declining with increasing distance from the invasion core. The results suggest that trophic interactions of an invading species vary strongly along an invasion gradient, demonstrating that ecological impacts are spatially dynamic rather than fixed attributes of invasive species. Consequently, invasion impacts should be viewed as dynamic processes that evolve continuously as invasions progress through space and time.