Patterns and drivers of fish trophic trajectories over time

Biological communities are facing profound upheaval induced by global environmental change. While changes in community composition over time are now well documented, much less is known about whether concomitant shifts in trophic structure also manifest. Here, we leveraged a 10-year dataset of freshwater fish communities and stable isotope compositions in nine lakes to test whether compositional changes (i.e., changes in community structure) and local environmental factors drove trophic trajectories over time. We found marked changes in the trophic structure of fish communities across all lakes, with a general tendency toward narrower trophic niches dominated by trophically redundant species. The variations in trophic trajectories among lakes were primarily linked to differences in the temporal pace and directionality of change. Specifically, lakes exhibiting greater compositional changes displayed more irregularity in their trajectory, and communities dominated by non-native species displayed elevated trophic stability over time. Our findings reveal species turnover as the dominant factor shaping trophic dynamics, through the addition or removal of predatory species and trophic turnover. The trophic stability observed in communities that were already invaded at the start of the study could be driven by their reduced susceptibility to compositional change caused by subsequent invasions. These findings highlight the existence of strong changes in trophic niches and unveil the intricate interplay between compositional changes and biological invasions in governing the trophic trajectories of communities and food web architecture, with subsequent implications for ecosystem functioning.