Tropicalization interacts with other human stressors to mediate the diets of expanding and resident herbivores

In response to warming temperatures, species worldwide are expanding their range poleward. As these species move into new ecosystems, they interact with novel organisms and may alter food webs. Climate-driven expansions can thus lead to cascading changes in ecological interaction webs, affecting ecosystem function and services. However, climate change rarely acts in isolation. Other anthropogenic stressors, such as pollution and habitat conversion, act in concurrence with the indirect, biotic effects of climate change. In this study, we employed gut content analysis to investigate dietary similarity between sheepshead (Archosargus probatocephalus) and a tropical congener, sea bream (A. rhomboidalis), in the Indian River Lagoon, Florida (USA). We paired these data with long-term seagrass and macroalgae monitoring data to investigate the effects of a macrophyte cover gradient, as a proxy for eutrophication driven die-offs, on the diets of these fish species. Sheepshead and sea bream had highly similar diets, dominated by macrophyte material, though we also detected a significant effect of fish species identity. Furthermore, tropical sea bream consumed more seagrass at locations with higher seagrass abundance compared to low seagrass abundance, whereas sheepshead consumed a similar amount across seagrass abundance levels. These results suggest the potential for competition between these species, which may impede future recoveries of currently declining sheepshead populations. Invasion of sea bream may hinder recovery of imperiled seagrass populations by increasing grazing pressure in this eutrophic, light-limited system. We provide evidence that the indirect effects of climate change and other anthropogenic stressors can interact and influence ecological interactions.