Trophic Dominance by an Assemblage of Invasive Fishes in a Large River

Abstract

Invasive species and climate change are accelerating changes in biodiversity. Predicting these changes is difficult, especially in large rivers which are hotspots of freshwater biodiversity and important dispersal corridors for aquatic organisms. The lower Illinois River is a major dispersal corridor between the Mississippi River watershed and the North American Great Lakes. The mainstem of this river is highly modified for navigation and may serve as a refuge for invasive species. We compared native and invasive fishes within trophic groups at the scale of an entire river to determine how invaders have risen to dominance. During fall 2012-2017, mainstem river fishes were sampled using standard gears and side-aspect, split-beam echosounding. Biomass and individual body sizes of fishes were used to estimate consumption. Whole-assemblage biomass and consumptive demand of herbivores, insectivores, planktivores, and piscivores were extrapolated from river segments to the entire 315-km main channel. The consumptive demand of native and invasive planktivores was simulated as a function of increasing river temperature and declining food quality. Across the entire fish assemblage, invaders dominated biomass but not species richness each year. Invasive and native fishes reached a fall-season maximum of 6,370 thousand tons and 2,942 thousand tons within the mainstem river, respectively. The total biomass of all but the piscivore trophic group was dominated by invasive fishes. Regardless of total assemblage biomass, invasive fishes had lower energy demand than natives, likely providing them a competitive advantage. Invasive planktivores required less energy than natives to compensate for both increased temperature and declining food quality. Assemblages of invasive species with large bodies and efficient energy consumption allow them to dominate trophic levels and spread into novel ecosystems.