Trophic disruption by an invasive species linked to altered energy fluxes

Abstract

The Trophic Disruption Hypothesis (TDH) predicts that invasive species may cause native species to undergo trophic dispersion (change in trophic-niche area) and trophic displacement (diet switching), predictably altering food-web structure and biodiversity. In Everglades National Park, Florida, USA, African Jewelfish (Rubricatochromis letourneuxi) density has recently (2012–2017) undergone a boom-bust cycle, linked to declines of native taxa and altered aquatic-community composition that persist after the bust. Everglades restoration efforts seek to restore historic hydrologic conditions that may contribute to food-web changes unfolding coincidentally with the jewelfish boom. We used complementary datasets of stomach contents and stable isotopes (δ¹⁵N and δ¹³C) to quantify pre- and post-invasion consumer diets, trophic positions, trophic niches, basal energy use (autotrophic vs. heterotrophic), and energy fluxes to test assumptions of the TDH. The direction of change for these metrics from dry season to wet-season post-invasion (i.e., effect of adding water) was used as a proxy for the direction of effects from restored water delivery. For trophic shifts attributable to jewelfish invasion, we tested assumptions of the TDH. Comparing pre- versus post-invasion for native consumers, we observed trophic displacement in 42% of species size classes (based on stomach contents), trophic dispersion for 57% of species (based on stable isotopes) and 54% of species size classes (based on stomach contents), and overall greater reliance on autotrophic energy. Altered trophic dynamics were more frequent pre- versus post-invasion than among habitats or between seasons, and the direction of those responses was in the opposite direction of dry-season to wet-season differences and/or occurred at a higher frequency. Post-invasion food-web structure and function revealed increased relative abundance of mesopredators (including African Jewelfish) and reduced biomass and energy fluxes into and out of small fishes (e.g., Cyprinodontiformes). Our results show that African Jewelfish invasion is linked to altered spatiotemporal trophic dynamics and energy fluxes through declines in native fishes and invertebrates, which indirectly affected trophic relationships at the regional scale in the Everglades. As a result, we suggest extending the TDH to explicitly include the potential for invasive species to alter basal energy use, spatiotemporal trophic dynamics, and energy fluxes.