LB 22-229 - Fish invasions restructure aquatic food webs and trophic dynamics

Background/Question/Methods:

Invasive species are a leading cause of global biodiversity loss and a major economic burden. For instance, estimates of damage in the United States from invasive species since 1960 total at least $1.2 trillion. In Everglades National Park, two invasive species, African Jewelfish (hereafter ‘jewelfish’, Hemichromis letourneuxi) and Asian Swamp Eels (hereafter ‘swamp eels’, Monopterus albus) have undergone recent, drastic increases in density. Many native taxa have shown corresponding decreases in density. We studied food-web structure and function to quantify the effects of jewelfish and swamp eels in Florida wetlands. Complementary datasets of stomach contents and nitrogen and carbon stable isotopes allowed comparisons of pre- and post-invasion consumer diets, trophic positions, trophic niches, and basal energy use among habitats and between seasons. We use these data to test the Trophic Disruption Hypothesis (TDH) that predicts invasive species cause trophic dispersion (change in trophic niche size) and trophic displacement (prey switching). Furthermore, we predict trophic niche sizes will increase (consumers become more generalist), trophic positions will decrease, and that consumer will use more autotrophic energy post-invasion.

Results/Conclusions:

Diet data revealed that jewelfish and swamp eels are opportunistic carnivores eating fishes and invertebrates, and that jewelfish also consume detritus. Post-invasion, we documented trophic displacement in nearly half of native consumers, decreased trophic positions, increased reliance on autotrophic energy, and trophic dispersion, usually in the form of decreased trophic niche size, for the majority of consumers. Changes in trophic dynamics were more common when comparing pre- and post-invasion than among habitats and season post-invasion. Broadly, post-invasion changes were driven by decreased consumption of omnivorous zooplankton, decapods, and fishes combined with increased consumption of snails and aquatic mites. This study represents a rare comparison of pre- and post-invasion food webs stratified by habitats and seasons and supported the TDH. Fish invasions have altered trophic dynamics and declines in native fishes and invertebrates are likely caused by both consumptive (predation) and non-consumptive effects (shared prey groups, fear) from both jewelfish and swamp eels. Given time, the TDH posits changes to trophic dispersion and trophic displacement will change food-web structure, food-web function, and biodiversity. We observed changes to food-web structure (trophic displacement and dispersion) and function (elevated autotrophic energy use) with unknown consequences for diversity.