Do Low-Trophic Position Invaders Matter?: Exotic Grazing Fish in Novel Ecosystems

The rates at which biotic invasions are occurring are unprecedented and this is especially true with fish species. Fish introductions have been linked to declines in native species, but few studies have examined the effects of fishes invading at lower trophic levels. Low-trophic position organisms, including herbivores and detritivores, are perfectly positioned in food webs to have major impacts on community structure and ecosystem function. They have strong top-down effects that can cause profound changes in ecosystem functioning. These organisms can also have bottom-up effects that disconnect natural energy pathways that lead to higher-level consumers.  The objective of our study was to elucidate the impact of an exotic grazing fish, armored catfish (Siluriformes: Loricariidae), on epilithon content and abundance and macroinvertebrate community structure. We measured epilithon dry mass, percent organic matter, and stoichiometry and macroinvertebrate abundance and diversity in a series of in situ enclosures and exclosures in an invaded river in Chiapas, Mexico. Grazing by high densities of exotic armored catfish significantly reduced the abundance of epilithon compared to ungrazed treatments. Moreover, ungrazed treatments tended to have greater carbon and nutrient content than grazed treatments. Ungrazed treatments had higher densities and species richness of macroinvertebrates relative to grazed treatments. These results suggest that exotic grazing fishes may fundamentally alter the composition and chemistry of epilithon and macroinvertebrate community structure in benthic habitats.