Non-native grazers in novel environments: Consequences of introduced armored catfish in stream ecosystems

Background/Question/Methods

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 consumers (i.e., herbivores and detritivores) are well positioned in food webs to have major impacts on community structure and ecosystem function because they can have strong top-down effects that cause profound changes that ripple through ecosystems. 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 the structure of lower trophic levels. 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 southern Mexico. We also measured the same environmental parameters in high-density and low-density loricariid invasion sites in this river.

Results/Conclusions

Loricariid grazing significantly affected the stoichiometry and abundance of epilithon in the experimental manipulation and the invasion site survey. Grazing by high densities of exotic armored catfish significantly reduced the abundance of epilithon and the total amount of carbon and nitrogen stored in epilithon compared to ungrazed treatments in the exclosure experiment. Similarly, the low-density invasion site had larger amounts of total carbon and nitrogen stored in the epilithon and more epilithon per area than the site with high densities of loricariids. Additionally, loricariid grazing significantly affected the abundance and diversity of macroinvertebrates in the exclosure experiment and the invasion site survey. Ungrazed experimental treatments and the low-density invasion site had higher densities of macroinvertebrates and greater species richness of macroinvertebrates relative to grazed treatments and the high-density invasion site. These patterns were driven by large numbers of chironomids found in ungrazed fish exclosures and in the low-density invasion site. The results of the exclosure experiment and the survey suggest that exotic grazing fishes may fundamentally alter the composition and chemistry of epilithon and macroinvertebrate community structure in benthic habitats.