Effects of native and invasive species on stream ecosystem functioning

Native consumers play a vital role in maintaining ecosystem functioning, but species invasion may alter these processes. We hypothesized that the individual effects of large-bodied native and invasive consumers on ecosystem functioning and invertebrate community assemblages differ from the interactive effects. In the San Marcos River (Texas, USA), big claw river shrimp (Macrobrachium carcinus, Decapoda: Palemoides) is a large-bodied native freshwater shrimp species with low abundance, and armored catfish (Hypostomus plecostomus, Loricariidae) is an abundant, large-bodied invasive species. To examine natural distribution of these consumers, we conducted repeated field surveys of the river. Catfish had a wide range of distribution with high densities in the river, while shrimp with low densities had a patchy distribution in the open areas adjacent to openings and crevices. To investigate their individual and interactive effects, we performed a 2 x 2 factorial mesocosm stream channel experiment cross-classifying the presence and absence of catfish and shrimp for assessing their effects on ecosystem engineering and ecosystem functioning. Presence of invasive catfishes in mesocosms showed a significant influence on ecosystem functioning by decreasing periphyton biomass, altering periphyton nutrient ratios, and facilitating detrital decomposition. Catfish presence altered invertebrate community composition in leaf packs and produced ecosystem engineering effects by altering benthic habitat. The native big claw river shrimp had little effect on ecosystem functioning in mesocosms, but its presence appeared to mitigate the effects of catfish on periphyton and sediments. Our results suggest that native consumers can have the ability to mitigate the effects of invasive taxa, thus conservation of native species is critical to the preservation of ecosystem functioning in stream ecosystems.