Effects of terrestrial invertebrate reduction on three stream fishes in experimental mesocosms

Abstract

Terrestrial invertebrates (TIs) provide an important trophic subsidy for many stream fishes. However, more information is needed regarding the degree to which different species rely on this subsidy and the potential consequences of altering subsidy levels. Such information is important for understanding foodweb dynamics and predicting patterns of community change as subsidy levels vary among habitats and over time. I manipulated TI inputs to experimental streams and tested for effects on diet and body condition of fishes from 3 different trophic functional groups. Study species included a TI specialist (blackstripe topminnow Fundulus notatus), a benthic invertebrate specialist (orangethroat darter Etheostoma spectabile), and an invertebrate generalist (bigeye shiner Notropis boops). Both F. notatus and N. boops changed diets under TI reduction, but only F. notatus body condition decreased, probably because of its lesser ability to use autochthonous resources. Notropis boops body condition was unchanged by TI reduction. Unlike F. notatus, N. boops was able to incorporate autochthonous C into body tissues, as indicated by muscle δ13C values. Etheostoma spectabile diet and body condition were unaffected by TI manipulation. These results suggest that stream fishes in functionally diverse assemblages are not affected equally by TI alteration, and that effects may be predictable based on trophic functional group classification. Such information is useful in predicting patterns of assemblage change with terrestrial subsidy reduction, as often occurs with anthropogenic landscape alteration.