Effects of nonnative species on food web structure and variability in the Gila River drainage, New Mexico

Authors: Tyler J. Pilger, Keith B. Gido, David L. Propst

Year: 2009 (xli)

Abstract

The upper Gila River basin in southwest New Mexico, USA is one of the few unimpounded drainage basins remaining in North America and provides a stronghold for a unique and largely endemic fish fauna. However, coincident with introduction of nonnative predators, such as smallmouth bass, Micropterus dolomieu and yellow bullhead, Ameiurus natalis, distributions of native fishes have severely declined. We used diet and stable isotopes to elucidate trophic relationships and evaluate potential effects of nonnative piscivores in the upper Gila River basin during June-July, 2007 and 2008. Native large-bodied fishes were mainly algivore/detritivores and native small-bodied fishes were primarily insectivores. Small-bodied nonnative fishes fed on detritus and aquatic invertebrates. Nonnative predators preyed on small-bodied fishes and predacious aquatic invertebrates and had higher relative trophic positions than all native fishes. In addition, we calculated measures of food web structure, such as food-chain length and mean trophic position to evaluate variability in food web structure at two spatial scales, macrohabitat and reach scales. Food web structure was more variable among than within reaches and was significantly associated with fish species richness across macrohabitats but was weakly associated with abiotic reach-scale factors. Variation in food web structure was concordant with variation in fish community composition and suggested that factors that influence the distribution of fishes also influence food web structure.