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

Native fishes in the Gila River basin have experienced severe declines in abundance at sites occupied by several nonnative fishes, including smallmouth bass, Micropterus dolomieu and yellow bullhead, Ameiurus natalis.  There is currently little knowledge of food web interactions among these species, nor an understanding of the spatial scales of those interactions.  We quantified food web interactions both among habitats of a study reach and among eight study reaches using dietary analysis and stable isotopes.  Specifically, we characterized the roles of native and nonnative fishes in the food web and evaluated how these roles varied within and among study reaches.  Both diet and stable isotopes showed smallmouth bass and yellow bullhead adults to be piscivorous, but fish only occurred in 20.6% (± 4.1% SD) of individual’s diets.  Variability of trophic position of native Sonora sucker, Catostomus insignis, was greater among reaches than within (CV_among = 7.3, CV_within = 5.3), whereas native desert sucker, Pantosteus clarkii, had greater variability within reaches than among (CV_among = 3.0, CV_within = 6.2).  Overall, the association between relative abundance of nonnative predators and trophic position of native fishes was not significant.