Retrospective food web analysis of the Gila River: Do native and non-native interactions intensify during drought?

Authors: Reese, Rosalee*, Turner, Thomas, Propst, David, Gido, Keith

Year: 2015 (xlvii)

Abstract

The relatively pristine Upper Gila River in New Mexico is a stronghold for endemic native fishes despite the presence of nonnative fishes. In other, more severely human-impacted tributaries in the Colorado River basin, nonnative fishes are a major factor in native species extirpation. Our goal is to test whether negative effects of nonnatives on natives are compounded during challenging hydrologic conditions (i.e., drought). Stable isotope analysis (SIA) allows for estimation of trophic position and trophic niche width. Fish specimens were selected from natural history collections to represent a time series that encompasses wet and dry years, as well as varying non-native abundances. We estimated Á¢â‚¬Ëœisotopic niche spaceÁ¢â‚¬â„¢ by plotting ÁŽÂ´13C vs. ÁŽÂ´15N for native and nonnative fishes and statistically compared breadth and overlap in niches among species. Our prediction is that during drought periods, isotopic niches of nonnatives and natives will overlap, indicating competition, but that in wet periods, isotopic niches will diverge significantly as an indicator of resource partitioning. SIA of museum specimens offers the potential to test key hypotheses about the impact of non-native species on an endemic fauna, and provide understanding of the environmental context that non-native species negatively impact native fishes. Such understanding is important now more than ever for conservation of the Gila River, where climate change and pending water diversion could lead to further decline of native fish abundance and eventual extirpation.