Longer food chains and crowded niche space: understanding trophic interactions between non-native and endemic, riverine desert fishes

Authors: Timothy Walsworth, Phaedra Budy, Gary P. Thiede

Year: 2010 (xlii)

Abstract

Biologists studying community structure in lotic systems often argue that abiotic factors are most influential in shaping riverine communities, such that fish-based recovery projects often focus on restoring habitat and other physical features to a perceived historical state. However, the changes in abiotic factors are often accompanied by the introduction and establishment of non-native species, fundamentally altering the flow of energy and trophic structure in these degraded systems. The San Rafael River in southeastern Utah has experienced degradation representative of many desert streams, and is home to populations of the flannelmouth sucker (Catostomus latipinnis), bluehead sucker (Catostomus discobolus), and roundtail chub (Gila robusta), each subject to a range-wide conservation agreement and collectively known as the ‘Three Species.’ Downstream sections of the river have been invaded by many non-native species, which have established populations in the river, while the upstream sections of the river are almost entirely comprised of native fishes. In this study we examined the effects of the non-native fishes on the food web in the San Rafael River using stable isotope analysis of Á¤¹³C and Á¤¹⁵N; we used convex hulls to estimate the trophic niche space of species in both sections of the river to investigate the potential for competition. Four non-native species occupy a higher, previously unoccupied trophic level in the San Rafael River, resulting in a longer food chain. The Three Species occupied similar trophic niche spaces both in and out of the presence of non-native fishes. However, the trophic niche spaces of non-native fish overlap the bluehead and flannelmouth suckers’ trophic niche spaces completely, and 89% of the roundtail chub’s trophic niche space, suggesting a strong potential for competition for food and other limited resources. These findings suggest that efforts to restore populations of these native fishes need to include actions to mediate the effects of these non-native fishes, not only the loss of habitat.