Crowded Food Webs: The Impact of Novel Competitors and Predators on the Growth Rates of Endemic Desert Fishes

Native fishes of the Colorado River basin have suffered significant reductions in range due to multiple anthropogenic alterations.  The San Rafael River, Utah, is home to populations of the “three species”; flannelmouth sucker (Catostomus latipinnis), bluehead sucker (Catostomus discobolus), and roundtail chub (Gila robusta).  The three species are listed as state species of concern and are subject to a range-wide conservation agreement.  Alterations to flow and habitat alterations have facilitated the establishment of and exacerbated the impacts of non-native fishes in the river.  These fishes alter the flow of energy through the food web, and present novel competitor and predator archetypes to the three species.  A diversion dam and natural barriers prevent the upstream spread of invasive fishes, and result in two distinct food webs above and below.  In this study, we compare food web structure and growth rates of native fishes in areas of the river with and without established non-native species.  We analyzed food web structure using stable isotope analyses of δ¹³C and δ¹⁵N, and convex hull space was used to estimate trophic niche space and overlap between species.  We based growth on length at age from dorsal fin ray sections and the Frasier-Lee back-calculation method.  We found that non-native fishes lengthen the food chain in the river.  The highest trophic position in invaded reaches was 3.63 (green sunfish, Lepomis cyanellus), while the highest trophic position in uninvaded reaches was 3.42 (speckled dace, Rhinichthyes osculus).  In addition, several non-native species now occupy the upper end of the food chain, creating a “top-heavy” food web.  The trophic niche space of non-native fishes overlap almost entirely with each of the three species (i.e., 100% overlap with flannelmouth sucker and bluehead sucker trophic niche spaces, 96.86% with roundtail chub trophic niche space).  This new source of predation and competition could limit the activity of and resource availability to native fishes, potentially lowering growth rates.  Reduced growth rates could lead to lower survival, and reduced generation time and reproductive success, thus further imperiling the three species.  Protecting apparent source populations of the three species from invasions throughout their range, or removing established populations of non-native, could improve the three species’ likelihood of long-term persistence by maintaining trophic conditions to which they are evolutionarily adapted.