Are invasive and native trout functionally equivalent predators? Results and lessons from a field experiment

Non-native brook trout (Salvelinus fontinalis) have replaced native cutthroat trout (Oncorhynchus clarkii) in streams across the western USA. Brook trout have been assumed to be functionally equivalent to cutthroat trout, which could argue against extensive control of this species by government agencies. This assumption was assessed by comparing the diets of the two trout species and their effects on benthic invertebrates in a large-scale field experiment. Within a Rocky Mountain stream where brook trout had already become established, trout species and density were manipulated in 20 fenced sections to create four treatments: cutthroat trout at a density natural for un-invaded streams, brook trout at two densities, natural for invaded streams or reduced to match the natural density of cutthroat trout, and areas depleted of most trout. By the end of the experiment, populations of brook trout at natural density consumed seven times more benthic invertebrates than cutthroat trout. Brook trout also consumed more Apatania caddisflies and Heterlimnius riffle beetles per capita than cutthroat trout. However, the greater consumption did not cause detectable reductions in benthic invertebrate densities. A large-scale experiment of trout removal like this may be inherently limited in its ability to detect effects on benthic invertebrate densities. However, this and several companion studies indicate that stream ecosystems would not function in the same way if cutthroat trout are replaced by brook trout. Sustaining these functions, as well as biodiversity and socioeconomic values, are important reasons to conserve native cutthroat trout. Copyright (c) 2012 John Wiley & Sons, Ltd.