Assessment of the Yellowstone Lake food web during lake trout suppression and Yellowstone cutthroat trout recovery informs conservation benchmarks

Abstract

The collapse of native Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri (hereafter cutthroat trout) in Yellowstone Lake was caused by predation by invasive lake trout Salvelinus namaycush. As an ecosystem with a low-diversity fish assemblage and several longterm data sets, Yellowstone Lake provided a unique opportunity to evaluate the influence of an invasive salmonid population undergoing suppression beyond only predator-prey dynamics. Diet data for cutthroat trout and lake trout were evaluated at varying densities to determine the effects of density on diet composition and diet plasticity. During the lake trout high-density state, lake trout consumed fewer native cutthroat trout and switched to amphipods, which were also consumed by cutthroat trout, resulting in high diet overlap between the species. As suppression reduced invasive lake trout densities, lake trout returned to consuming cutthroat trout and diet overlap was released. A shift in lake trout delta 13C signatures from the high-density state to the moderate-density state also corroborates higher consumption of cutthroat trout and invasive lake trout diet plasticity. Beyond predator-prey dynamics of lake trout and cutthroat trout, the invasion of lake trout caused > or = 25% change in energy flux for all organisms in Yellowstone Lake except for copepods. Food-web functional state did not change among food webs, but percentage of functional state contributing to total flux did vary. Herbivory was the dominant food-web functional state for all years, with the greatest percentage of flux from herbivory in 2011. In addition, by using a whole-ecosystem model that accounted for whirling disease and historical (natural) lake-level variation, I show that suppression of the lake trout population is necessary for cutthroat trout recovery, but the amount of suppression effort needed for cutthroat trout to reach recovery benchmarks is linked to severity of climate change. Additionally, if climate change increases the frequency and severity of reduced lake levels in the future, cutthroat trout recovery benchmarks may need to be adapted. With this research, I demonstrate how the feedbacks among predator-prey dynamics, disease, and climate change can complicate the suppression of invasive species and the conservation of invaded ecosystems and must be considered for establishing realistic conservation benchmarks.