Long-term analysis of body condition reveals species coupling and the impacts of an invasion

Abstract

Predator–prey coupling can result in oscillations of predator–prey densities. These oscillations in predator–prey densities correspond to oscillations in intraspecific competition where a high population density causes high intraspecific competition. Strong coupling of native species can however be disrupted by the introduction of invasive species into food webs. Here, we investigated how the body condition (body mass relative to body length) of a predator, lake trout, and its primary prey, cisco, changed as their respective population densities shifted. We found that the body condition of lake trout and cisco was strongly influenced by their respective population densities, that is, density dependence. The body conditions of lake trout and cisco were also inversely related, which highlights strong predator–prey coupling. Further, we were able to detect the impacts of a recent invasive species, Bythotrephes, as we saw size-specific shifts in the body condition of prey following the invasion. Overall, this study highlights how the long-term study of a simple measure, body condition, can reveal predator–prey coupling and yield new insights into the impacts of an invasive species.