Epidemics in native species influence the outcome of a species invasion

Abstract

Invasive species can have large effects on native communities. When native and invasive species share parasites, an epidemic in a native species could facilitate or inhibit the invasion. We sought to understand how the size and timing of epidemics in native species caused by a generalist parasite influenced the success and impact of an invasive species. We focused on North American native and invasive species of zooplankton (Daphnia dentifera and Daphnia lumholtzi, respectively), that can both become infected with a fungal parasite (Metschnikowia bicuspidata). In a laboratory microcosm experiment, we exposed the native species to varying parasite concentrations (none, low, high) and two invasive species introduction times (before or during an epidemic in the native species). We found that none of our treatments affected densities or infection (measured as number infected or prevalence) of the invasive species, but there were more resting eggs of the invasive species in the absence of the parasite. The density of the native species was lowest with a combination of the parasite present, and the invasive species introduced during the epidemic; infection prevalence in these treatments was also higher than prevalence in treatments where the invasive species was introduced before the epidemic. Therefore, the timing of an invasion during an epidemic can affect the native species, even when there is no measurable effect of invasion timing on the invasive species. Our results suggest that the size and timing of epidemics in native species can influence the impacts of a species invasion.