Outbreaks of disease in native species can reduce invasion success

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Paradyse E. Blackwood, Kacie L. Jonasen and Catherine L. Searle, Department of Biological Sciences, Purdue University, West Lafayette, IN

Background/Question/Methods
Invasive species can have large effects on native communities. When native and invasive species share parasites, the presence of this shared enemy should favor the less susceptible host species, facilitating or inhibiting the invasion. We sought to understand how the size and timing of epidemics in native species influence the ability of an invasive species to successfully establish. We focused on a native and invasive species of zooplankton (Daphnia dentifera and Daphnia lumholtzi, respectively), that can both become infected with a fungal parasite (Metschnikowia bicuspidata). Previous work has shown that the native species experiences larger reductions in density when exposed to the parasite compared to the invasive species. In a laboratory microcosm experiment, we exposed the native species to varying parasite levels and two invasive species introduction times (early epidemic, late epidemic), and measured the density of the invasive species as a measurement of invasion success. We predicted that the invasive species would be more successful when the native species had recently experienced a large epidemic (i.e., high density of the parasite, later introduction time).
Results/Conclusions
We found that the invasive species was more successful in the absence of the parasite, while the timing of the invasion seemed to have minimal effects on their success. Both species experienced reduced densities in the presence of the parasite and the invasive species reduced the total density of the native species. However, densities of the invasive species remained low in all treatments, indicating that the parasite might be especially harmful for species at low densities, particularly in the presence of an alternate host (here, the native species). More broadly, our results suggest that epidemics in native species can be detrimental to invasive species and the timing of the invasion during an epidemic might not have a large effect on invasion success