Field experiment to investigate potential positive effects of invasive species on native species: a test of the Dilution Effect Hypothesis, A

The invasive New Zealand mud snail (Potamopyrgus antipodarum) has reached extremely high densities in rivers of the Greater Yellowstone area. Even though P. antipodarum has some demonstrated negative effects on native species, under some conditions this invasive snail could also benefit native snails. The dilution effect hypothesis asserts that species that cannot serve as hosts to parasites, such as P. antipodarum, can act as decoys to parasites, thus benefiting host species by reducing their rates of infection. We addressed this hypothesis with parasitic flatworms known as trematodes, which obligately use snails as one of multiple hosts in their complex life cycle. Infected snail hosts often suffer decreased survival and growth, and complete cessation of reproduction. Hence, reduced rates of infection in native snails will increase individual fitness and positively affect population growth rates. Along with PiE student Michele Larson, and my faculty mentor Amy Krist, I conducted a field experiment, with two different native snail species, to learn whether the presence of P. antipodarum decreases trematode infection rate in native snails. We had three experimental treatments: native snails with low, ambient, or high biomass of invasive P. antipodarum. We predicted that with more invasive snails acting as resistant decoys to parasites, infection rates of native snails would be lowest with a high biomass of invasive snails. Surprisingly, we found very high infection rates in all treatments and hence no differences in infection levels among treatments. Unexpectedly, we also found that native snails housed with a higher biomass of P. antipodarum harbored fewer species of trematodes. Because we found very high infection rates our experiment was not an adequate test of the dilution effect hypothesis. Hence my mentors will repeat this experiment this summer in sites with lower infection levels.