Can Invasive Snails Reduce Parasitism in Native Snails? An Evaluation of the Dilution Effect Hypothesis
The dilution effect hypothesis states that any non-target host species can act as a decoy or resistant host to disease or parasite transmission stages, thereby reducing the negative effects of the diseases or parasites on the coevolved host (Prenter et al. 2004, Johnson and Thieltges 2010). Invasive species are often resistant to parasites in the new ecosystem because native parasites have not evolved to be able to successfully infect them (Prenter et al. 2004). In laboratory experiments, Kopp and Jokela (2007) found the presence of the invasive snail, Lymnaea stagnalis, acted as a resistant host for trematode infection resulting in reduced infection rates in the native snail. Similarly, the presence of the invasive American slipper limpet, Crepidula foricata, and invasive Pacific oysters, Crassostrea gigas, reduced trematode infection load on native mussels, Mytilus edulis, in both single species and mixed species (both invasive species present) treatments (Thieltges et al. 2009). However, both of these experiments were conducted in mesocosms with simplified biotic interactions (Kopp and Jokela 2007, Thieltges et al. 2009).