Field evidence for a parasite spillback caused by exotic mollusc Dreissena polymorpha in an invaded lake.

This study provides field evidence in support of the "parasite spillback hypothesis" that predicts a positive numerical response of native parasites to the presence of an exotic host that is competing for these parasites with native hosts. We examined the role that the exotic mollusc Dreissena polymorpha (zebra mussel) plays as the second intermediate host of the echinostomatid trematode Echinoparyphium recurvatum in Lake Naroch, Belarus. We found that due likely to the disproportionately high abundance of its novel exotic host, the metacercarial stage of this parasite has become substantially more abundant than that of other echinostomatid trematodes recorded in native molluscs. This finding suggests an increased load (the "spillback") of E. recurvatum on native waterfowl that serve as its final hosts. The risk of aquatic birds becoming infected with this trematode via feeding on D. polymorpha was found to vary both spatially and seasonally. This risk was particularly high in the autumn at a depth of 2 m and at sites of bird gatherings.