Alien predators are known to potentially strongly affect their prey populations. We studied the impact of raccoon dogs (Nyctereutes procyonoides) on waterbird breeding success in eight semi-urban wetlands in Finland. We manipulated raccoon dog density in two wetlands by removing individuals (2002 protection year, 2003 and 2004 removal years). We additionally performed nest predation experiments. We monitored raccoon dog density, estimated hunting bag size and observed waterbird breeding success. Our hypothesis predicts that the omnivorous raccoon dog plays a role in waterbird breeding success by depredating nests. Our experiments shown that the raccoon dog hunting bag in eutrophic wetlands may be large, as we removed 8.6–20.0 animals per km². Both our nest predation experiment and field data indicated that raccoon dogs affect the breeding success of waterbirds. We found a significant relationship between raccoon dog density index and predation rate of the artificial nests, but not between red fox (Vulpes vulpes) density and predation on artificial nests. We did not find an association between raccoon dog abundance and the breeding success of mallards (Anas platyrhynchos) and great crested grebes (Podiceps cristatus). However, our study shows that birds species with different breeding strategies – e.g. great crested grebe, mute swan (Cygnus olor), mallard, Eurasian wigeon (Mareca penelope), coot (Fulica atra), lapwing (Vanellus vanellus) and marsh harrier (Circus aeruginosus) – when considered together showed higher breeding success both in 2003 and 2004 when compared to breeding success before removal. There was, however, variation in how strongly the species responded to raccoon dog removal. Our results indicate that the removal of alien raccoon dogs can be an important tool in wetland management.

Keywords: ducks, invasive species, lapwing, nest predation experiment, Nyctereutes procyonoides, predator removal