Impact on bird fauna of a non-native oyster expanding into blue mussel beds in the Dutch Wadden Sea

Intertidal mussel beds are important for intertidal ecosystems, because they feature a high taxonomic diversityand abundance of benthic organisms and are important foraging grounds for many avian species. After the introductionof the Pacific oyster (Crassostrea gigas) into the EuropeanWadden Sea,manymussel beds developed intooyster dominated bivalve beds. Despite the fact that oysters have been colonizing many European intertidal areasfor about two decades, their impact on the ecosystemis still poorly understood. Here,we investigated the impactof oysters on the condition of mussels and on the spatial distribution of birds on 18 bivalve beds with differentgrades of oyster occurrence throughout the Dutch Wadden Sea.Moreover, in comparing bird densities on bivalvebeds with densities expected on the total intertidal area, we could detect which species exhibit a preference forthe structured habitat. Overall, 50 different bird specieswere observed on the beds, of which about half regularlyfrequent intertidal flats. Most of these species showed a preference for bivalve beds. The condition of mussels decreasedwith the oyster dominance, whereas the majority of bird species was not affected by the oyster occurrence.However, three of the four species that were negatively affected depend on intertidal mussels as foodsource. Even though the Pacific oyster is a nonnative species, attempts to fight it may do more harm to avian biodiversitythan good