Introduced species are often considered to bea threat to residents, but not all reciprocal trends mayreflect species interaction. In the northern GermanWadden Sea, native mussel Mytilus edulis beds aredeclining and overgrown by introduced Pacific oystersCrassostrea gigas and slipper limpets Crepidula fornicata.We review the population development of thethree species and analyse whether the invading speciesmay be responsible for the decline of native mussels.The Pacific oyster predominately settles on musselbeds in the intertidal and the slipper limpet dominatesaround low water line. We compare the developmentof mussels and invaders in two subregions: musselbeds near the islands of Sylt and Amrum decreasedboth in the presence (Sylt) and absence (Amrum) ofthe two invading species and more detailed investigationscould not confirm a causal relationship betweenthe increasing invaders and decreasing mussel beds.There is evidence that the decline of mussel beds ismainly caused by failing spatfall possibly due to mildwinters, whereas the increase in slipper limpets andoysters is facilitated by mild winters and warm summers,respectively. We conclude that changing speciescomposition is a result of the climatic conditions inthe last decade and that there is no evidence yet thatthe exotic species caused the decline of the natives. Itremains an open question whether the species shift willcontinue and what the consequences for the nativeecosystem will be.