Invasion and Extinction: The Last Three Million Years of North Sea Pelecypod History

Biotic interchange occurs when two or more biotas with separate evolutionay histories come together as barriers between them become less effective. The idea that extinction of species often accompanies invasion of species during biotic interchange is rooted in the view that the success of invasion is controlled largely by competition or predation among newcomers and native species. An analysis of the stratigraphical ranges of pelecypod species from the Pliocene and Pleistocene of the North Sea Basin revealed that the magnitude of extinction is not cowelated with the absolute number of species invading from the North Pacific following the opening of the Bering Strait during the Middle Pliocene or from the Mediterranean during the Early Pleistocene, and that North Pacific invaders and native North Sea species were equally susceptible to extinction during the Early Pleistocene. I suggest that, although invasion may result in populational and ecological shifts in the recipient biota, it should rarely bring about extinction among marine species because marine competitors and predators are almost never 100 percent successful in locating and killing their victims, and because marine populations tend to be large. The fact that many invaders from the North Pacifc and Mediterranean first became established in the North Sea during the Pleistocene points to the possibility that the reoccupation of the North Sea by marine organisms following periods of emergence provided unusually good opportunities for the successful invasion of foreign species.