Endemic species are losing some of their geographical distribution area through competition with exotic species. The numbers of some exotic species have even grown into plague proportions; in these cases we describe the species as invasive, the common slipper shell for example. This shellfish came over from America with oysters in the late 19th century and became a real pest for local oyster farmers. However, as yet we know of no species endemic to the North Sea that has become extinct due to the introduction of an exotic species. But the phenomenon has been observed in fresh water and estuaries. Exotic species can also have an adverse effect on the reproduction of our endemic species. The American and endemic crab can mate, for example, but their offspring are sterile.

Exotic species can have the effect of increasing biodiversity. But the newcomers are mostly fast-growing species, more resistant to disturbance and pollution. They are at home in harbour and coastal areas with artificial, hard substrates and in areas that have become impoverished through overfishing. There is a danger that the world's marine flora and fauna will become homogenous and uniform. So, although introductions might enrich local biodiversity, they could also bring about worldwide impoverishment.

There is also a real danger that exotic species bring organisms and disease along with them, which harms endemic species . The oysters from Japan, for example, brought a single cell parasite that damages the endemic oysters. Introduced phytoplankton species can cause a toxic plankton bloom, rendering the oysters and mussels unfit for consumption.