Credit: D. PLATVOET/J. DICK/I. EWART

In 1683, the Ottoman Turks' advance up the Danube was turned back at Vienna. A contemporary crustacean invader, however, is having more success. Jaimie Dick and Dirk Platvoet have discovered that, in the freshwater ecosystems of the Netherlands, the native shrimp Gammarus duebeni is being wiped out by a menace from the east, Dikerogammarus villosus (Proc. R. Soc. Lond. B 267, 977–983; 2000).

This species is native to eastern Europe and the Ukraine. But D. villosus has spread to Western Europe through the Danube–Main canal (which opened in 1992) and appeared in the Netherlands about five years ago. Females, shown here on zebra mussels, are about 15 mm long, males being twice that size.

The alien's method of takeover is nothing if not direct — it eats the natives. In particular, male D. villosus consume female G. duebeni , which are smaller than males of the same species and less able to resist attack. The invader is especially destructive because it can feed on its prey between moults, when the exoskeleton is tough, and not just on soft-skinned, recently moulted shrimps.

In some places, G. duebeni has already been displaced by a fast-breeding North American species, G. tigrinus, which probably reached Europe in ship ballast-water. To an extent it could withstand this assault, because the two species prefer different habitats and salinities which helps to keep them apart. But D. villosus can tolerate a wide range of different conditions, and it is also thought to be responsible for the recent sharp declines in populations of G. tigrinus.

As it becomes ever easier for human beings to traverse the globe, exotic animals and plants will be introduced into new environments, both on purpose and as unseen hitchhikers. We face the prospect that ecosystems will become increasingly drab and homogeneous, dominated by a few super-competitive species.