Previous biodiversity changes in the North Sea

Introduced species

In the North Sea, the introduced seaweed (Sargassum muticum) and the Pacific oyster (Crassostrea gigas) are significantly expanding their range. Both new species form extensive new habitats which host a unique epiflora and epifauna and appear at first sight to enhance local biodiversity. If, and how, such increases in biodiversity affect existing communities and their structure is still unclear, but local competition is already changing the natural habitats considerably.

Competition by introduced species

The North Sea suffers from competition. Local competition is particularly high by fast-growing oyster beds which suppress the natural mussel banks. The seaweed Sargassum, in turn, floats in extensive strands at the surface and impacts intertidal rocky-shore communities simply by shading and by competing for nutrients. In the open sea, many small flagellates replace the indigenous diatoms. On the next trophic level, gelatinous plankton replaces copepods, resulting in changes in the pelagic food web and thus its productivity. Such changes or transition phases have occurred quite abruptly, and have been identified as regime shifts.