Many species are declining in the Wadden Sea, only a few are thriving

In a unique collaboration between the University of Groningen and the Carl von Ossietzky Universität Oldenburg, researchers studied population trends across 3,058 populations of various species throughout the entire Wadden Sea. Twenty-four per cent of the populations showed a significant decline in size, and this trend is occurring simultaneously across various groups. Only fourteen per cent of the populations showed an increase in size. These findings were published in the journal Global Change Biology on June 18, 2025. 

The ‘losers’ are mainly native species, often related to each other, while the ‘winners’ are various non-native species. At first glance, birds seem to be doing well, but a number of species have been declining significantly since the late 1990s. Phytoplankton and plants are also strongly represented among the losers, as these are species at the base of the food web. But bivalves, marine worms, and some fish such as the Atlantic cod are also struggling. 

It is striking that the declines in the numbers of the losing species often occur simultaneously, which points to a common underlying cause. ‘In follow-up research, we aim to investigate the specific causes of these dramatic changes,’ says Britas Klemens Eriksson, Professor of Marine Ecology at the University of Groningen. ‘In any case, it is a warning signal of potential local extinction.’