Wetenschappelijk artikelSignals from the Wadden Sea: population declines dominate among waterbirds depending on intertidal mudflats.

The Wadden Sea, shared by Denmark, Germany and the Netherlands, is one of the world's largest intertidal wetlands. Waterbirds are an important element of the Wadden Sea ecosystem. By their migratory behaviour they connect the Wadden Sea with other sites, ranging from the arctic to the western seaboards of Europe and Africa, forming the East-Atlantic Flyway. The Joint Monitoring of Migratory Birds (JMMB) project of the Trilateral Monitoring and Assessment Program (TMAP) follows the changes in population size within the Wadden Sea. In this paper we describe and analyse population trends over the years 1991-2009 for 22 waterbird species using the Wadden Sea in internationally important numbers and depending on intertidal mudflats. Population declines predominated in this 18-year period. More populations decreased in Schleswig-Holstein and Niedersachsen than in The Netherlands and Denmark. This is the case particularly for species feeding on polychaetes. In contrast, waterbirds feeding on bivalves are in decline in all regions except Denmark. On the finer spatial scale of tidal basins, these patterns in trends are still apparent, although much variation in trend directions exists within the Dutch Wadden Sea, especially in bivalve specialists. For those species for which we could compare the trend in the Wadden Sea with the trend of their entire flyway population, we found that the former were more negative. This finding and the contrasting trends between regions and tidal basins within the Wadden Sea suggest that causes of the population changes are to be sought within the Wadden Sea itself. These causes, which may act in combination, could be related to factors operating within the Wadden Sea only or with factors operating on a larger scale but having an intensified or differentiated effect within the Wadden Sea. Interestingly, the Wadden Sea regions where negative trends of benthivorous waterbirds predominate are characterized by a large tidal amplitude, whereas areas where bird numbers have generally increased are characterized by a small tidal amplitude. An inventory of possible causes indicated climate change, eutrophication, shellfish fisheries, invasive species and increasing numbers of avian predators as the most important candidates to be investigated further to explain the observed trends.