Mudflat species: Threatened or hidden? An extensive seed bank survey of 108 fish ponds in Southern Germany

Mudflats are the most endangered habitats in Central Europe. In Germany, 60% of mudflat plant species are listed as endangered on the national Red List. The main causes are human activities such as the straightening and construction of barrages along large rivers and the abandoning or changing management of fish ponds. Species from ephemeral habitats, such as mudflats, typically have a persistent soil seed bank. However, it has not yet been examined whether mudflat species are still present but hidden in the sediment seed bank. To answer this question, we studied the seed bank of 108 ponds in Southern Germany. More than 300,000 seeds from mudflat species germinated from the sediment samples. Species listed on the national or regional Red Lists were found in all but three ponds, with all ponds containing up to 11 species. Some Red List species were present at very high density (up to nearly 3000 seeds/1). Although the pond locations in this study had been intensively floristically monitored for over half a century some species with > 20 records were determined to be new in two study locations. Based on the last drainage date or record we conclude that seeds of mudflat species may survive between 50 years or even a century. It is, therefore, important that this hidden diversity should be considered in future conservation management practices. Furthermore, Red Lists should revise their entries on species with extremely long-term persistent seed banks by stating that they may be not extinct but hidden.