Large-scale effects of a small herbivore on salt-marsh vegetation succession - A comparative study on three Wadden Sea Islands

Grazing by livestock is used as a management tool to prevent the dominance of a single tall-growing species during succession on European salt marshes. The effects of natural small herbivores are often neglected by managers. Long-term exclosure experiments on the island of Schiermonnikoog show that hares retard vegetation succession at the early stages of salt-marsh development. In the present study we test whether we can scale-up these exclosure studies to a whole salt-marsh system. We compared 30 years of undisturbed vegetation succession at the Wadden Sea islands of Schiermonnikoog, Rottumerplaat (both The Netherlands) and Mellum (Germany). Salt-marsh development started at all sites in the early 1970s. Hares have been present only on Schiermonnikoog. At each site an area was selected covering a gradient from high to low salt marsh. Surface elevation and clay thickness were measured and a vegetation map was made on the three islands. The areas showed similar clay thickness at low surface elevation, indicating similar sedimentation rates and hence nitrogen inputs. Rottumerplaat and Mellum showed a higher dominance of the late successional species Atriplex portulacoides in the low marsh and Elymus athericus in the high marsh compared to Schiermonnikoog. Typical mid-successional, important food plant species for hares and geese had a higher abundance at Schiermonnikoog. Patterns of vegetation development in the absence of hares followed the observed patterns in the small-scale exclosure experiments at Schiermonnikoog. Without hare grazing, vegetation succession proceeds more rapidly and leads to the dominance of tall-growing species in earlier stages of succession. The present study shows that next to large herbivores, small herbivores potentially have large-scale effects on salt-marsh vegetation succession during the early successional stages