Lange-termijn veranderingen bij broedvogels in Wapserveld-Berkenheuvel

Wapserveld-Berkenheuvel is a nature reserve in the northern part of the Netherlands, presently consisting of heathland with fens, coniferous forest and (former) farmland in the river valley of the Vledder Aa. Annual breeding bird censuses in 1968-2006, in combination with speciesspecific research on reproduction, food choice and dispersal, were used to evaluate the impact of changes in land use and management. It was shown that the number of breeding waterbirds on heaths largely was correlated with rainfall in the preceding winter (October-March), and not with measures taken to raise the water level. Bird densities and diversity on average improved after diversifying heathland through grazing (sheep and cattle), burning, mowing and sodcutting, but declined again when management slackened. Large-scale set-aside in farmland resulted in the complete loss of already dwindling populations of Lapwing, Black-tailed Godwit and Curlew, following habitat changes and steep declines in earthworm biomass. During the first years of set-aside many species profited from the heterogeneous weed-rich vegetation, only to decline again (but not to levels typical of present-day farmland) after this pioneer stage. The removal of non-indigenous coniferous trees (mainly larch, Norway spruce and Douglas fir) resulted in the temporary settlement of Woodlarks and Nightjars on clear-fellings. As many presumed impacts of management on bird life were correlational, we stress the importance of long-term research using replicated, randomized experiments. Moreover, in order to understand underlying mechanisms in population changes it is crucial also to collect data on reproduction, mortality, dispersal and food resources, rather than just censusing breeding numbers.