Dutch sedge warblers Acrocephalus schoenobaenus and West-African rainfall:empirical data and simulation modelling show low population resilience in fragmented marshlands

Sedge Warbler populations in The Netherlands have declined dramatically ove r the last 25 years, especially between 1973-75 and 1982-85. Population dec lines correlate with yearly rainfall in the western part of the Sahel-Souda n zone, the most probable wintering area of West-European birds (after remo ving effects of autocorrelation, r(s) = 0.58). However, populations in rela tively unfragmented landscape showed a clear recovery after 1984, while tho se in heavily fragmented landscapes showed no recovery. A spatially explici t population simulation model was used to test the general hypothesis that bird populations in fragmented landscapes show stronger declines and less r esilience than populations unfragmented habitats in response to a catastrop he such as winter drought. The simulations demonstrated that in fragmented landscapes (less than 1% marshland), the relative decrease in numbers was 5 0% higher than in less fragmented habitats (more than 1% marshland). Furthe rmore, after a decrease, the recovery to initial numbers in landscapes with less than 1% marshland would take about five times longer than in areas wi th more than 15% suitable habitat. We tentatively conclude that Sedge Warbl ers breeding in fragmented marshland habitats are more vulnerable to drough t events in West-Africa than those in unfragmented habitats.