Differential response of migratory subpopulations to winter habitat loss

1. In a previous paper (Goss-Custard et al. 1995), a demographic model suggested that different breeding subpopulations of a migrating shorebird, the oystercatcher Haematopus ostralegus, that shared the same wintering area might be affected in different ways by the progressive reduction of their winter habitat. Initially, as winter habitat was removed in simulations, all subpopulations decreased in parallel. However, as habitat loss continued, the subpopulations with lower fledgling production began to be disproportionately affected. This paper explores what caused this difference in response between subpopulations. 2. A simplified version of the model was used in which two or four subpopulations wintered in one area. Two simple density-dependent functions were included, one for breeding territoriality and one for winter mortality, with density dependence starting only above a certain threshold density. Each pair that acquired a breeding territory reared a constant number of hedged young each year. 3. As winter habitat was reduced, the reduction in population size of the subpopulations with lower reproductive rates began to accelerate as soon as the point was reached when all adults were breeding. Before this point, breeding birds that had died in the winter had been replaced by non-breeders. After this point, the subpopulation reproductive output was not sufficient to replace birds dying in the winter. At the same time, winter mortality rates remained high as winter densities were sustained by birds from the other subpopulations. 4. The results suggest that, in order to detect the first effects of winter habitat loss, conservationists should pay special attention to monitoring breeding numbers in local subpopulations with below average rates of reproduction. The results also suggest that the cause of a decline in some local breeding subpopulations, when others remain stable, may need to be sought on the wintering grounds and not in the breeding localities themselves.