Populations in agricultural landscapes often occur in source-sink situations: small patches of marginal habitat (sinks) are supported by an immigration flux from larger patches of high-quality habitat (sources). We sought to demonstrate that this situation occurs for Reed Warblers (Acrocephalus scirpaceus) in a riverine, mainly agricultural landscape in the Netherlands. We collected data on occurrence and habitat features in a large number of mostly small marshlands. We used a stochastic model to simulate the population dynamics in a metapopulation with sinks and sources. A statistical analysis of the field data, using regression techniques with occupation probability and abundance index as dependent variables, showed that habitat quality was less favourable in small habitat patches (e.g., ditches with reeds) than in larger patches ( large, heterogeneous marshlands). The spatial cohesion of the landscape also played an important role: abundance of breeding Reed Warblers in regions with low spatial cohesion was low. Local extinctions and recolonizations occurred, and their rates depended on the spatial parameters of the patch. This supports the hypothesis that metapopulation theory is applicable here. The results of the modeling study demonstrated that, besides the trivial dependence of sinks on sources, a larger amount of sink area and increased exchange of individuals increased the stability of source patches. This was shown not only by the larger size of the source population but also by increased resilience after a catastrophe. The area of the sink seemed less important than its distance to the source. The simulation indicated an optimal area of a few hectares (compared to a 10-ha area of the source) and a maximum distance of 2–5 km from the source. In creating sustainable landscapes, for example, by setting up an ecological network consisting of a limited number of high-quality patches, these small and seemingly insignificant habitat patches could play an important role and should be taken into consideration.
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