Consequences of parasite invasion and land use on the spatial dynamics of host populations

1. Conversion of natural habitats to human use can affect the abundance and distribution of predators and parasites, create population sinks, and reduce the viability of valued prey and host species. We asked how the distribution of brown-headed cowbirds Molothrus ater, a generalist brood parasite, has influenced the source-sink dynamics of song sparrow Melospiza melodia populations and their regional population trends. 2. We intensively studied 17 host populations subject to varying levels of parasitism for 1-36 years. We linked these data to spatial and demographic models to predict growth rate in song sparrow populations in the Southern Gulf Islands, BC, Canada. 3. Patterns of growth in song sparrow populations were closely related to cowbird distribution, which in turn depended on land use patterns at landscape scales. Locally, sparrow populations were expected to increase in areas far from cowbird feeding areas, where parasitism was low, but to decline where parasitism exceeded 20%. The predicted population trends were similar to those recorded locally and via the North American Breeding Bird Survey. 4. Synthesis and applications. We show that the distribution of habitats favourable to brood parasites can affect whether host populations grow or decline regionally. In highly sedentary hosts like the sparrows in this study, density-dependent juvenile dispersal and marked spatial variation in the probability of parasitism can give rise to source-and-sink dynamics. Our results illustrate how the application of spatial models and empirical data can predict how land use decisions may influence host dynamics. We identify ways in which applied ecologists might influence land use to enhance the persistence of valued hosts, and suggest that our approach provides a promising framework for exploring regional-scale spatial dynamics of species in order to identify critical habitat and prioritize investments in conservation.