A seasonal shift in habitat suitability enhances an annual predator subsidy.

1. Entry of substantial numbers of natural enemies from outside a habitat can have profound impacts on food web structure in the recipient habitat, but underlying mechanisms are poorly understood, including the role of relative predator fitness in source and recipient habitats. We studied a naturally occurring annual movement of the salt-marsh spider Pardosa littoralis across habitats in an attempt to clarify factors enhancing and impeding movements of predator populations. 2. Marsh vegetation is dominated by two cordgrass species, Spartina patens, a complex-structured grass with a well-developed litter layer, and Spartina alterniflora, a sparse-structured grass with little thatch accumulation. Pardosa hunts across both habitats and can drastically reduce densities of planthoppers and leafhoppers, the most abundant marsh herbivores. 3. We found an annual subsidy of Pardosa from S. patens, extending hundreds of meters into S. alterniflora made possible by a winter refuge provided by S. patens. As a result, the strength of the subsidy is correlated with the severity of the preceding winter, with the largest subsidies following the coldest winters. 4. Higher Pardosa fitness in the recipient habitat following winter, as indicated by higher growth rates associated with greater prey availability, enhanced the strength of this subsidy. Conversely, lower structural complexity in S. alterniflora, which is associated with higher rates of cannibalism in this spider, may impede the subsidy. 5. The mechanistic underpinnings of the predator subsidy demonstrated here can improve our understanding of subsidies in other contexts, such as conservation biological control. In addition, identifying such subsidies is key to preserving food webs in recipient habitats when source habitats are threatened.