Resource subsidies to productive habitats: How important are they?

Studies of the effects of cross habitat resource subsidies have been a feature of food web ecology over the past decade. To date, most studies have focused on demonstrating the magnitude of a subsidy or documenting its effect in the recipient habitat – ecologists have yet to develop a satisfactory framework for predicting the magnitude of these effects. We used 115 datasets from 32 studies to compare consumer responses to resource subsidies across recipient habitat type, trophic level, functional group and productivity contrasts in donor and recipient habitats. Contrary to expectation, the magnitude of consumer response was not affected by recipient habitat productivity or the ratio of productivity between donor and recipient habitats. However, consumer response was significantly related to the ratio of subsidy resources to equivalent resources in the recipient habitat. We confirm this general result through a large scale field manipulation of a subsidy in a strongly productive recipient habitat. Riparian web-building spiders responded strongly to subsidy exclusion despite the high level of primary production in the recipient habitat. The amount of subsidy in the recipient habitat was always greater than local resources despite the lower productivity of the donor habitat. Broad contrasts in productivity are modified by subsidy type, vector, and the physical and biotic characteristics of both donor and recipient habitats. For this reason, the ratio of subsidy to equivalent resources is a more useful tool for predicting the possible effect of a subsidy than coarser contrasts of in situ productivity.