Understanding food-web dynamics requires knowing whether species assemblages are compartmentalized into distinct energy channels, and, if so, how these channels are structured in space. We used isotopic analyses to reconstruct the food web of a Kenyan wooded grassland. Insect prey were relatively specialized consumers of either C3 (trees and shrubs) or C4 (grasses) plants. Arboreal predators (arthropods and geckos) were also specialized, deriving c. 90% of their diet from C3-feeding prey. In contrast, ground-dwelling predators preyed considerably upon both C3- and C4-feeding prey. This asymmetry suggests a gravity-driven subsidy of the terrestrial predator community, whereby tree-dwelling prey fall and are consumed by ground-dwelling predators. Thus, predators in general couple the C3 and C4 components of this food web, but ground-dwelling predators perform this ecosystem function more effectively than tree-dwelling ones. Although prey subsidies in vertically structured terrestrial habitats have received little attention, they are likely to be common and important to food-web organization.