Ecologists have long observed resource transfers between neighboring habitats. But they assumed that such transactions are largely asymmetrical, with resources flowing from the more productive habitat to the less productive one. In the case of adjacent forest and stream habitats, for example, forests were thought to sustain streams in times of need. But a new study, published yesterday in the Proceedings of the National Academy of Sciences, reveals that the relationship is far more reciprocal than previously thought. The results of a two-year study conducted by Kyoto University ecologist Shigeru Nakano, who died last spring, and his colleague Masashi Murakami of Hokkaido University show that whereas the forest feeds the stream in the summer, the stream feeds the forest during the spring and fall. Specifically, stream fish prey on forest invertebrates when the aquatic food supply is at its lowest, during the summer. In the spring and fall, however, when the foliage is sparse, the forest birds must turn to aquatic invertebrates for food. "The importance of reciprocal resource subsidies between habitats," the authors conclude, "indicates that the loss or degradation of one habitat may have more detrimental effects on neighboring communities than we have previously recognized."