The term "keystone species" is used to describe organisms that exert a disproportionately impor­tant influence on the ecosystems in which they live. Analogous concepts such as "keystone mutualism" and "mobile links" illustrate how, in many cases, the interactions of two or more species produce an effect greater than that of any one species individually. Because of their role in transporting nutrients from the ocean to river and riparian ecosystems, Pacific salmon ( Oncorhynchus spp.) and brown bear (Ursus arctos) have been described as keystone species and mobile links, al­though few data are available to quantify the importance of this interaction relative to other nutrient vectors. Application of a mass balance model to data from a southwestern Alaskan stream suggests that nitrogen (N) influx to the riparian forest is significantly increased in the presence of both salmon and bear, but not by either species individually. The interactions of salmon and bear may provide up to 24% of riparian N budgets, but this percentage varies in time and space according to variations in salmon escapement, channel mor­phology and watershed vegetation characteristics, suggesting interdependence and functional redundancy among N sources. These findings illustrate the complexity of interspecific interactions, the importance of linkages across ecosystem bound­aries and the necessity of examining the processes and interactions that shape ecological communi­ties, rather than their specific component parts.