The project will examine the effects on seabird populations from introduction of Arctic foxes on some Aleutian Islands. The hypothesis that the foxes introduced onto the islands have restricted the distribution and abundance of breeding and roosting seabirds will be tested. That restriction may have reduced or eliminated the role of seabirds from producing the nutrient supply that nourishes the plant community. The study will be conducted in parallel with a U.S. Fish and Wildlife program of removing foxes from the islands. The USF&W conservation program provides an opportunity to apply the results of the research in a before and after situation so that the role of foxes on community structure may be examined more thoroughly. The research addresses three topical areas in ecology: 1) the role of keystone predators in terrestrial communities; 2) the interplay between top-down and bottom-up processes in food webs; and 30 the importance of subsides between ecosystems.