SURVIVING AS AN ORGANIZED INTERESTIn order to survive, an organization must effectively exploit the resources in its environment. Because many organizations are dependent on the same resources as other, similar, organizations, they must find a way to share resources or eliminate other organizations through competition. In the case of interest groups working toward the same goal or collective good, however, competition seems illogical. How do interest groups resolve this conflict between selfpreservation and the achievement of a collective good? Lowery and Gray (1995) provide at least part of the answer in their analysis of interest groups at the aggregate level. They draw on biological concepts from population ecology theory to explain the number of groups within specific issue areas, but they do not use the theory to explain how or why membership levels in a group might vary across political jurisdictions. Because population ecology theory outlines the nature of an ecosystem where groups within an issue area must struggle for resources and members to survive, the purpose here is to test the implications of the theory for the membership levels of interest groups within an issue area. I review population ecology theory and other literature addressing