This examination of lobbying communities explores how interest group populations are constructed and how they influence politics and public policy. By examining how populations of interest groups are comprised, this work fills an important gap between existing theories of the origins of individual interest groups and studies of interest group influence. The population ecology model of interest communities developed here builds on insights first developed in population biology and later employed by organizational ecologists. This book should be of interest to political scientists working on lobbying and interest groups and social scientists interested in the development of organizations.