Many organizations are made up of other organizations that have decided to act collectively as with research and development consortia, industrial alliances, trade associations, and formal political coalitions. These collective organizations can be characterized by their differing strategies: some are general in scope, while others specialize on a more narrow purpose. What explains the prevalence of generalism and specialism among collective organizations? We develop an ecological model in which collective organizations compete over member organizations. Assuming that an organization joins a collective when its objectives match that of the collective, our model predicts a generalism bias in the ecology of founding and growth among collective organizations. This outcome is predicted to be path dependent, however, emerging over time according to relatively minor differences in initial conditions. These predictions are supported in an analysis of founding and growth rates among US R&D consortia, and the model helps to account for the numbers, sizes, and strategic diversity of these consortia.