An insular geography approach to equilibrium numbers of physician specialties across urban centers.

Abstract

Abstract In this study we apply the theory of island biogeography to the distribution of physician specialties among large American cities. Drawn from the population and community ecology of isolated habitats, the theory provides a predictive model for determining equilibrium numbers of physician specialties from observations of rates of immigration and emigration of specialties at a particular city. The theory assumes that increased specialty density results in increased negative interaction or competition between specialties, and that this leads to decreased immigration and increased emigration rates. The theory also assumes that increased isolation decreases immigration rates. Key elements of the theory are city size, degree of isolation, number of specialties present, rate at which new specialties arrive, and rate at which existing specialties leave. Our analysis showed that city size was strongly related to specialty number, as predicted by the theory, but that isolation, contrary to the predictions, was unrelated to number. During the 6 year period studied, emigration appeared to decrease as density increased, and immigration appeared to increase. These reversals of the theory's assumptions indicate that some form of non-negative interaction between specialties may develop as specialty number becomes large. Alternative specialty interactions are discussed as possible explanations of the reversal of immigration and emigration relationships. The reversals indicate that specialty number is likely to grow to the maximum possible in large cities, resulting in intensification of the problem of duplication of health care services. Lack of any relationship between degree of isolation and number of specialties suggests that relocation of physician training may have little effect on specialty distribution across large cities.