Ethological Isolation and Competition of Allospecies in Secondary Contact

Abstract

A geographic model is presented for the competition between allospecies whose breeding ranges are in secondary contact. It is assumed that the density of the combined adult populations is fixed, and that selection operates locally through differential recruitment rates of young of the two allospecies. Dispersal is modelled as a standard diffusion process. Random or limited crosspairing, but no recruitment of hybrids, is assumed. The competition differs from that of ethologically isolated species because reproductive potential is lost through crosspairing. The model predicts stable geographic displacement clines even when each species has only a slight competitive advantage in its preferred range. The major prediction of the model in this case is that the displacement cline will be at least four times as steep with extensive crosspairing as without it. An abrupt geographic shift in competitive balance also produces a steep displacement cline, as expected. The predictions of the model are tested by means of a review of the North American avifauna. Those allospecies pairs with historically stable displacement clines and reasonably balanced competition are identified. Although field data are not sufficient for a conclusive test, eight out of nine relevant cases seem to conform to the model.