The presence of other fish species affects speciation in threespine sticklebacks

Competition for shared resources is typically invoked as the most important ecological interaction promoting diversification and speciation, whereas the roles of abiotic variables and natural enemies have been largely neglected. To improve our understanding of the ecological factors involved in the evolution of sympatric stickleback species pairs (Gasterosteus aculeatus), I examined whether lakes in the same region with only a solitary population of sticklebacks had unique features not shared with lakes containing stickleback species pairs. Sixteen lakes with a solitary population of sticklebacks were identified as potential candidates for two invasions of marine ancestors. Several physical and chemical characteristics of these candidate lakes were found to be not significantly different from those of lakes with sympatric species pairs. However, when the community of other fish species was considered, a striking pattern emerged: all but one of the 16 candidate lakes contained other fish species in addition to cutthroat trout (Oncorhynchus clarki), whereas sympatric species pairs were found only in lakes with cutthroat trout and no other fish species. The two other fish species most often encountered, prickly sculpin (Cottus asper) and rainbow trout (Oncorhynchus mykiss), are potential competitors and predators of threespine sticklebacks. The results of this survey suggest that our understanding of the role of ecology in diversification and speciation will be furthered by a consideration of interactions other than interspecific competition between focal populations.