Habitat Segregation in Ground Crickets: The Role of Interspecific Competition and Habitat Selection

Habitat segregation of closely related species has been well documented among insects, but the factors responsible are poorly understood. We have examined the influence of habitat selection and interspecific competition on the habitat associations of the closely related ground crickets Allonemobius allardi and A. fasciatus. Allonemobius fasciatus is found in wet pastures and low—lying grassy areas; A. allardi occurs primarily in dry pastures and grasslands. In a large—scale field experiment, crickets were introduced into the centers of runways that extended from wet pasture at the edge of a pond to dry pasture on a south—facing slope. Each runway had been sprayed with insecticide, so that no Allonemobius were present prior to the introduction. We stocked the runways with either A. fasciatus alone, A. allardi alone, or a mixture of the two species. Twenty—four days after release, we collected from each of 15 segments within all runways. Analysis of distribution patterns in mixed— and single—species runways showed that neither species' distribution was influenced by the presence of the other. Thus there is no evidence that current interspecific competition is important in determining distributions. In single—species runways, A. fasciatus moved into wetter areas, whereas A. allardi displayed no clear tendency to move either up or down the runways. It appears that habitat selection by A. fiasciatus can account for its association with wet pasture. We discuss several scenarios for the evolution of habitat selection and its relationship to the processes of adaptation and speciation.