Parapatry and Encounter Competition between Chipmunk (Tamias) Species and the Hypothesized Role of Parasitism

Abstract

-TWO parapatric species of chipmunks occur in the Front Range of Colorado: Tamias quadrivittatus only below 2130 m in lower montane life zones and Tamias umbrinus from 2130 m upward in upper montane life zones. The smaller and ecologically distinct Tamias minimus occurs sympatrically in nearly all life zones at elevations above 1750 m. Of the three species, which all display intraand interspecific territoriality, T. umbrinus is the most aggressive. Populations of T. umbrinus at the lower-elevation margin of its distribution in the Front Range and of T. quadrivittatus can suffer heavy infestations by larval cuterebrid bot flies (Cuterebrafontinella), whereas T. minimus is virtually resistant to bot myiasis. Since bot flies are virtually limited to elevations below 2200 m, I propose that parapatry between the two common host species results from a balance between aggressive dominance by T. umbrinus above 2130 m and increased sensitivity to the adverse effects of bot parasitism by the dominant host species below this elevation. Tamias quadrivittatus shares more biogeographical similarities with the warmer climate-adapted parasite than does T. umbrinus and so has had greater evolutionary opportunity to become coadapted as a host. There is a vital need in community ecology for hypotheses concerning the interaction of parasitism and competition and its effect on species distributions to be developed and tested.