The Indivisible Niche of Tamiasciurus: An Example of Nonpartitioning of Resources
1981
Smith, Christopher C.
Tree squirrels in the genus Tamiasciurus are adapted to exploit conifer seeds and fungi in boreal conifer forests. The ranges of the two species in the genus are parapatric, suggesting competitive exclusion. Where their ranges are contiguous in the Cascade Mountains of southern British Columbia, Canada, the squirrels differ in at least five characters which adapt each species to be competitively superior in its range. Two of the characters (alarm calls and fur color) adapt the squirrels to avoid predation and the other three (jaw strength, body size, and reproductive rate) adapt them to exploit the resources typical of their ranges. Two or three of these differences also allow each species to be competitively superior on its side of other sections of the species boundary in coastal British Columbia and northeastern Oregon. The two species from rare hybrids, especially where logging has created disturbed habitats. The pattern of carrying food to storage in a central cache is the basis for both the evolution of territoriality in Tamiasciurus and the absence of sympatry for the two species. In most cases where congeneric territorial species are sympatric and carry food to the center of the territory, they spend most of their time searching for and catching food and relatively little time carrying it. For Tamiasciurus, on the other hand, food is easy to locate and most of the foraging time is spent in carrying food to the cache. Splitting the food niche of Tamiasciurus would involve too large an increase in time and energy for carrying food to be balanced by any gain in feeding and digestive efficiency that could result from specializing on a narrower range of food.
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