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3. Habitat, Home Range, and Distribution
- University of New Mexico Press
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21 Habitat, Home Range, and Distribution Few animals are as closely associated with a single habitat type as these squirrels, they are found only in the dry ponderosa pine forests of the interior Southwest. Even here they are not universal and are restricted to forests where winter snows are moderate and continuous snow cover generally lasts less than three months. —D. E. Brown, Arizona’s Tree Squirrels, 1986 Introduction T he natural habitat of the tassel-eared squirrels is characterized by ponderosa pines. The home range is the area that includes all the travels of an individual squirrel daily and throughout the year. The geographical distribution of these squirrels is that of specific stands of ponderosa pine forests found in the southwestern United States and in Mexico. In 1853, Woodhouse described the area where tassel-eared squirrels were first located and identified: “This truly elegant squirrel I procured in the San Francisco Mountain, during the month of October, where I found it quite abundant, and after leaving which place I did not see it again. I have beeninformedlately...thattheyarequitenumerousnearFortDefiance,in the Navajoe country” (ref. 1). Indeed, the Sitgreaves expedition never again passed through the ponderosa pine habitat of the squirrels. Habitat Habitatincludestheresourcesrequiredbyaspeciesforsurvivalandreproduction . Ponderosa pine forest cover for Abert’s squirrels is described as the vegetative shelter including the nest, the nest tree, and the vegetation surrounding the nest tree (ref. 2). 3 22 C H A P T E R T H R E E A live-trapping study with S. a. ferreus in Colorado examined five habitat types and found 92% of the squirrels in the ponderosa pine forest , 7% between grassland and ponderosa pine forest, and 1% in ponderosa pine–Douglas fir. No squirrels were trapped in the grassland or deciduous habitats (ref. 3). Good squirrel habitat can provide sources of squirrels that move into poor habitats or “sinks,” which are characterized by low survival rates. “Source areas” for tassel-eared squirrels are characterized by high basal areas, high level of interlocking canopies, large trees, and stable populations of squirrels. Characteristics of sink habitats are opposite of those of source areas, with low basal areas, low level of interlocking canopies, small trees, and fluctuating squirrel populations (ref. 4). Basal areas of ponderosa pine forests and tassel-eared squirrel population densities have been shown by numerous researchers to be linked (ref. 4, 5, 6, 7, 8). The basal area of a stand of trees is the cross-sectional area of all trees within the stand and is expressed as m2 /hectare (ha). Higher basal areas have better recruitment of squirrels. Norris Dodd, a research FIGURE 3.1 Distribution of ponderosa pine forests in the southwestern United States and Mexico. Used with permission from David Patton. [18.116.15.31] Project MUSE (2024-04-17 23:13 GMT) H a b i t a t , H o m e R a n g e , a n d D i s t r i b u t i o n 23 FIGURE 3.2 Distribution of the six subspecies of tassel-eared squirrels in the southwestern United States and Mexico. Redrawn by Diane Iverson from Hoffmeister and Diersing, 1978. 24 C H A P T E R T H R E E biologist with the Arizona Game and Fish Department (AZGFD), compared squirrel habitats across nine landscape-sized (280 ha) ponderosa pine forested areas and found that a basal area > 35 m2 /ha qualified as optimum squirrel habitat (ref. 9). Examinations of squirrel diets with respect to mycophagy was found to have significant relationships with basal area, denser canopy cover, and fungal biomass (ref. 10, 11). A habitat quality study on S. a. kaibabensis in a virgin ponderosa pine stand of the Kaibab National Forest found an association between the number of Kaibab squirrels and the number of nests, concluding that the number of nests was a good indicator of habitat quality (ref. 5). A “status of knowledge” paper on ponderosa pines compiled by Gilbert Schubert of the U.S. Forest Service in 1974 estimated that ponderosa pine forests in Arizona, Colorado, New Mexico, and Utah cover approximately four and one-half million hectares. New Mexico has the most hectares and Utah the fewest (ref. 12). Assessments of the presettlement conditions around 1870 that existed in the ponderosa pine forests of northern Arizona found that forests had stands of older trees with open areas separating those stands, in contrast to the current conditions of forests consisting of dense young trees (ref...