Academic journal article The American Midland Naturalist

A Comparison of Habitat Use and Demography of Red Squirrels at the Southern Edge of Their Range

Academic journal article The American Midland Naturalist

A Comparison of Habitat Use and Demography of Red Squirrels at the Southern Edge of Their Range

Article excerpt

ABSTRACT.

Populations at the edge of their geographic range may demonstrate different population dynamics from central populations. Endangered Mt. Graham red squirrels (Tamiasciurus hudsonicus grahamensis), endemic to southeastern Arizona, represent the southernmost red squirrel population and are found at lower densities than conspecifics in the center of the range. To determine if differences are due to conditions at the southern periphery of the range, we compared habitat characteristics, demography, body mass, space use and nesting behavior with another subspecies located at the southern edge of the range, the Mogollon red squirrel (T. h. mogollonensis). We found that mean and minimum daily temperatures were higher at Mt. Graham whereas maximum temperatures were higher in the White Mountains, male Mogollon red squirrels were heavier than male Mt. Graham red squirrels in all seasons and female Mogollon red squirrels were slightly heavier than female Mt. Graham red squirrels in spring, proportion of squirrels in reproductive condition was lower in female Mogollon red squirrels, Mogollon red squirrels had smaller home ranges, used different types of nests and traveled less distance to nest than Mt. Graham red squirrels. There were no differences in annual rainfall, seedfall, habitat characteristics or survival between mountain ranges. Localized conditions appear to account for the disparity between populations. These differences demonstrate the importance of evaluating attributes of peripheral populations for maximizing persistence and intraspecific diversity.

INTRODUCTION

The extent of a species' range is determined by many factors, both biotic and abiotic. For wide ranging species, populations at die edge of dieir range are frequendy exposed to suboptimal and more variable conditions dian populations in other parts of the range, tiius local abundance is often lower at the periphery (Shelford, 1911; Kendeigh, 1974; Hengeveld et al, 1979; J.H. Brown, 1984; Lawton, 1993; Safriel et al, 1994; Hochberg and Ives, 1999; Gaston, 2003) . Populations at die range periphery are also more likely to spedate because of increased likelihood of barriers isolating populations, and may be more prone to extinction due to this isolation and lower resource quality (Rosenzweig, 1975; Terborgh and Winter, 1980; Gaston, 2003). For these reasons, evaluating behavior of individuals in peripheral populations and available resources may help identify key resource requirements necessary for conservation prescriptions (Lesica and Allendorf, 1995; Lomolino and Channell, 1995; Fraser, 1999).

The red squirrel ( Tamiasciurus hudsonicus) is a wide ranging species diat inhabits a diverse array of environmental conditions in different parts of its range (Steele, 1998). Red squirrels are small (<300 g), diurnal tree squirrels ranging throughout northern Canada and Alaska, the northern United States and soudi along the Cascade, Rocky and Appalachian Mountains (Hall, 1981). This species generally inhabits conifer forests above 2,000 m elevation (D.E. Brown, 1984; Froehlich, 1990; Steele and Koprowski, 2001) and deciduous and mixed forests above 500 m in the eastern United States (Linzey and Linzey, 1971).

Red squirrels are active year round and most store conifer cones when available within a central cache or midden, that is vigorously defended from conspecifics and other species (CC. Smith, 1968; Finley, 1969; Gurnell, 1984). Middens are necessary for survival as they provide cool, moist conditions diat prevent cones from drying and opening, thus furnishing a reliable food supply over winter (M.C. Smitii, 1968; Hurly and Lourie, 1997). Forest structure around middens is important in creating a microclimate necessary for cone preservation in addition to providing nesting sites, cover and escape routes from predators, and access to foraging sites (Hatt, 1929; Layne, 1954; CC. Smith, 1968; Finley, 1969). Appropriate conditions for nest and midden placement may be limited at the southern extent die red squirrel's range due to relatively high levels of solar radiation ttiat may increase vulnerability to desiccation. …

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