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Determinants of Spatial Distribution of Rocky Mountain Bighorn Sheep

1975; Wiley; Volume: 39; Issue: 2 Linguagem: Inglês

10.2307/3799919

1937-2817

Norman H. Shannon, Robert J. Hudson, V. C. Brink, W. D. Kitts,

Ruminant Nutrition and Digestive Physiology

Temporal and spatial distributions of ewe-juvenile groups of Rocky Mountain bighorn sheep (Ovis canadensis) in southeastern British Columbia were studied throughout one annual cycle. With simple and partial correlation analyses,, seasonal distribution was related to 11 environmental variables: slope, changes in slope, distance to escape terrain, elevation, aspect, forest cover, shrub productivity, biomass of palatable grasses, nitrogen content of palatable grasses, distance from salt, and snow depth. The influence of individual environmental factors on distribution varied throughout the year. Together the variables accounted for approximately 65 percent of the observed areal variation in counts of bighorn sheep. Mapping of the residuals of the multiple regression equations revealed discrete areas of extreme overor underestimation. LJ WILDL. MANAGE. 39(2):387-401 Animals living in a seasonally varying frequently disperse or migrate in order to avoid or better endure periodic hardships. This is well demonstrated by Rocky Mountain bighorn sheep and other mountain ungulates which show distinct seasonal changes in distribution. Although these movements have been documented and compared in a number of studies (Simmons 1961, Blood 1963), fewer attempts have been made to quantify the predictability of distribution and the relative importance of the numerous variables which affect it. Such information is basic to understanding the animal in relation to its environment. Habitat selection is an expression of a complex response of animals to a large number of often interdependent variables which make up the Umwelt or relevant environment of the animal. These variables may be intrinsic, dependent on the physiological and behavioral status of the animal, or extrinsic, dependent on cues from abiotic and biotic features of the environment. The objective of the present study was to describe seasonal changes in the spatial distribution of a population of Rocky Mountain bighorn sheep, to determine the extent to which these were related to the seasonally changing environment, and to infer how seasonal changes in physiological and behavioral characteristics may be reflected in a biasing of the response of animals to each of the environmental variables. Since variables existed in complex inter-connection, multiple correlation coupled with residual mapping techniques were employed to analyze areal variations (Berry and Marble 1968). We thank the Fish and Wildlife Branch, the Grazing Division, and the Department of Agriculture of the Government of British Columbia for financial support. R. Orich and L. Mather provided valuable technical assistance. This study is part of a Master of Science thesis by the senior author. STUDY AREA AND POPULATION The study area, described in detail by Demarchi (Unpublished data, British CoPresent address: Department of Animal Science, Faculty of Agriculture and Forestry, University of Alberta, Edmonton, Alberta T6G 2E3. J. Wildl. Manage. 39(2):1975 387 This content downloaded from 157.55.39.100 on Thu, 25 Aug 2016 06:31:31 UTC All use subject to http://about.jstor.org/terms 388 SPATIAL DISTRIBUTION OF BIGHORN SHEEP Shannon et al. lumbia Fish and Wildlife Branch, Victoria), was centered on an important big game winter range in the Rocky Mountain Trench region of southeastern British Columbia. It was bounded on the south and east by the Rocky Mountains and on the west by the Kootenay River Valley. The range of 6,000 ha was bordered on the south and west by a deeply dissected canyon formed by the Wigwam and Elk Rivers (Fig. 1). Climatically, the region is considered continental with an annual mean temperature of about 6 C. Minimum and maximum temperatures recorded at 933 m elevation were -34 C and +39 C. The area receives about 64 cm precipitation annually, distributed rather uniformly throughout the year. Snowfall recorded during the study (winter of 1971-72) was unusually heavy, 209 cm (British Columbia Dept. Agric. 1972). The area included large glacierdeposited alluvial terraces, low rolling rocky knolls, and rugged cliffs and moraines. The winter range was on the terraces (elev. 1,060 m) and lower slopes; the summer range extended to the alpine areas of Mount Broadwood (elev. 2,500 m). Much of the area used by wildlife in winter was created by a large fire in 1931. Removal of ponderosa pine (Pinus ponderosa) -Douglas fir (Pseudotsuga menziesii) forests resulted in the production of relatively stable seral grasslands on the river benches and seral shrub stages on the south and westerly slopes. Saskatoon (Amelanchier alnifolia), redstem ceanothus (Ceanothus sanguineus), snowberry (Symphoricarpos albus), dogbane (Apocynum androsaemifolium), and buffaloberry (Shepherdia canadensis) were the dominant shrubs. Douglas maple (Acer glabrum) was also abundant on these slopes. Important grasses included beardless wheatgrass (Agropyron inerme), Kentucky bluegrass (Poa pratensis), junegrass (Koeleria cristata), and Idaho fescue (Festuca idahoensis). Yarrow (Achillea millefolium), phlox (Phlox caespitosa), and horsemint (Monarda menthaefolia) were the principal forbs. The summer range supported a varied growth of grasses, most abundant of which were alpine bluegrass (Poa alpina), sheep fescue (Festuca ovina), and slender wheatgrass (Agropyron trachycaulum). Sedges (Carex spp.), rushes (Juncus parryi), and the forbs, sulphur eriogonum (Eriogonum umbellatum), white pussytoes (Antennaria alpina), white dryas (Dryas octopetala), and yarrow, were abundant. Buffaloberry, snowberry, and willow (Salix spp.) were also important in the subalpine region. The study area supported large numbers of bighorn sheep, particularly in winter (ca. 200), and elk (Cervus canadensis) and mule deer (Odocoileus hemionus) were common. The range had not been grazed by domestic livestock since 1954 except for a few horses which occupied the area annually from April to February.