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Stone sheep and their habitat in the northern Rocky Mountain foothills of British Columbia Luckhurst, Alan John

Abstract

Stone sheep (Ovis dalli stonei) and a representative, undisturbed habitat for this species were studied in the northern Rocky Mountain Foothills from May 1969 through May 1971. A highly descriptive and holistic approach was taken in this introductory study, with physiography, soils, climate, and vegetation and the native sheep all being assessed. The study was concerned primarily with the alpine sheep habitat with emphasis on the critical winter range. Vegetation in this northern environment, reflecting physiographic, climatic and edaphic diversity, presents a complex, heterogeneous pattern locally to a degree seldom observed in more southern latitudes. Local variations in climate, on different slopes and aspects, have produced striking floristic differences within short distances. Moreover, soils developed over different bedrock formations and distrubed little by glaciation contributed considerably to diversity in the alpine habitat. Extremely acid soils characterized by impeded drainage and low temperatures limited forage production over much of the habitat. However, soils developed over calcareous parent materials on southern exposures had the favourable characteristics of moderately coarse texture, good drainage and an adequate nutrient status. These soils supported relatively productive plant communities and high quality forage for the sheep. The vegetation was also characterized by stability especially in the alpine zone; this zone is largely free of a fire history and is characterized by climax or long-term disclimax communities. Apparent deteriorating climate over long periods may be very significant, however, because in all likelihood they reduce favourable sheep habitat. Stone sheep were almost entirely dependent on the herbaceous alpine vegetation for their nutritional requirements. Even though plant succession proceeds slowly at these latitudes, seral grasslands tended to be invaded quite rapidly by shrubs which reduced the herbaceous cover and caused drifting snow to accumulate in winter. Grasses and sedges made up 95.6 percent of the winter diet and 78.5 percent of the fall diet of sheep collected in the Nevis Creek study area. Vegetative diversity contributed largely to a balanced habitat for the sheep and the fortuitous combinations of factors of climate, soils and vegetation provided critical winter range on largely snow-free slopes with southern exposures. Three plant communities provided winter forage but one, the Elymus - Agropyron community, provided almost 60 percent of the forage utilized by wintering sheep. About 80 percent of the standing crop in this community, which made up only about 20 percent of the winter range and four percent of the total productive habitat by area, was utilized by the sheep. Although productivity was typically low in this alpine ecosystem, forage quality was relatively high and was maintained in the cured stage by hard fall frosts and the persistent winter cold. Counts conducted in summer and winter over an extensive portion of the northern foothills and Rocky Mountains showed stone sheep populations averaged 35 percent mature ewes, 28 percent mature rams, 15 percent yearlings and 22 percent lambs (n=981). Early summer counts for two seasons in the study area showed an average ratio of 74 lambs per 100 ewes two years of age or older indicating a high birth rate and low mortality in the first few weeks of life. The lambs experienced almost 50 percent mortality by the end of their first year; however, most of it occurred early in the first winter. Classified counts of the ram segment of the population indicate a period of low mortality during adult life to age 8 or 10 years. Of course, intraspecific competition and malnutrition during severe winter conditions, disease and parasites, injury, predation and hunting all contributed to mortality to some degree. Stone sheep populations reflect the stability of their relatively undisturbed alpine habitat. Actinomycosis and lungworm were common in sheep at Nevis Creek but there is no record of large-scale enzootic die-offs in this or other stone sheep populations such as occur in bighorn populations due to lungworm-pneumonia disease. Stone sheep habitat, once remote, is rapidly becoming more accessible and subject to man's influence with development of the north. Interference in this northern environment by man must consider its sensitivity to abuse and its slow recovery. Any change or destruction of this northern sheep habitat, particularly the important and restricted elements such as the Elymus - Agropyron plant community on the Nevis Creek winter range, would unquestionably seriously reduce the sheep populations dependent upon it.

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