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Foraging ecology and nutrition of Stone’s sheep Seip, Dale R.


This study compared the foraging behaviour and nutrition of Stone's sheep (Ovis dalli stonei) which had access to burned range to that of sheep on unburned range throughout the year. In winter, sheep were restricted to areas with less than 25-30 cm of snow. For most sheep this meant windswept, alpine ridges, but one small population used a burned, subalpine slope. Grazing pressure was high on these restricted winter ranges, particularly during a severe winter, resulting in removal j of all the forage from alpine range, and leaving only very poor quality forage on the subalpine range. The poor winter nutrition was reflected in low fecal nitrogen and DAPA levels and high Protostronqylus counts. In spring, sheep used either natural or fire-induced subalpine clearings to obtain early new plant growth. Forage quantity was much lower on unburned ranges, but biting rate and foraging time were the same as on burned ranges with high forage biomass. Quality of available forage and food intake were also similar. The foraging behaviour of Stone's sheep appeared to be similar to domestic sheep in that intake rate was primarily dependent on grass length rather than forage density. Sheep moved to higher elevations as the season progressed which provided them with access to the higher quality alpine forage. Regular use of mineral licks in summer apparently satisfied their sodium requirement. Grasses and sedges were the most important foods throughout the year although forbs and browse were important in the summer. Adult mortality rates were low. Lamb production was low following severe winters, presumably due to poor nutrition. An explanation for changes in lamb production based on density-dependent selective feeding was developed. Burned ranges were of limited value to Stone's sheep because they were usually unavailable as winter range, did not provide superior spring range and were not used in summer. Horn growth, however, appeared to be slightly better on burned ranges. A model of nutrient intake is developed to investigate the seasonal pattern of nutrition on burned and unburned ranges.

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