Dall's Sheep - Life in the Mountains is Changing
Dall’s sheep, a subspecies of thin-horn sheep, are one of the most iconic animals of northern mountainous landscapes. These completely white sheep only occur in alpine pockets of northwest North America, extending from B.C. through the Yukon/NWT and into Alaska. The majority of their global population, estimated at less than 110,000, occur mostly in the Yukon and Alaska. They are almost entirely subarctic and are relatively rare compared to other ungulates. These sheep have evolved to endure extreme cold, persistent snow, sparse food, precarious terrain, and multiple predators. Playing an integral role in their ecosystem, Dall’s sheep are also of great importance to the past and present culture of Indigenous peoples and are highly valued by hunters and wildlife enthusiasts.
Dall’s sheep naturally go through population ups and downs, regulated by a variety of factors. Predation from coyotes, golden eagles, grizzly bears, and wolves – in addition to subsistence hunting by Indigenous peoples – have always played a role in the evolution of Dall’s sheep. Predation on its own has not been linked to serious, long-lasting sheep declines, likely due to their superb adaptations for rugged habitat. In the last century, direct and indirect effects of anthropogenic activities such as sport hunting, aircraft noise, roads, and backcountry recreation have become relatively new stressors for sheep. Although nowadays the negative effects of these disturbances can often be mitigated through landscape management tools such as parks. Protected areas encompass large swaths of Dall’s sheep habitat across their range and do a good job of managing impacts to sheep within their boundaries by regulating or prohibiting sport hunting, access, and land use in general. However, parks and protected areas are not immune to changing environmental conditions associated with climate change.
Research is showing freeze-thaw and rain-on-snow events are happening more often, in addition to increasing variability in snowpack properties such as depth, density, and seasonal coverage. Warmer winters are responsible for these conditions which create ice crusts on top of the snowpack and can encrust vegetation. As a result, Dall’s sheep are forced to spend extra energy while foraging and travelling during the time of year when they are already nutritionally deprived. Recent studies have found sheep survival rates were higher when winter temperatures were mild, but once temperatures were too high, freeze-thaw events and winter rain became more common, which in turn reduced adult sheep survival. In another study, lamb survival was linked to the timing of spring snow melt; when snow persisted longer in the spring, lamb survival was poor. Results from some Yukon sheep surveys have shown that low temperatures in fall and/or spring have been correlated with lower-than-average lamb counts per 100 nursery sheep. Nursery sheep include ewes, yearlings, and rams younger than two years old. This ratio of lambs per 100 nursery sheep, known as “recruitment”, by biologists, is a standard index for monitoring ungulate species. Tracking recruitment ratios over time provides biologists with insights into long term population health.
Within the last decade in particular, range-wide Dall’s sheep surveys have found evidence of population instability and in some cases, serious declines. Recent research suggests that climate change may be impacting sheep in ways they haven’t had time to adapt to yet, especially when layered on top of predation pressure and human impacts. We know climate change is happening faster in the north than other parts of the world, particularly in winter and at high altitudes. Given their excellent adaptations for life in the harsh northern alpine, when Dall’s sheep struggle it is a signal of change affecting far more than just sheep.
Although the impacts of different climate factors and the results from research may only apply to specific regions or populations of this species, it is apparent that climate change is altering subarctic, mountain ecosystems at unprecedented rates. Dall’s sheep have proven their resilience to predation, hunting pressure, and other human-caused impacts in the past. However, their capacity to adapt fast enough to rapid change in the mountains remains unclear.
Read MoreDall’s sheep naturally go through population ups and downs, regulated by a variety of factors. Predation from coyotes, golden eagles, grizzly bears, and wolves – in addition to subsistence hunting by Indigenous peoples – have always played a role in the evolution of Dall’s sheep. Predation on its own has not been linked to serious, long-lasting sheep declines, likely due to their superb adaptations for rugged habitat. In the last century, direct and indirect effects of anthropogenic activities such as sport hunting, aircraft noise, roads, and backcountry recreation have become relatively new stressors for sheep. Although nowadays the negative effects of these disturbances can often be mitigated through landscape management tools such as parks. Protected areas encompass large swaths of Dall’s sheep habitat across their range and do a good job of managing impacts to sheep within their boundaries by regulating or prohibiting sport hunting, access, and land use in general. However, parks and protected areas are not immune to changing environmental conditions associated with climate change.
Research is showing freeze-thaw and rain-on-snow events are happening more often, in addition to increasing variability in snowpack properties such as depth, density, and seasonal coverage. Warmer winters are responsible for these conditions which create ice crusts on top of the snowpack and can encrust vegetation. As a result, Dall’s sheep are forced to spend extra energy while foraging and travelling during the time of year when they are already nutritionally deprived. Recent studies have found sheep survival rates were higher when winter temperatures were mild, but once temperatures were too high, freeze-thaw events and winter rain became more common, which in turn reduced adult sheep survival. In another study, lamb survival was linked to the timing of spring snow melt; when snow persisted longer in the spring, lamb survival was poor. Results from some Yukon sheep surveys have shown that low temperatures in fall and/or spring have been correlated with lower-than-average lamb counts per 100 nursery sheep. Nursery sheep include ewes, yearlings, and rams younger than two years old. This ratio of lambs per 100 nursery sheep, known as “recruitment”, by biologists, is a standard index for monitoring ungulate species. Tracking recruitment ratios over time provides biologists with insights into long term population health.
Within the last decade in particular, range-wide Dall’s sheep surveys have found evidence of population instability and in some cases, serious declines. Recent research suggests that climate change may be impacting sheep in ways they haven’t had time to adapt to yet, especially when layered on top of predation pressure and human impacts. We know climate change is happening faster in the north than other parts of the world, particularly in winter and at high altitudes. Given their excellent adaptations for life in the harsh northern alpine, when Dall’s sheep struggle it is a signal of change affecting far more than just sheep.
Although the impacts of different climate factors and the results from research may only apply to specific regions or populations of this species, it is apparent that climate change is altering subarctic, mountain ecosystems at unprecedented rates. Dall’s sheep have proven their resilience to predation, hunting pressure, and other human-caused impacts in the past. However, their capacity to adapt fast enough to rapid change in the mountains remains unclear.
