Weather Impacts on Dall's Sheep

Ewes and lambs on rocky cliffs in Noatak National Preserve.

NPS/Marci Johnson

Dall's sheep live in rugged environments and are vulnerable to extreme weather events such as late spring snow and rain when they are lambing. As weather patterns are changing, how do weather events impact Dall's sheep across Alaska? This paper looks at the relationship between when snow season ends and sheep numbers in different parts of the state--in the northern, Arctic parks and in the southwest mountains of the Alaska Peninsula. The authors found that later springs (the longer snow stayed on the ground) led to fewer lambs and overall population declines.

Delayed spring onset drives declines in abundance and recruitment in a mountain ungulate

Abstract

Changes in climate are driving widespread landscape changes in northern ecosystems, altering the size and distribution of wildlife populations over multi‐decadal time scales. Extreme weather events are also expected to become more common over time, affecting a variety of species, and mountain ungulates may be particularly susceptible because they occupy habitats with particularly harsh winter weather conditions. To explore the impacts of weather conditions and adverse weather events as population drivers, we surveyed Dall's sheep throughout their latitudinal range in Alaska and assessed lamb production and population trend in relation to end of the continuous snow season (CSS) as a measure of spring onset. In 2013, spring onset was extraordinarily late, providing an opportunity to directly assess the impacts of variability in weather on sheep population dynamics. We hypothesized that the timing of the end of the CSS would have greater impacts in arctic areas where conditions are presumably most limiting. We found that lamb production was negatively related to the annual timing of the end of the CSS and was near 0 in arctic areas in 2013. The 2013 event was also associated with ~40–70% declines in overall sheep numbers in arctic areas, indicating adult survival was also impacted. Overall, our results suggest that expected increases in adverse weather events may have direct, lasting impacts on mountain ungulate populations and that these impacts can be expected to be most extreme for populations occurring at northern range limits where growing season conditions are most restricted.

Rattenbury, K. L., J. H. Schmidt, D. K. Swanson, B. L. Borg, B. A. Mangipane, and P. J. Sousanes. 2018. Delayed spring onset drives declines in abundance and recruitment in a mountain ungulate. Ecosphere 9(11):e02513. 10.1002/ ecs2.2513