Dall's Sheep in Alaska's Arctic Parks: Steep Declines and What Drives Them

Dall's sheep live on some of Alaska's most rugged alpine ground. In the Brooks Range they reach the northern edge of their range — and in the parks of the Arctic Network, their numbers have dropped sharply.

The five parks of the Arctic Network. NPS.

How many sheep are left

Yearly surveys of smaller areas show the pattern in more detail:

• Itkillik subarea (GAAR): steady at about 1,600 sheep from 2009 to 2012, then down about 60% across 2013 and 2014. It has held low but stable since, at about 670 (2015–2019).
• Western Baird Mountains (WEAR): a steep, continuing slide — down 73% from 2011 to 2019, to a low of 174 sheep (141–230).

Survey results in the western Baird Mountains, 1986–2019, and the Itkillik subarea, 2009–2019. Older western Baird figures come from a sightability model applied to minimum counts; recent ones, and all Itkillik figures, from distance sampling. NPS.

Lambs are being born — but numbers are not recovering

The number of lambs per 100 "ewe-like" sheep (ewes, yearlings and young rams) is the usual gauge of breeding success. The average is about 30.

• In 2013 and 2014, almost no lambs were seen in either the Itkillik or the western Bairds.
• From 2015 to 2019, the ratio was above average: 35 in the Itkillik and 33 in the western Bairds.

Even so, those better years have not turned into population growth. Every hunt in WEAR was closed by emergency order in 2014 and stayed closed through 2020.

Dall's sheep search for grasses and lichens under snow. NPS/Matt Cameron.

What climate has to do with it

At the edge of their range, these sheep are especially sensitive to weather. Recent research by the Park Service and other scientists points to two effects:

• Late snow kills lambs. One study found lamb survival closely tied to how early snow melts in spring. When snow lingers, more lambs die.
• Mild winters help adults — up to a point. Another study found adults survived better in milder winters. But when temperatures swing too high, freeze-thaw cycles and freezing rain seal plants under ice, and the sheep cannot reach their food.

The Arctic parks fit that picture: the crash came in 2013 and 2014, two years of cold, snowy winters and late, icy springs. How the herds will respond to continuing climate change is unclear, and the surveys will go on to find out.

Why these sheep matter

• Habitat: Gates of the Arctic and Noatak cover most of the sheep habitat in the central and western Brooks Range. In the 1980s they were estimated to hold 13–15% of all the world's Dall's sheep.
• People: the sheep are an important subsistence species for local residents and are prized where sport hunting is allowed in the preserves.
• Visitors: they are among the most visible large mammals for wildlife watching in northern Alaska.
• A signal: sheep numbers, spread, age mix and health reflect how alpine conditions are changing.

Big, widespread declines have happened twice — in the early 1990s and again in 2013–2015 — and both times led to emergency closures of subsistence and sport hunting in WEAR. Tracking the trend is how managers conserve the sheep and keep hunting possible.

Ewes and lambs huddle in steep terrain, seen from a Super Cub during an aerial survey. NPS/Jared Hughey.

How the Park Service counts them

The Arctic Network's Dall's sheep monitoring uses aerial distance sampling at two scales:

1. Park-wide surveys of GAAR and WEAR, every four years.
2. More frequent surveys — every one or two years — in the key hunting areas: the Itkillik and Anaktuvuk subareas in GAAR and the western Baird Mountains in WEAR.

Bayesian models turn the sightings into estimates of total numbers and of each sex and age class. The work is shared with the Central Alaska Network, which monitors sheep in Denali, Wrangell-St. Elias and Yukon-Charley Rivers.

Dall's sheep survey areas in the Arctic Network. NPS/Dylan Schertz.

What the monitoring is for

• Long-term trends in how many sheep there are and where they live.
• Long-term trends in the mix of sexes and ages.
• Changes in regional and local numbers, caught early.
• Demographics and health where hunting is managed.
• Understanding alpine conditions and how environmental change affects local herds.

Dall's sheep rams head for escape terrain. NPS/Dylan Schertz.

Sources

Based on "Dall's Sheep Resource Brief for the Arctic Network," Arctic Inventory and Monitoring Network, National Park Service; a work of the United States government in the public domain. Two close-up portraits of rams credited to an individual photographer are not reproduced here.