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Refuge Notebook: Dall sheep in a changing world

Posted: November 21, 2013 - 2:45pm  |  Updated: November 21, 2013 - 2:58pm
Photo courtesy Tom Lohuis, Alaska Department of Fish and Game  A really cute Dall Sheep lamb boasts a radio collar. A warming climate may be contributing to declining Dall Sheep populations in the Kenai Mountains.
Photo courtesy Tom Lohuis, Alaska Department of Fish and Game A really cute Dall Sheep lamb boasts a radio collar. A warming climate may be contributing to declining Dall Sheep populations in the Kenai Mountains.

There are few places in Alaska where you can see mountain dwelling ungulates from the roadside or on the casual day hike. We are fortunate to have such an opportunity in our back yard. The Kenai Mountains are truly a magical place providing unique opportunities for wildlife watchers, photographers and hunters.

One such mountain ungulate, the Dall sheep (Ovis dalli dalli), is of particular interest to me. I am a sheep hunter and, like others with my affliction, risk life and limb year after year in pursuit of this animal. I have a closet full of expensive technical gear which I use once a year for this sole purpose. Some would say I am obsessed, my wife says I’m crazy. Both are true. Needless to say, when I moved to the Peninsula a short time ago I was excited by the prospect of hunting sheep close to home.

Yet, due to declining sheep populations on the Peninsula, our resident sheep hunters (including me) are often forced to try their hand elsewhere in the state. The declining trend in sheep numbers is not only a peninsula-specific phenomenon but appears to be occurring in other parts of the state as well. Sheep populations tend to fluctuate over time with severe winters, climactic events appearing to be a strong driver when populations are low.

The Kenai Peninsula represents the southernmost range for Dall sheep in Alaska. Sheep habitat, only a small portion of the Kenai Mountains, likely restricts sheep distribution. Sheep population monitoring efforts have been under way since before statehood. The Alaska Department of Fish and Game and U.S. Fish and Wildlife Service records show that sheep steadily increased from 1949 to 1968, before sharply declining during the 1970s. Only two peninsula-wide censuses have been completed. In 1968, the peninsula-wide population was estimated to be 2,200—2,500. In 1992, the estimate dropped to 1,600 sheep. The Alaska Department of Fish and Game currently estimates 800—1,200 sheep on the Peninsula based on minimum count aerial surveys.

Several factors play roles in sheep population dynamics. Above all it seems that recent changes in climate are having a large impact on sheep populations not only on the Peninsula but statewide. Sheep are a good indicator of changes in mountain ecosystems as they are sensitive to environmental change. Severe winter conditions including heavy snow and mid-winter rain (and subsequent icing events) can be rough on sheep. Deep snow can restrict food availability and force sheep to expend precious energy to find food. Mid-winter rain events followed by freezing events can have similar negative impacts on sheep as food becomes frozen underneath a layer of ice, making it hard if not impossible to access.

Dr. Tom Lohuis, an ADF&G researcher, has observed what he considers “hallmarks of severe nutritional restriction” in collared animals within his study areas in Game Management Units 13D and 14C. The high prevalence of poor body condition and low pregnancy rates is, in his opinion, probably related to difficult winter foraging conditions brought about by ice layers and wet, heavy snow. 

Whether you are a believer or a denier of climate change, it is hard to argue with the bizarre and often erratic weather patterns this past decade. Take this year’s painfully drawn out spring as an example. This was a statewide phenomenon and, based on 2012 aerial surveys by National Park Service staff, looked to be pretty hard on sheep.

Preliminary results from Gates of the Arctic National Park and Preserve show as much as a 50 percent decline in total sheep numbers, low to no lamb production, and a decline in ewes, yearlings, and immature rams.

The long winter and very cold May were likely factors. Other data from Denali National Park road sheep surveys this past summer also showed noticeably low lamb numbers as compared to 2008-2012.

Another relevant trend in the Kenai Mountains is rising treeline in response to a warming climate. Roman Dial and colleagues (2007) estimated that treeline has crept into alpine tundra at the rate of 1 meter per year over the last 50 years. While this may not seem like much, it equates to about 300,000 acres of lost alpine tundra (aka sheep habitat). Given that two thirds of their diet is composed of tundra plants, sheep are losing their groceries.

It’s not a reach to predict that if this trend continues, sheep populations will continue to decrease.

Disease may also impact sheep populations. The good news is that despite sampling more than 100 adult sheep in two areas of the Chugach and on the Kenai, Lohuis hasn’t been able to attribute any population-level mortality effect to viral or bacterial disease. In fact, results from a fairly comprehensive screening of blood from 14 sheep (10 rams, 4 ewes) that Lohuis captured on the Kenai Peninsula were negative for all viruses tested.

The bad news is that a warming climate could lead to increased transmission rates of some diseases due to range expansion of transmission vectors or the introduction of new vectors.   

None of this bodes well for Dall sheep on the Kenai Peninsula. If a changing climate is the root cause of sheep declines, then we may have to become more conservative in our management. Sheep have inhabited Alaska since the Pleistocene and let’s hope they stay, lest the sheep hunter becomes extinct.

 

Nathan Olson is the wildlife biologist-pilot at Kenai National Wildlife Refuge. You can find more information about the Kenai National Wildlife Refuge at http://kenai.fws.gov or http://www.facebook.com/kenainationalwildliferefuge.

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