Series: Alaska Park Science - Volume 16 Issue: Science in Alaska's Arctic Parks

By Kyle Joly, National Park Service

Caribou (Rangifer tarandus) are an iconic Arctic species. With a circumpolar distribution ranging from the temperate rain forest to polar deserts, the species is highly adaptable both physiologically and behaviorally. Yet, caribou populations face many challenges, such as climate change and industrial development, and are in decline in many portions of their range.
two caribou swimming in a river
Caribou, such as these two bulls in Kobuk Valley National Park, are excellent swimmers.

NPS Photo / Kyle Joly

Numbering nearly 500,000 caribou in 2003, the Western Arctic Herd (WAH) was the largest herd in Alaska and one of the largest on the planet. By 2016, the herd had declined to 201,000 (ADFG 2016). Habitat, climate, predation, human influences, density-dependent factors, insects, parasites, diseases, competition with other species, and other factors can influence caribou populations (Joly and Klein 2011).

It remains unclear which of these drivers is most important in the decade-long decline of the WAH; particularly difficult winters may have contributed. Population crashes and irruptions in the WAH have been linked to a long-lasting, large-scale climate cycle known as the Pacific Decadal Oscillation (PDO; Joly et al. 2011). Declines are associated with the “negative” phase of the PDO (colder years), while increase with the “positive” phase (warmer years; Figure 1).
a chart illustrating how caribou herd populations fluctuate in tune with the pacific decadal oscillation
Fig 1. Population of the Western Arctic Herd (black line), 1970-2013, and the strength of the Pacific Decadal Oscillation (PDO, colored bars).  Large declines in the herd coincided with negative (“cold”) phases of the PDO and rapid growth with the positive (“warm”) phase.
Caribou are known to have the longest terrestrial migrations on the planet. WAH caribou are no exception, with individuals traveling up to 2,737 miles (4,404 km) per year (Joly and Cameron 2015). As the herd has declined, its home range (roughly the size of Montana) has also shrunk. This phenomenon has been documented in other herds as well (e.g., Messier et al. 1988). Despite having a smaller home range, travel by individual caribou increased during the decline (Joly and Cameron 2015). One possible explanation is that the quality of the herd’s range has declined.

Harvest of WAH caribou is dominated (>90%) by subsistence hunters that live within the range of the herd. Hunting likely had limited impact on the herd when it numbered 500,000 caribou, however, as the herd continues to decline, its influence has increased. High numbers of harvested cows could accelerate the herd’s decline. Cautious management of the harvest is essential until the trajectory of the herd reverses.
Caribou running on snow in winter

NPS/Kyle Joly

The WAH faces an uncertain future. Will the decade-long decline continue, causing hardship across this vast and wild region? Is the strong positive PDO of the past couple of years a harbinger of herd recovery? Caribou populations are known to naturally oscillate at the decadal scale (Gunn et al. 2003, Joly et al. 2011), however, climate change and rapid industrial development may hinder the natural recovery of the WAH and other herds around the Arctic.

While tolerant of an extreme range of temperature, climate change could negatively impact caribou in myriad of ways. Warmer temperatures could lead to more wildfires, reducing the abundance of lichens, the primary winter forage of caribou (Joly et al. 2012). Warmer temperatures combined with early successional habitats promoted by increased fire may also allow for more shrubs and moose (Alces alces), and thus predators such as wolves (Canis lupus), which could affect caribou populations (Joly et al. 2012). These conditions may also enhance insect populations that torment caribou during the short Arctic summer. Not all impacts of climate change may be detrimental to caribou. Warming temperatures could lengthen the growing season in the Arctic and increase the abundance of summer forage.
aerial view of hundreds of caribou crossing a tree-less landscape
Caribou often form large aggregations in July, seen here in Noatak National Preserve, in response to intense insect harassment.

NPS Photo / Kyle Joly

Industrial development continues to expand across the Arctic and the pace of that development is predicted to increase as warming renders the region more accessible. Currently, the only major development in the range of the WAH is the Red Dog Mine, which includes the mine itself, a port and related facilities, and an industrial road connecting the two. Initial anecdotal evidence suggested that the impacts of the operation were limited; however, more contemporary, quantitative studies have revealed otherwise. Dust trailing behind ore-hauling trucks is carried by the wind, affecting habitat for miles, including into Noatak National Preserve (Hasselbach et al. 2004). Disturbance associated with the road has also been implicated in altering the migratory patterns of the WAH. Substantial numbers of WAH caribou can be delayed for more than a month on their fall migration south when they encounter this lone, well-controlled road (Wilson et al. 2016). These effects need to be considered as agencies analyze a proposal by the State of Alaska to construct a 200-mile long road through Gates of the Arctic National Park and Preserve from the existing contiguous road system into northwest Alaska, one of the world’s largest remaining roadless areas in the world, to mine prospects in the Ambler region. Long-distance migrations are imperiled globally, therefore protecting the migratory corridors for the WAH is critical. What does the future hold for the WAH?

As the millennia-old Inuit saying goes...
“No one knows the way of the wind and the caribou”


Alaska Department of Fish and Game (ADFG). September 2016.
Alaska Fish and Wildlife News: Western Arctic Caribou Herd Update. Available at: (accessed November 18, 2016)

Gunn, A. 2003.
Voles, lemmings and caribou – population cycles revisited? Rangifer Special Issue 14:105-111.

Hasselbach, L., J. M. Ver Hoef, J. Ford, P. Neitlich, E. Crecelius, S. Berryman, B. Wolk, and T. Bohle. 2004.
Spatial patterns of cadmium and lead deposition on and adjacent to National Park Service lands near Red Dog Mine, Alaska: NPS Final Report. National Park Service Technical Report NRTR-2004-45. 59 pp.

Joly, K. and M. D. Cameron. 2015.
Caribou vital sign annual report for the Arctic Network Inventory and Monitoring Program: September 2014-August 2015. Natural Resource Report NPS/ARCN/NRR—2015/1090. National Park Service, Fort Collins, Colorado. 25 pp.

Joly, K., P. A. Duffy, and T. S. Rupp. 2012.
Simulating the effects of climate change on fire regimes in Arctic biomes: implications for caribou and moose habitat. Ecosphere 3(5):1-18. Article 36.

Joly, K. and D. R. Klein. 2011.
Complexity of caribou population dynamics in a changing climate. Alaska Park Science 10(1):26-31.

Joly, K., D. R. Klein, D. L. Verbyla, T. S. Rupp and F. S. Chapin III. 2011.
Linkages between large-scale climate patterns and the dynamics of Alaska caribou populations. Ecography 34(2):345-352.

Messier, F., J. Huot, D. le Henaff, and S. Luttich. 1988.
Demography of the George River Caribou Herd: Evidence of Population Regulation by Forage Exploitation and Range Expansion. Arctic 41:279-287.

Wilson, R. R., L. S. Parrett, K. Joly, and J. R. Dau. 2016.
Effects of roads on individual caribou movements during migration. Biological Conservation 195:2-8.