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Taking a Bite Out of Caribou Herds

Barren-ground caribou are in decline across Canada. Are insects and parasites the cause of their drastic population decline, or is something bigger to blame?

Excerpt from Winter 2023 Fair Chase Magazine
By Benjamin Juan Padilla, Postdoctoral Research Associate - University of Calgary

Big game species like ungulates have captured the imaginations of humans for tens, if not hundreds, of thousands of years. From the artists of the Lascaux Caves to modern hunters and naturalists, humans are drawn to their beauty, spectacle, and thrill of the chase. In North America, caribou stand apart as one of the continent’s most charismatic yet underappreciated ungulates—one that is uniquely suited for extremes. From the mountains of the Canadian Rockies in the south to the furthest northern reaches of the Arctic Archipelago, caribou (which are North American counterparts to Eurasia’s reindeer; both are subspecies of Rangifer tarandus) make their living in some of the most extreme resource-poor habitat, especially migratory barren-ground and island-dwelling subspecies.

Herds of barren-ground caribou withstand long winters with temperatures routinely as low as -70 degrees Celsius. Their seasonal migrations amount to more than 1000 km each year. Caribou migrations across the landscape have thoroughly transformed and maintained the ecology and culture of the region. By foraging and moving nutrients across the tundra, caribou have helped to maintain plant diversity and have been integral to the survival and culture of Indigenous peoples for 10,000 years.

And yet, despite having survived ice ages, glaciations, and mass extinctions, the future of North America’s caribou is uncertain as changing climate alters parasite dynamics, pushing caribou to their limits. Indigenous peoples and Western researchers search for answers as barren-ground caribou populations decline across the North. As a postdoctoral research scientist at the University of Calgary, I worked alongside Indigenous communities and government management agencies in Canada’s Northwest Territories to try and understand how the combined impacts of insect harassment, parasites, and changing forage quality may have contributed to declines in what was once Canada’s largest herd.

Ancient Migrations

Ever the taxonomists, scientists have categorized caribou into herds depending on where they calve. For most Indigenous communities, caribou is caribou, regardless of where they calve. The migratory Bathurst caribou herd, named for the Bathurst Inlet on the Arctic Ocean where females gather to calve, occupies a range of more than 100,000 square km in Canada’s Northwest Territories and Nunavut. Once possibly the largest herd of migratory caribou in Canada, hundreds of thousands of caribou from the Bathrust herd wandered thousands of kilometers across the barren lands following circuitous migration routes that some Indigenous Elders say is written in the sinews and fissures of the caribou’s skull. Local Indigenous peoples have had deep spiritual and physical connections to caribou since time immemorial. Archaeology and oral traditions extend the history of caribou hunting back at least 8,000 years, and place names across the region reflect this deep relationship. Kwįìka, or “fence narrows” in English, is a natural movement corridor used as an ambush site by hunters into modern times. But times are not always good, and in such an extreme environment, people and caribou must live and move in rhythm with the seasons.

The barren-ground caribou’s spring migration from their wintering grounds along the edge of the boreal forest to the calving grounds on the barren lands near the Arctic Ocean typically begins in mid to late April, long before the spring thaw. Females reach the calving grounds in early or mid-June, and calves are born soon after. Like most other northern ungulates, caribou are considered capital breeders, which means that parturition (birth of calves) and the start of milk production happens before new spring forage is available. New spring growth is the most nutrient-rich forage a caribou can find, but it doesn't become abundant in the far north until several weeks after calving. This means that the significant nutritional demands of migration, gestation, and lactation are drawn almost entirely from energetic reserves gained from the previous summer. As a result, cows' energy balance and body condition reach a low point in mid-July. The brief Arctic summer, when abundant new forage is rich in energy and protein, is critical to replenish depleted energy reserves and ensure survival over winter. All of this put caribou on a knife’s edge energetically, where small changes in energy expenditure or energy intake throughout the year could result in individual reproductive failure and reduced population-level parturition rates.


Selected North American Caribou Herd Locations and Seasonal Energy Map.

Cycles of Boom and Bust

Aerial surveys of the Bathurst herd’s calving grounds in the mid-1980s estimated a population of well over 200,000 breeding females and more than 500,000 in total! Not only was the herd abundant, but they were also healthy and big. The Boone and Crockett World’s Record Central Canadian barren-ground caribou was taken in September 1994 by Donald J. Hotter III near Hottah Lake, in the heart of the Bathurst herd’s late summer and early fall range. But since this hunt, things have changed. Total population estimates went from 360,000 in 1996 to 31,000 in 2009 and down to 8,200 total in 2021—a decline of approximately 98 percent in less than 30 years.

Knowledge of the local Tłįchǫ People, who live in relationship with the land and with caribou on the Bathurst herd’s range, corroborates some scientific findings. Tłįchǫ Elders and harvesters reported “good times” in the 1990s, when caribou were fat, abundant, and accessible. They began noticing changes in caribou location, body condition, health, and taste in 1998 and 1999. The year 2000 is seen as a turning point. Caribou harvesters reported large mortality events that had never been seen before. By 2005, caribou were very hard to find. Changes in caribou health were also observed, including skinnier animals, watery bone marrow, white “spots” or cysts in muscles or liver, and “bad tastes” in the meat. To better understand why the caribou had not returned to their communities, the Tłįchǫ initiated an on-the-ground monitoring program called Ekwǫ Nàxoèhde K’è (ENK) to monitor the caribou, their health, their predators, and their home. In recent years, as numbers of the Bathurst herd have continued to decline, the Tłįchǫ Elders, harvesters, and youth who participate in ENK have reported changes in weather, insect activity, forage quality, relative calf abundance, and caribou body condition.

Precisely what is causing these declines has been the subject of much research, observation, conversation, and debate. For some, perhaps those with a deeper historical perspective, a sudden drop in caribou numbers isn't all that unusual. Like many other species of Arctic wildlife, caribou experience cycles in populations. Scientists have pointed to decadal trends in climate, such as the North Atlantic Oscillation (NAO), that change temperature and precipitation patterns. The story goes something like this. When weather conditions favor plant growth, caribou gain body fat and have higher survival rates, pregnancy, and calf recruitment. Populations grow, eventually reaching a peak. As populations approach the peak, the large caribou population begins to reduce forage biomass through overgrazing, which reduces body condition, survival, and fecundity, making the population more susceptible to environmental changes. When the NAO shifts and weather patterns change, a harsh winter or particularly hot summer will substantially impact the population.

Multigenerational Indigenous knowledge and oral histories confirm the scientific hypothesis of cyclic populations. Stories of times when caribou were so numerous hunters barely needed dog teams and sleds to reach them are balanced by times when communities survived by switching to fish, muskoxen, or moose. A collaborative study between Western researchers and members of the Łutsël K’e Dëne First Nation looked at trample scars on the roots of spruce trees along caribou migration routes and found evidence of changes in scar frequency that matched Dëne knowledge of population cycles reaching back 200 years.

If caribou populations always cycle, should we expect numbers to jump back up sometime soon? According to some, the answer is yes. Some Alaskan herds that have seen peaks and valleys have been on the rise recently. Some in Canada, like the Blue-Nose East herd, are beginning to show signs that they might be on the rebound. But, the severity of the crash experienced by many herds and the extended length of the low-phase without recovery suggests something more may be happening.

Compounding Threats

The effects of global climate change impact Arctic ecosystems and caribou in both positive and negative ways. Earlier springs and longer summers are extending the growing season and increasing the biomass of caribou forage. But it's not only quantity but quality of forage that matters, and the energy content of forage declines with longer growing seasons. Winter rain-on-snow and freeze-thaw events can leave an impenetrable layer of ice between caribou and their food, causing massive die-offs. These events appear to be more frequent in the new climate reality.

The Arctic tundra is notorious for its swarms of insects in spring and summer. Harassment from mosquitoes, black flies, and parasitic warble flies dramatically changes caribou behavior. Instead of replenishing depleted body fat by foraging on energy-rich new plant growth, caribou focus on escaping the bugs. Indigenous Elders and harvesters have told me of a group of a few hundred caribou running in a circle together on one bad bug day. Hot, dry summers, which are becoming more common, exacerbate insect harassment, which means caribou have less time to focus on regaining fat for winter. Other parasites, such as gastrointestinal nematodes, are also expected to increase as warmer temperatures speed up development rates


A diagram of the bioenergetic caribou model. Insect harassment, parasitic botflies, and gastrointestinal nematodes all reduce energy intake and increase energy expenditure. This leads to less body fat and lower rates of survival and reproduction.

On-going Studies and Collaboration

In the real world, however, no one threat—climate change, predation, or disease—is ever the sole cause of population decline. Instead, the cumulative impact of several interacting stressors is likely to blame. Because body condition and nutrition are fundamental drivers of caribou population dynamics, my collaborators and I developed a bio-energetic model to evaluate the effects of these stressors on caribou populations through their impact on body condition, reproduction, and survival. This work showed that reduced time foraging caused by nematodes and insect harassment, combined with the energetic costs of avoiding insect harassment and growing parasitic warble flies, can reduce caribou body condition and population size. And when combined, their effects are magnified. But, even when using weather data to parameterize insect intensity and nematode development, our model couldn't recapture the full extent of population decline over the past 40 years. The real world is much more complicated. When combined with predation, extreme weather, mining development, and other infectious pathogens, the energetic costs of insects and nematodes may have been the bugs that broke the caribou’s back.

ENK spends months on the barren lands each summer using traditional Tłįchǫ hunting practices and Elders’ knowledge to observe caribou at a critical time of year missed by government surveys, which can help to improve caribou conservation and management in the region.

Led by Dr. Susan Kutz, the research team I’ve been a part of at the University of Calgary has worked alongside Northern Indigenous communities on the health and conservation of caribou and muskoxen for years. By bridging Western science and veterinary knowledge with Indigenous knowledge and understanding through community-based monitoring and hunter-based sampling, the Kutz Lab is working to improve Western conservation and management by shifting the balance of power back toward Indigenous communities. In the coming year, we are initiating a new project for Central Canadian barren-ground caribou to see whether Indigenous knowledge of caribou health, coupled with harvester-based sampling and local field observations, can anticipate population shifts for proactive management.

Projects like ENK and the growing number of collaborative monitoring and management projects bridging Western and Indigenous knowledge offer a ray of hope for the future of caribou. Through efforts like these, we are beginning to recognize that one mine, road, or predator can’t be pointed to in isolation as the cause of wildlife declines. Each stressor is compounded over time at the population level, so we, as wildlife advocates and hunters, must be aware of the potential impacts of small changes in habitat or human activity on threatened wildlife.

In the end, I think there is a lot to learn from Tłįchǫ Elders about how to relate to the world around us. Indigenous cultures like the Tłįchǫ see everything in nature, including humanity, as integrated, interdependent, and inseparable. In the words of one Tłįchǫ Elder, “We have to respect the caribou, and they give us respect. You treat them good, and they will treat you good.”

References and Resources

Beaulieu, D. 2012. Dene traditional knowledge about caribou cycles in the Northwest Territories. Rangifer 32:59.

Dokis-Jansen, K. L., B. L. Parlee, Ł. K. D. F. Nation, D. S. Hik, B. G. Berthiaume, E. Macdonald, and C. Stinn. 2021. “These trees have stories to tell”: Linking dënesǫ́łıné oral history of caribou use with trample scar frequency on black spruce roots at ɂedacho kué. Arctic 74:290–305.

Ehlers, L., G. Coulombe, J. Herriges, T. Bentzen, M. Suitor, K. Joly, and M. Hebblewhite. 2021. Critical summer foraging tradeoffs in a subarctic ungulate. Ecology and Evolution 00:1–38.

Gordon, B. C. 2005. 8000 years of caribou and human seasonal migration in the Canadian Barrenlands. Rangifer 25:155.

Gunn, A. 2003. Voles, lemmings and caribou - population cycles revisited? Rangifer 23:105.

Gunn, A. and Russell, D. 2022. Update on the status of wild reindeer and caribou. IUCN Deer Specialist Group.

Jacobsen, P. 2022. Ekwǫ̀ Nàxoèhdee K’è: 2021 Results. Pages 1–45. Dedats’eetsaa: Tłįcho Research and Training Institute.

Koltz, A. M., and L. E. Culler. 2021. Biting insects in a rapidly changing Arctic. Current Opinion in Insect Science 47:75–81.

Leffler, A. J., H. A. Becker, K. C. Kelsey, D. A. Spalinger, and J. M. Welker. 2022. Short-term effects of summer warming on caribou forage quality are mitigated by long-term warming. Ecosphere 13:e4104.

Parlee, B. L., M. Manseau, and L. K. É. D. F. Nation. 2005. Using Traditional Knowledge to Adapt to Ecological Change : Denésǫłıné Monitoring of Caribou Movements. Arctic 58:26–37.

Post, E., E. Kaarlejärvi, M. Macias-Fauria, D. A. Watts, S. Bøving, S. Cahoon, R. C. Higgins, C. John, J. Kerby, C. Pedersen, M. Post, and P. F. Sullivan. 2023. Large herbivore diversity slows sea ice–associated decline in arctic tundra diversity. Science 380:1282–1287.

Tyler, N. J. C. 2010. Climate, snow, ice, crashes, and declines in populations of reindeer and caribou (Rangifer tarandus L.). Ecological Monographs 80:197–219.

Ekwǫ̀ Nàxoèhdee K’è Results and Resources:

Caribou Ecotypes in Canada:

Archaeology of Dogrib Culture in Northwest Territories:



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