Caribou Decline and the Effect of Snow Depth on Calf Recruitment

It is a generally accepted truth among hunters that heavy winters with deep snow are bad for deer, making it especially difficult for them to evade predators like wolves and coyotes. As I was looking for some recent science that might be interesting to hunters, I came across a new study about boreal woodland caribou that sheds some new light on the effects of snow depth and wolf predation on calf recruitment. While interesting on its own, there is also a larger North American conservation story that caribou are part of now. As hunters, wildlife managers, and conservationists, the role we play in that story will be an important part in writing the legacy that we leave with regards to wildlife conservation in North America.

Caribou in North America

Caribou (Rangifer tarandus) are a truly North American species, with a range that extends from Alaska straight across Canada to Newfoundland and Labrador.  The oldest evidence of caribou on this continent puts this ancient species at around 1.6 million years old. Since 1937, the caribou has been on the Canadian 25 cent piece as a representative of Canadian wildlife, next to another of North America’s noble creatures, and our national animal, the beaver.

Caribou in North America are divided into a number of groups, or subspecies, based on the type of ecosystem they live in. The Canadian Species At Risk Act focuses on three subspecies of caribou: woodland (R. t. caribou), barren-ground (R. t. groenlandicus), and Peary (R. t. pearyi). Grant’s, or porcupine, caribou (R. t. granti), which live in the Yukon and Alaska, comprises a fourth subspecies. Dawson’s caribou (R. t. dawsoni), a fifth subspecies that lived on Haida Gwaii, is extinct. Between all the subspecies, caribou range across the continent and at one time this range extended south into the United States, across the Great Lakes and into New England. The southerly limit of caribou range has progressively moved north since the early 1900s. The Southern Selkirk caribou herd is estimated to number about a dozen animals and still crosses the border between Idaho and British Columbia periodically.

(For an explanation of the system of Latin names for species, referred to as Linnean binomial nomenclature, click here.)

Source: SARA Recovery Strategy

Boreal Woodland Caribou

The boreal woodland caribou is the widest ranging, though not the most numerous, of the caribou subspecies. As a widely distributed species that occupy massive areas of land, caribou management is a complicated task. Generally, wildlife management divides species into distinct units, or populations, that are managed at more localized scales. For management and conservation purposes, boreal woodland caribou are divided into 52 somewhat discreet populations which as of 2002, together comprised an estimated 33 000 individuals (though this number is likely lower now).

Boreal woodland caribou inhabit the vast boreal forest ecotype. The boreal forest encircles the globe and is the most wide-ranging forest ecosystem in the world, constituting 14% of the Earth’s land cover. In Canada, the boreal zone spans the country from the Yukon and Northwest Territory down and across British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, and Newfoundland and Labrador, forming a contiguous band across the country.

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The boreal forest has 75% of Canada’s forests, spreading over more than 526 million hectares of the Canadian landscape, including forested and non-forest areas and vast amounts of wetlands. It is one of our most iconic ecosystems, composed largely of black and white spruce, tamarack, balsam fir, jackpine, white birch, and aspens. The boreal ecozone is home to a large suite of wildlife, including an estimated 300 bird species and a number of large mammals, including black bears, grizzly bears, wolves, moose, and woodland caribou.

The range of the boreal zone across Canada. Source: NRCAN

Caribou and Habitat Decline

Unfortunately, both woodland caribou populations and the quality of their boreal forest habitat are in decline throughout the country. Habitat fragmentation, largely as a result of industrial development and other land use changes, is a serious threat to the health of boreal forests across the country. Caribou occupy huge areas, so connections between different parts of their habitat is a vital part of caribou population health. Organisations like the Yellowstone to Yukon Conservation Initiative focus on increasing habitat connectivity for all species. Caribou were listed as threatened under the Species at Risk Act in Canada in 2003, with habitat loss, mainly due to human disturbance related to industrial development, being the primary threat to their populations, as well as hunting and predation.

Assessing caribou population numbers is a difficult task. According to the Canadian Parks and Wilderness Society’s (CPAWS) 2016 annual report on boreal caribou, caribou now occupy less than half of their historic range. Of the 52 recognized boreal caribou populations in Canada, SARA reports only 6 as stable, with 12 decreasing, and 33 unknown.

Estimates of local abundance are somewhat more available, though still quite difficult to achieve. For example, the Newfoundland and Labrador Department of Fisheries and Land Resources reports declines in the Red Wine Mountain population from roughly 700 individuals in the 1980s to less than 100 currently. The George River caribou herd in Labrador is estimated to have declined from 800 000 individuals in the 1990s to less than 9 000 currently.

On the other side of the country, in the Northwest Territories, the NWT Species at Risk Committee (SARC) reports that 53% of caribou are found in areas where populations are either declining or stable, though precise numbers or rates of decline are uncertain.

An iconic boreal forest ecosystem. Source: Ducks Unlimited

Does Snow Affect Calves?

Developing effective conservation strategies will continue to depend on the availability of current information on caribou. Healthy habitat and successful calf recruitment are certainly two of the most important factors in caribou recovery and conservation. Recruitment, broadly defined as the point at which the young of a species survives long enough to become part of the population, depends on three main factors: reproductive success, calf survival, and adult female survival. These, in turn, are dependent on a species having access to healthy habitat with enough food resources, particularly to make sure adult female body condition is sufficient to support successful reproduction. New research from the Government of Northwest Territories and the University of Alberta gives us some new information on factors affecting caribou recruitment.

The authors of a 2017 paper titled, Snow depth does not affect recruitment in a low-density population of boreal woodland caribou (Rangifer tarandus caribou), published in the European Journal of Wildlife Research, examined the impacts of snow depth on calf recruitment and wolf predation in a population of caribou in the Northwest Territories. The researchers studied radio-collared caribou and conducted aerial observations over an 11-year period to test the hypothesis that years with increased snow depth would lead to lowered calf recruitment in either the current or following year.

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This particular population ranged over a fairly pristine environment with little human disturbance and relatively low density of wolves, the main predator of caribou. Over the 11 years of the study, the researchers observed 2 505 individual caribou in 456 groups. The number of calves per adult cow in the population was 0.23-0.45. For perspective, the Yukon Government suggests that a range of 0.25-0.30 calves per adult cow is considered stable.

The study found that calf recruitment was not inversely related to snow depth in either the current or previous year. In other words, years with deeper snow did not seem to lead to a reduction in a number of calves that survived to reach adulthood. Further, wolf predation efficiency was not enhanced by greater snow depth. Typically, it is thought that in years with deep, soft snow, wolves are more efficient at hunting ungulates because they are better equipped to travel on the surface of the snow where ungulates sink into the snow and have difficulty escaping predators.

The researchers are careful to point out, however, that these results are not necessarily applicable across study areas. They identify the need for more studies, particularly in areas with different densities of caribou and wolves and different ranges of snow depth and characteristics (e.g. snow density or resistance) to better understand how these results might vary in different regions.

In at least one other example, a paper published in 2003 reported on a comprehensive study that examined ungulate population responses to an experimental wolf reduction program in the Yukon. This study also found that snow depth did not seem to affect caribou calf recruitment.

Although snow depths between these two studies varied, neither study observed a relation between snow depth and caribou demography (population increases or declines). The 2003 study did, however, find that wolf reduction led to “sharp reversals in the Aishihik caribou and moose populations from declining to rapidly increasing”.

Implications of Science

Any discussion of caribou science really needs to come back to conservation. We are at a precipice with caribou conservation in North America. Organisations focused on caribou conservation and recovery, such as CPAWS, consistently highlight the need to protect habitat and mitigate the effects of anthropogenic disturbance. CPAWS has also criticised the federal government for lack of prompt and decisive action on caribou recovery strategies. Under species at risk legislation in Canada, the Minister of Environment and Climate Change is required to prepare species recovery strategies that include identifying critical habitat for a listed species and a plan of action to protect that habitat.

In April 2017, CPAWS filed a lawsuit against the Minister of Environment and Climate Change for the Ministry’s lack of action in identifying measures to protect critical habitat for caribou. According to CPAWS, critical habitat for caribou was identified in 2012, and in the four years since then, there have been no reports describing what is being done to enhance protection of boreal caribou habitat.

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Species at Risk Legislation should be used as a mechanism to protect species and take action that will lead to their recovery. Lawsuits against federal agencies by environmental non-government organizations (ENGOs) have been misused in other cases, such as those to keep the Yellowstone grizzly bear population listed under the U.S. Endangered Species Act despite successful recovery; however, in cases such as the boreal woodland caribou, the CPAWS lawsuit will hopefully be a powerful push for the Ministry of Environment and Climate Change to use the SARA as the conservation tool that it should be.

At the end of the day, we need to use the tools at our collective disposal to protect wildlife and I think CPAWS made a difficult, but positive, decision in launching this lawsuit.

Difficult Decisions

Nunatsiavut, an Inuit self-government region in Labrador, made the difficult decision in 2013 to implement a five-year moratorium on caribou hunting to allow the George River herd, which has declined an estimated 98% in the last 20 years, to rebound. Caribou are an important country food for Labrador Inuit and a vital part of their traditional diet. However, the Nunatsiavut Government recognized the long-term importance of at least temporarily eliminating harvest pressure on the herd. In January 2017, Nunatsiavut President Johannes Lampe issued the following statement: “As Labrador Inuit, we must be committed to ensuring the future sustainability of this herd, or we run the risk of losing another important part of our culture and way of life.” Unfortunately, others have not followed suit, and caribou continue to be hunted in areas where dramatic declines have been observed. Moreover, industrial activity and habitat destruction remain a severe threat to caribou populations.

The decision in Nunatsiavut should be an example and an inspiration to all of us, particularly hunters, who face difficult decisions to protect wildlife. In Ontario, recent declines in moose populations have prompted a series of changes to hunting regulations which have elicited emotionally and politically charged responses from various interest groups. As Ontarians, we may find ourselves in a place where we need to make difficult decisions for conservation and we need to be willing to set aside partisan differences to put moose first. As we continue to see large-scale climatic and other environmental changes, we may find ourselves at these kinds of crossroads with increased regularity. How we choose to respond will decide the conservation legacy we leave on this continent.

One Comment on “Caribou Decline and the Effect of Snow Depth on Calf Recruitment

  1. Pingback: Canada and Alberta Sign Caribou Conservation Agreement | Landscapes & Letters

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