Bathurst caribou calf following cow (Photo:  GNWT / A.Gunn) Bathurst caribou calf following cow (Photo: GNWT / A.Gunn)

Shifting Winter Home Range of the Bathurst Caribou Herd

It’s hard not to think of snow when we think of the North.  In many ways, snow defines the North.  Snow cover is connected to the Northern water cycle, regional and global climate, permafrost distribution, and the functioning of Northern ecosystems. 

Changes in climate can affect how much snow falls and influence the timing of the winter snow season.  Community members and scientists have observed changing winter conditions in caribou habitat across the North.  These changes involve warmer southern air moving into the area throughout the winter that can reduce snowfall, and cause earlier snow melts and shorter snow cover seasons.  These “incursions” of warm air can also change the snowpack structure.  Warmer spring weather can cause more frequent melting and refreezing of snow.  These icing events can seal the ground beneath a sheet of ice, making it difficult for caribou to break through to reach the lichen below.   As a result, caribou have to use more energy to forage for food. 

How is the snowpack changing, and what could those changes mean for caribou and their habitat?  Over the last 25-30 years, many barren-ground caribou populations across the circumpolar North, including the Bathurst herd, have undergone dramatic declines.  There are likely many factors or a combination of interacting factors that are contributing to these declines, such as the impacts of climate change, predators, harvest, natural and human-related disturbance to habitat.  Changes in the annual snowpack may also play a role, impacting how caribou move and use their winter ranges. 

Map showing the North Circumpolar Region (2008). Natural Resources Canada

Research led by Mike English, a professor in the Department of Geography and Environmental Studies at Wilfrid Laurier University, has been investigating changes in the snowpack and climate patterns.  The team is particularly interested in what is driving the “synchronous” decline in most of the barren-ground caribou herds found around the circumpolar world—changes that have been happening at about the same time.   Have climate conditions around the Northern hemisphere possibly played a role in these declines? 

The study brought together a team of scientists from different disciplines to investigate these questions. Using satellite sensing, researchers were able to track changes in the snowpack over large areas on a daily basis.  They also studied changes in the snowpack near Wekweètì from 2012 to 2016, working with local residents on taking snow surveys to measure snowpack characteristics –depth, density and crystal formation.  The density of snow is related to how much water it contains.  New fallen powder snow has a low density, for example, whereas wet or heavy snow has a high density.  They also recorded snow crusts and ice lenses in the snow, formed when the top surface of snow melts, then pools into a hard, shiny ice layer.  

Community members have often observed that the snowpack is changing, especially in late winter.  Similarly, researchers found that widespread warming in early March 2016 resulted in a hard, dense snow crust 10-20 cm thick.  When the snow is deep, dense or icy, travel can be difficult for the caribou and they may not be able to reach the lichen they need. 

Using caribou collar data, the study also identified a change in the Bathurst caribou range –a shift northward after 2005.  Before that time, the Bathurst caribou spent the winter months in the boreal forest across a large area mostly south of the tree line between Great Bear and Great Slave lakes.  But in recent years, a greater number of caribou have been spending more time near the tree line and northwards into the tundra.  It’s possible that their movements may be related to the changing snowpack and that this shift may move the caribou into areas that are not as impacted by the incursions of warm air.  Mike English also stressed that the current shift in habitat range doesn’t mean that it will stay that way in future. 

Is the decline of many of the Northern caribou herds tied into the northward shift observed in Bathurst caribou?  The answer may be linked to how often warm, southerly air masses move into the North.  To examine this, the research team looked at climate patterns over the last several years.  Specifically, they looked at the pattern of the Arctic Oscillation and related it to the pattern of decline of the Bathurst caribou herd. 

Arctic Oscillation (AO)  is a climate pattern characterized by varying pressure and winds circulating in a counter-clockwise direction around the Arctic.  When the AO is in its positive phase, a ring of strong winds circulating around the North Pole acts to keep colder air across the North.  This belt of winds becomes weaker in the negative phase, allowing colder arctic air to move south.  The phase can have an important effect on weather in the North.  In the negative phase, the AO brings warmer weather to the North; the positive phase can make the weather wetter and colder. 

Interestingly, the research team found that there appears to be a relationship between the Arctic Oscillation pattern and the Bathurst caribou decline.  In his presentation, Mike English showed a graph that depicted changes in the Bathurst herd’s estimated population from 1986-2015.  Where the graph showed periods when the population was in steep decline, the AO was negative.   Where the graph was more or less horizontal, indicating a more stable population trend with little change, the AO was positive.

The results are intriguing and lead to another question:  Is the synchronous circumpolar decline in caribou herds related to a common climatic driver?  In other words, is there something in the climate that all the circumpolar North is experiencing that is behind the declines?  It’s a question that is perhaps only the “tip of the iceberg” as new questions emerge and researchers continue to look for answers on this critical wildlife matter.

(Photo:  Susan Beaumont, WRRB)

Fact Box:  Snow

  • The snowpack forms from layers of snow that accumulate.  It is made up of the total snow and ice on the ground, including both new snow and the previous snow and ice which have not melted.
  • The annual snowpack in the winter foraging area of the Bathurst caribou herd covers the ground for 7-8 months of the year.
  • The snowpack helps regulate the temperature of the Earth’s surface by reflecting solar energy back into space.  On a smaller scale, variations in snow cover can affect regional weather patterns.
  • Once snow melts, the water helps fill lakes, rivers, and other surface water bodies and recharges the moisture in soils.
  • Snow cover may also play a role in the probability of forest fires.  Snow helps insulate the ground below, holding in heat and preventing moisture from evaporating into the atmosphere. Earlier snow melt typically causes soils to be drier for longer, increasing the likelihood of drought and a longer wildfire season.