CARIBOU/REINDEER (Rangifer tarandus)

RANGE: Arctic and subarctic.

Most caribou today roam the Arctic tundra, although a few herds inhabit the boreal forests of North America. In the last remaining large-scale ungulate migrations in the northern hemisphere, migratory caribou herds move seasonally along traditional pathways to reach areas with the most plentiful food and fewest predators and insects. Caribou play an important role in Arctic ecosystems by helping to cycle nutrients and structure plant communities in addition to supplying food to subsistence hunters. Many caribou herds are in decline across the Arctic. [1] At least four-fifths (34 out of 43) of the major herds that have been monitored during the past decade are declining, and caribou numbers have fallen an average of 57 percent from historical maxima. [2] Climate change, habitat fragmentation, and mineral and oil extraction are thought to be the primary causes for these declines.

Climate change is threatening caribou in numerous ways. Caribou time their migration to spring birthing grounds so that their arrival corresponds with the flush of nutritious plant growth. Ample food in spring and summer is important for the entire herd. Calves need nutritious food to put on enough weight to survive the winter, females need to produce milk for young calves and conceive in autumn, and males need to bulk up to compete for females. Although the spring growing season is starting earlier due to rising temperatures, caribou are not changing the timing of migration to keep pace, causing a mismatch between caribou and their food. In West Greenland, the onset of plant growth has advanced by two weeks as mean spring temperatures have risen by more than 4 degrees Celsius (7.2 degrees Fahrenheit). Because caribou time their migration based on changes in day length and not temperature, they are arriving past prime plant growth. Due to lower food availability, more calves are dying and offspring production has dropped fourfold. [3]

As temperatures warm, biting flies and mosquitoes that harass caribou are emerging earlier and increasing in abundance and activity. As insect harassment increases, caribou spend less time foraging and more time trying to escape, which results in reduced physical condition. Biting insects do make a difference. Scientists have found that severe insect harassment can lead to decreased pregnancy rates among females and increased mortality during winter. [4]

Caribou, especially at the northernmost latitudes, are also being hard hit by the increasing frequency and intensity of extreme winter weather events that are characteristic of climate change—including freezing rain events and deeper snow. “Freezing rain” or “rain-on-snow” events, where rain falls on snow and then freezes, create ice crusts that can lock the caribou’s lichen food under impenetrable frozen layers. Deeper snow makes it more difficult for caribou to find lichen and more energy-costly to move, and also increases their vulnerability to predators. Extreme weather events have been the primary cause of catastrophic population losses of Peary caribou from the Canadian High Arctic islands and of the Svalbard reindeer. [5]