The caribou are the most common large land mammal of the Arctic and subarctic. Some of the major mainland herds have populations of tens of thousands while gathering at the calving grounds. Some herds, however, are not as fortunate. The Peary caribou, (shown below) the only members of the deer family to have established themselves in the Canadian High Arctic, have been classified as "Endangered" since 1991 [1].
It has been reported [2] that the Peary caribou population in the western Arctic Islands has declined 95% in under 40 years, from an estimated population of 26,000 in 1961 to only 1100 in 1997.
The cause of this decline is climate change - more specifically, unfavourable snow and ice conditions [1].
During the winter, caribou forage for food by scraping away at the snow to find moss and lichen. Any climate change that would consistently make foraging more difficult for the caribou would threaten the herd. A season of heavy snow and ice would increase winter starvation and decrease the caribou's spring body fat, reducing lactation and calf survival rates [2].
Early ice break-up in the summer and late ice formation in the winter is further endangering several caribou herds. For example, the Dolphin and Union herd that migrate from the mainland to Victoria Island in the spring and return to the mainland again in the fall, experience drownings due to unsafe ice conditions [3]. The potential for large-scale drownings is much greater if there is an appreciable delay in the formation of ice in the fall, or earlier ice break-up in the spring.
Caribou are affected by climate change in the summer as well. Insect harassment by mosquitoes and parasitic flies is associated with air temperatures of 13° C or higher [2]. Thus, if the summer air temperatures reach 13° C or higher earlier in the season due to climate change, the increased energy required to forage and avoid insects will cause a decrease in body fat.
- Why is sea ice important to the polar bear?
-
Polar bears depend on sea-ice first and foremost as a platform from which they stalk ringed seals, their main source of food [4]. The seals are caught as they emerge from breathing holes in the ice - they are rarely caught in open water. According to Stirling and DeRocher [4], there has been only one recorded instance of a polar bear capturing a seal in open water. Thus, the time between ice freeze-up and break-up is critical for the bears as they must be able to deposit enough fat on their bodies to survive the open-water period. The ability for females to successfully produce and wean cubs is dependant on the time of break-up. Because of the loss of their main source of food during the yearly open-water period, if ice breaks up only one week earlier, the bears come ashore 10 kilos lighter [5]. Stirling and DeRocher [4] have also stated that if break-up occurs two or more weeks earlier than present, eventually cub production would not balance mortality, and the population would decline.
To illustrate the importance of freeze-up and break-up, the birthrate suddenly increased for the polar bears of Hudson Bay in 1992, when break-up occurred three weeks later than in 1991. Stirling [6] noted that this was a consequence of the eruption of Mt. Pinatubo, which resulted in significant global cooling, especially in the northern hemisphere.
Although 1992 was a fortuitous year for the polar bears, the summer of 1999 came too quickly for the bears of Western Hudson Bay. The CBC's Eve Savory noted that polar bears were forced off the ice and into their summer fast almost four weeks earlier than normal [5]. Savory's interview with Stirling also revealed that of all mother and cub families caught in the spring of 1999 and found again in the fall of that year, fewer than 2/3 of the cubs survived.
- Why is precipitation important to the polar bear?
-
Increased snowfall during the winter might be beneficial to the survival of seal pups before they are weaned as predators would be less successful at attacking seal lairs. However, increased precipitation in the form of rain would make these lairs much more vulnerable to attack, and could devastate the seal population [4], thus causing a decline in the polar bear population. There is also an increased probablility that rainfall in late winter could collapse the bears' maternity dens while the mother and cubs are still inside [4].
Polar bears are an obvious subject of study regarding climate change because they are at the top of the small arctic food chain. However, there are many other species who are ice dependent, including the walrus, spotted seal, harp seal, ribbon seal, and the bearded seal [4].