THAWING OUT NORTHERN MAMMALS
For mammals in the northern regions of the world, global warming must seem a little more real than it does to humans below the Arctic Circle. In 2004, the Arctic Climate Impact Assessment released a report called Impacts of a Warming Climate that revealed dire and immediate consequences of climate change for species ranging from polar bears to seals .
Temperatures in the Arctic are increasing at twice the rate of the rest of the world, according to the report. Sea ice is rapidly melting and breaking up at a rate of about 3% per decade, winters are shortening, and every year brings more precipitation.
All of these climatic changes are working together to change the landscape of the Arctic. Longer and warmer growing seasons are helping the treeline to shift northward, replacing tundra, and bringing insects and forest fires to the North.
Making the situation direr, other kinds of pollution might cause the Arctic ice to melt even more quickly. The carbon dioxide that is released when fossil fuels are burned acts as a greenhouse gas to warm the atmosphere, but soot, another byproduct of the same process, affects the ice directly. NASA scientists have found large accumulations of soot in Arctic ice, much of which can be traced back to worldwide industry. Soot in the air traps heat, and the dark colour of ice-bound soot absorbs heat, resulting in warmer temperature and melting ice.
Arctic mammals, specially adapted to the cold and ice, are witnessing the transformation of their habitat. Global warming and pollution will unquestionably change their lives and alter their species’ futures.
In the short-term, some mammals may be able to adapt to the warming temperatures—red squirrels, for instance. A population of the animals near Kluane Lake, Yukon, has seen average temperatures rise by 2oC in the last three decades. The growing season of white spruce, the cones of which are the squirrels’ staple food, has lengthened dramatically, and the trees are producing up to 35% more cones within each squirrel’s lifetime.
As the food supply steadily increased, the squirrels’ behaviour began to change, too. In the last 10 years, they sped up breeding at an average rate of six days per generation, meaning that squirrels are giving birth 18 days earlier than they were a decade ago.
Most of the change in red squirrel breeding (about three and a half days per generation) comes because the animals have an innate ability to change their behaviour to suit a certain range of environmental conditions, a phenomenon known as phenotypic plasticity.
To a lesser degree, though, red squirrel behaviour is genetically evolving to keep up with their changing environment. About a day’s worth of the change in birthing dates of each successive generation is actually a result of genetic changes that are being passed from mothers to daughters.
But most of the changes to the Arctic that are caused by global warming aren’t quite as beneficial as an increase in food supply.
Polar bears are predicted to suffer in several ways from a warming arctic climate. The bears are listed as “of special concern” by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC), because hunters over-harvest adult females and melting sea ice is threatening their access to food.
Bears roam the ice searching for air holes where seals come to rest and raise their young. In the 20 years preceding the 21st century, sea ice declined by 14%, and some researchers have predicted that, within the next 50 years, ice will no longer be connected to the mainland. If the prediction holds, it will become more difficult for polar bears to catch and kill seals while they use the ice.
The seals will be available to polar bears for a shorter time, too. Compared to 30 years ago, sea ice is breaking up more than two and a half weeks earlier.
Declining sea ice is already affecting the health of polar bears. Their average mass is steadily declining, and some researchers predict that, within 100 years, female bears won’t have enough food to reach the mass required for reproduction. Of course, there are annual fluctuations in sea ice that would allow adequate feeding in some years, but those years will become fewer and farther between.
Female polar bears also use sea ice as a path to reach the sites they use as dens while they give birth. Females return to the same denning sites year after year, and they swim and walk over the ice to get to them—and many of them build their dens on sea ice, which is becoming increasingly disconnected from land. Thaws and break-up of ice mean that traditional dens will be harder to reach, and may be in precarious positions.
Sea ice isn’t the only thing that’s melting. Further south, many female polar bears build their dens on land, digging them into snow or permafrost soil. In the past few years, warming temperatures have thawed several permafrost dens while polar bears are inside, causing the collapse of the dens and killing the female within6.
For polar bears, like many large mammals, survival as a species depends upon high numbers of fertile adult females. Many of the problems caused by global warming (declining sea ice, collapsing) are disproportionately affecting females. These females are difficult to replace because polar bears take several years to reach sexual maturity.
It should be noted, however, that some parts of the Arctic are changing in ways that may have nothing to do with global warming, and seem to run counter to the effects of climate change. Even though ice cover is decreasing in most of northern Canada, some marine animals are in danger because of increases in sea ice in other parts of the North.
Since the late 1970s, the extent of sea ice in the eastern Canadian Arctic—specifically Baffin Bay, Davis Strait, coastal West Greenland, and Lancaster Sound—has actually increased. That means that there are fewer cracks and gaps in the ice to allow marine mammals like narwhals and bowheads to come up for air[10, 11]. At the same time, animals like polar bears that hunt those underwater creatures aren’t able to catch them as easily—or in some cases, marine animals become trapped in the areas around limited numbers of air holes, making them easy prey for predators.
Arctic mammals won’t only be affected by changes in climate and their abiotic environment. The northward shift in ecosystems will also bring southern organisms farther north, increasing competition. In one extreme example, wolves, which tend to stay below the treeline, may move farther north with the shifting of the boreal forests, bringing them closer to polar bear cubs, an easy source of food.
Although most of the impacts of global warming remain predictions rather than facts, there is no question that the climate of the North is changing. Arctic mammals will be forced to deal with warming temperatures and altered environments, and their responses will shape the futures of both their species and their ecosystems.
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