Ours blanc : chronique d'une extinction annoncée
Adapté aux conditions extrêmes de la vie sur la banquise, l'ours polaire voit son existence menacée par le recul de la glace de mer en Arctique. La situation est d'autant plus alarmante que ce carnivore serait incapable de retourner à la vie terrestre.
Polar bears and ice go together in our minds as automatically as salt and pepper. The largest of the terrestrial carnivores, polar bears are uniquely adapted to live on Arctic sea ice. The significance of that relationship can be expressed quite simply: polar bears came into existence in the first place because of sea ice - if the ice disappears, so will the polar bears.
Only a million or so years ago - a blink of an eye in evolutionary time - the barren ground grizzly bears roamed the coast of the Arctic mainland, adjacent to a vast new habitat: sea ice. And the ice was home to an unexploited abundance of prey – seals. At some point, grizzlies ventured out onto the sea ice and, as they learned to prey upon the ice-breeding ringed seals, they rapidly evolved into the present day polar bears.
Body shape comparison between polar bear (up) and grizzly (down). Narrower head, longer neck and lower shoulder hump differentiate the polar bear morphology from its closest relative the grizzly - © Julien Norwwod - Le Cercle Polaire
For undisturbed millennia, the sea ice was not a harsh place for the newly-evolved polar bear but, simply home, and a very comfortable home at that. More recently however, because of rapid climate warming in the Arctic, sea ice is disappearing at an alarming rate. In some areas, there are also significant trends toward earlier breakup in spring and later freeze-up in fall. As a result, the long term survival of the polar bear now appears uncertain. Over the next few decades, the prospects for their future will become clearer and will almost certainly depend on our collective ability to stabilize and then reduce our production of greenhouse gases.
Sea ice, Home to the Ice Bear
I have paused to marvel at the very existence of the polar bear countless times, and in many places in the Arctic, while standing on annual ice several feet thick, at the edge of an ice floe, beside an open lead (a crack in the ice with water in it), or on a vast unbroken expanse of frozen ice. In early April, there is nothing living in sight. In fact, there is not much of anything at all in sight but ice, snow, sometimes a bit of ice-cold blue water between nearby floes, or a few wisps of mist rising up from a lead. The ice occasionally cracks and creaks beneath your feet as the currents or winds make it shift slightly, possibly causing floes to grind against each other. The temperature may be -30 C (-22o Fahrenheit), sometimes with a wind of 30-50 kilometers an hour (19-31 mph) with drifting snow.
A large adult male climbs out on the ice after swimming between floes to walk along the edge of the landfast ice looking for seals - © Ian Stirling
In these conditions, the dry snow squeaks with even the slightest movement of your mukluks, as if you were walking over hundreds of mice with each step. Gusts of wind may batter you back and forth as you stand there. For as far as you can see, everything is white until the white eventually blends with the blue sky on a distant horizon. If you climb to the top of a pressure ridge of buckled ice, you can see slightly farther, but the view is the same.
At such special moments, I am often overwhelmed by several things, but especially by how tiny and ineffectual my presence is in the vastness of the frozen Arctic Ocean. While I watch a distant polar bear striding purposefully across the sea ice in such circumstances, oblivious to me or anything else, it is easy to understand why it became the powerful iconic animal of the Arctic sea ice in the first place, yet difficult even now to comprehend its vulnerability.
Polar Bear, the Arctic superpredator
|Male polar bears weigh about 375-600 kilograms (825-1320 pounds) while occasional individuals may reach 800 kilograms (1760 lb). They sometimes exceed 250 centimeters (10 feet) in length, measured in a straight line from the tip of the nose to the tip of the tail, although most male polar bears are a bit shorter. They are roughly twice the size and weight of adult females, which weigh 200 to 350 kilograms (440-750 lb) and achieve an adult body length of about 190-220 centimeters (up to about 7 ft). Females first breed at four to six years of age and most often give birth to two cubs in snow dens on land (some cubs are born in dens on the sea ice). Cubs stay with their mothers for two and a half years before weaning which means that unless cubs die prematurely, females do not breed more frequently than every three years. Both sexes live twenty to twenty-five years and sometimes to over 30 years. Their primary prey is ringed seals and, to a lesser degree, bearded seals.|
Sea Ice, a World in Perpetual Motion
Arctic sea ice is a tough environment for any animal to survive, let alone prosper in. The enormous variability, both predictable and unpredictable, in the distribution and abundance of the ice itself requires animals living there to be able to make large adjustments to their movements on a regular basis. The extent of the ice cover or the distribution of leads and pressure ridges may vary enormously between seasons or years. On a large scale, in the Beaufort or Chukchi Seas adjacent to northern and western Alaska respectively, an individual ice floe may move hundreds of miles in various directions throughout the year in response to wind and currents. A polar bear that goes into a maternity den on the drifting ice in November may exit several hundred miles away only a few months later. On a smaller scale, ice may be absent or abundant through summer or fall, sometimes changing overnight because of wind or currents. When the ice is relatively thin during freeze-up, a fall storm may compress 80 or more km (50 plus miles) into a virtually impassable rubble field. The location of the best habitat for overwintering seals, or the reproductive productivity of the seals themselves, may vary greatly between years for reasons that are not currently understood.
A refrozen lead in the pack ice makes an ideal hunting habitat for polar bears as there are always more seals and breathing holes to hunt at here - © Ian Stirling
Such extreme variability exists in few other habitats and is the reason why polar bears, unlike the terrestrial bears, are not territorial. Resources that black or grizzly bears depend on, such as berry bushes or salmon streams, will be in the same place every year. In contrast, the availability of prey or the location of the best sea ice habitat can fluctuate so much between years that it doesn’t make sense to try to defend a territory.
Both the surface and the underside of sea ice are vital to polar bears. The surface provides the critical platform on which the bears travel and from which they are able to hunt seals. Less apparent, on the underside of the ice, is a unique and biologically rich, if transient, seasonal community of algae and marine organisms. This is the epontic, or “under-ice,” community. In response to the sunlight penetrating the ice in spring, a flourishing community of algae and single cell organisms becomes established on the underside and within the ice itself. The rapid increase in day length and intensity of sunlight in spring stimulates a bloom in phytoplankton (plant cells) that supports small invertebrates, which in turn are fed upon by larger invertebrates and small fish that live in cracks and on the underside of the ice, thus providing food for the ringed seals.
The total polar bear population was estimated at between 20.000-25.000 in 2005, all living in the Arctic Ocean and adjacent seas covered by sea ice in winter. They are subdivided into 19 geographic populations with little or no interbreeding - Source IUCN : Polar Bear Specialist Group
It has been estimated that in some areas up to a third of the primary productivity of the ocean in ice-covered waters over the continental shelf is produced on the underside of the sea ice. Most important, because the epontic community is anchored in and beneath the ice, its biomass remains near the surface for several months until the ice eventually melts, after which most of the biomass sinks to the seafloor. It is the retention of an abundant and accessible food source on the underside of the ice that both attracts seals and keeps them there.
Subadult and non-breeding ringed seals are more abundant along the floe edge and in the drifting pack ice - © Pierre Taverniers
The seals hunted from the ice by polar bears are mammals, so they must breathe air. Thus, they breathe at open cracks in the ice or in breathing holes they maintain by abrading the ever-freezing ice with the heavy claws on their foreflippers. However, when they surface to breathe, they are vulnerable to capture by the bears. When summer comes, the most important polar bear habitat, the annual ice over the continental shelf, simply disappears. At this point, the bears must either retreat onto land and survive on their stored fat reserves or migrate north into the multiyear ice of the central Arctic Ocean where they probably also mainly live on their fat but are still able to hunt a certain amount while they await freeze-up over the continental shelf again in the fall.
Sea Ice Nourishes a Florishing Ecosystem
To the uninitiated observer looking down from an airplane, the seemingly endless sea ice on the Arctic Ocean may appear much the same. However, there is as much variety in types of ice and the relative abundance of associated life, as there might be if you were to compare a coastal Douglas fir forest with the tundra. Not surprisingly, some types of ice are more important to polar bears than are others.
Two kinds of sea ice have the greatest influence on the ecology of polar bears, mainly because of how they influence the abundance and accessibility of the ringed seals that primarily sustain them. Multiyear ice may be several years old, partially melting in summer, but refreezing and replacing the melted portion through the winter. It may be 10-15 feet thick (3-5 meters) and snow-covered, thus allowing a limited amount of sunlight to pass through it to stimulate biological productivity. It is predominantly found in the middle of the Arctic Ocean, lying over very deep and generally unproductive water with low densities of seals.
When a polar bear wants to cross a lead but the ice is not thick enough to hold him, he swims underneath, making holes at intervals to breathe through until he reaches the other side and climbs out - © Ian Stirling
Annual ice forms in winter, melts completely in summer, and occurs mainly along the southern coastlines of the Arctic Ocean, in channels between islands, and in more southerly areas such as Hudson Bay. Annual ice rarely exceeds about 2 meters (6.5 feet) in thickness and is more easily penetrated by sunlight in the spring. Most annual ice lies over the shallower and more biologically productive waters of the continental shelf where seal densities are highest. Consequently, that is where polar bears do most of their hunting. Within the annual ice, there are two main categories: landfast ice, as the name suggests, is attached to the land while pack ice is made up of moving ice floes that are not attached to land. Within these categories are several subcategories of ice types, but three are of greatest importance for seals, and thus for the bears that hunt them: stable landfast areas with snow drifts deep enough for seals to make their birth lairs (usually in bays or near the coast); the floe edge, which is the outer edge of the landfast ice with associated leads; and drifting pack ice dense enough to cover 75% or more of the surface of the ocean.
An adult female and her cub feeding on a seal she has killed on the stable land fast ice - © Ian Stirling
The preferred breeding habitat of ringed seals is the stable landfast ice. There, territorial adults maintain breathing holes in the ice beneath snow drifts deep enough for making haul-out lairs. Their breathing holes are distributed at fairly low densities, which makes them difficult for the bears to find. Meanwhile, the subadult and non-breeding ringed seals are most abundant along the floe edge and in the drifting pack ice. Bearded seals also prefer areas along the floe edge and in the moving pack where it is easier to find natural openings in the ice to breathe. Adults are several times larger than ringed seals and consequently are mainly killed by adult male polar bears. (Although bearded seals are capable of maintaining breathing holes with the claws on their foreflippers, like ringed seals, they tend not to).
An adult male polar bear in Svalbard scavenging by diving down to the remains of a dead whale to pull up bits to eat - © Ian Stirling
The preferred hunting habitat of polar bears of most ages and both sexes is along the floe edge or on the drifting pack ice where seals are more abundant and accessible, where there is more likely to be carrion to scavenge on, and where seal breathing holes are likely to be less protected by consolidated snow drifts. In late March or early April, when female bears exit their dens with three- to four-month-old cubs, they hunt at first mainly in the stable ice areas closer to the coast and away from the floe edge. The hunting may be more difficult there, but adult males, which are known to kill and eat cubs if they have a chance, are also encountered less frequently.
Moving from land onto Ice
The simple existence of a vast expanse of Arctic sea ice, home to millions of seals but unoccupied by a large surface predator, made it possible for the polar bear to evolve from the land-dwelling grizzly bear in the first place. The ecological circumstances that originally gave rise to the polar bear were probably not very different from what we see today along the southern coast of the Beaufort Sea in northern Alaska and western Canada. There, barren-ground grizzly bears den on land, sometimes as close as a few miles from the coast and the adjacent sea ice. In spring, when the grizzlies first emerge from their dens, hungry after a winter of existing only on their stored fat reserves, it is often difficult to find the vegetation they seek at first because the ground is still covered with snow. For years, it has been well known to the local Inuvialuit hunters, who live along the coast of the Beaufort Sea in Canada, that some of the first grizzlies to leave their dens go out onto the sea ice to either scavenge on the remains of seals killed by polar bears or possibly prey on seals themselves. The grizzlies have been seen feeding on seal carcasses but whether they kill them themselves or are simply scavenging is uncertain. What is known, however, is that some bears do this every spring and probably have for a long time.
Long hair rooted between the toes extends beneath the bare pads of the very large paws and feet giving added traction when walking on ice - © Ian Stirling
There is a lot of variation in coat color of the barren ground grizzlies, from dark brown or black to a light brown trending to almost blond. It is not hard to imagine that the lighter colored bears might have been slightly less noticeable to the initially naive seals than their dark-colored brethren and so were more successful at stalking and killing seals that had hauled out to rest on the ice near their breathing holes. Those bears that were more successful at killing seals would have survived better, produced more cubs, passed on the genes for lighter coat color, and gradually turned white, to become the polar bears of today. However, polar bears adapted in many more ways that just coat color.
Highly adapted for living on the ice
As polar bears adapted to life on the ice, their morphology changed quickly from that of their stocky, mainly dark-colored, long-clawed grizzly bear ancestors with a distinctive shoulder hump. In only a little over a million years, polar bears have become sufficiently distinct from grizzlies to be instantly recognizable. The skull became less heavily boned than a grizzly’s and more elongated, as did the neck, which may be an adaptation enabling a polar bear to lunge farther into a breathing hole in the ice when trying to capture an escaping seal. During winter, rock-hard wind-blown snow drifts form over seal breathing holes. Ringed seals come up through their breathing holes and scoop out small haul out lairs in the overlying drifts where they can rest. In spring, pregnant females give birth to their pups in such lairs. When bears detect a seal in a lair below a drift, they may smash on the top with their forepaws or dig furiously to create a hole large enough to quickly reach down through and attempt to capture the occupant. Having a longer, narrower head and neck than a grizzly might enable a polar bear to more quickly reach deeper into a lair or breathing hole and increase the chances of catching a seal.
A maternity den in western Hudson Bay after the female and her cubs have left for the sea ice to return to hunting seals. They go into them by August in Western Hudson Bay and remain there until late February or early March. The mother and cubs remain at the den site for 7-10 days after breaking out before leaving. This allows the cubs to get some exercise and acclimatize to the cold weather - © Ian Stirling
Compared to the long claws on the forepaws of the grizzly, polar bear claws are short, stubby, and sharp-pointed, probably for maintaining a firm grip on the ice. Long hair rooted between the toes extends beneath the bare pads giving added traction when walking on ice. Interestingly, the large hairless pads on their feet also have small vacuoles on the surface that may function like tiny suction cups to give additional traction on the ice although, this is only speculative. What is not speculative is that, compared to the feet of grizzlies or black bears, polar bear feet are much bigger relative to the rest of the body. These large paddle-like feet are remarkably effective for swimming but also act like snow shoes for walking across freshly forming ice without breaking through. Since ringed seals also like these areas of newly forming thin ice - in part possibly because they are more difficult for polar bears to reach - it seems clear the polar bears’ large feet increase their access to seals.
On the western coast of Hudson Bay, large males often dig pits in the gravel to lie in while they wait for the ice to re-freeze. The pits are usually on the top of a beach ridge where the breezes off the water can help keep them cool while they fast in summer - © Ian Stirling
Lastly, when thinking about how polar bears evolved to a life of hunting seals on the sea ice of the Arctic, the question of the Antarctic sometimes comes up. After all, there are many seals of several species there and a lot of ice, so why aren’t there polar bears or a similar surface predator there as well? The answer is quite straightforward. The gap of cold, stormy, inhospitable ocean between the southern tip of South America (the closest continental land mass to the Antarctic) and the continent itself is simply too wide for terrestrial predators of any species to be able to cross. But that doesn’t mean there is no predator of seals to fill the ecological niche of the polar bear. That role is very successfully shared by leopard seals and killer whales, the top level marine predators.
Ian Stirling removing a satellite collar from a female polar bear on the sea ice in the Beaufort Sea - © Ian Stirling
The Sea Ice is its Hunting Ground
Polar bears depend on sea ice for everything that matters to their survival, including hunting, seasonal movements, finding mates and breeding, and, in some areas, even digging maternity dens in snow drifts by ice ridges and giving birth to their cubs far from land. Their most important prey species, the ringed seal, is distributed at low densities over huge areas of sea ice, as well as being almost absent in others, for reasons that are not well understood. For polar bears, the most essential aspect of a frozen polar sea is that it provides the substrate over which they move in search of seals, as well as the essential platform from which to hunt. Although a few bears have been known to kill seals in open water near the shoreline, the number of successful hunts is likely tiny relative to the hunting on the sea ice. Furthermore, extensive swimming in cold water to hunt would be energetically inefficient compared to walking on ice.
The most important prey species for the polar bear is the ringed seal. Its preferred part of the ringed seal is the fat and skin. If the pups are very young (top), they have little body fat and are often ignored by the bears. On the adult ringed seal (below), all the fat and skin have been removed and eaten but the meat has been left behind - © Ian Stirling
To appreciate how important it is to have access to a huge area of ice habitat and be able to travel easily over it in search of prey, consider that the average polar bear requires approximately forty-five ringed seals (or ringed seal equivalents) per year to survive. Limited hunting of other species reduces the number of ringed seals needed to some degree. However, in crude numbers, the currently estimated 20,000 to 25,000 polar bears worldwide would require more than a million ringed seals (or ringed seal equivalents) annually. There is no other marine mammal in the Arctic that could replace ringed seals for sustaining the majority of the world’s polar bears.
The high degree of variability in the distribution and abundance of sea ice between seasons and years presents serious problems for the bears, not the least of which is navigation in an environment with few visual clues.
Regardless, it has been well demonstrated by tracking adult female polar bears with satellite radio collars, and recapture of tagged animals of all ages and sex classes, that they do not move at random. Nor do they just drift passively in the direction that wind or currents may take the ice if that is not where they want to go. Instead, many studies have shown that individual bears return predictably to the same areas at the same seasons, such as to those used for spring hunting, maternity denning, or summer sanctuary during the open-water period. For example, some bears that spend most of their time along the southern coast of Baffin Island walk several hundred miles south over the drifting ice of the Labrador Sea each spring to feast on abundant unwary harp seal pups near the coast of Newfoundland. After a few weeks of active feeding, the bears walk back north to Baffin Island against the current on a treadmill of ice floes that are steadily moving south before disappearing altogether in the North Atlantic Ocean.
Bearded seals weigh 3-400 kg and are the second most preferred prey of polar bears. Because of their large size, most are killed by adult male polar bears - © Ian Stirling
Changing Ice Conditions impacts Polar the Bears' life
At first glance, the status of polar bears might appear secure. They are widely distributed throughout the seemingly unlimited, ice-covered seas of the circumpolar Arctic. They still inhabit the majority of their original habitat, and their worldwide abundance, in nineteen subpopulations, was estimated by the IUCN Polar Bear Specialist Group to exceed 20,000 in 2005. However, the incredible success of the polar bear in adapting to life on the sea ice is also its greatest vulnerability. A large carnivorous specialist such as the polar bear has few options if its habitat disappears. It cannot revert to a terrestrial life and eat berries or other vegetation. By feeding captive grizzlies, researchers have confirmed that their large size cannot be attained solely by eating vegetation. Achieving their massive body mass is dependent on animal matter (salmon) in the diet. Similarly, polar bears are large animals and they got that way by eating seals, not vegetation. Consequently, their survival in anything like today’s numbers require large and accessible seal populations and huge areas of ice from which to hunt.
The diet of the polar bear is comprised of more than 95 % seal, mainly ringed seals or other smaller sized seals, as in Davis Strait where the harp seal is much more abundant than ringed - © Ian Stirling
Aboriginal hunters throughout the Arctic and polar scientists agree that climate warming is having a significant negative impact on sea ice. Especially worrisome is the conclusion of scientists at NASA and elsewhere that the total amount of ice still present in the polar basin at the end of summer has been declining at a rate of 9 per cent or more each decade since satellite monitoring became possible in 1979. In September 2007, the minimum ice extent of the Arctic Ocean in the polar basin reached a much greater-than-predicted all-time low and, in 2008, fell to the second smallest minimum area. The possibility of a totally ice-free Arctic Ocean, in summer at least, has been predicted to be possible in less than forty years. In 2006, NASA also reported that the total amount of ice in the Arctic was declining during winter as well as at the end of summer. These changes, and the speed with which they are happening, exceed the worst scenarios that any of us studying polar bears could have imagined twenty, or even ten, years ago.
We are facing the previously unthinkable – the loss of summer sea ice in the Arctic in the foreseeable future, mainly because of human-induced climate warming. During our thirty year study of polar bears on the western coast of Hudson Bay, we have seen things I can still hardly believe possible in a period as short as a human’s working lifetime. Hudson Bay is completely covered through the winter by annual ice that, until recently, melted completely by about mid-July. Open water prevailed for about four months through the summer and fall. However, the average date of breakup of the sea ice is now a full three weeks earlier than it was only thirty years ago! Freeze-up is slowly getting later as well.
Because an early breakup (1998) reduces the ringed seal's availability, polar bears of the western Hudson Bay have to adapt their usual diet (1994): female polar bears hunt more harbour seals which have a similar body weight to ringed seal. Males capture more bearded seals, too large to be hunted by females - © Ian Stirling
This extraordinarily rapid change has had immediate and severe consequences for polar bears. The most critical time for polar bears to feed on ringed seals is from late spring to breakup, when newly weaned ringed seal pups are abundant, their body composition is up to 50 percent fat, and they are still naive about predators. In fact, the polar bears of western Hudson Bay probably accumulate 70 per cent or more of the energy they survive on for the entire year during the spring and early summer feeding period. Thus, over the last thirty years, the polar bears in Western Hudson Bay have been forced to abandon hunting seals on the sea ice for progressively longer periods because they lose their essential hunting platform (ice) at the single most important time of year. The earlier breakup occurs, the poorer the condition of the bears when they come ashore to fast through the open water period.
As a consequence of steadily declining body condition, the average weight of lone (and suspected pregnant) adult female polar bears in the fall has declined from approximately 290 kilograms (640 pounds) in 1980 to about 230 kilograms (507 pounds) in 2004. Our past records indicate that no female polar bear weighing less than about 190 kilograms (420 pounds) in the fall has been recorded with cubs the following year, suggesting that although females below that weight can survive, they don’t reproduce. At the present rate of decline in their fall weights, it seems likely that in a few more decades, few adult females will be capable of reproducing in western Hudson Bay.
In the western Hudson Bay, the spring sea ice melt occurs 3 weeks earlier than 25 years ago. This results in a shorter ringed seals hunting season, thus a decline of an average of 60 kg in body weight of the adult female polar bear in the fall - © Ian Stirling
The decline in the annual survival rate of cubs, subadults, and the oldest bears is also significantly related to timing of breakup: the earlier the breakup, the poorer the survival of bears of those ages. Thus, the negative effects of progressively earlier breakup of sea ice in western Hudson Bay on reproduction and survival probably caused the polar bear population in western Hudson Bay to begin to decline and then ensured it continued. In the northern half of western Hudson Bay, which is part of Nunavut (the northernmost territory in Canada) hunting of an annual sustainable quota of polar bears by Inuit is legal. However, after the western Hudson Bay population of polar bears began to decline, the former annual quota would no longer have been sustainable, resulting in a continued trend of overharvesting that, when added top the negative effects of climate warming, helped to cause that population to decline from about 1200 animals in 1987 to 935 in 2004.
An adult female and her triplet cubs. She is anesthetized for tagging and her cubs are tied to one of her legs by rope leashes so they will not run away. They will be immobilized with an injection given by hand and tagged and measured as well - © Ian Stirling
New research in southern Hudson Bay has confirmed that the ice is breaking up earlier than it used to and that there has been a corresponding decline in the condition of polar bears of all ages and both sexes, between the mid-1980s and 2005. The similarity of these trends to those in western Hudson Bay suggests a decline in the polar bear population in southern Hudson Bay will follow, if it has not already started.
In the Chukchi, Southern Beaufort, Laptev, and Barents seas – some of the Arctic Ocean’s circumpolar seas - breakup is also occurring earlier while freeze-up is later. Thus, both the extent and duration of open water north of the coast through summer and fall are increasing. Consequently, more bears from those areas are now spending extended periods on land. At the same time, the southern edge of the offshore pack ice, where the rest of the bears from those regions spend the summer, now lies over deep unproductive water far from the coast, where there are fewer seals to hunt.
Ian Stirling measuring the size and thickness of a hole dug through the ice by a polar bear trying to capture a seal. In the Beaufort Sea in the last couple of years, bears have been having a harder time finding seals in spring because of the strong winds that are breaking up the ice more frequently in winter and compressing it against the shore. Some bears are so hungry they actually claw their way through solid ice to try to capture seals that may have a breathing hole below. This is rarely successful and costs the bear a lot of energy. The existence of such holes in the ice is a measure of how hungry the bears are becoming - © Ian Stirling
In the southern Beaufort Sea, the polar bear population shared by northern Alaska and northwestern Canada appears to have declined from about 1800 in the 1980s to about 1500 in 2006. The major known ecological change during that period is that the southern edge of the remaining ice in summer now retreats farther to the north, away from the productive continental shelf area along the coast where seals are more abundant, and the ice remains farther north for longer periods than it used to. In 2001 and 2002, the ice-free period was relatively short (a mean of 92 days) and the annual survival of adult female polar bears was approximately 99 per cent. In 2004 and 2005 however, because of unusually warm summers, the ice-free period averaged 135 days and survival of adult female polar bears was only about 77 percent. Further evidence of nutritional stress in the Southern Beaufort sea, coinciding with years of extended open water, includes several instances of starvation, cannibalism, and bears desperate enough for seals that they claw through solid ice in vain attempts to capture seals that might have breathing holes below. As noted earlier, 2007 set a new record for retreat of the ice, which likely had further negative effects on the bears. If the Arctic continues to warm, we can probably expect to lose ice at a rapid rate.
The Polar Bear will Disappear with its Habitat
The Fourth Assessment Report of the Intergovernmental Panel on Climate Change of the United Nations, published in January 2007, came to the “unequivocal” conclusion that the world’s climate is warming rapidly and that the primary cause is human activity. What this means for polar bears, is that large-scale loss of ice will continue, with severe negative impacts on the bears’ survival, reproduction, and population size. The effects of climate warming will vary in timing and rate of change in different regions but, over the long term, the effects will be negative.
Ian Stirling examining a cannibalized adult female : while returning to the sea ice with its newborn from its maternity den, the female was killed and eaten by a large adult male. The skull shows that bites to the head resulted in her death - © Ian Stirling
In Hudson Bay-Foxe Basin and in the eastern Canadian Arctic (Baffin Bay and Davis Strait-Labrador Sea), the sea ice melts completely each summer. Since the most important feeding period for bears there is from mid-April until breakup, they are already among the first populations to be negatively affected by climate warming. Bears there survive the summer mainly on their stored fat, although they scavenge opportunistically, sometimes feed on vegetation, and may attempt to hunt other marine mammals. However, polar bears obtain almost all their annual energy requirements by hunting seals from the sea ice surface, not from other sources. Speculation that polar bears might sustain themselves from alternate food sources on land is fanciful.
The main area where multiyear ice presently remains in late summer and fall, and will in future, is along the coastline and between some of the most northerly islands of Canada and Greenland. If the relatively unproductive multiyear ice there is replaced by annual ice, over the continental shelf at least, it is possible that the biological productivity, and its suitability for seals and polar bears, may increase, at least in the short-term. However, if the climate continues to warm unchecked, as is currently predicted, this possible respite may be brief.
An adult female and her cub on shore in Svalbard. The ice is gone so now, like the bears in summer in Hudson Bay, they must live on her stored fat reserves for several months before they are able to return to the sea ice to hunt seals, a critical season witch duration may increase with global warming effects on arctic sea ice - © Ian Stirling
Despite the enormous changes in the Arctic that have already been documented, humans still can and must respond. It will likely be several decades before the most negative predictions are realized. Thus, time is critical for both polar bears and the sea ice that dominates the Arctic’s marine ecosystem. Polar bears evolved into existence because of a large productive habitat, unoccupied by a terrestrial predator. As that habitat disappears, so will the bears that live and depend on it. It is vital that all humans use whatever time remains to reduce greenhouse gas emissions sufficiently for sea ice and polar bears to persist for our children and grandchildren to marvel at. Nothing less is defensible.
For more information
• Recent observations of intraspecific predation and cannibalism among polar bears in the southern Beaufort Sea. Steven C. Amstrup, Ian Stirling, Tom S. Smith, Craig Perham and Gregory W. Thiemann, 2006. Polar Biology
• Possible Effects of Climate Warming on Selected Populations of Polar Bears (Ursus maritimus) in the Canadian Arctic. Ian Stirling and Claire L. Parkinson, 2006. Arctic, 59 (3)
• Effects of Earlier Sea Ice Breakup on Survival and Population Size of Polar Bears in Western Hudson Bay. Eric V. Regehr, Nicholas J. Lunn, Steven C. Amstrup and Ian Stirling, 2007. The Journal of Wildlife Management, 71(8)
• Unusual Predation Attempts of Polar Bears on Ringed Seals in the Southern Beaufort Sea: Possible Significance of Changing Spring Ice Conditions. Ian Stirling, Evan Richardson, Gregory W. Thiemann and Andrew E. Derocher, 2008. Artic, 61(1).
Voir l'Appel des Pôles de Ian Stirling
Visit the IUCN Polar Bear Specialist Group website
By Ian Stirling, Emeritus Scientist with Environment Canada and Adjunct Professor at the University of Alberta. He participates in a number of national and international committees on polar bears and marine mammals. In recognition of his work, he has received the Canadian Northern Science Award, been made an Officer in the Order of Canada, and a Fellow of the Royal Society of Canada.
Ian Stirling © Juillet 2009- Le Cercle Polaire - Tous droits réservés