Alternations in Arctic ecosystems force polar bears to change feeding habits

Alternations in Arctic ecosystems force polar bears to change feeding habits

12 January 2012

Global climate change has altered the Arctic ecosystems and affected the habitat of all wild animals inhabiting the Arctic region. The melting of the Arctic Ocean’s ice layer is causing changes in the seasonal dynamics and reduces the area and thickness of the drift ice, while the melting permafrost results in the erosion of coastal areas and landslides, and damages the tundra’s vegetation cover. Long-term research and monitoring of natural processes at Arctic nature reserves, which are reference ecosystems, have generated sufficient data for comparison and assessment of the current changes. With civilization development and environmental changes, The informational value of nature reserves as reference ecosystems is increasing.


Research conducted at these important sites shows that changes in the Arctic biota are occurring at all levels, affecting ecosystems as well as the animal community and population. The available data shows that the changes in the region have had a negative impact on some Arctic species, such as the polar bear, the arctic fox and the snow owl. Yet, the warming process has led to the expansion of some species that had not been found in the Arctic earlier. For example, the snow goose only benefits from such climatic metamorphoses. As the Arctic climate gets warmer, new pathogens or viruses may appear or become more active, and pollutants may spread faster.


The rapid habitat changes make it possible to test the polar bear’s ability to adapt to a new environment. For researchers this provides material for a natural experiment and a chance to understand the whole range of the animal’s behaviour and environmental abilities. While exploring the Arctic habitat, ancestors of the present-day polar bear developed their new morphological, physiological and behavioural characteristics, which are linked to a better adjustment to the severe Arctic climate, surviving on drifting ice and hunting ocean mammals. Today, in their search for food polar bears constantly find themselves in conditions that force them to ‘recall’ or once again to learn skills and techniques inherent to their terrestrial ancestors.


Wrangel Island Arctic nature reserve has conducted long-term observations of polar bears forced to live on land, which reveal new data on the animal’s behavioural adaptability and capability to find and adjust to new food sources.


Female bears are known to feed on dry grass after leaving their den in spring. Previously, they were thought to eat a small amount of grass to clean their stomach and intestines after hibernating, or, possibly, to obtain certain vitamins. Now, polar bears are regularly observed grazing grass meadows for long periods in summer and autumn, eating grass in amounts that clearly exceed those required to meet vitamin demand or maintain normal intestinal peristalsis.


Along with eating plant food, polar bears also feed on sea animals. When meat is unavailable, polar bears hunt along the shoreline, eating fish, seaweed and invertebrates that had been washed ashore. In 2003, a massive amount of polar cod washed ashore on Wrangel Island, providing polar bears with a month’s supply of food. 


The salmonidae are not commonly found in Wrangel Island’s rivers due to the lack of spawning beds, or redds, and for a long time this source of food was unavailable to polar bears. Yet, in 2008 and 2009, for the first time in decades, a large number of pink salmon entered the large rivers’ estuaries and lagoons. Polar bears immediately added salmon to their diet, starting to gather dormant fish along the shore. Also, a young female bear was spotted successfully hunting salmon in a lagoon, catching the fish and carrying it away to eat in the tundra. This polar bear was not as skilful at hunting salmon as the brown bears in Kamchatka and Alaska, but the video filmed by German documentary maker Uwe Anders clearly shows the animal exploring the new catch and learning as it hunted.


Another object of polar bears’ interest is bird colonies, or large congregations of birds. Bird colonies in the Arctic attract polar bears all year round: in summer during the coastal period and in spring during the ice season. Under the rocks, dead birds can be found, and in summer vegetation is abundant due to the birds’ excrements constantly enriching the soil. Polar bears can often be spotted at bird colonies exploring accessible areas, enjoying naps at nearby cliff peaks and on slopes between rocks, feasting on food found in the grass under the ledge rocks, and seeking out remnants of dead birds at cliff bottoms. In the bird colonies that host a massive number of little auks (small sea birds from the Alcidae family that nest in rock crevices), such as colonies in the Franz Josef Land archipelago, polar bears regularly feed on these seabirds and their eggs, digging them up from under heaps of small stones and moving large blocks aside.


Arctic hoofed mammals, such as the reindeer and the muskox, are rare prey for the polar bear as they are fast runners. Muskoxen can be dangerous if they defend themselves when attacked. A muskox can pierce an attacker’s chest or belly with its pointed horns. Both the reindeer and the muskox can be found on Wrangel Island. They attract polar bears, which use them as food when possible. We have observed several cases when polar bears followed muskoxen and even ran some distance pursuing a flock. Throughout the entire period of observation, there was only one successful case of hunting a grown muskox by a polar bear. The circumstances remained unclear as the case was registered after the hunting was over; yet, the signs of fighting showed that the muskox had not been dead already but was killed by the polar bear. Polar bears feast on carcasses of all hoofed animals they find dead on the island’s coast or in its inland areas.


Lemmings are at the basis of the tundra’s ecological pyramid. All kinds of tundra carnivores actively feed on lemmings during the years of the rodent’s mass reproduction, or so called population peaks. Lemmings are hunted not only by small and medium miophag predators, i.e. the predators that predominately feed on rodents, such as the Arctic fox, the snow owl and the skua, but also by large animals such as the wolverine and the wolf, whose common prey in the Arctic tundra’s ecosystem is the reindeer. Once, it was difficult to imagine a polar bear hunting mice in the tundra. Until recently, only a few cases of polar bears hunting lemmings had been observed, and those were random. No data was available on lemmings becoming popular prey for polar bears. But in 2009 in 2010, during a lemming population peak on Wrangel Island, many polar bears started hunting these rodents. In 2009, researchers in the tundra would often see polar bears that dug out lemming holes in the same manner as brown bears do when hunting ground squirrels. At many sites in the tundra, we have found large pits around 1.5m in diameter in place of lemming holes. A young male polar bear learned to catch the lemmings that hid under barrels scattered near an old village. The animal rolled away the barrels that lay on the side or overthrew an upright barrel, seizing a lemming that hid under it. In the summer of 2010, even more cases of hunting lemmings were observed throughout the island. A greater number of bears hunting the rodents was observed as compared to 2009, and there were numerous pits in the tundra, with some sites literally dug over. In 2011, the lemming population on the island plummeted, but polar bears had gained experience in finding food in the tundra. This fact is probably linked to the bears’ earlier appearance in the island’s inland areas as compared to the previous years.


These facts indicate the polar bear’s high environmental adaptability and intellect, as well as its capability to find new solutions under new circumstances. However, additional sources of food provide only temporary support, helping to survive in the periods when the bears’ main source of food is not available, and do not guarantee that the species will flourish in the long term. When available, as well as during the coastal periods, walruses and earless seals remain the polar bear’s main and most preferable source of food. During the ice-melting season, sick or dead Arctic sea animals washed ashore serve as easily available food. However, the numerous cases of deaths among ringed seals and walruses caused by a recently occurring unknown disease have caused great concern.


An unusually high mortality rate among walruses on Wrangel Island and the coastal areas of Chukotka and Alaska was observed in the summer and autumn of 2007, with hundreds of dead animals found in various coastal areas. On Wrangel Island, the number reached one hundred. In 2007, the high number of walrus deaths could be linked to their exhaustion after spending a long period in the open sea, unable to reach the shore to take a break. In 2011, the same situation occurred, with some 30 dead animals found on Wrangel Island’s coast, while the total number of walruses that have reached the coast was considerably smaller as compared to 2007.  In 2011, along with a high number of deaths among walruses, massive deaths among ringed seals were observed throughout the region, namely on Wrangel Island, Chukotka and Alaska. In the central part of Wrangel Island’s southern coast, remnants of 20 dead ringed seals were found. We also found dead seals in all other coastal areas we inspected. Most dead animals were found as remnants after being eaten by polar bears. Some of the ringed seals that had reached the shore showed clear signs of illness, such as exhaustion, lack of fur on their bodies and uncontrolled movements. These sick seals were soon eaten by polar bears as well.


According to the reports by Russian and American researches, the same situation occurred in Chukotka and Alaska. Since mid-July, 60 dead and 75 sick ringed seals have been found on Alaska’s coast, with reports on sick animals still being received at the time the information was released by the media. The symptoms of the illness among ringed seals in Alaska were similar to those observed among the animals on Wrangel Island and Chukotka, with bald spots on the body, sores usually found on the head and hind flappers, shallow rough breathing and the lack of the typical reaction to escape into the water on seeing a human being. The same symptoms were found in walruses.


The autopsies performed on dead ringed seals by American researchers revealed an abnormally enlarged brain. Some seals and walruses had smaller than normal lymph glands, which can indicate an immune malfunction, among other things. The researchers’ lab testing of samples taken from the dead animals revealed no bacterial, mycotic or viral infection when tested for 18 known pathogens.


The cause of the disease and its triggers remain unknown, with examination still underway. While the actual cause is being determined, a wide range of disease-producing factors can be assumed, including bacteria, viruses, radioactive or toxic agents, environmentally induced stress and pollutants. No similar symptoms have been observed in polar bears or humans despite their consumption of sea mammals. However, the risks of contracting the disease by humans and polar bears, as well as wild and domestic animals still remain.


Obviously, the situation with the high number of deaths among ringed seals and walruses requires a thorough and prompt examination and monitoring, as does the fact that the current changes in the Arctic ecosystems can bring new and unknown threats to its wildlife, calling for our close attention and research.


Nikita Ovsyanikov is Deputy Director for Research, Wrangel Island Nature Reserve; PhD in Biology; Member of the Polar Bears Specialist Group of the International Union for the Conservation of Nature (IUCN) Species Survival Commission, and member of a working research group at the US-Russia Polar Bear Commission

(Photos copyright © Nikita Ovsyanikov and Irina Menyushina)