One of the consequences of the ongoing climate change is the melting of polar ice. The extent of sea ice in the far north has been on the decline since 1979 at the average rate of 11.3% per decade.
In 2012, the minimum summer extent of sea ice set a new record low (which stands to this day), while the 2021 minimum constitutes the twelfth lowest result in nearly 43 years of observation. Satellite records present a similar picture, with the last 15 years marking 15 smallest sea ice extent values. This downward trend raises serious concern about Arctic species which need sea ice to survive.
Of all marine mammals of the Arctic, the polar bear (Ursus maritimus) relies on sea ice the most heavily. It is an apex predator, which means it stands at the very top of the food chain and has no natural enemies. The polar bear inhabits areas which remain locked in ice for much of the year and uses the ice as a hunting platform. Although often considered to be a specialist predator of ringed seals, the polar bear is capable of a flexible foraging strategy, depending on the availability of prey. And so, while the main item on its menu is the ringed seal, the bear may also feed on other seal species – such as the bearded seal (Erignathus barbatus), the harbour seal (Phoca vitulina) or the harp seal (Pagophilus groenlandicus) – if they are numerous enough and available in a given area. Polar bears may also hunt other species of marine mammals, such as beluga whales (Delphinapterus leucas) and narwhals (Monodon monoceros). They are also known to hunt the walrus (Odobenus rosmarus), although very few such cases have so far been documented. If an opportunity arises, polar bears won’t say no to carrion either, which is how more impressive species like the bowhead whale (Balaena mysticetus) also sometimes make it onto the bears’ menu.
Much of the fat reserves, which help the bear survive summer and winter, are accumulated in spring, when seal pups lounging on the ice are relatively easy prey. Seals spend most of their lives under water, but the pups are born on the ice, where they rest and gain weight before they enter the realm of the Arctic Ocean. A seal pup is not entirely helpless – its mother watches over it between hunts and the pup itself will seek safety in water whenever it feels threatened. Polar bears seek out seals using their acute senses – they can smell a potential meal from a distance of up to 9 kilometres. Locating prey, however, is not quite the same as actually making a kill. To do the latter, the bear must get very close and to do that it needs ice strong enough to support its massive body. But there’s a problem.
The analysis of ice data recorded over the last 42 years makes it clear that the average thickness of sea ice in the Arctic has diminished from 3.64 metres in 1980 to 1.89 metres in 2008, a reduction by a metre and 75 centimetres! What does it mean for polar bears? That the season during which they can successfully hunt seal pups is getting shorter and shorter. Can’t they hunt seals in water instead? After all, Ursus maritimus means “marine bear” and the animals are no doubt excellent swimmers. A few have even made it to Iceland! And yet they are no match for the seal, which swims faster, dives deeper and – in most cases – can easily escape even the most agile polar bear. That’s why to gain an advantage the polar bear needs sea ice.
During summer, polar bears use more energy than they gain and what keeps them from starving are fat reserves accumulated in spring along with whatever food the animals manage to find on land. This extra food includes seaweed, grass, berries, fish and – less commonly – reindeer (Rangifer tarandus), seals, various bird species and eggs.
In recent years, Canada, Greenland and Svalbard have seen an increase in documented cases of unusual predation attempts, drowning and emaciated individuals, and cannibalism. And although cannibalism has long been observed within the polar bear population, it used to be limited to situations in which adult males killed and ate young individuals. Recent analyses suggest that these days it is no longer the case and the increase in the incidence of cannibalism is generally ascribed to a decline in the availability of seals and the resulting hunger.
According to recent reports, polar bears are gradually learning to find sustenance on land. Compared to a few decades ago, they eat more eggs and chicks from bird colonies and make increasingly frequent attempts to hunt larger land mammals, such as the reindeer. This indicates that seals are the main item on the polar bear’s menu not so much because of the bear’s specific hunting adaptations but simply because up until relatively recently they were more readily available than other prey. While polar bears were believed to be specialist predators, related species – like, for example, the brown bear – are generally considered opportunistic feeders, which means they feed on a wide variety of foods, depending on what’s currently available. And now it seems that polar bears are just as able to adapt to the changing availability of nutrient sources as their cousins from further south.
High genetic diversity within the species gives polar bears an edge in terms of their adaptive skills. Studies of changes in the size of polar bear population in the context of changes in sea ice extent do not yield conclusive results, which implies that ice availability is not the only factor determining the size of the population. In the southern part of the Beaufort Sea, a decline in polar bear numbers correlated not only with the availability of sea ice, but also with the decline in the population size (and not just the availability) of seals.
More and more often, hunger drives polar bears to attack people. Research shows that such attacks are most frequently carried out by undernourished adult males. Adult females are much less likely to exhibit similar behaviour unless they’re protecting their cubs.
In some regions, the flexible foraging strategy of polar bears may help to alleviate the consequences of sea ice loss caused by climate changes. What is more, in places like Foxe Basin in Canada, the summer loss of sea ice has led to an increase in the number of killer whales and other whale species, which the killer whale hunts. Whale carcasses which wash ashore as a result constitute a valuable source of nutrition for local bears. Researchers believe, however, that this may not be enough to sustain the bear population if sea ice disappears also in winter, making seal hunting impossible.
The retreat of sea ice beyond the continental shelf and longer ice-free periods in summer are predicted to negatively affect foraging efficiency, increase foraging stress levels and the distances polar bears will have to cover to reach suitable seasonal habitats. In some parts of the Arctic, sea ice melts completely on an annual basis, forcing polar bears to spend summer seasons on land. During this period, the animals have little or no access to adequate food and survive on fat reserves accumulated the previous year.
All in all, in some areas of the Arctic climate warming forces changes in polar bear diet. To survive, polar bears must exhibit foraging flexibility and adapt to adverse conditions. It is worth pointing out, however, that the ongoing climate changes affect not only the bears’ diet, but also their reproductive success and migration routes. For this reason, systematic monitoring of the entire polar bear population is necessary to better understand these magnificent predators and more accurately predict their future in the warming world.
Text: Katarzyna Tołkacz
English translation: Barbara Jóźwiak
Literature:
Aars, J. (2021) ‘Polar Bear Behavior in Response to Climate Change’, in Davis, R.W. and Pagano, A.M. (eds) Ethology and Behavioral Ecology of Sea Otters and Polar Bears. Cham: Springer International Publishing (Ethology and Behavioral Ecology of Marine Mammals), pp. 311–323.
Galicia, M.P. et al. (2016) ‘Dietary habits of polar bears in Foxe Basin, Canada: possible evidence of a trophic regime shift mediated by a new top predator’, Ecology and Evolution, 6(16), pp. 6005–6018.
Michaux, J. et al. (2021) ‘New insights on polar bear (Ursus maritimus) diet from faeces based on Next Generation Sequencing technologies’, Arctic, 74(1).
Wilder, J.M. et al. (2017) ‘Polar bear attacks on humans: Implications of a changing climate’, Wildlife Society Bulletin, 41(3), pp. 537–547.
Photo 1. A polar bear spotted in Svalbard. Photo credit: Katarzyna Greń
