Walruses coming ashore
23 August 2017
Diminishing sea ice means polar bears and walrus are spending more time on land. Will this lead to more conflict between the two species? GEOFF YORK examines the evidence.
IT HAS LONG BEEN surmised that as the Arctic warms and sea ice recedes, more polar bears will spend longer time on shore throughout their range. This is supported by recent research using movements data from polar bears in the Southern Beaufort and Chukchi Seas along with anecdotal reports of increased sightings and encounters.
Much thought and some new projects have focused on what this likely means for conflict between people and polar bears, while other research has also highlighted the potential impacts to shore dwelling birds and bears. Less thought has been given to the potential impacts on other marine mammals such as walrus who are also increasingly forced to spend more time on land. Pacific walrus have shown signs of shifting distribution with historically large or new haulouts reported in both Chukotka and Alaska. While use of land is nothing new, traditionally these haulouts were composed of males, the haulouts were much smaller in size, and were not dominated by females and calves as is now the case. This is entirely new and likely tied to the retreat of sea ice beyond the productive continental shelf – a key feature of the vast and relatively shallow Chukchi Sea that defined this largely benthic system for decades if not millennia. When walruses are at sea and using the ice as a resting and rearing platform, they tend to occur in lower numbers and are continuously in motion with the sea ice, distributing their foraging across the Chukchi. As lack of sea ice forces the animals ashore, the new haulouts can number 20,000- 70,000 individuals or more.
On land and in herds of this size, walruses are prone to stampede events that endanger young and old, resulting in increased mortality of females and new calves. The presence of walrus and of carcasses is a strong attractant for polar bears. These species have evolved together in a sea ice dominated system but an adult male walrus can more than match most polar bears with its size, strength, incredibly tough hide, and dangerous tusks. They can and do mortally injure polar bears so it is common to see bears showing deference to adult walrus. As with seals and small whales, in open water the advantage goes clearly to the walrus. Young, injured, or sick walrus are another matter and can easily fall prey to a savvy bear while carcasses from stampede events or other natural mortalities can provide a veritable bounty for polar bears lucky enough to encounter them along the coast.
I have had the fortune to observe polar bears and walrus within meters of one another on more than one occasion while working in the Russian Arctic. In one situation, an incredibly well fed male polar bear was feasting on carcasses adjacent to a haulout estimated at over 80,000 animals. Neither the bear or the living walrus appeared to pay one another much attention, though I imagine the walrus, mostly females and calves, were keeping a watchful eye. In both other cases, the herds were smaller and the haulouts more traditional: adult males dominated the use of land while females and younger walrus stayed close to or in the water, at least when a bear was present. At both sites, an adult male polar bear could simply walk up to within a few meters of the male walrus and lie down and observe, likely assessing the situation for any easy targets, but an attempted predation was not observed. So what can we expect as more polar bears and more walrus spend more time ashore? Likely more skirmishes as either inexperienced bears, or those in poor shape, take more risks in attempts to secure food.
A small number of polar bears will also benefit greatly from increased carcass availability. However, the greatest threat to both species will remain the loss of sea ice habitat. As the Arctic continues to warm at a pace more than double that of the global average, the fundamental ecosystem that walrus and polar bears evolved to exploit is being pulled out from under them. This exposes both species to increased foraging costs, factors that decrease reproductive success, and that increase mortality: fundamental shifts to the ecosystem itself; new disease vectors; and competition from what were historically southern species. The last walrus will not likely be taken by the last polar bear, rather they will both be greatly reduced in range and numbers unless warming and subsequent loss of sea ice can be stabilized or reversed.
GEOFF YORK is Polar Bear International’s Senior Director of Conservation
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