Unstable thinking about polar-bear habitat

polar bearsWe are told that summer sea-ice extent is declining and that this threatens the health and population size of polar bears. Ringed seals, the primary prey of polar bears, are likely to also be affected.

What we are not told is that thick sea ice in late winter and early spring have caused greater devastation to polar-bear populations than recent summer sea ice declines. Wide variations in spring sea-ice habitat are a natural phenomenon and some have been known about for hundreds of years. They occur independently of any summer sea-ice changes that may be due to human-caused global warming.

Fast ice, where many ringed seals have their pups, can get extremely thick during especially cold winters. This phenomenon has been documented in northeastern Greenland, for example, and during thick ice winters, breeding ringed seals are forced to move south to areas with ice shallow enough to maintain breathing holes. To survive, most polar bears move south also. The local population drops.

Strong winds can also drive sea ice onshore, as happens about every 10 years or so off eastern Alaska where it meets the Northwest Territories of Canada. This leaves a wide swathe of intensely buckled ice that is too thick in most places for ringed-seal breathing holes and birthing lairs, so they move elsewhere. Female polar bears emerging from onshore dens with new cubs find no newborn ringed seals to eat and cannot relocate fast enough to prevent the tiny cubs from dying of starvation. Under such conditions, young bears on their own for the first time also have a hard time finding enough to eat and many eventually die of starvation. The local population drops dramatically.

Deep snow over sea ice in spring can effectively hide ringed seal birth lairs from hungry polar bears when they need abundant food the most. The fat baby seals are there but the polar bears have trouble finding them. Again, many cubs die and adult females may not be able to eat enough to maintain a pregnancy the following year. The local population drops.

However, none of these naturally devastating effects are included in models that forecast the future for polar-bear populations, which model only summer ice changes. They are also not permanent: when local conditions improve, polar-bear population sizes rebound.

In my opinion, a flawed and out-dated ecological concept – that sea ice, under natural conditions, provides a stable, predictable habitat for polar bears and ringed seals – has fatally skewed the conservation biology of these Arctic species.

Since the late 1960s, Arctic marine-mammal conservation has been based on the assumption that sea ice provides a stable, predictable environment for polar bears and Arctic seals: today, it underpins their ‘threatened with extinction’ status. A stable environment, this oversimplified theory proposes, should support populations at relatively high levels over time, without marked variation in size due to habitat change.

This idealised ecological concept was strongly promoted during the 1970s and was embraced by early polar-bear biologists. Observations since then, however, have shown the assumption of sea ice as a stable habitat over short time scales is false. Spring sea-ice thickness has been naturally variable over time scales of a few years to decades in the Beaufort Sea, East Greenland and Hudson Bay; spring ice extent has been naturally variable in the Barents Sea for centuries and spring snow depth on sea ice is known to vary over short periods – all with devastating effects on local polar bear populations.

Marked declines in polar-bear and ringed-seal survival in response to thick spring sea ice and reduced snow depth have been documented by Arctic biologists. These two variables are closely tied because spring (April-June) is the period of on-ice birth, nursing, and mating for ringed seals and is also when polar bears consume two-thirds of their annual prey and seek their mates. For example, the last time that thick spring ice conditions developed in the Beaufort Sea (2004-2006) biologists estimated the local population dropped 25-50%; similar conditions in 1974-1976 were described as equally devastating.

Apparently expecting stable or increasing populations, despite strong evidence to the contrary, Arctic biologists now attribute virtually every downturn in population size of polar bears to declines in summer sea ice blamed on human use of fossil fuels. They have shifted the blame for the devastation caused by thick spring ice or heavy snow onto recent summer ice declines, allowing them to claim that summer ice changes are manifestations of unprecedented, human-caused habitat instability. In fact, there is strong evidence that a longer ice-free season benefits polar bears in some regions because it allows ringed seals a greater time period for feeding.

I conclude that the assumption that Arctic sea ice is a naturally stable habitat over short time-frames is a biological fallacy. Predictive population models based on this myth are flawed, their results illusory. They do not take critical spring sea-ice changes into account. Yet, the International Union for the Conservation of Nature and the US government have, for the first time, accepted modeled (future) population declines of Arctic species based on modeled (future) summer sea ice changes as valid threats to their survival, all built upon this fallacy.

Given what we now know about the animals and their naturally changing habitat, it is time to concede that existing data do not support predictions that polar bears are threatened with extinction due to summer habitat instability.

The author is a PhD in zoology. She blogs at www.polarbearscience.com

This is a previously unpublished summary, written exclusively for The Arctic Journal, of a peer-reviewed, fully referenced essay on this topic that was published earlier this year by the Global Warming Policy Foundation in their “Briefing Paper” series (#16, June 8, 2015: The Arctic Fallacy: Sea Ice Stability and the Polar Bear), which includes a foreword by Dr Matthew Cronin, a professor of animal genetics at the University of Alaska Fairbanks. Pdf here