Polar bears move around as sea ice habitat changes – this is what resilience looks like

Oddly, it seems some people expect polar bears to sit around and suffer (or die) when local conditions deteriorate, rather than move elsewhere.

PolarBear_2008_USGS

While there are perhaps a few places where moving is not really an option over the short term, over the long term (more than one season) polar bears are free to shift to another locale if ice conditions change (either too much ice or too little).

An announcement by the WWF last week (10 April) caught my eye, as it talked about bears moving from one area to another because of changing ice conditions — as if this was surprising, extraordinary and newsworthy. That said, at least they weren’t suggesting the bears are all going to die because of declining ice, which is a huge improvement.

See what you think of this part of the press release (below), in the context of what we know about the movement of bears between regions:

When the polar bears of Svalbard venture from their dens this year, a joint expedition of WWF-Canon and the Norwegian Polar Institute will be there to observe how the bears are adapting to changing sea ice conditions.

The WWF-Canon expedition comes only weeks after the maximum extent of Arctic sea ice was found to be at its fifth lowest level in the satellite record. A recently published paper also suggests that the Svalbard expedition area will be ice-free in summer by 2050.

“We don’t know what the future holds for these bears,” said Geoff York, WWF lead on polar bears. “We do know that bear populations deprived of sea ice for significant amounts of time are less likely to survive or breed successfully.”

There is some evidence that the Svalbard population is moving away from traditional denning sites on the Norwegian islands. The bears need to be close to sea ice to hunt when they emerge from their dens. One possibility is that they are moving further east where the ice survives longer. [my bold]

Further east” would be towards Russia, to the islands of Franz Josef Land in the North Kara Sea, ~260 km (160 mi) east of Svalbard (Fig. 1).

Figure 1. Top, Primary denning areas in the Svalbard Archipelago circled (Kong Karls Land and Hopen Island); Middle, “Barents Sea” popular bear subpopulation boundaries, courtesy PBSG; Bottom, Sea ice extent around Svalbard (circled) as of April 10, 2014, courtesy NSIDC.

Figure 1. Top, Primary denning areas in the Svalbard Archipelago circled (Kong Karls Land and Hopen Island), map from Wikipedia; Middle, “Barents Sea” popular bear subpopulation boundaries, courtesy PBSG; Bottom, Sea ice extent around Svalbard (circled) as of April 10, 2014, courtesy NSIDC. Click to enlarge.

Franz Josef Land looks like a fine alternative – up close (Fig. 2), you can see there are a multitude of islands for female bears to choose from, if they wanted to make maternity dens on land.

Figure 2. Close-up of Franz Josef Land, Russia, a short polar bear jog of 260 kilometers (160 mi) east of Svalbard. See cool National Geographic video clip of polar bears there.

Figure 2. Close-up of Franz Josef Land, Russia, a short polar bear jog of 260 kilometers (160 mi) east of Svalbard. See cool National Geographic video clip of polar bears there. Click to enlarge.

From an expedition update posted earlier this year (07.01.2014 / Expedition Logbook The Polar Bears of Franz Josef Land) which includes a very cool National Geographic video clip of polar bears that already live there. The introduction to it says:

During the Franz Josef Land Expedition, National Geographic Explorer-in-Residence Dr. Enric Sala and his team experienced very profound encounters with Polar Bears and were able to confirm that the greatest terrestrial predator on earth is still abundantly populating this remote arctic ecosystem.” [my bold]

[There is still no population estimate from this region — according to the Polar Bear Specialist Group (PBSG), no bears that live in the Kara Sea subpopulation contribute to the global polar bear count]

However, some work has been done. Research in the region (Fig. 3 below) of 105 adult female bears equipped with satellite radio collars from 1988 to 1999 (Maurizen et al. 2002:87), showed that bears move rather freely between these areas (Svalbard and the Kara Sea):

Hence, our results suggest no sharp population boundaries between Svalbard and the Barents and Kara Seas, indicating one continuous population within this area.”

It seems there is good reason for this flexibility: variability in spring ice coverage in the Barents Sea is now acknowledged as being due to natural variations in the Atlantic Decadal Oscillation (AMO). Since spring to early summer is the most critical time for polar bears, they need a strategy for dealing with years (or decades) of low spring ice coverage.

The other option for bears currently denning around Svalbard is to switch to denning on the sea ice, as many females in the area already do (Maurizen et al. 2001). For example, of the 105 bears tracked for the above study, at least 11 out of the 74 bears (based on dots marked on the map) captured around Svalbard and in the Barents Sea were taken on the first-year pack ice in spring (March/April), and were described as ‘pelagic bears.’

Figure 3. This is Fig. 6 from Maurizen et al. (2002:85), which shows as part of their results, the overlap in the ranges of radio-collared females from 1988-1999, between Svalbard (Norway) and Franz Josef Land in the North Kara Sea (Russia).

Figure 3. This is Fig. 6 from Maurizen et al. (2002:85), which shows as part of their results, the overlap in the ranges (of radio-collared females from 1988-1999), between Svalbard (Norway) and Franz Josef Land in the North Kara Sea (Russia).

Not Just a Barents Sea Phenomenon
Another area where polar bears are known to move around, and where substantial numbers of females make their dens on the sea ice (Amstrup and Gardner 1994) is off Alaska and western Canada (i.e., the Southern Beaufort, Chukchi Sea and Northern Beaufort subpopulations), see previous posts here and here.

Figure 3. This is Fig. 3 from Amstrup et al. (2005), showing the overlap of collared western North American polar bears – the Southern Beaufort, Chukchi Sea and Northern Beaufort subpopulations. When heavy spring ice conditions periodically develop in the Eastern Beaufort, many bears in that area probably move west into the Chukchi Sea, as ringed seals did during the winter of 1974/75 (Burns 1975).

Figure 4. This is Fig. 3 from Amstrup et al. (2005), showing the overlap of collared from Southern Beaufort polar bears into adjacent Chukchi Sea and Northern Beaufort subpopulations. When heavy spring ice conditions periodically develop in the Eastern Beaufort, many Southern Beaufort bears probably move west into the Chukchi Sea, as both ringed seals and bears did during the winter of 1974/75 (Burns 1975).

There is evidence of movement of ringed seals and polar bears out of the Beaufort Sea when spring ice conditions were especially heavy in 1974/1975, as they were the previous year (1973/1974).

Marine mammal biologist John Burns noted the following in 1975 (Burns 1975:77 pdf below):

“During the period 9 to 19 June [1975] extensive aerial surveys of ringed seals were undertaken in areas of land fast ice between Barter Island and Point Lay. ..The necessity of conducting these surveys was based on observations during the winter of 1974-1975, which indicated unusually high densities of ringed seals in the Chukchi Sea and the reverse in the Beaufort Sea. Observations for the Beaufort Sea were obtained by [Jack] Lentfer (personal communication).

Survey results substantiated that a major, short-term shift in density had indeed occurred. Density of seals in the Beaufort Sea was down 10-fold with a corresponding increase in the Chukchi Sea of between 10 and 15 fold. Communications with Canadian investigators indicated that the decrease of seals in the Beaufort Sea was not restricted to the areas north of Alaska, but were also similar in the Canadian sector including Amundson Gulf.

From past experience it was obvious the prevailing sea ice conditions in the Beaufort Sea were, by and large, unfavorable for breeding ringed seals whereas they were excellent in the Chukchi Sea. The distribution of ringed seals directly affected the distribution of their most significant predator, the polar bear.” [my bold]

Bears that could move out of the Beaufort (adult males, subadults and adult females without cubs) would be the most likely to head west toward more fertile hunting grounds in the Chukchi. Most females with newborn cubs would have had to tough it out in the Beaufort: we know from reports that many lost their litters (and a few died themselves). Overall, however, the population rebounded quickly over the next few years when spring ice conditions improved.

Such totally predictable (and well documented!) adaptation and resilience — why would anyone, let alone a polar bear biologist, think that a species that had survived several glacial to interglacial transitions would be done in by changes as slight as we have seen over the last 30 years?

References
Burns, J.J. 1975. Quarterly report. Pg. 77-78 in Environmental Assessment of the Alaskan Continental Shelf, Principal Investigators’ Reports, July-September 1975. Boulder. Pdf here.

Amstrup, S.C. and Gardner, C. 1994. Polar bear maternity denning in the Beaufort Sea. The Journal of Wildlife Management 58:1-10.

Amstrup, S.C., Durner, G. M., Stirling, I. and McDonald, T. L. 2005. Allocating harvests among polar bear stocks in the Beaufort Sea. Arctic 58:247-259. http://arctic.journalhosting.ucalgary.ca/arctic/index.php/arctic/article/view/426
Pdf here.

Mauritzen, M., Derocher, A.E. and Wiig, Ø. 2001. Space-use strategies of female polar bears in a dynamic sea ice habitat. Canadian Journal of Zoology 79:1704-1713. http://www.nrcresearchpress.com/doi/abs/10.1139/z01-126#.U0spVlda9lo

Mauritzen, M., Derocher, A.E., Wiig, Ø., Belikov, S.E., Boltunov, A.N., Hansen, E. and Garner, G.W. 2002. Using satellite telemetry to define spatial population structure in polar bears in the Norwegian and western Russian Arctic. Journal of Applied Ecology 39:79-90. http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2664.2002.00690.x/abstract

Comments are closed.