While the polar bear is an Ice Age species, genetic and fossil evidence suggests it barely survived the profound sea ice changes associated with the Last Glacial Maximum, one of the most severe glacial periods of the Pleistocene.
A map of sea ice extent at the climax of the Last Glacial Maximum (both perennial and seasonal ice), prepared with the help of a colleague, makes it possible to discuss what genetic and fossil evidence can tell us about the probable effects of glacial conditions on polar bears and ringed seals.
Sea ice extent during the LGM
We’re used to seeing the Last Age (also known as the Last Glacial Maximum (LGM, 26-11.5 thousand calendar years ago) depicted exclusively as ice sheets over continents and land bridges exposed by lower sea level, such as the image below (Fig. 1).
But there was also much sea ice over the northern oceans in areas where ice does not form today – indeed, ice in the Arctic Basin was so thick (perhaps 1 km thick) it would have excluded the marine mammals that use it today. Much of what is now productive polar bear, ringed seal and bearded seal habitat was either thick perennial ice, dry land, or covered by a continental ice sheet (like Hudson Bay).
Geologist colleague Jim Baichtal, who has much better mapping skills than I do, modified a version of the map above (from Wikipedia), adding perennial (year-round) and seasonal (winter/spring) sea ice to the LGM Northern Hemisphere landscape.
The sea ice additions are based on a large number of studies that document the extent of perennial sea ice and the limits of annual sea ice at the climax of the LGM (see “sea ice references” below). While this is an approximation (and may change somewhat with additional research), it gives a reasonably good picture of the sea ice habitat available to polar bears and Arctic seals at that time.
Habitable sea ice area during the LGM
Seasonal or first year ice that melts every summer is the best all-round habitat for polar bears and Arctic seals (Derocher et al. 2004). While some ringed seals and polar bears do occupy the Arctic Basin today (NIPCC summary, “Bears in the deep Arctic Basin”), multiyear ice is thinner now than it was during the LGM.
During the LGM, the perennial ice in the Arctic Basin undoubtedly excluded all marine mammals (Fig. 2). Perennial ice in Baffin Bay, Davis Strait, and East Greenland may have been relatively thinner but still thick enough to exclude ringed seals and polar bears. It’s likely, in my opinion, that if there was potential polar bear and ringed seal habitat in the southern-most perennial ice areas, densities were not very high.
As a consequence, it’s not hard to see (Fig. 3 left panel) that the approximate area covered by seasonal ice during the height of the LGM would have been considerably smaller than present today in March (Fig. 3 right panel). Polar bear and seal habitat was also split into Atlantic and Pacific segments that could not mix because it was blocked by perennial ice in the Arctic Basin.
What little useable annual sea ice habitat that remained formed at the edges of the continents and ice sheets (similar to East Greenland polar bear habitat today). For the sake of simplicity, let’s call these areas North Atlantic, Eastern North America, Western Europe, Norwegian Sea, Western Bering Sea, Outer Aleutians, and Western North America.
Habitable LGM sea ice positioned over deep water
A glance at where shallow (< 300m) continental shelves are in modern oceans (Fig. 4, below) shows that virtually all LGM seasonal sea ice was located over deep water. However, although there was seasonal ice in the Sea of Okhotsk, there’s no evidence to suggest polar bears lived there during the LGM (and none live there now).
A global drop in sea level of ~125 m (410 feet) (due to water tied up as ice) meant that a large proportion of coastal continental shelves in the Northern Hemisphere were above water. However, in most cases (except Beringia), these were covered or partly covered with continental ice sheets (e.g., Northern Russia, Eastern North America) or with perennial sea ice, such as the shallow areas in the Bering Sea, the Gulf of Alaska, and Davis Strait/Baffin Bay.
In contrast, the deep water in the western Bering Sea and off the southern tip of Greenland (Fig. 4), appears to have been covered in seasonal (annual) sea ice that would have melted every summer. That means virtually no seasonal sea ice was positioned over shallow continental shelves, said to be the preferred habitat of modern polar bears.1
While virtually all of the available seasonal sea ice habitat for polar bears and ringed seals was positioned over deep water, this may or may not have had lower productivity than sea ice in the Arctic Basin that’s over deep water.2 Oceanic conditions would have been so much different than today (especially currents from the south) that it would be imprudent to assume that productivity was necessarily low just because the water under the sea ice was relatively deep.
Habitat for maternity dens and summer refuges
Not surprisingly, polar bears had very little land to retreat to in summer when the annual sea ice melted. Coastal shorelines available to polar bears would have been limited to SE Alaska and Northern British Columbia (which were part of a coast ice-age refugium, see discussion here), the south shore of the Alaska Peninsula, and southern France.
In all other areas, when the annual sea ice melted during LGM summers, polar bears would have been forced onto the edges of continental ice sheets that formed coastal glaciers in Western Europe, Eastern NA, and Western NA, or onto the margins of the perennial sea ice, such as in the Gulf of Alaska and the Norwegian Sea. Such conditions almost certainly required polar bears to endure a summer fast of a month or more (depending on the location) as they do today, while seals foraged in open waters to fatten up for the next winter.
Coastal glaciers and perennial sea ice regions may have served as the primary denning areas for pregnant females, although dens may also have been made in the few regions mentioned above where land was available. It is more likely that annual sea ice reformed so quickly and thickened so early in the season that most polar bear females made their dens on the sea ice, as many still do today in the Southern Beaufort and the Barents Sea (see previous posts here and here).
Effects of sea ice conditions on polar bear numbers
Charlotte Lindqvist and colleagues (2010) suggested that the extreme conditions of the last Ice Age could have led to a severe population decline that would account for the “genetic bottleneck” evident in modern polar bear DNA about that time.
In other words, modern polar bears are very similar genetically now but were not so previously, which suggests that their numbers were formerly higher and more genetically diverse, but were subsequently reduced to fewer bears with similar DNA (a “population bottleneck”). After this population decline, numbers re-bounded, generating a modern population with much less genetic diversity than before (which has happened frequently in a number of species and does not necessarily lead to a survival issue).
The dates for the LGM (26-11.5ky) suggest it is a likely candidate for the population bottleneck that Lindqvist et al. (2010:5054) suggest occurred:
“Even more surprising, the age of the modern polar bear crown group (the clade containing the last common ancestor of all extant members) is estimated to be less than 45 ky, slightly older than the age of the ABC bears (Fig. 3A), a date that is also found with the expanded dataset of control-region sequence fragments (Fig. S5). These estimates suggest a very recent and rapid expansion of modern polar bear populations throughout the Arctic since the Late Pleistocene, perhaps following a climate-related population bottleneck, although data from more modern and Holocene polar bear specimens will be required to establish this.” [my bold]
Their conclusion seems to be confirmed by a later study done by Miller and colleagues (2012), who also found genetic evidence for a pronounced population decline at about the end of the last Ice Age (Fig. 5), with a rebound in numbers afterward (discussed previously here).This genetic evidence suggests that sea ice conditions during the LGM had a huge impact on polar bear numbers, larger than previous glacial periods (even-numbered MIS) and the warm conditions of interglacials (odd-numbered MIS), including the last Interglacial (the Eemian or MIS 5e, ca. 130-110 thousand years ago).
During the Eemian, summer temperatures were 5-8 degrees C warmer in parts of the Arctic, annual global mean temperature was about 1 degree warmer than today, and there was no sea ice in the Bering Sea during the winter (see “Eemian excuses”). Yet, polar bears seem to have survived the Eemian with only a moderate drop in population size.
But when and how did the polar bear population decline occur – at the beginning, the end, or the climax of the LGM?
Recent devastating effects of thick spring ice on polar bears and their prey (discussed here, here, and here; also Chambellant et al. 2012), hint at what must have happened during the early stages of the last Ice Age:
1) Sea ice would not have melted completely by summer’s end in Western and Southern Hudson Bay, and within a few years (or a few decades), the sea ice that formed would have been too thick for ringed seals to maintain breathing holes and have their pups in the spring and many bears would have starved. Savvy bears that moved out of Hudson Bay into Davis Strait early in this process would likely have survived. The thickening of annual ice would have eliminated Hudson Bay as polar bear habitat well before it transitioned into perennial multiyear sea ice, which was eventually over-run by the Laurentide Ice Sheet.
2) Multiyear sea ice would eventually have replaced annual ice in the Arctic Basin as well as its marginal seas (Beaufort, Chukchi, East Siberian, Laptev, Kara, and Barents Seas), making all but occasional use by polar bears and ringed seals impossible. As it got even thicker, marine mammals would have been totally excluded — those that survived would have had to move south.
This suggests that the largest decline in polar bear population numbers likely took place at the beginning of the LGM.
However, it’s also possible that severe conditions later in the LGM caused polar bear numbers to decline further. It is also possible that polar bears were extirpated from the Pacific region, suggested by the fact that no subfossil polar bear bones that date to the end of the LGM or early Holocene have yet been found. In contrast, there are a number of subfossil remains of polar bears in the North Atlantic/Western European region that date to the very end of the LGM and very early Holocene.
If polar bears disappeared in the Pacific, it would mean that modern polar bears all descend from a small remnant population of bears that survived the LGM in the North Atlantic and recolonized the Arctic rapidly as suitable sea ice habitat became available.
All told, it’s hard for me to envision a slightly warmer Arctic being more deadly for polar bears than the Last Glacial Maximum.
Footnote 1. For example, the Fish and Wildlife document that supports the listing of polar bears as ‘threatened’ with extinction (US Fish & Wildlife 2008) states:
“Durner et al. (2004, pp. 18-19; Durner et al. 2007, pp. 17-18) found that polar bears in the Arctic basin prefer sea ice concentrations greater than 50 percent located over the continental shelf with water depths less than 300 m (984 feet (ft)).”
Footnote 2. Derocher and colleagues (2004) had this to say about polar bears and sea ice over deep water:
“If as projected by Comiso (2002b), a large amount of the pack ice in the polar basin retreats to the north and lies over the deep polar basin, then it is likely that productivity will be less than over the continental shelves. However, with thinner ice and more open water, productivity may be greater than it presently is. This dichotomy makes accurate predictions difficult.”
Chambellant, M., Stirling, I., Gough, W.A. and Ferguson, S.H. 2012. Temporal variations in Hudson Bay ringed seal (Phoca hispida) life-history parameters in relation to environment. Journal of Mammalogy 93:267-281.
Derocher, A.E., Lunn, N.J. and Stirling, I. 2004. Polar bears in a warming climate. Integrative and Comparative Biology 44:163–176.
Lindqvist, C., Schuster, S.C., Sun, Y., Talbot, S.L., Qi, J., Ratan, A., Tomsho, L., Kasson, L., Zeyl, E., Aars, J., Miller, W., Ingólfsson, Ó., Bachmann, L. and Wiig, Ø. 2010. Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proceedings of the National Academy of Sciences USA 107:5053-5057. Open access http://www.pnas.org/content/107/11/5053.abstract
Miller, W., Schuster, S.C., Welch, A.J., Ratan, A., Bedoya-Reina, O.C., Zhao, F., Kim, H.L., Burhans, R.C., Drautz, D.I., Wittekindt, N. E., Tomsho, L. P., Ibarra-Laclette, E., Herrera-Estrella, L., Peacock, E., Farley, S., Sage, G.K., Rode, K., Obbard, M., Montiel, R., Bachmann, L., Ingolfsson, O., Aars, J., Mailund, T., Wiig, O., Talbot, S.L., and Lindqvist, C. 2012. Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. Proceedings of the National Academy of Sciences USA 109:E2382-E2390. doi: 10.1073/pnas.1210506109. see also: Paper plus supplemental data open access: http://www.pnas.org/content/early/2012/07/20/1210506109.full.pdf+html
US Fish and Wildlife Service. 2008. Determination of threatened status for the polar bear (Ursus maritimus) throughout its range. Federal Register 73(95):28212-28303.
Sea ice references
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Caissie, B. E., Brigham-Grette, J., Lawrence, K. T., Herbert, T. D. and Cook, M. S. 2010. Last Glacial Maximum to Holocene sea surface conditions at Umnak Plateau, Bering Sea, as inferred from diatom, alkenone, and stable isotope records. Paleoceanography 25:PA1206, doi:10.1029/2008PA001671.
Clark, P. U., Dyke, A. S., Shakun, J. D., Carlson, A. E., Clark, J., Wohlfarth, B., Mitrovica, J. X., Hostetler, S. W., and McCabe, A. M. 2009. The Last Glacial Maximum. Science 325:710-714.
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