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Some things we know-and don't know-about Polar Bears
Susan J. Crockford, Ph.D.
Oct. 14, 2008
INTRODUCTION
Much of what you hear about polar bears these days - their status, their plight - is distilled from a literature dominated by studies done within very limited portions of the Arctic: those that are accessible to researchers. Logistical and technical difficulties prevent scientists in all disciplines from traveling to, and working within, the ever-changing sea ice that exists well offshore. As a consequence, the picture that gets painted of polar bear existence sounds more completely understood than it really is. Due to the nature of the beast and the habitat in which it lives, there is in reality a profound uncertainty regarding polar bear population status, some of its life history features and conditions of its habitat, and the status of its primary prey, the ringed seal. However, it is clear from their long-term success surviving within this habitat that the tight association polar bears and arctic seals have with moving sea ice gives them tremendous flexibility and adaptability to changing climatic conditions.
My purpose here is to address some of the bias that mars virtually all general information sources one might consult on polar bears and ice-dependent Arctic seals, in point form for easy reference. Most references cited here are available on request as pdf files. This document was compiled from several papers published on associated topics (Crockford 2004, 2006; 2008; Crockford and Frederick 2007; Crockford and Frederick, in review) and material collected in the course of reviewing the January 2007 draft of the Report for Congress on Polar Bears prepared by Library of Congress researcher Eugene H. Buck, filed April/07. This update incorporates information amassed since that date.
Note that in regard to ice:
1) pack ice and sea ice both refer to large sheets or broken chunks of ice that drift with the
currents and wind as the seasons change (Rigor and Wallace 2004) - most Arctic ice is sea ice
(Ferguson et al. 2000) and the ice edge is the southern-most limit of the drifting pack.
2) fast ice, grounded fast ice, landfast ice and shorefast ice all refer to ice attached to land,
although shorefast ice is perhaps the least ambiguous terminology.More.......
DISTRIBUTION AND STATUS OF THE POLAR BEAR
- Polar bears world-wide are divided into 19 subpopulations for management purposes (Figure 1).
- Two of these populations, genetically indistinguishable from each other (Cronin et al. 2006), occur within US territory
1) the Southern Beaufort Sea population (SB, shared with Canada, half in US territory) is
estimated at 1,526 animals (Regehr et al. 2007b, “1211-1841 at a 95% confidence interval”);
2) the Chukchi/Bering Sea population (CB, shared with Russia, half in US territory) is tentatively
estimated at 2,000 - no population survey has yet been done (Aars et al. 2006).
Figure 1. The nineteen designated polar bear subpopulation boundaries (courtesy IUCN Polar Bear Specialist Group, see Aars et al. 2006)
- Globally, less than one third of the nineteen populations are currently estimated as declining, more than one third are increasing or stable, while the remaining third have insufficient data available to estimate population trends: the SB population is currently declining, based on presumed consequences of some bears in poor condition, not an actual decline in numbers over time ( Regehr et al. 2007b; Rode et al. 2007), the CB trend is unknown (Aars et al. 2006).
- Four out of the five subpopulations listed as declining in 2006, as well as several others (including CB), are considered at risk from over-harvesting (i.e. hunting), not reduced sea ice (Aars et al. 2006).
- Some population estimates are based on “mark/recapture” methods, others on aerial survey; due to fog and cold, aerial surveys seldom extend beyond 125km north of the sea ice edge (e.g. Aars et al. 2006, 2008, Barents Sea), with at least one exception (Fischbach et al. 2007, Southern Beaufort Sea).
- The status of the polar bear in the central Arctic Basin (see Fig. 1 above), the largest of the nineteen designated regions, is completely unknown (Aars et al. 2006), although bears have been reported there (e.g. Van Meurs and Splettstoesser 2003).
DENNING AND OTHER LIFE HISTORY HABITS
- Most of what we know about polar bear biology is based on the easily-accessible animals of Western Hudson Bay (WHB), see references below, which comprise only 3-5% of the global population and are anomalous for a number of reasons (Aars et al. 2006; Dyck et al. 2007, 2008; Mauritzen et al. 2001; Regehr et al. 2007a; Schliebe et al. 2008; Stirling et al.1977):
1) WHB is the most second most southerly subpopulation worldwide, so their ice always melts earlier in the year than most of the others (however, the Southern Hudson Bay (SHB) subpopulation is the furthest south and its population has remained stable over the last 20 years (Aars et al. 2006).
2) WHB population is the most easily accessible and has been under scrutiny since the late 1960s.
3) WHB is the only subpopulation, out of the five considered to be declining, where the population trend is based on a statistically significant decrease in population estimates over time (Aars et al. 2006).
4) WHB is one of the most geographically constrained subpopulations, so they easily get trapped ashore - usually for about four months at a time - when summer sea ice retreats (however, this also happens to the SHB subpopulation, without an associated population decline).
5) most of the females prefer to den on land or shorefast ice rather than on offshore sea ice
(compared to the Southern Beaufort, where about 40-60% den offshore (Fischbach et al. 2007).
-Virtually the only areas studied in any detail for polar bears and ringed seals, are the coasts of Hudson Bay in Canada (e.g. Amstrup et al. 2007; Derocher et al. 2004; Ferguson et al. 2005; Holst et al. 1999; Lennox and Goodship 2008; Lunn et al. 1997; Regehr et al. 2007a; Stirling and Derocher 2007; Stirling et al. 2008a), the Beaufort Sea off Alaska and Northwestern Canada (e.g. Amstrup 1995; Frost et al. 2004; Regehr et al. 2007b; Schliebe et al. 2008; Stirling 2002; Stirling et al. 2007, 2008b), and the Svalbard region in the Barents Sea, off Norway (e.g. Aars et al. 2008; Derocher et al. 2002; Holst et al. 2001; Krafft et al. 2006; Labansen et al. 2007; Lydersen and Gjertz 1986; Mauritzen et al. 2001; Wiig et al. 1999). Some studies have also been undertaken in the Canadian Arctic Archipelago (e.g. Ferguson et al. 2000; Hammill and Smith 1991; Kelly and Wartzok 1996; Kingsley et al. 1985; Smith and Hammill 1981; Smith et al. 1991; Stirling and Øritsland 1995) and the Davis Strait/Baffin Bay region of Canada (e.g. Ferguson et al. 2000; Finley et al. 1983). Information on populations elsewhere in the Arctic, including regions north of Greenland and Russia, is very limited or nonexistent (e.g. Aars et al. 2006).
- Polar bears are capable of fasting for more than four months at a time while fully awake and mobile, regardless of the season (they do not need to den or hibernate as other bears do - only pregnant female polar bears hibernate over the winter in true bear fashion): as a consequence, polar bears are known to biologists as walking hibernators (Lennox and Goodship 2008; Stirling and Øritsland 1995).
- While polar bears that spend extensive time on land during the summer months (such as those in WHB) may fast for up to four months, previous research has shown (Stirling and Øritsland 1995) that bears in most regions are at their lowest body weight in spring (i.e. March). This suggests that winter fasting leading to starvation may be a more limiting factor for polar bears and this may be particularly true if winters are associated with development of especially thick shorefast ice. Such cold winters in the past, as occurred during the mid-1960s, mid-1970s, mid-1980s, and early 1990s, led to marked reductions in polar bear numbers (Stirling 2002; Stirling and Lunn 1997) due to dramatic declines in availability of young ringed seals. In Greenland, ringed seals are known to move offshore when shorefast ice becomes too thick for them to maintain their breathing holes (Vibe 1967).
- Over most of their range, most polar bears remain on the sea ice year-round or at most spend only short periods on land. Schliebe et al. (2008) found that from 2000-2005, on average 3.7% of all Southern Beaufort Sea polar bears in Alaska spent time on land between mid-September and the end of October. While nearshore-dwelling Davis Strait bears were found to spend two-three months on Baffin Island (Ferguson et al. 1997), polar bears in WHB are unique in routinely spending about four months on land from summer through fall (Regehr et al. 2007a; Schliebe et al. 2008).
- In October and November, male polar bears head out on the sea ice where they spend the winter. Pregnant females either seek sites on offshore ice, or on shorefast ice/shoreline areas (snow covered land), to dig large dens in snow where they give birth and spend the winter.
- Den locations chosen by female polar bears in the Southern Beaufort Sea region have varied since the early 1980’s: 62% of dens were on offshore sea ice from 1985-1994 but only 37% of dens were offshore from 1998-2004 (Fischbach et al. 2007). It is possible that world wide, the general pattern for polar bear dens is an almost equal number on offshore sea ice and shorefast ice/land (with WHBay being anomalous). Dens are known to be difficult to spot from the air (e.g. Ferguson et al. 1997).
- Polar bear females appear to have individual habitat and denning preferences: females do not require mainland or shorefast ice sites for denning but some individuals prefer them. (Mauritzen et al. 2001):
1) bears that choose “pelagic” habitats generally live on offshore drifting sea ice year round.
2) bears that choose “nearshore” habitats generally live on shorefast ice year round.
- When seasonal ice recedes north in summer, as it does every year in most areas, pelagic-dwelling bears stay on the drifting sea ice while nearshore-dwelling bears move to land. Both pelagic-dwelling and nearshore-dwelling individuals of both sexes are known in all subpopulations studied (Mauritzen et al. 2001; Ferguson et al. 2000; Schliebe et al. 2008).
- The fact that pelagic-dwelling bears not only exist but behave differently than nearshore-dwelling bears to reduced sea ice is critical to predicting how polar bears as a species might react to changes in ice conditions: unfortunately, we simply do not know how many bears den out of study range.
- While there is extensive evidence that virtually all Arctic marine mammal populations are negatively impacted by increased sea ice conditions (Stirling 2002; Laidre et al. 2008; Harington 2008), evidence for how these animals react to decreased sea ice is extremely limited, coming from extensive studies in the anomalous WHB region and a few short term studies in the southern Beaufort Sea. In other words, what we know for sure is that increased sea ice is associated with a decline in polar bear and ringed seal numbers; we don’t really know what impact decreased summer sea ice might have on polar bears and ringed seals that inhabit other regions of the Arctic.
- Computer models that predict extinction of polar bear populations within this century due to human-induced global warming (e.g. Derocher et al. 2004) do not take into account adaptations of bears and their prey to reduced sea ice extent (Armstrong et al. 2008; Bodkin et al. 2007), even though both bears and their prey have clearly done so in the past (e.g. Kochnev 2006; Vibe 1967). Such adaptation would likely involve living year round within the mobile offshore sea ice that is now beyond study range (wherever it occurs), without a shift to land.
- Even if substantial declines in polar bears and their prey do occur because of anthropogenic global warming, as predicted by Amstrup et al. (2007) and others (e.g. Laidre et al. 2008; Stirling and Derocher 2007), this does not doom them to extinction: many species have recovered from far more dramatic declines in population than predicted by even the most pessimistic scenarios conceived of by climate models, including humpback whales (Dalton 2008), gray whales (Reeves et al. 2002), northern fur seals (Reeves et al. 2002), Atlantic cod (Bigg et al. 2008), and sea otters (Doroff et al. 2003; Estes 1990), among others. Contrary to common biological assumption, small populations often retain sufficient genetic variation for significant recovery (e.g. Aguilar et al. 2004; Kaeuffer et al. 2007).
- Adaptation of a species is not the same as adaptation of individuals: the death of some individuals during changing conditions is likely inevitable but this does not mean the species (i.e. the entire population) is not adaptable (e.g. Grant and Grant 2002; Grime et al. 2008). Polar bear populations may have declined and recovered many times in the past in response to changing sea ice conditions, without us knowing.
PREY SPECIES DISTRIBUTION AND STATUS
- Survival of polar bears is dependent on available prey, which consists primarily of ringed seal, Phoca hispida and (depending on region and/or season) bearded seal, Erignathus barbatus (Derocher et al. 2002, 2004; Stirling and Øritsland 1995). They occasionally take walrus, Odobenus rosmarus and small whales (such as beluga, Delphinapterus leucas, and narwhal, Monodon monoceras) and scavenge large whale carcasses (such as bowhead, Balaena mysticetus).
- Ringed seals have a circumpolar distribution and are associated with ice year round. They give birth and mate on ice and are not known to haul out on land. Some ringed seals prefer to over-winter and give birth on shorefast ice while others live their lives well offshore in the drifting sea ice (Born et al. 2004; Davis et al. 2008; Ferguson et al. 2000; Finley et al. 1983; Wiig et al. 1999), similar to the known “pelagic-dwelling” and “nearshore-dwelling” preferences of individual polar bears (see discussion above). Ringed seals feed throughout the darkness of the Arctic winter and are available prey for polar bears wintering in the offshore pack ice (Kelly and Wartzok 1996).
- Most marine mammal researchers working in the Arctic assume that ringed seals breed primarily in shorefast ice habitats (e.g. Burns 1970; Derocher 2004; Frost et al. 2004; Hammill and Smith 1981, 1991; Holst et al. 1999, 2001; Kingsley et al. 1985; Krafft et al. 2006, 2007; Lydersen and Gjertz 1986; Smith and Hammill 1981; Stirling 2002; Stirling and Øritsland 1995), despite several well-documented studies that conclude a significant portion of all ringed seals must live and breed well offshore, out of study range (Born et al. 2004; Davis et al. 2008; Ferguson et al. 2000; Finley et al. 1983; Wiig et al. 1999).
- Ringed seals eat primarily young polar cod, Boreogadus saida, which live under the ice (e.g. Born et al. 2004; Labansen et al. 2007), although they eat other types of fish as well as the amphipods and small copepods (shrimp-like invertebrates) that polar cod themselves eat.
- Both polar cod and their prey live under ice of all types, including multi-year and first year drifting sea ice regardless of the ocean depth (Lønne and Gulliksen 1989): in other words, cod do not require ice that is positioned over shallow, continental shelf waters and therefore, neither do ringed seals or polar bears, contrary to common assumption (e.g. Derocher et al. 2004). While Arctic deep water is often assumed to be of low productivity (e.g. Fischbach et al. 2007), this has not be demonstrated. If offshore sea ice over deep water is suitable habitat for polar cod, it should be suitable for ringed seals and polar bears also. This assumption is supported by reports at the North Pole of “small fish” (estimated as 5-8cm, presumably young cod,) thrown up by ice-breakers, algal growth noted on the underside of broken ice, and the presence of ringed seal (Todd et al. 1992), as well as reports of polar bears themselves (Van Meurs and Splettstoesser 2003).
- As for polar bears, much of ringed seal habitat, especially the drifting sea ice that lies well offshore, has not been surveyed, leading to much uncertainty regarding population size and status of ringed seal: the current estimate used for the global population numbers for ringed seal is about seven million (Davis et al. 2008; Wiig et al. 1999; Nowak 2003; Reeves et al. 2002).
Although climate models predict that future summer pack ice declines will decimate polar bear populations (e.g. Laidre et al. 2008; Stirling and Derocher 2007), forecasting a loss of from 66% of the world total population by 2050 (Amstrup et al. 2007) to outright extinction (Derocher et al. 2004), such conclusions do not take into account the fact that polar bears can fast for more than four months when required and are capable of living entirely at sea, in the ice that lies well offshore where there are substantial numbers of seals, without ever setting foot on land. Nor do such dire prophecies take into account the kind of adaptability described by a Russian researcher: “our investigations on Wrangel Island have shown that the polar bear is a very plastic animal: it can rapidly change its way of life, spatial distribution and behavior according to new ecological conditions” Kochnev (2006:163).
HOLOCENE AND PLEISTOCENE HISTORY
-Polar bears evolved from brown bears (Ursus arctos) during the last Ice Age and while they are thus a relatively new species (no more than 200,000 years old, probably much younger), ringed seals and bearded seals have been around for at least two million years (Arnason et al. 1995, 2006; Davis et al. 2008; Kurten 1988; Harington 2008).
-Polar bears are close genetically to brown bears although they are a distinct species (Cronin et al. 1991; Talbot and Shields 1996). Mitochondrial DNA sequences of polar bear are closer to one particular population of brown bear from Southeast Alaska than some dogs are to wolves (Crockford 2004, 2006). Although we know polar bears can successfully interbreed with brown bears (Duff-Brown 2007), this reflects their recent common ancestry - it does not call into question their status as a distinct species or detract from their divergent ecological, morphological and physiological features (Crockford 2004, 2006; Cronin 2007).
-The polar bear survived two major warm periods over the last 11,000 years (The Holocene):
1) The Early Holocene. At the end of the last Ice Age, the Northern Hemisphere in particular
entered an extended period of rapid warming, with temperatures in Arctic regions eventually
reaching levels several degrees warmer than today. At that time, the sea ice above western
North America is known to have retreated substantially, allowing arctic species such as bowhead
whales and walrus to move northward into areas of the Canadian arctic they cannot reach today
(Dyke et al. 1999, Dyke and Savelle 2001; Fisher et al. 2006).
The Early Holocene Climatic Optimum peaked at about 11,000-9,000 years ago near Alaska and at 8,000-5,000 years ago near Greenland & northern Europe: in both areas, temperatures rose rapidly 10-150C to a point significantly warmer than present (about 2.50C warmer) in most places and up to 70C warmer in Northern Russia
(MacDonald et al. 2000)
about 5-100C of that warming
took place within 30 years or less
(Alley 2000; Bennike 2004; Dahl-Jensen et al. 1998; Jennings et al. 2002; Kaufman et al. 2004; Steffensen et al. 2008).
The rate of warming that took place in the early Holocene far exceeds any climate model predictions of warming over the rest of this century.
2) The Late Holocene. Another significant but shorter warm period occurred about 1000 years
ago, when arctic temperatures were slightly warmer than today. This warming, known as the
Medieval Warm Period, also triggered sea ice reductions in arctic regions and was accompanied
by significant reductions in Greenland glaciers, that created so much arable land that Viking
farms established in west Greenland were occupied for 400 years. During the Medieval
Warm Period, ca. 800-1200 A.D., temperatures in Greenland rose about 10C above modern levels
(Fagan 2000; Soon and Baliunas 2003), allowing establishment of Viking settlements in areas of
western Greenland that today are covered in glaciers; in Finland, pine forests existed further north than they do today, with temperatures ca. 0.50C warmer than present (Kultti et al. 2006).
- There is no evidence to suggest that ice in the Arctic Basin disappeared entirely during either the early or late Holocene warm periods or that any ice-dependent species disappeared: polar bears (and their known prey species, ringed seals, bearded seals and walrus) existed before the last Ice Age and significant populations of them remain today (although we don’t know how large any of the ancient populations actually were).
- Based on the evidence of extensive polar ice (Bradley and England 2008) and fossil remains of seals and polar bears found outside the Arctic, most Arctic populations appear to have been displaced south during the last Ice Age (Dyke et al. 1999; Harington 2008; Kurten 1988).
- Note that during previous Holocene warm periods mentioned above, skeletal remains of bowhead whales and walrus on shorelines mark the their prior distributions (Dyke et al. 1999, Dyke and Savelle 2001; Dyke and England 2003; Fisher et al. 2006): there are no bones of polar bears found amongst these (Art Dyke, pers. comm., 2007).
- Fossil and subfossil remains of polar bears (who presumably died of natural causes, not killed by humans) are exceedingly rare: there are exactly 6 (six) Pleistocene age specimens of polar bear worldwide (Harington 2001; Kurten 1988) and one major Holocene deposit from a natural trap cave on the Pribilof Islands, in the Bering Sea, that is about 4,500 years old (Veltre et al. 2008). More polar bear remains are found in Late Holocene archaeological deposits in the Arctic than in natural-death contexts, although they are still quite rare (e.g. Murray 2008; Harington 2001, 2008).
- In other words, the suggestion that polar bears would have moved to land during early Holocene warm periods (in response to reduced ice cover worldwide) is pure conjecture and not supported by any evidence. Virtually all polar bears must die on the ice where their remains sink to the bottom of the ocean: fossil finds are rare because the Arctic sea ice habitat is not conducive to discovery.
SEA ICE THICKNESS AND EXTENT (ESSENTIAL POLAR BEAR HABITAT)
- Many statements made regarding sea ice thickness in the Arctic do not acknowledge the incompleteness of this data: one frequently cited study (Laxon et al. 2003) surveyed (via satellite) only ½ of permanent sea ice and did not include ANY of the region in the central Arctic Basin (above 810 N).
-Another frequently cited reference (Lindsay and Zhang 2005) concludes that Arctic sea ice is experiencing a continual decline that cannot easily be reversed, but this is not a data-based paper - it is a model based on what is now considered old, substandard data from coastal submarine surveys.
- Sea ice thickness in the huge Arctic Basin region is based on very few actual measurements that have been extrapolated to represent the entire region and used in various climate models to predict future conditions (Rothrock et al. 2003; Yu and Rothrock 1996); ice extent data from satellites used in these models have been available only since 1979; these data are insufficient for assessing long-term trends.
- Limited coverage of some of these surveys, in addition to the fact that the models do not take effects of wind into account, have almost certainly led to overestimates of sea ice reduction and ice thinning (Holloway 2001; Holloway and Sou 2002): wind can temporarily concentrate ice in areas that are not surveyed.
Polar bears that live in regions of extensive sea ice routinely hunt on newly-formed ice that is less than 30 cm thick (about 1 ft.) and are quite capable of utilizing “thick” first year ice (more than 120 cm thick, or about 4 ft.) for over-wintering activities, including denning: they do not require thick multi-year ice (Ferguson et al. 2000). First year ice in March of this year was about 1.6m thick (NSIDC 2008).
- Recent predictions of future sea ice conditions, as they might impact polar bears, are adaptations of unverified “general circulation models” intended to forecast global temperatures (Amstrup et al. 2007; Armstrong et al. 2008; Koutsoyiannis et al. 2008). Such models are conditional on global temperatures being amplified by an hypothesized amount over the entire Arctic (Polyakov et al. 2002; Serreze and Francis 2006). None of these models take into account the fact that Arctic climate is subject to profound regional variation and influenced by a host of little-understood drivers of wind and weather patterns, including the Arctic Oscillation (e.g. Overland and Wang 2005; Polyakov et al. 2002) and Pacific Decadal Oscillation (Biondi et al. 2001; Newman et al. 2003), which are known to shift precipitously on decadal and multidecal time scales.
- Models of future climate change in the Arctic predict sea ice reductions to occur primarily in winter, while all observed sea ice changes so far reported have occurred in spring and summer (NASA 2007; NSIDC 2008; Overland and Wang 2005) and most of these reductions are not Arctic-wide but confined to the western Arctic (Rigor and Wallace 2004; Rigor et al. 2002).
- So far, there is no firm evidence that there has yet been “unidirectional” warming in the Arctic over the last 100 years (e.g. Fisher et al. 2006; Kahl et al. 1993), nor an unprecedented, irreversible decline in either sea ice extent or thickness (Holloway and Sou 2002) — neither is there firm evidence that an “Arctic amplification” effect is markedly and uniformly magnifying circumpolar Arctic temperatures (Polyakov et al. 2002; Serreze and Francis 2006).
- Note that the computer model results presented late last year (Amstrup et al. 2007), which forecast dramatic declines in polar bear numbers based on predicted reductions in seasonal sea ice thickness and extent due to human-generated increases in atmospheric CO2, have not yet been tested against even a single years worth of independent data.
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Contact:
Dr. Susan Crockford
Bio:
Susan Crockford (Ph.D., University of Victoria, Canada) is an evolutionary biologist with more than 30 years experience in the specialized field of archaeozoology and is a world-renowned expert in the identification and analysis of animal bone (including fish, birds and marine mammals) recovered from archaeological sites and animal digestive tracts. She is particularly interested in vertebrate evolution, especially of dogs, polar bears, and humans. She has written a book for non-scientists based on her dissertation topic ("Rhythms of Life: Thyroid Hormone and the Origin of Species") and in 2006, appeared prominently in the PBS NATURE documentary "Dogs That Changed the World." She has many peer-reviewed academic publications (see www.pacificid.com and www.rhythmsoflife.ca ) and recently published a paper with colleague Gay Frederick on the effects of climate cooling on marine mammal distributions in the North Pacific Ocean within the past 5,000 years (Sea ice expansion in the Bering Sea during the Neoglacial: evidence from archaeozoology. 2007. "The Holocene" 17:699-706). She runs a private research firm (Pacific Identifications Inc.) with two colleagues and holds an adjunct faculty position at the University of Victoria.
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