Does Climate Change Pose a Significant Threat to Polar Bears?

Jules Galligan – Geology
Olivia Smith – Pre-Veterinary Science
Kathy Tran – Animal Science

A polar bear carries away it’s meal for the day–a young cub.
Image retrieved from: http://www.trvl.com/cache/img/c-1024-768/wp-content/uploads/2012/09/047-130136481-11.jpg

One cold, icy December afternoon, researchers scanned the Arctic ice below, hoping to track a polar bear they tagged a few months earlier expected to recently give birth to cubs. In the distance, a splash of red caught their eye–a polar bear presumably feasting on a previously killed seal. As they got closer they could not believe their eyes. To their dismay, the mother bear they had been tracking was ripping apart a clump of white fur–her own cub. The bear looked up as they approached, the cub’s head dangled from the mother’s mouth. This polar bear cannibalism was not their first encounter. In fact, studies show that more polar bears are practicing cannibalism more frequently for the purpose of securing reliable sources of nutrition (Amstrup, Stirling, Smith, Perham & Thiemann, 2006).

Arctic Sea Ice

When scientists try to answer the question, “Why have polar bears resorted to consuming other polar bears?” they tend to study not only the polar bear populations themselves, but all aspects of their environment, including their habitat. Normally polar bears hunt seals, their primary prey, almost exclusively from sea ice (Rode et al., 2011). When hunting seals, sea ice is a necessary platform from which polar bears watch for seals coming up to breathe. When the seal is near or at the surface, the polar bear will then grab it with either it’s claws or jaws. Additionally, polar bears may hunt seals who are resting on sea ice (Derocher, 2012). Arctic sea ice became a primary focus of much of the research because when reviewing all of the major factors that impact polar bear health, it all led back to one problem: Arctic sea ice.

The Arctic sea ice is melting, and fast. In 2016 NASA reported an approximate loss of 13.4% per decade. Melting sea ice is primarily due to increased global temperatures over the past century, seen in the doubling of the rate of temperature increase over the last 50 years (Riebeek, 2010). To get an idea of the magnitude of the melting, consider September, which is the month when new ice growth begins each year (Steele, Dickinson, Zhang, & Lindsay, 2015). In 2015, average sea ice for September was 1.87 million square kilometers (720,000 square miles) below the 1981 to 2010 average extent (Steele et al., 2015). The 10 warmest years in the 134-year record all have occurred since 2000, with the exception of 1998. The year 2015 ranks as the warmest on record (NASA, 2015). To further predict future warming at it’s current rate, NASA and NOAA designed models to try and simulate the responses and interactions due to climate warming. Based on a range of plausible emission scenarios, average surface temperatures could rise between 2°C and 6°C by the end of the 21st century (Riebeek, 2010). Although one could argue that warming is natural as the Earth does experience fluctuations in temperature, in the past century alone, the temperature has climbed 0.7°C, which is roughly ten times faster than the average rate of warming expected (Riebeek, 2010).

As sea ice continues to melt thanks to the continued warming, we see a positive feedback loop that increases the rate at which melting occurs. Because sea ice is white, it reflects heat well. When melted, it exposes the dark bodies of water underneath, which readily absorb heat from the sun, in turn encouraging more melting. Several studies based on knowledge of bear ecology, population dynamics, and model projections suggest that populations experiencing fluctuating, unpredictable sea ice will experience nutritional and eventually demographic effects in the near future (Rode et al., 2011). Scientists have already seen this in polar bears who have been forced to reside on land longer than usual in their non-hunting season because of the increasing ice melt (Ferguson, Taylor, Born, & Messier, 1998). In order to maintain polar bear populations, we should address the reduced access to prey, body condition, and reproductive success by addressing habitat protection.

 Accessing Appropriate Nutrition

Food accessibility is the foundation for the health of any population. Without access to a sustainable source of energy, any organism can expect to suffer. In polar bears, researchers have observed adjusted feeding habits (Amstrup et al., 2006), primarily because hunting from sea ice has become less available (Rode et al., 2011). As mentioned previously, cannibalism has become more commonly observed, albeit still rare in the broad scope of observed hunting methods (Amstrup et al., 2006). Cannibalism is more commonly practiced by males, who specifically travel to dens to search for females who are about half their size and easily accessed when restricted inside dens (Amstrup et al. 2006).

When it comes to hunting prey, seal populations are healthy, stable, and readily available for polar bears to hunt–the accessibility is the problem. In fact, conditions and reproduction of ringed seals and barbed seals, which comprise almost 80% of polar bear diets, have either increased or remained stable since the 1970s (Rode et al., 2014). This helps demonstrate that seals are plentiful and not the problem, emphasizing the problem is sea ice.

The actual act of hunting is most prominent in the late spring, when fat, recently weaned and naive ringed seal pups are abundant. Polar bears reach their lightest weights of the year in late March, just before the birth of this cohort of seal pups. This suggests that it is the success of their hunting in spring and early summer that enables them to maximize the body reserves necessary for survival, reproduction, and nursing of cubs through the rest of the year (Stirling, Lunn, & Iacozza, 1999). Normally, summer months bring slight warming to the Arctic, causing the phenomenon known as “breakup”, which refers to the loss of all or almost all sea ice. During this time, all bears must depend on their stored fat resources while fasting on land until the freeze-up again in the fall (Rode 2011). Bears depend on the ice to refreeze each winter, allowing them access to the ice and therefore their hunting platforms. Studies show that the melting of sea ice is 2.5 weeks sooner than it was 30 years ago and polar bears are losing hunting time (Derocher, Lunn, & Stirling, 2004). Limited time to hunt means storing enough fat for the summer months becomes less and less achievable. It would be very much like going to your pantry everyday, only to find that one day you return and your shelving has raised itself higher and higher. Sooner or later, that shelving is completely out of reach, and you’ll find yourself with nothing but crumbs and the old bag of peanuts at the bottom.

Alternatives to Traditional Hunting

Without access to seal populations via ice, polar bears are left with two options–hunt in deeper waters or on land. Two studies in 2011 and 2014 on polar bear hunting had the same conclusion: polar bear populations forced into hunting in deeper waters exhibit continued nutritional stress and declines in population (Rode et al., 2011; Rode et al., 2014). Research shows that deep water locations have low oceanic prey density, making them unsustainable hunting locations for meeting the nutritional requirements of polar bears (Amstrup et al., 2006).

When it comes to hunting on land, studies have shown that terrestrial foods, such as muskox and caribou, do not meet the high fat energy requirement of polar bears that seals do. Few terrestrial foods worldwide are as energetically dense as marine prey (Rode, Robbins, Nelson, & Amstrup, 2015). The land that polar bears are being forced to retreat to as more and more ice melts is also home to brown bears that are already known to prey on the terrestrial foods around. Polar bears moving in would be competition for the available food supply (Rode et al., 2015). If it was a small population of polar bears turning to these terrestrial foods, the local prey populations might be a sufficient enough amount without creating a problem for their numbers as well. Unfortunately the community of polar bears is vast and there’s no way that the addition of these numbers of predators wouldn’t decimate the terrestrial prey that would be the plausible food source (Rode et al., 2015).

Body Condition as an Indicator of Health

In polar bears, scientists measure body condition based on a few visual factors, then each bear is assigned a rating on a scale. The scale varies from skinny to very fat; any bear who is referred to as having poor body condition is considered skinny or thin on this Body Condition Index. This rating can be improved by palpation of the amount of fat polar bears have, after sedation of course (Polar Bears International, 2016). Overall, a ubiquitous use of the same body condition scale helps researchers establish uniformity when judging body condition of polar bear populations. Reductions in body fat, an indicator of reduced body condition, is often measured through skull width measurements, which can be an indicator of fat thickness (Rode, Amstrup, & Regehr, 2010). Spring skull width and body mass of yearlings were greater following years with shorter ice free periods (Rode et al., 2010). Mean skull width and body mass of male populations in this study performed by Rode et al. in 2010 declined between 1982 and 2006, indicating a general reduction in body condition of males over the age of 5 (Rode et al., 2010).

In one study on body condition, scientists argue that polar bears’ reduction in body condition and size are mechanisms to cope with climate change (Rode et al., 2014). Seeing as energy sources have become increasingly sparse due to inability to hunt, the reduction in body size may be an adaptive response to reduce the general energy requirements of the animal (Rode, 2014). Research shows that reduced body condition in any species appears to be one of the most proximate mechanisms by which climate change affects species and may be a useful indicator of organismal responses to climate change (Rode et al., 2014). Reducing necessary energy input may seem like an excellent adaptive trait, however it has not been improving polar bear conditions.

This involuntary fasting or reduced intake is also reflected in seal populations, where scientists are seeing 50% less seal biomass killed in 2005-2006 compared to 1985-1986, which correlates with a significant increase in the frequency of polar bears in a fasting state (Pilford, Derocher, Stirling, & Richardson, 2015). These bears aren’t dieting by choice, and have proven desperate for food at times. In one study, polar bears preyed on young pups, which requires digging through thick rafted ice sheets to access birth lairs, an energy inefficient hunting strategy (Pilford et al., 2015). Desperation for food indicates that reduced body condition is not something that the bears have adapted to willing.

Reproductive Success & Survivability

Ultimately, what we have discussed so far does not bode well for successful reproduction in any organism. Polar bear populations exhibiting signs of nutritional stress also displayed signs of reproductive stress, which affects the survivability of the species (Rode et al., 2014). Survivability in general is crucial to a healthy population, and growing cubs do not thrive if malnourished. Studies show that reproductive output and mass of polar bears are directly influenced by fluctuations in marine productivity and availability and accessibility of seals (Derocher & Stirling, 1995). The general of polar bears has population has dropped 22% since 1987 (Stirling, Lunn, & Iacozza, 1999, p. 301) and research shows that cub mortality increased from 25% in 1980-1984 to 50.9% in 1987-1992 (Derocher & Stirling, 1995, p. 1657). Not only are more cubs dying, but less are being produced. Litter size decreased from 0.48 to 0.34 litters per female between 1987 and 1992, reflecting poorer health in reproducing females. Loss of whole litter increased from 10% to 34.1% in that same time period (Derocher & Stirling, 1995, p. 1659). Bears that do survive through their cub age are finding themselves unable to live by themselves. Derocher and Stirling’s (1995) study also indicated that proportion of yearlings independent of their mother declined from 81% in 1966-1979 to 34% in 1980-1992 (Derocher & Stirling, 1995, p. 1660). Yearlings unable to thrive on their own are unlikely to be able to properly provide for themselves in the future.

Refuge vs. Wilderness

Melting sea ice is not something that can be easily fixed like a broken window or flat tire. Understanding that with current warming trends, sea ice melting will continue to occur, our most critical actions must be focused on preventing further damage to polar bear habitats. In the Arctic, accessory damages to habitats usually include elements such as logging, mining, oil and gas drilling, building roads, off-road-vehicle use, and development. Currently, two wildlife refuges in Alaska–the Arctic Refuge and Alaska Maritime Refuge–are hotspots for polar bear denning. Refuges are lenient when it comes to human interactions, and many refuges allow raw material collection, such as oil drilling (ANWR Assessment Team, 1999). Our proposal is to convert these two denning hotspots into wilderness preservations that run under The Wilderness Society ruling, that maintains the rules under which a wilderness preservation must remain. The big difference between a refuge and a wilderness is that a wilderness must remain free of roads and structures (Brown, 2008). Conversion to a wilderness would ultimately protect these areas from human interactions, giving the polar bears an extra step of protection when it comes to their denning habitats.

We can see an example of this success with the story of the California Condor. In 2013, President Obama converted Pinnacles, a previously volcanic-spired national monument, into a national wilderness called the Hain Wilderness (The Wilderness Society, 2013). These birds, which were almost extinct 25 years ago, can now boast being about 400 strong. This is a prime example of the conversion to a national wilderness helping a species re-establish itself in a location where its habitat was previously threatened. Although turning the land into a wilderness preservation won’t stop the melting of sea ice, it will help act as a buffer to give the polar bears an opportunity to do their best with these changes until we can find a more permanent solution to the problem.

Impacts of Drilling

One could argue that the loss of drilling in Alaska may negatively impact the communities that depend on the industry for economic support. Although keeping the location open to drilling may stabilize the oil industry, the fish industry could potentially suffer a significant loss. As established before, the marine ecosystem is a complex system requiring balance. Fish populations have been of particular concern in the last decade, with cod in the spotlight, as they are preyed on by both larger species of fish as well as seals (Hansen & Harding, 2006). Seal populations have been on the rise in the past few decades, especially the harbor seals in the North Sea, which has made a significant impact on the fish populations and fisheries stability (Hansen, 2006). Although this bodes well for seal populations, fishermen have expressed frustration with seals who interfere with their fishing and fishing equipment (Harwood, 1987). As seal populations continue to increase, fishermen will have continued competition for the already limited fish populations. These potentially increasing seal populations are seen by many fishermen as a real threat to their livelihoods, especially at a time when competition amongst fishermen is also increasing (Harwood, 1987). Although fishermen have tested various methods of seal population control, a natural predator exists that historically has helped stabilize seal populations–polar bears. Without them, fishermen can expect to experience continued frustration with seals, if not more. Without enough fish supply, the fishing industries in these seal-dense locations could potentially crash. Although sustaining the fish populations through these means may mean a hit to the oil industry, the potential impacts that crashed fisheries have on the overall ecosystem may prove to be far more troublesome than what we can anticipate.

In order to maintain polar bear populations, we should address the reduced access to prey, body condition, and reproductive success by addressing habitat as the pinnacle of the problem. Reduced access to prey because of sea ice losses is the first domino in a long line of them. Problems resulting from it are reduced body condition and reduced reproductive success, and with each polar bear experiencing poor health, the hope that this population will survive through the century becomes more questionable. The problems we can attack now are fixes that rely completely on human choice, with a focus on switching prime polar bear habitats from refuges to wilderness to protect them from the potential impacts of industry raw material collection, particularly that of oil drilling.

References

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