The effects of climate change are most acute in the Arctic, where rapid warming of the ocean’s surface has caused a drastic reduction in sea ice. This changing habitat has wide-ranging effects on the Arctic’s top predator, the polar bear, which spends a large portion of its time living and hunting on sea ice. The changing conditions in the Arctic also affect the survivability and spread of pathogens (infectious fungi, bacteria, viruses and parasites) as well as the cycling of toxic environmental pollutants that accumulate at high latitudes. As arctic warming pushes polar bears southward off sea ice and expands the range of certain pathogens northward, it’s plausible that the health of polar bears may become affected. A recent study looked into exactly this, by seeing how the changing Arctic is affecting the exposure of polar bears to both infectious pathogens and toxic environmental chemicals.
Polar Bears and the Changing Arctic
The International Union for the Conservation of Nature (IUCN) Red List of Threatened Species lists polar bears (Ursus maritimus) as “vulnerable” primarily due to loss of arctic sea ice caused by climate change. Over the last three decades, temperatures in the Arctic have increased faster than anywhere else in the world at a rate of 1.6°C per decade causing sea ice extent to decline at a rate of 8% over a similar period
Polar bears are the apex predators of the Arctic and sea ice provides a rich hunting ground for their preferred prey of seals and beluga whales. However, the rapid reduction in sea ice has changed the behavior of polar bears in the summer and fall leading to more time spent foraging on land. In the Beaufort Sea region of the Arctic the percent of the polar bear population that comes ashore has increased from 6% to 20% since 2000, and the average duration spent on land annually has increased by over a month in the same time period.
Polar bears that spend time on land have less access to seals and beluga whales, and instead primarily forage on carcasses of bowhead whales left behind by Alaskan natives. The changes in habitat and diet caused by changes in foraging grounds due to the warming Arctic likely alter the levels of contaminants that polar bears are exposed to. In addition, polar bears that spend time on land are likely to come in contact with other terrestrial animals such as foxes, grizzly bears, gulls, and ravens potentially increasing the likelihood of interspecies transmission of disease.
How do you study polar bears?
To explore how the amount of time polar bears spend on land might affect their health, researchers examined the prevalence of five infectious pathogens as well as certain persistent organic pollutants (POPs) in the blood of polar bears between 2007 and 2014. To collect the blood, polar bears were first spotted from helicopters flown over the Arctic sea ice and on land and then tranquilized from a distance with a dart gun. To keep track of each unique polar bear, researcher applied ear tags and lip tattoos.
Ice Bears vs. Land Bears – Pathogen Exposure
Polar bears that were found on land were seven times more like to be exposed to a specific pathogen, Toxoplasma gondii, than bears found on sea ice. T. Gondii is a parasite infamously found in domestic cats that can lead to fatal illness, taxoplasmosis, in humans with weakened immune systems. Overall prevalence in polar bear blood of T. Gondii has increased by 81% over the past decade.
This study also reported the first detection of two other pathogens in an arctic marine mammal (F. tularensis and N. caninum) and the northern most detection of a third (C. burnetii). These pathogens are all thought to be land-based as they typically infect rabbits, deer, and bears, and can all lead to negative health outcomes including fevers, ulcers, and spontaneous abortion. Taken together the evidence suggests there are notable differences in the pathogen exposures of land foraging polar bears compared to sea ice polar bears, and that these differences are rapidly changing in time as the Arctic habitat changes.
Ice Bears vs. Land Bears – Pollutant Exposure
While there were increased exposures to certain pathogens in land foraging polar bears compared to ice bears, there was a simultaneous decrease in exposures to certain toxic chemicals, specifically a class of carcinogenic pesticides named chlordanes. This difference is likely due to differences in diets. Land polar bears feed primarily on the carcasses of bowhead whales, which eat tiny plankton in the ocean, whereas ice polar bears feed on beluga whales, which eat larger organisms such as fish and invertebrates. Certain contaminants like chlordanes tend to accumulate up a food-chain in larger prey through a process known as bioaccumulation. Therefore, the diet of the sea ice bears likely contained higher concentrations of contaminants that bioaccumulate.
This study demonstrates the complicated tradeoffs of climate change impacts on polar bears in the Arctic. As polar bears are forced off of sea ice and onto land, they are increasing their exposures to a variety of pathogens that they have likely never encountered before. On the other hand, changes in their diet may actually reduce their contaminant burden.
Why This Matters
Polar bears are the apex predators of the Arctic and therefore can act as a sentinel for the overall health of the ecosystem. As climate change causes drastic changes to the Arctic habitat, monitoring the health of polar bears can give us insight into the resiliency of the overall system and inform us on how other species in the food web might respond. The Arctic polar bears are one of the few large land mammals left and understanding the threats to their survivability now may help us mitigate future risks.
Source: Todd C. Atwood, Colleen Duncan, Kelly A. Patyk, Pauline Nol, Jack Rhyan, Matthew McCollum, Melissa A. McKinney, Andrew M. Ramey, Camila K. Cerqueira-Cézar, Oliver C. H. Kwok, Jitender P. Dubey & Steven Hennager. 2017. Environmental and behavioral changes may influence the exposure of an Actic apex predator to pathogens and contaminants. Scientific Reports 7, Article 13193. doi:10.1038/s41598-017-13496-9