Polar bears (Ursus maritimus) are in the news again because of dire predictions for the coming decades on their population numbers. The effects of climate change have been predicted to impact the polar regions first and most dramatically according to most models, and indeed, it is at the poles where we are recording some of the most dramatic examples of climate change. Polar bears, being the largest of the living bear species, are charismatic and popular, and because of the likely impact that climate change will have on them they have become a poster species for the climate issue.
In a recent interview bear expert Andrew Derocher predicted that one population of polar bears (western Hudson Bay) could see its numbers drop too low to be viable within three decades (Yale Environment 360 2010). We have explored polar bears and their populations in other posts. Here I want to examine why changes in sea ice and warmer periods are such a concern for polar bears.
Polar bears evolved relatively recently, diverging from an ancestral brown bear population about 150,000 years ago (Lindqvist et al. 2010). There is a unique population of brown bears that live on Admiralty, Baranof, and Chichagof (ABC) islands of southeastern Alaska’s Alexander Archipelago. This population, called the ABC bears, is the closest brown bear relatives of the polar bears—early members of this population split off to live full time on the sea ice, evolving into the modern polar bear species. Thus, polar bears are an example of rapid mammalian evolution, undergoing morphologic changes such as elongated snouts, overall size changes, furry padded feet, and color changes, as well as social and metabolic changes to adapt to the rhythms of the arctic seasons.
It is their complex adaptations to living on the rugged ice that makes them most susceptible to changes in that habitat. They use the ice as a platform for hunting seals, as a habitat for finding mates and mating, and for traveling long distances. As the ice breaks up earlier in the spring, and re-freezes later in the winter, several weeks of prime hunting time are taken away from the polar bears. Today, they are able to spend almost three weeks less on the ice hunting than they were able to several decades ago. This is critical because after the ice breaks up for the year, the bears must fast until the next season, and longer times of open water means long fasting periods.
This can be critical for a female bear that must gestate her young, birth them, and begin to nurse them to a size large enough that they can accompany her onto the ice for hunting the next season. So, she is expending a great deal of energy in contributing to the growth of her young while fasting. If she did not build enough fat reserves the year before to withstand this metabolic marathon, she and her offspring will not survive into the following year. A few additional weeks of having to fast can be the difference between life and death.
The intimate connection that polar bears have evolved with their arctic habitat means that they are finely tuned to changes in that world. And with the effects of climate change appearing in the arctic regions first, they are in fact akin to the “canary in the coal mine,” a harbinger of things to come.
Lindqvist, C., S. C. Schuster, Y. Sun, S. L. Talbot, J. Qi, A. Ratan, L. P. Tomsho, L. Kasson, E. Zeyl, J. Aars, W. Miller, Ó. Ingólfsson, L. Bachmann, and Ø. Wiig. 2010. Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proceeding of the National Academy of Sciences.
Yale Environment 360. 2010. For Hudson Bay polar bears, the end is already in sight. http://e360.yale.edu/content/feature.msp?id=2293.
Additional related posts:
Polar Bear Populations
Public Opinion and a Geologic Perspective on the Effects of Global Warming