In this video segment excerpted from the “Arctic Wanderers” episode of the PBS series NATURE, guide and host Chris Morgan examines the challenges faced by polar bears as global climate change melts their fragile pack ice habitat. Rising temperatures are reducing this ecosystem by 11% per decade, with consequently dire impact on polar bears’ overall health and population.
- Considering that they are excellent swimmers, why is thin, melting ice such a problem for polar bears?
- Given current rates of pack ice loss, how much longer until the entire polar ice cap has melted?
- Considering how effectively polar bears have evolved to survive in their harsh arctic environment, why is it unlikely that they’ll be able to adapt to their new, warmer world?
The polar bear is both the world’s largest bear and largest land carnivore. Although born on land, polar bears are excellent swimmers, spending so much of their time at sea and on sea ice that their scientific name—“Ursus maritimus”—translates to “maritime bear.” While closely enough related to the brown bears more common in temperate zones that rare interbreeding has been documented, polar bears have evolved unique adaptations to their arctic and largely aquatic environment: a longer nose helps warm cold air; smaller ears reduce radiated heat loss; large, scooped feet distribute weight when walking on ice and provide better propulsion when swimming; and a thick layer of blubber provides insulation, buoyancy, and sustenance when food is scarce.
Polar bears possess an extremely keen sense of smell, and are able to detect their primary food source—ringed and bearded seals—up to a mile distant. They primarily hunt at and around the edge of pack ice—the ice shelf that extends in winter months from land—often sneaking up and catching their prey as they surface in ice holes to breathe. When the pack ice recedes in summer and early fall and polar bears are unable to hunt seals, they live primarily off their fat reserves. Polar bears have been known to supplement their diet with a wide variety of plants and other animals, but their digestive systems are specially adapted to seal meat and blubber, and cannot derive sufficient caloric intake from other sources.
Because of their reliance on seals, polar bears are especially vulnerable to the polar warming associated with global climate change. The relatively thin pack ice shelf from which polar bears hunt seals is climatically fragile; rising temperatures melt this ice earlier and further with each passing spring, forcing polar bears to land before they have amassed sufficient fat reserves to survive the lean summer and early fall. Reductions in the thickness and area of pack ice also force bears to swim more frequently and for longer distances in search of food, further depleting their energy and occasionally leading to drowning.
While some may insist on debating the causes of climate change, its consequences on the worldwide polar bear population are undeniable. For example, polar bears’ feeding season in the western Hudson Bay is three weeks shorter than it was 30 years ago; in the same period, the polar bear population has declined over 22%, with the average weight of pregnant females dropping 20%. This insufficient nourishment also leads to lower reproductive rates among adults and lower survival rates among cubs.
Life Science, Content Standard C
As a result of their activities in grades 9-12, all students should develop understanding of
- Biological evolution
- Species evolve over time. Evolution is the consequence of the interactions of (1) the potential for a species to increase its numbers, (2) the genetic variability of offspring due to mutation and recombination of genes, (3) a finite supply of the resources required for life, and (4) the ensuring selection by the environment of those offspring better able to survive and leave offspring.
- Interdependence of organisms
- Living organisms have the capacity to produce populations of infinite size, but environments and resources are finite. This fundamental tension has profound effects on the interactions between organisms.
- Matter, Energy, and Organization in Living Systems
- The distribution and abundance of organisms and populations in ecosystems are limited by the availability of matter and energy and the ability of the ecosystem to recycle materials.
- Behavior of organisms
- Organisms have behavioral responses to internal changes and to external stimuli. Responses to external stimuli can result from interactions with the organism’s own species and others, as well as environmental changes; these responses either can be innate or learned. The broad patterns of behavior exhibited by animals have evolved to ensure reproductive success. Animals often live in unpredictable environments, and so their behavior must be flexible enough to deal with uncertainty and change. Plants also respond to stimuli.
- Like other aspects of an organism’s biology, behaviors have evolved through natural selection. Behaviors often have an adaptive logic when viewed in terms of evolutionary principles.
Science in Personal and Social Perspectives, Content Standard F
As a result of activities in grades 9-12, all students should develop understanding of
- Environmental quality
- Materials from human societies affect both physical and chemical cycles of the earth.
- Many factors influence environmental quality. Factors that students might investigate include population growth, resource use, population distribution, overconsumption, the capacity of technology to solve problems, poverty, the role of economic, political, and religious views, and different ways humans view the earth.
- Natural and human-induced hazards
- Human activities can enhance potential for hazards. Acquisition of resources, urban growth, and waste disposal can accelerate rates of natural change.
- Some hazards, such as earthquakes, volcanic eruptions, and severe weather, are rapid and spectacular. But there are slow and progressive changes that also result in problems for individuals and societies. For example, change in stream channel position, erosion of bridge foundations, sedimentation in lakes and harbors, coastal erosions, and continuing erosion and wasting of soil and landscapes can all negatively affect society.
- Natural and human-induced hazards present the need for humans to assess potential danger and risk. Many changes in the environment designed by humans bring benefits to society, as well as cause risks. Students should understand the costs and trade-offs of various hazards—ranging from those with minor risk to a few people to major catastrophes with major risk to many people. The scale of events and the accuracy with which scientists and engineers can (and cannot) predict events are important considerations.
- Science and technology in local, national, and global challenges
- Humans have a major effect on other species. For example, the influence of humans on other organisms occurs through land use—which decreases space available to other species—and pollution—which changes the chemical composition of air, soil, and water.