On the cover, a mother polar bear (Ursus maritimus) and her cub explore an open channel in the ice in North Baffin National Park, Canada. The polar bear is listed as a vulnerable species by the IUCN/World Conservation Union. Both biological and social factors threaten the survival of polar bears. These factors include low reproductive rates and exposure to toxic pollutants. Pollutants become increasingly concentrated as they gradually pass up the food chain to reach these predators. In addition, humans often perceive polar bears as competitors or threats. Finally, the circumpolar geographic range of this species crosses national boundaries and includes international waters, making enforcement of protective legislation extremely difficult.