The American Black Bear

American black bears, Ursus americanus, are the quintessential generalists.  Their ancestors colonized the New World about 3.5 million years ago, following the Bering Land Bridge route that the Tremarctine (short-faced) bears had followed about 12 million years earlier.  These early black bears found themselves in a world already well-populated with large, dangerous predators.  In addition to at least two species of very large short-faced bears, there were cats; big cats.  The first cats arrived in North America about 36 million years ago and had evolved into many species, some of which were larger than any living lions and tigers.   At the time the black bears arrived there were three main groups of cats, the sabre-toothed cats, the scimitar-toothed cats, and the true cats which included mountain lions, cheetahs, and jaguars.  Some of these cats evolved in North America, but others such as the lion arrived from Asia.  Coyotes probably arrived at about the same time as black bears and wolves may have arrived slightly later.  From the beginning, black bears could not compete with these specialists for large prey species and they were not large and strong enough to drive them away from their kills.  However, there was a wide variety of other food sources available, and like their cousins in southern Asia, black bears were able to utilize fruits, insects, honey, plants, carrion, and small prey successfully.  They climbed trees to escape some predators.  They survived, reproduced, adapted, and spread out across North America.

During the last few ice ages, black bears and many other species of the Pleistocene fauna were forced south by the expanding ice sheets.  As less habitat became available, the competition for resources intensified.  In times like these, generalists often have an advantage, and this proved true for the black bear.  The race against extinction goes not always to the swift or the strong.  During the glacial periods, and afterwards as humans and other new species invaded from the Old World, many of the larger mammals disappeared.  The Pleistocene lion, the sabre-toothed cat, the Florida short-faced bear, the giant short-faced bear, and other species winked out of existence in North America along with their prey: mammoths, mastodons, giant sloths, horses, camels and others.  For a brief period the black bear was probably the only bear left in what is now the contiguous United States.  They shared this land with dire wolves, cheetahs and coyotes about a million years ago.   Wolves arrived about 700,00 years ago.  Mountain lions and Pleistocene lions arrived a few hundred thousand years after that.  The black bear’s monopoly of the bear niche was short-lived however, for as the ice sheets receded at the end of the Pleistocene about 10,000 years ago, two new and formidable predators arrived on the scene: grizzly bears, and humans.

Black bears inhabit forested areas throughout their range.  They have adapted to open forests such as chapparal in the Southwest and spruce bogs in the north, to dense rainforests in the Pacific Northwest, to swamps in the Southeast, to the thick expanses of the boreal forest across Canada.  One of the major components of black bear habitat is a heavy understory of shrubs.  The understory provides food and cover.  Although they are very adept at climbing, they seldom forage in trees.  They use trees primarily for security; to climb away from danger and to leave their cubs while foraging. 

The coastal subspecies may have been isolated on separate island refugia during the last Pleistocene glaciation.  The subspecies carlottae is found on the Queen Charlotte Islands (Haida Gwai), while vancouveri is found on Vancouver Island.  The subspecies kermodei, or the spirit bear, is found primarily on Princess Royal Island (see below) and may have survived the ice age on the westernmost part of this island in the Hecate Strait lowlands.  Other possible refugia on the Northwest Pacific coast include the islands of Southeast Alaska (see the brown bear chapter), the Queen Charlotte Strait lowlands, the coastal fringe of mainland British Columbia, and western Washington.  Of the 16 current black bear subspecies, 8 are likely to be found in British Columbia.  This suggests that conditions during the Pleistocene isolated black bears in this region into many ‘islands’ of habitat.  On the coast they were probably islands surrounded by water and ice; in the interior they may have been islands of forest surrounded by open grasslands.  The present day variation in black bear populations represents local adaptations to climate and food sources in these various ‘islands’.

Picture courtesy of Harriet Corbett, Rox Graphics, 866 Rd. 7RP, Powell, WY 82435, 307 645 3202, crowhart@wtp.net