North American animals in U.S. zoos

The Nearctic realm

Biogeographers call most of North America the Nearctic realm. It runs from Greenland and the Arctic through Canada and the United States to the highlands of Mexico. Southern Florida, coastal and southern Mexico, Central America and the Caribbean fall outside it. North and South America were separated for about 180 million years and evolved very different animals, while a former land bridge across the Bering Strait let many species move between North America and Asia. The two northern realms are sometimes combined as the Holarctic.

The dog family, horses and camels all originated in the Nearctic. Today only two mammal families are endemic to it, and the pronghorn is the sole survivor of one of them, Antilocapridae. It resembles an antelope only through parallel evolution, and its closest living relatives are the giraffe and the okapi. North America lost much of its megafauna, including mammoths, mastodons, ground sloths and sabre-tooth cats, at the end of the Pleistocene.

Tundra, forest, prairie and desert

The north is arctic tundra and boreal forest. Temperate broadleaf forests cover the east, the Great Plains grasslands fill the center, and coniferous forest, cold deserts and shrublands spread across the west. The south-west and northern Mexico hold deserts, dry shrublands and pine-oak forest.

Each habitat has its specialists. Prairie animals include the American bison, the heaviest herbivore on the continent, and the black-tailed prairie dog. The black-footed ferret depends on prairie dog colonies, which make up as much as 90 percent of its diet. In the south-western deserts, the Gila monster is the only venomous lizard native to the United States. The eastern hellbender, North America's largest salamander, needs clear, fast, rocky streams, and the American alligator lives in southern wetlands from Texas to North Carolina.

Losses and comebacks

Several North American species have come close to vanishing. Commercial hunting and cattle diseases cut the bison from an estimated 60 million animals to 541 by 1889. The California condor, the continent's largest land bird, fell to 27 wild birds in 1987, poisoned by DDT and lead. Every one was taken into captivity, and releases began in 1991. The black-footed ferret was declared extinct in 1979, rediscovered in Wyoming in 1981, and then bred in captivity for reintroduction across the West, Canada and Mexico.

Not every story is finished. The red wolf is Critically Endangered, with only around 30 thought to remain in the wild, all descended from a reintroduced population in eastern North Carolina. Nearly 80 percent of eastern hellbender populations are declining or gone, mainly because the forest along their streams is being cleared. The alligator shows that recovery is possible: listed as endangered under the Endangered Species Act in 1973, it was removed from that status in 1987.