The Nearctic is one of Earth’s eight ecozones. The Nearctic covers most of North America, Greenland, and the highlands of Mexico. Southern Mexico, southern Florida, Central America, and the Caribbean are part of the Neotropic ecozone. The World Wildlife Fund divides the Nearctic into four bioregions. These four regions are the Canadian Shield, Eastern North America, Western North America, and Northern Mexico.
The Canadian Shield bioregion extends across northern North America from the Aleutian Islands to Newfoundland. The Arctic Tundra and Boreal forest ecoregion are part of the Canadian Shield.
The bioregion of Eastern North America includes the temperate broadleaf and mixed forests of the eastern US and southeastern Canada, the Great Plains temperate grasslands of the central US and south-central Canada, and the temperate coniferous forests of the southeastern United States.
Western North America includes temperate coniferous forests of the coastal and mountainous regions of southern Alaska, western Canada, and the western US from the Pacific Ocean to the Rocky Mountains. The colder intermountain deserts and shrublands and temperate grasslands of the western US are also included in this bioregion.
The bioregion of Northern Mexico includes the mild-to-cold winter deserts and shrublands of northern Mexico and southwestern US. The Chihuahuan, Sonoran, and Mojave deserts are part of this region. The California chaparral and woodlands, temperate and subtropical pine-oak forests, Arizona Mountain forests, Sierra Madre Occidental, Sierra Madre Oriental, and Sierra Juarez and San Pedro Martir pine-oak forests are also part of the Northern Mexico bioregion.
Although North and South America are now joined together by the Isthmus of Panama (a narrow strip of land that was formed around 3 million years ago), these two continents were separated for nearly 180 millions years, and each land mass had very different animal and plant families. When the ancient supercontinent Pangaea split in two about 185 million years ago, North America remained joined to Eurasia as part of the supercontinent Laurasia. South America was part of Gondwana. Since North America split off from Eurasia, it has been joined to South America and Asia by land bridges. These land bridges allowed for the exchange of plant and animal families between the continents. This was known as the Great American Interchange.
A land bridge once existed across the Bering Strait linking North America to Asia which allowed many plants and animals to move between these two continents as well. The Nearctic and Palearctic share many common species. These two ecozones are sometimes grouped together as one known as the Holarctic ecozone. Many large animals (camels, mammoths, mastodons, sloths, and saber-tooth cats and short-faced bears) became extinct in North America at the end of the Last Glacial Maximum. Around the same time, humans appeared, which is believed to be one of the major causes for the extinction of many of the animals that no longer exist here. Climate change is the other factor. Along with humans, arrived the bison, brown bear, and elk. These animals adapted well to the new land and spread out across the region like wildfire.
Many endemic animal species are found in the Nearctic, including Canidae (dogs, wolves, foxes, coyotes), Camelidae (camels), Equidae (horses), and others. There are four endemic bird families of the Nearctic: Divers (Gaviidae), Grouse (Tetraoninae), Auks (Alcidae), and Waxwings (Bombycillidae). Two scarab beetle families are also endemic to the Nearctic, as well as the fly species Cynomya cadaverina.