This region is divided into the Palearctic and the Nearctic. The palearctic is Northern Africa and all of Eurasia, with the exception of Southeast Asia and the Indian subcontinent. The Nearctic is North America north of southern Mexico.
These are further subdivided into a variety of ecoregions. Many ecosystems, and the animal and plant communities that depend on them, are found across multiple continents in large portions of this ecozone.
The continuity of these ecosystems results from the shared glacial history of this ecozone. Also, during glacial periods, mammal fauna was able to cross from Asia to North America via the Bering Strait.