This is a list of the native wild mammal species recorded in Mexico. As of September 2014, there were 536 mammalian species or subspecies listed. Based on IUCN data, Mexico has 23% more noncetacean mammal species than the U.S. and Canada combined in an area only 10% as large, or a species density over 12 times that of its northern neighbors. Mexico's high mammal biodiversity is in part a reflection of the wide array of biomes present over its latitudinal, climatic and altitudinal ranges, from lowland tropical rainforest to temperate desert to montane forest to alpine tundra. The general increase in terrestrial biodiversity moving towards the equator is another important factor in the comparison. Mexico includes much of the Mesoamerican and Madrean pine-oak woodlands biodiversity hotspots. From a biogeographic standpoint, most of Mexico is linked to the rest of North America as part of the Nearctic realm. However, the lowlands of southern Mexico are linked with Central America and South America as part of the Neotropical realm. Extensive mixing of Nearctic and Neotropical mammal species commenced only three million years ago, when the formation of the Isthmus of Panama ended South America's long period of isolation and precipitated the Great American Interchange. Twenty of Mexico's extant nonflying species (opossums, armadillos, anteaters, monkeys and caviomorph rodents) are of South American origin. Most of the megafauna that formerly inhabited the region became extinct at the end of the Pleistocene about 10,000 years ago, shortly after the arrival of the first humans. Increasing alteration and destruction of natural habitats by expanding human populations during the last several centuries is causing further attrition of the region's biodiversity, as exemplified by the "hotspot" designations (by definition, such areas have lost over 70% of their primary vegetation).