Mexico is considered a "megadiverse" country, since it is part of the select group of nations that possesses the greatest diversity of animals and plants, almost 70% of the world's diversity of species (considering the best known groups: amphibians, reptiles, birds and mammals and vascular plants). This concept is different from the concept of biodiversity.
The main criterion for belonging to the group of megadiverse countries is endemism. To be megadiverse, a country must have at least 5,000 endemic plant species. Other criteria included in the concept are: diversity of species, diversity of higher taxonomic levels (genera, families, etc.), and diversity of ecosystems, including the presence of marine and tropical forest ecosystems (Mittermeier et al. 2004).
Some countries are in contact zones between two biogeographical regions where faunas and floras with different histories are mixed. In Mexico, the Nearctic and Neotropical zones converge.