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Causes of Megadiversity in Mexico
The most important causes of megadiversity in Mexico are its topography, its variety of climates and its complex geological, biological and cultural history. All these important factors have contributed to the formation of a mosaic of environmental conditions that enabled evolution of a large variety of habitats and lifeforms.

The complex topography of the country, together with the changes running along the latitudinal continuum create an enormous number of environmental variants. The changes in altitude produce other climatical variations in many other dimensions, such as the intensity of solar radiation, atmospherical humidity, diurnal oscillation of temperature and amount of  oxygen available.

The biogeographical are the most important among the biological factors. Nowadays, the transition zone dividing two of the world's major biogeographical zones runs through Mexico forming a bridge in between the Nearctical (North American) and Neotropical (Central and South American) realms (FIG.1). This transition zone appeared approximately 6 million years ago, when the land masses of North and South America first made contact.
Due to this past Mexico represents a compound biogeographical area, where the contact in between two rich ancestral zones have given origin to a rich mixture of fauna and flora with different biogeographical histories.

Apart of all the, so far, mentioned factors, other important historical element is the one related with the severe climatological changes during the Pleistocene, when the glaciers extended to such an extent that the area of what would eventually be known as Mexico was under the influence of cold and temperate climates.
This enabled immigration of  many cold climate species and at the same time caused the extinction of  many tropical species in a large part of their original areas of distribution forcing them to restricted zones where the conditions remained favorable. This isolation, in effect, resulted in origination of new species, which, in many cases extended the areas of distribution after the glaciers receded.
This process is considered to have produced a considerable increase in number of species which are relatively new and of endemic nature.
Biogeographical zones
FIG.1

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