
In fact no other country on the planet has such variety. Measured in terms of number of species per 10,000 sq km Costa Rica tops the list of countries at 615 species, compared to a wildlife-rich country such as Rwanda that has 596, or to the comparatively impoverished USA with its 104 species. This simple fact alone (not to mention the ease of travel and friendly residents!) makes Costa Rica the premier destination for nature lovers from all over the world.
Along with its diverse geography, the large number of species in Costa Rica is also due to the relatively recent appearance of the country. Roughly three million years ago Costa Rica rose from the ocean and formed a land bridge between North and South America, and as species from these two vast biological provinces started to mingle and mix, the number of species was essentially ‘doubled.’
Read more: http://www.lonelyplanet.com/costa-rica/wildlife/overview#ixzz2XAPGvn8x