Ascending from the valley floor in Tucson to the top of Mt. Lemmon is equivalent to traveling from Mexico to Canada — in terms of the biological diversity and the number of ecological zones one passes through.
These high elevation mountain zones are called Sky Islands:
Weldon Heald coined the term “sky islands” in 1967 to denote mountain ranges that are isolated from each other by intervening valleys of grassland or desert. The valleys of this basin and range country act as barriers to the movement of certain woodland and forest species, somewhat like saltwater seas isolate plants and animals on oceanic islands – hence the common association with the archipelago phenomenon. Other species, such as mountain lions and black bears, depend on movement corridors between mountain islands to maintain genetic diversity and population size. (from The Sky Island Alliance website)