The Madrean Archipelago Ecoregion is located in southeastern Arizona and southwestern New Mexico. It is characterized by isolated forested mountain ranges that are surrounded by a virtual sea of intervening deserts and grasslands, hence the term you’ve heard more frequently the "Sky Islands.” This amazing ecoregion is located within the madrean pine-oak woodlands, a globally significant biodiversity hot spot, harboring the highest diversity of mammals, birds, bees, and ants of anywhere in the continuous United States. (This information was gleaned from the Bureau of Land Management Madrean Archipelago Rapid Ecoregional Assessment, September 2012).
Please read this very comprehensive and brilliantly written article by Matt Skroch, Sky Islands of North America, a Globally Unique and Threatened Inland Archipelago.
Well done Matt!