When the ice sheets started to retreat, some 20 000 years ago, most of the temperate flora and fauna slowly migrated back into higher latitudes and the Desert Biome gradually expanded across the mid-latitudes to its current extent. A subset of the temperate biota, however, stayed behind in the rugged and cool mountain ranges that emerge from the desert plains. Establishing higher-up with each passing generation as the climate warmed, the ice-age organisms were able to persist in the cool mountain environments, where conditions are similar to the ones they had enjoyed in the lower plains during the Ice Ages. As they ascended into the isolated desert mountains, the communities of the desert "skyislands" became separated from other mountains by harsh desert plains. Like prehistoric castaways, the Ice Age species now survive high-up in the cool refuges of the desert mountains; a biological memory of past evolutionary history subsisting high-up in the mountains like ghosts of climates past. And, because they have been reproducing in isolation for 15 000- 20 000 years, many of their populations have developed unique genetic traits and have evolved into new species. Thus, in a similar fashion to evolution in remote oceanic islands, the biota of the desert sky-islands is composed by a large number of endemic species and has immense value for biological conservation (Axelrod 1950).

As the effect of the Ice Ages was more severe in the northern hemisphere, which is mostly covered by continental land masses, than in the more oceanic southern half of the globe (Figure 1.9), most of these Pleistocene montane relicts are found north of the equator. In North America, where the desert relief is highly folded, mountainous sky-islands dapple the central part of the Sonoran and the Chihuahuan Deserts, and of the Great Basin. All these ranges contain endemic pines, oaks, madrones, and chaparral species, remnants of the "Madro-Tertiary" flora, a unique temperate ecosystem that covered much of the now-dry North American deserts during the last six million years.

In Africa, similar relict mountains emerge from the harsh Saharan plains: near the Mediterranean coast, the Atlas Mountains in northern Morocco shelter rich pine and oak forests. Further south, the Ahaggar and Tassili-n-Ajjer ranges of south-eastern Algeria and the Aïr massif in northern Niger harbour a number of endemic and rare Mediterranean species such as the tarout, the wild olive, and the Saharan myrtle. To the east, the Tibesti mountains in southern Libya hold some Mediterranean as well as some tropical relicts. These Saharan mountains also provide prime habitat for migratory birds and a key refuge for threatened wildlife.

In the Somali Peninsula, and across the Gulf of Aden in the southern part of the Arabian Peninsula, high mountain ranges shelter similar temperate relicts: along the northern Somalian coast, in the tip of the Horn of Africa, the Somali Montane Woodlands thrive along the coastal ranges fed by moisture brought in from the sea. A biogeographic refuge and centre of endemism, these mountain habitats harbour many endemic species of both plants and animals. The higher parts of the ranges contain an evergreen scrub quite similar to the Mediterranean maquis, with species such as kadi, mountain box, Ethiopian pistachio, and remnants of ancient juniper forests.

Across the Aden Straits, and overlooking the coast of the Red Sea, the high Asir mountains of Yemen and Saudi Arabia give shelter to an even richer relict montane ecosystem with juniper, bramble, and zaytoon olive. Fed by the coastal fogs of the Red Sea, the region shelters more than 170 endemic plants, and is a critical refuge for a large number of endangered animals such as the Arabian leopard and the Hamadryas baboon. Finally, in the eastern part of the Arabian peninsula, near the coast of the Gulf of Oman, the Al Hajar Mountain range forms a spectacular wall of mountains that rise almost 3 000 m from the surrounding deserts, providing an important refuge for endemic and relict species of plants of Mediterranean and Indo-Iranian origin, often growing in the vicinity of tiyu (a carob relative) and other trees of African ancestry. The endemic Arabian tahr, a type of wild goat, is still common on these precipitous slopes.

These mountains, and many others, play an immensely important role in the maintenance of desert diversity and in the conservation of desert biota. Sharp ecotones exist between arid plains and island-like, desert-surrounded mountains. In these slopes, altitudinal change reflects evolutionary time and in their intense transitions it is possible to explore the evolutionary history of deserts during the Pleistocene (Figure 1.10).