True deserts are not the final stage of a process
of desertification; they are unique, highly-adapted
natural ecosystems. The desert biome includes
arid and hyper-arid drylands, providing ecosystem
services and supporting human populations that
may degrade the land resources in much the
same ways as in other ecosystems. The deserts
of the world occur in six biogeographic realms. A
summary of the main traits of the desert ecoregions
within those realms is given in Appendices I and
II, including desert type, area, area converted and
under protection, species richness for various
taxa, human density, and human footprint. A more
detailed description of each ecoregion can be
found at the National Geographic "Wild World"
wildworld/terrestrial.html) and at the World Wildlife
Fund terrestrial ecoregions website (http://www.
see also Olson and others 2001). A short narrative
description is given below, while a short description
on the evolutionary origin of desert biodiversity is
given in Box 4.1.