A desert ecosystem is made up of the non-living elements affected by, and the living organisms adapted to, a climate where less than 10 inches of rain fall a year. These are harsh ecosystems with generally poor soil. The most important adaptations of organisms involve the scarcity of water.
The relatively few plants and animals that survive in the desert also must be able to deal with extreme temperature fluctuations between day and night, since heat is retained very poorly by the dry, cloudless air. Many of the species that live in deserts require rain for a burst of feeding and reproduction, and then go back into dormancy for the dry periods. Many plants have seeds that can survive to sprout only when rain occurs, and many animals live in burrows and emerge only at night when it is cooler.
Deserts generally occur either because of global wind patterns or rain shadows. Dry winds blowing from the poles to the equator not only hold little moisture for these areas, but also tend to evaporate any water present. Rain shadows occur, not because of general wind patterns, but because the wind happens to pass over a mountain range. As air is pushed up into colder altitudes, it tends to shed all the precipitation it carries, leaving little or nothing to fall on the other side of the range.