Located on the west coasts of tropical and subtropical climes, these have average temperatures over 20 °C and annual rainfall under 250 mm. Deserts are too dry for most plant species except for xerophytes. Desert insects, reptiles, mammals, and birds are all adapted to drought. See M. Evenari et al., eds (1985). See also Nash (2000) PPG 24, 3 on geomorphological processes in arid areas.