Extreme desert is arid wasteland, with practically 

 no vegetation. In the ecological sense, however, des- 

 erts also include arid regions which contain consider- 

 able vegetation in the form of bushes, shrubs, and 

 trees especially adapted to tolerate hot, dry climates. 

 Deserts are unique in possessing a large number of 

 different life-forms among the dominants ( Shreve 

 1942). 



Deserts, like any other biome, occur in belts at 

 similar latitudes north and south of the equator 

 around the world and they cover about one-fifth of its 

 surface. Prominent deserts occur in southwestern 

 United States and northwestern Mexico ; in Sahara, 

 southern Africa, Arabia, central Asia, Australia, and 

 in a narrow strip along the west coast of South 

 America. All these areas are at low elevations. In 

 spite of its arid character, the desert, like other 

 biomes, has a fascination and charm for one who 

 becomes familiar with its inhabitants and their prob- 

 lems (Jaeger 1955, 1957). 



CLIMATE 



26 



Geographic 



Distribution of 



Communities: 



Desert Biome 



Deserts around the world generally occur 

 on the lee side of high mountains and continents, 

 with respect to the prevailing winds. Average annual 

 precipitation in the desert scrub of North America is 

 usually not more than 5 in. (13 cm) and snowfall is 

 slight (Jaeger 1957). Because of the high rate of 

 evaporation and lack of penetration into the soil. 

 Weather Bureau statistics are not indicative of how 

 much moisture of precipitation is actually available 

 to organisms. Long drought periods are typical, and 

 the little rainfall that occurs is often in the form of 

 short, violent storms or cloudbursts. The ground 

 surface is generally baked hard, and most of the rain 

 runs off: flash-floods are not infrequent (Lowder- 

 milk 1953). Precipitation is slightly greater and 

 evaporation less in the Great Basin. Where rainfall 

 is so slight, dew formation assumes great significance. 

 In the deserts of Israel, dew forms 120-240 nights 

 of the year (Duvdevani 1953). 



The yearly evaporation from a pan of water may 

 be 7 to 30 times the actual amount that falls on an 

 area of similar size (Buxton 1923). The high evapo- 

 ration rate correlates with the low relative humidity 

 which at noon averages less than 25-30 per cent. 



There is little cloudiness, and the actually re- 

 ceived percentage of possible annual sunshine aver- 

 ages close to 90. Ultraviolet radiation reaches the 

 ground in high concentrations. Winds are more or 

 less continuous. 



The mean annual temperature in the Great Basin 

 is approximately 10°C (50°F), but it is over 20°C 

 (68°F) in parts of the desert scrub. Daily maximum 

 summer temperature in the desert is 40°C (104°F) 



332 



