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The Biology of the Deserts
Introduction
Deserts are defined by their arid conditions. A consequence of this aridity is that most of the area occupied by desert is barren and monotonous, leading many people to view it as a wasteland. In contrast, biologists have long seen deserts as laboratories of nature, where natural selection is exposed at its most extreme. Generations of scientists have focused on the numerous unique adaptations of plants and animals for surviving the harsh desert environment.
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while the abiotic environment defines deserts and imposes strong selection pressure on the organisms that live there, the biotic interactions among the organisms in deserts are no less exciting or intricate than those of other environments
Desert
•definition :An arid biome occupying approximately 20% of the land surface of the earth in which water loss due to evaporation and transpiration by plants exceeds precipitation during most of the year. (molles, 2010- ecology concept and applications)
•There are different forms of deserts: Cold and Hot desert
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CLASSIFICATION
In 1961 Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received:-
1- Extremely arid lands have at least 12 consecutive months without rainfall
2- Arid lands have less than 250 mm(10 in) of annual rainfall
3- Semiarid lands have a mean annual precipitation of between250 and 500mm (10-20 in)Arid and Extremely arid lands are deserts, and Semi arid grasslands are generally referred to Steppes
How old are deserts?
One may be tempted to assume that deserts have always beenso. However, fossils found in deserts such as those discovered in the Gobi desert by Roy Chapman Andrews in the 1920s, the Lystrosaurus fossils in the Karoo desert (Kitching 1977; Rubidge 2005), the ammonite fossils in the Arabian desert (Parnes 1962), and the soft- bodied Ediacaran fossils of the Great Basin desert (Hagadorn and Waggoner 2000)
indicate that these were once shallow seas, deltas or even, inthe case of the Arabian desert, areas of the former Tethys Sea when the world was a single continent known as Pangaea (Parnes 1962). Plate tectonics has resulted in major changes in the positions of the continents and, consequently, in the positions of the deserts (Wegener 1966).Many, if not most, deserts are reasonably young, although they do vary considerably in age.
Although the Sahara is the largest desert in the world(about 9 million km2), it was formed only about 7 million years ago (Schuster et a!. 2006). In contrast, the Namib desert is believed to be the world’s oldest desert. It is claimed to have been arid for at least 55 million years and, perhaps, as much as 80 million years a (Ward et a!. 1983).
1.5 Deserts are created by a lack of precipitation and not high temperatures
•What makes a desert is not a particular temperature but rather a lack of precipitation.
•Deserts make up about 40% of the world’s biomes (Ezcurra 2006).
1.7 WHAT DENIES RAINFALL TO DESERTS?
FOUR FACTORS
INFLUENCE THE LACK OF RAINFALL IN DESERTS (PAGE 1984; MILICH 1997);

1. THE MOST CONSTANT OF THESE IS THE GLOBAL CIRCULATION OF THE ATMOSPHERE, WHICH MAINTAINS TWIN BELTS OF DRY, HIGH-PRESSURE AIR OVER THE FRINGES OF THE TROPICS, KNOWN AS HADLEY CELLS (MILICH 1997). (MET OFFICE VIDEOS: WHAT IS THE GLOBAL CIRCULATION? (3 PARTS)(1-1)AIR IS FLUID(VOGEL 1994), AND IS KEPT IN CONTINUOUS MOTION BY SOLAR ENERGY WHEN THE SUN’S RADIATION REACHES THE EARTH, MOST PASSING THROUGH THE ATMOSPHERE, IT IS ABSORBED BY LAND AND WATER AND IS THEN RE-RADIATED AS HEAT. MOST SOLAR RADIATION IS ABSORBED IN THE TROPICS, WHERE THE SUNIS VIRTUALLY DIRECTLY OVERHEAD IN SUMMER AND WINTER. PAGE-7-
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