الفهرس 
acknowledgmentOnly 14 pages are availabe for public view
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Abstract
More than one third of the surface of the earth is occupied by arid lands.
These arid lands and their plant and animal life provide food, feed and many other
useful products for more than 785 million people or 17.7% of the world’s
populations that is expanding at a faster rate than the environment can effectively
support (Bell, 1985; McNeely, 2003). Arid ecosystems are inherently of low
stability and resilience. They have limited potentials for material cycling and
energy flow, and limited abilities to withstand changes. If land use practices
exceed these capacities, the ecosystem functions will be impaired, and damaged
leading to severe degradation (Ayyad and Le Floc’h, 1983). Arid land degradation
is the direct result of a long history of mis-use of its natural resources. This
involves, overgrazing, mechanical removal and uprooting of woody plants
together with ploughing out of herbaceous species (Ayyad, 1995). Added to that,
the expansion of cultivation into marginal areas, the fragmentation and destruction
of natural habitats, which leads to reduction in land productivity and erosion of
genetic resources. Habitat degradation and fragmentation may restrict gene flow
and result in genetic differentiation among populations. In turn, this may affect the
life history traits and the probability of species extinction (Bawa et al., 1991;
Geburek, 1997).