On the basis of the Jornada desertification study, the authors suggest that any process that leads to an increasing heterogeneity of soil resources in space and time is likely to lead to the degradation of semiarid regions, especially grasslands, and to the increasing spread of arid regions dominated by shrublands. Grazing increases soil heterogeneity in semiarid lands, and the conversion of these areas to desert-shrub is aided when cattle disperse the seeds of desert species such as mesquite. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. It is the authors' view that when people and livestock are concentrated into small areas, in which their impact leads to an increased heterogeneity of soil resources, permanent degradation of the production capacity of the land occurs. Satellite images show that distribution of arid land with low productivity has expanded in southern New Mexico. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.
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