Overgrazing and desertification in northern Mexico: highlights on north eastern region.

Land degradation has recently been exacerbated in rangelands of Mexico by heavy grazing pressures. In the country, the grazing stock (cattle, goats, sheep and hogs) increased from 22 million in the early 1950's to approximately 50 million in the mid 1980's. Recorded local grazing pressures are surpassing from two to six times the recommended stocking rates of most rangelands of northern Mexico. Signals of deterioration of natural resources vary in time and space mainly because of ecosystem diversity, recurrent drought spells of different magnitudes, and economic and political issues. Estimates of carrying capacities and overgrazing rates are out of date, and very few quantitative parameters of desertification processes are available. However, there are qualitative and perceptual evidences of changes in soil and vegetation patterns, as well as socioeconomic issues, such as land tenure and forms of organization, that deserve to be discussed. Management practices, research, and social issues are addressed in this review from the desertification perspective, with the objective of highlighting the main causes of overgrazing and desertification. Needs for research and future tasks to achieve the sustainable management of rangelands of northern Mexico, with especial emphasis on those of the northeastern region, are also pointed out. Keywords; Desertification, livestock, overgrazing, rangelands, land tenure, ejido, Tamaulipan thornscrub. ( Mexico is truly a rangeland country. Around 55% of its total area is covered by forage vegetation, and currently 100% of the land with available forage is used for livestock grazing (INEGI 1998a,b; PEF, 1995). Spaniards first introduced livestock into the country early in the 16th century. Following mining, numerous herds of cattle were moved from southern and central areas of the country into the northern lands where abundant forage was only used by wild herbivores (Sanchez, 1969; Sluyter, 1998). Since then, northern Mexican lands had been used essentially for livestock production. Land tenure changed in Mexico after the 1910's revolution. The Agrarian Reform, funded by the Constitution of 1917, provided large tracts of land to a community and formed the ejidos, -a communal land tenure system with legal, but partial rights on the land ownership. The ejidatario (landholder) held the land, but was not allowed to sell, rent, or mortgage it. Currently, ca 60% of the country land belongs to the ejido system of land tenure. Although some areas are used individually for cropland or intensive grazing, range is usually managed collectively. Pastoralism shifted from