Livestock Grazing

Gary Paul Nabhan and Andrew R. Holdsworth

Views of the entire earth from space highlight the overall climatological and ecological irrelevance of political boundaries. The usual message is "everything is connected." However, recent satellite images of the Arizona-Sonora border graphically show a different story. Especially in the grasslands along the eastern edge of the Sonoran Desert, the U.S. side of the border is darker from greater vegetation densities while the Mexican side is brighter due to lower vegetation densities. This large-scale ecological experiment shows that such natural factors as climate change cannot be causal but that differing land-tenure and land-use systems are (Bahre and Bradbury 1978 in Bahre 1991). More specifically, different livestock and fuelwood cutting intensities have been implicated (C. Huthchinson pers. comm.) We discuss the former in this section.

Livestock grazing in the Sonoran Desert has fluctuated greatly in the last several centuries from being relatively confined and intensive to being extensive and intensive. In the 19th century repeated Apache raids on ranchers and the paucity of water limited cattle production to relatively small areas (Bahre 1991). However, since the late 19th century's largest stocking rates in history, extensive cattle production has played a major role in the transformation of grasslands to scrublands, down-cutting of arroyos, the spread of exotic plants, and degradation of riparian areas. Stocking rates are now much lower than in the 1890s because such regulations as the Taylor Grazing Act of 1934 helped improve range land quality. However, overstocking still continues on public and private lands in Arizona (Bahre 1991), and Mexico's CODECOCA statistics confirm that 2 to 5 times the recommended stocking rates occur with regularity on the Sonoran side of the border (R. Aguirre, pers. comm.) Sonora's higher stocking rate is likely due to its greater amounts of private and ejidal (communal) land, less regulation, and the greater dependence on ranching and farming in Mexico (C. Hutchinson pers. comm.).

The comments above are particularly relevant to the semidesert grassland along the eastern edge of the Sonoran Desert. Robert Humphrey rephotographed the "two grazing treatment" transect provided by the 1890 boundary markers between the Rio Grande and the Colorado River. Ninety years later, after the initial photos were taken, he found significant vegetation degradation in the semidesert grassland sections in eastern Sonora. However, there were no significant changes in the species composition or lifeform dominance in the desertscrub along the U.S.-Mexico border west of Tucson (Humphrey 1987). Nevertheless, desertscrub vegetation structure can be dramatically altered. Blydenstein et al. (1957) found that in Sonoran desertscrub perennial grasses and the palatable shrub, range ratany, declined precipitously under grazing. Fleischner (1994) has provided a thorough analysis of the ecological costs of livestock grazing in many regions, including Sonoran desertscrub, semidesert grassland and riparian areas.

Figure 8 charts the dramatic fluctuations in Sonora cattle numbers and the relatively modest increase in livestock in Arizona and Baja California Norte. Búrquez and Martínez-Yrízar (1997) consider the cumulative impacts of cattle grazing in Sonora to be irreversible. Livestock grazing was ranked among the five most serious stressors by the surveyed scientists. However, 20% of the scientists surveyed thought that livestock grazing practices have diversified and in some places improved. Between 1982 and 1992, cattle numbers in Pinal, Santa Cruz and Maricopa counties dropped by 11 %, 18% and 45%, respectively (U.S. Dept. of Commerce 1996). However, some scientists reported that over-grazing has become more severe in the Lower Colorado River Valley, Central Gulf Coast, Foothills of Sonora Thornscrub/Coastal Thornscrub, Semidesert Grassland, and Riparian Deciduous Forests. It is most severe west of Hermosillo where cattle densities are as high as 22 head/kml in areas with low productivity. Overall, cattle densities are highest (>5 head/km2) towards the more mesic eastern and southern portions of the Sonoran Desert (Figure 9).

Many scientists consider that the most drastic cattle-related land transformation in Sonora has been the introduction of the extremely invasive African buffelgrass (Pennisetum ciliare). This phenomenon is described below in the section on exotic species.

From:
Nabhan, Gary Paul and Andrew R. Holdsworth. 1998. State of the Sonoran Desert Biome: Uniqueness, Biodiversity, Threats and the Adequacy of Protection in the Sonoran Bioregion. p.36-38. Tucson, Ariz.: The Wildlands Project.