Abstract
Native grasslands worldwide have been degraded by grazing, but the processes involved have been studied insufficiently. Grasslands were a major habitat on the southern section of the Mexican Plateau when the Spaniards arrived 500 years ago. Since then, they have been impacted heavily through grazing, but the details and history of such impacts have not been established. We aimed at untangling almost 500 years of grazing of these grasslands, based on extensive searches of published information and of documents in historical archives. We identified three periods of ranching: Cattle, from the mid-16th to the mid-17th Centuries; sheep, reflecting a change from tall to short grasses; and, finally, goats and horses, along with sheep, asses and some cattle, after serious grassland degradation by the mid-20th Century. Overgrazing has impacted grassland composition in the region and its capacity to support livestock, but also, strongly affect some 20 species of native vertebrates and an unknown number of plant and invertebrate species. The current condition of some ranges and a livestock exclosure indicate that grassland improvement is possible, but realistic objectives based on biodiversity conservation and livestock production should be targeted, rather than utopic pre-livestock frameworks. Grasslands in the region at the time of Spanish arrival possibly had a mixture of grasses and herbs, but buffalo grass and the central Mexico tobosa grass are potential initial range management targets for grassland recovery of the ranges in worst condition, while blue gramma and the Mexican plateau gramma are good targets for ranges that still have some grass cover.
Highlights
Inasmuch as the data available allowed, we focused on three objectives: (1) Synthesize the history of livestock grazing since their first introduction to the region, (2) link this history with that of the degradation of the regional grasslands, and (3) based on that and the current knowledge of the habitat requirement of grassland wildlife species, infer some of the impacts of overgrazing on them
Concluding remarks Landscape changes in other arid and semiarid regions in Mexico indicate that the introduction of settler non-indigenous livestock was an ecological turning point. This was documented thoroughly in the Valle de Malpaso, some 130 km northwest of our study region, in which Elliott et al (2010) concluded that “[t]he most dramatic changes detected in the valley resulted from the erosion associated with Spanish Colonial grazing and deforestation that began in the 16th century.”
Likewise, grazing by sheep was a major driver of severe environmental degradation in the Valle del Mezquital in central Mexico (Melville, 1994)
Summary
The dark patch is a 1-hectare plot from which all grazing by livestock has been excluded since 1980. Those outside it had a ground cover of 53% and were v isually dominated by jimmyweed (Isocoma veneta) and cespitose forms of gramma grasses, whereas inside the exclosure 99% of the ground was covered by a tall (~60 cm high) and dense herbaceous community of plants, in which purple muhly (Muhlenbergia rigida) was the major grass component, followed by gramma grasses (Bouteloua spp.) and forbs The contrast between plant cover and composition inside and outside the exclosure suggests strongly that the grasslands found in the area before livestock was brought in the 16th Century were very different from those found today
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