"Pastoralism had no recognizable social counterparts in the preconquest world outside of the Andes, and its introduction involved not only the addition of exotic species but also a completely alien perception of the natural resources and their use; indeed, it involved the formation of completely new systems of production."
Melville surveys the geography, geology, demographics, and history of the Valle del Mezquital. It was densely settled by native agriculturalists, the Otomí, who "had evolved a very successful approach to living in this high region of little rainfall and frequent frosts". By 1600, disease and virgin soil epidemics had reduced the indigenous population to perhaps 10% of its size at contact. The introduction of grazing animals followed the classical pattern of ungulate irruptions: rapid increase, overshoot, crash, and equilibrium at reduced levels.
A chapter then turns to Australia, describing the introduction of sheep in the eastern and southeastern tablelands. This is interesting but not particularly illuminating of the Mexican situation. (It also relies entirely on secondary sources, one of which is presumably responsible for a confused reference to kangaroos being in danger of extermination and "several subspecies" having been lost.)
Returning to Mexico, Melville presents an area by area survey of the Valle del Mezquital. She presents some quantitative evidence for the numbers and density of sheep in the 16th century, but for environmental change (sheet and gully erosion, and changes in vegetation cover) she uses qualitative evidence culled from a diverse range of records.
"Contrary to folk wisdom, which consigns sheep grazing to areas of poor natural resources, the better the resources in the Valle del Mezquital, the more intense the grazing and the more extensive and profound the degradation. Land became fit only for sheep — it was not inherently poor."
The final two chapters turn to the political economy of pastoralism in the Valle del Mezquital. The first looks at changes in land holdings and the process by which land came under the Spanish system of land tenure.
"By 1600 pastoralists controlled the means of production in the Valle del Mezquital. It is true that some Indian communities managed to retain control over some of the best lands in the region, and the demographic collapse meant that they needed less land and water to irrigate it; but they had lost land for future expansion, and the degradation of the water regime mean that many communities had also lost the means to irrigate it."
The second covers the concentration of landholdings and the monopolization of pasture — the end of the "open access" commons — and the role of environmental change, markets, and viceregal policy in the development of haciendas.
"Between 1580 and 1620 domination of regional production shifted from large numbers of animal owners who based their operations on small landholdings, to a small number of landowners with large tracts. The appearance of latifundia in the Valle del Mezquital indicated the extent to which the primary means to obtain wealth had shifted from animals to land. In a seeming paradox, land became more valuable as its value declined. Underlying this paradox was environmental change: as the productive potential deteriorated, more land with restricted access was necessary, so individual pastoralists moved to monopolize land and its value as a commodity increased."
There's some repetition in A Plague of Sheep and it could have been better structured. The use of statistics is also clumsy in places, with some calculations carried out with over-precise figures and no indication of what are clearly considerable error ranges. These are minor points, however. A Plague of Sheep offers a good mix of local detail and broader theory, and is an important contribution to understanding the dynamics surrounding the introduction of pastoralism into new environments.