Once the Europeans arrived, however, one of the greatest ecological revolutions in the history of the world transformed both lives and landscapes. Biological historian Alfred Crosby calls this revolution the Columbian Exchange--that flow of genes, microbes, plants, and animals between the "Old World" of Eurasia and Africa and the "New World" of the Americas. The first revolutionary wave may even have preceded the Europeans themselves: Indians infected with Eurasian diseases like measles, influenza, and smallpox may have unsuspectingly unleashed contagions as they traveled up ancient trade routes from central Mexico. Some archaeologists and ethnohistorians even contend that these diseases may have contributed to the demise of Hohokam civilization itself.

What we do know is this: thousands of Yaquis and Mayos were perishing when the first Jesuit missionaries ventured into Cahita territory in the early 1600's.The pattern repeated itself again and again at five- to eight-year intervals as Spanish settlers pushed northward in to the Opatieria and Pimeria Alta. Epidemiologically virgin populations had no genetic resistance or cultural mechanisms to resist the microbial onslaught. Like native peoples all over the Americas, the number of Indians in the Sonoran Desert declined by as much as ninety-five percent over the next two centuries.

For the Indians who endured, however, the Europeans brought new crops, new tools, and new animals that revolutionized their economies and their means of transportation. Winter wheat filled an empty niche in their agricultural cycle because it could be planted in November, when frosts at higher elevations in the Sonoran Desert would have withered corn, beans, or squash. Mules and oxen enabled them to cultivate their fields with wooden or iron plows, intensifying their reliance upon agriculture. Cattle, sheep, and goats allowed them to convert non-edible forbs and grasses into beef, mutton, cheese, milk, leather, and tallow. Horses expanded their ranges and shrank distances. A Euro-American agropastoralist economy--irrigation agriculture along the floodplain, animal husbandry in the uplands--supplemented, complemented, and slowly replaced digging-stick agriculture and wild food gathering.

Stock raising was the most land-extensive Euro-American transformation of the Sonoran Desert. Cattle, horses, goats, and sheep searched for forage from river floodplains to mountain crests. In more settled areas like central Sonora, overgrazing became endemic during the Spanish colonial period. The presido (military garrison) and town of Pitic (modern Hermosillo) alone ran 5000 head of cattle, 3422 sheep, 435 goats, 2138 horses, and 367 mules (Radding 1997, p218).

The discovery of silver in 1683 south of the Rio Mayo led to the development of Alamos, a city of wealth and social stratification based upon the enormous capital investment required to extract and process silver ore. Cieneguilla in northwestern Sonora, on the other hand, was a desert boom town where gambucinos (prospectors) dry-winnowed alluvial deposits for particles of gold. Vein-mining operations like that of Alamos depended upon large, stable labor forces organized into hierarchies of occupations. Placers like Cienguilla attracted restless, mobile congregations of Spaniards, mestizos, Yaquis, Opatas, and Pimas, most of whom worked for themselves. No other economic activity so thoroughly rearranged social relationships on New Spain's and then Mexico's northern frontier.

These social and ecological patterns replicated themselves after the United States wrested away more than half of Mexico's national territory during the Mexican War of 1845-48. Gold and silver lured Anglo-Americas and Europeans to the northern Sonoran Desert. Precious metals were the only commodity worth the enormous transportation costs on such an isolated and dangerous frontier.

Because Apaches, Yavapais, and River Yumans resisted European and Euro-American conquest so successfully for two centuries, Euro-American impact upon the desert environment was intermittent rather than sustained.