The vegetation of the Gran Chaco is intimately associated with the pattern of soils and reflects the same general east-west division. The eastern Chaco is noted for its parklike landscape of clustered trees and shrubs interspersed with tall, herbaceous savannas. To the west, a wide transition zone grades into the espinal, a dry forest of spiny, thorny shrubs and low trees. Chaco vegetation is adapted to grow under arid conditions and is highly varied and exceedingly complex. The climax vegetation is called quebrachales, and consists of vast, low hardwood forests where various species of quebracho tree are dominant and economically important as sources of tannin and lumber. These forests cover extensive areas away from the rivers; nearer the rivers they occupy the higher, better-drained sites, giving rise to a landscape in which the forests appear to be islands amid a sea of savanna grasses growing as high as a person on horseback. In the more arid western Chaco, thorn forests, the continuity of which is occasionally broken by palm groves, saline steppes, and savannas induced by fire or deforestation, are dominated by another quebracho tree that has a lower tannin content and is used most often for lumber. There is also a marked increase in the number and density of thorny species, among which the notorious vinal (Prosopis ruscifolia) was declared a national plague in Argentina because its thorns, up to a foot in length, created a livestock hazard in the agricultural lands it was invading.

Animal life

True to its name, the Gran Chaco has an abundance of wildlife. Among the larger animals are the jaguar, ocelot, puma, tapir, giant armadillo, spiny anteater, many foxes, numerous small wildcats, the agouti (a large rodent), the capybara (water hog), the maned wolf, the palustrian deer, the peccary, and the guanaco (a camelid related to the llama). The Chaco is one of the last major refuges for the rhea (or nandu), a large, flightless South American bird, and it has long been noted for its abundant and varied bird population. The streams are host to more than 400 fish species, among which are the salmonlike dorado and the flesh-eating piranha. Countless travelers’ tales complain of the pestilent insects. Reptiles also are abundant, with numerous lizards and at least 60 known species of snakes, including many pit vipers and constrictors, while at least six species of poisonous tree toads have been identified.

Early settlement

The indigenous peoples of the Chaco were numerous. Because of their subsistence as hunters, gatherers, and fishermen, tribal units were not much larger than extended families. Nevertheless, from among the diverse dialects, anthropologists have described a few major linguistic associations: the Guaycurú, Lengua, Wichí, Zamuco, and Tupí-Guaraní. Most of these people lived under extremely primitive conditions; settlement depended on the availability of fresh water, making stream courses the most coveted sites. Implements were fashioned largely from wood and bones because of the absence of stones, while the spiny leaves of the pineapple-like groundcover carraguatas served as a universal source of fibre. The Chaco forest, despite its harshness, contained more plant sources of human sustenance—e.g., edible pods, fruits, berries, and tubers—than surrounding areas, and this factor was well exploited by the native peoples. Game was gathered by trapping, netting, clubbing, and spearing, often in conjunction with large group drives. For those Indian groups still living outside the limits of European settlement, conditions are only slightly modified today, although these people now have domesticated animals and metal tools. Most tribes, however, exist as sort of a peasant pioneer fringe and practice some form of shifting subsistence agriculture.

European colonization and economic activity

Aside from the scattered (although successful) agricultural communes (reducciones) of the Jesuits and the settlement of Asunción, Paraguay, on its eastern fringe, the Chaco defied effective European occupancy until well into the 19th century. Hostile Indian groups, in concert with the forbidding nature of the Chaco itself, limited European influence in the colonial period to a situation much like a state of siege.

The limited early colonization in the Argentine and Bolivian Chaco was based on exploitation of the longhorn criollo (or Creole) cattle that roamed half wild throughout the region. The western Chaco Austral, near Salta, also was exploited as a source of heavy timbers for the mines in the highlands of Bolivia and Peru. In the late 19th century, the Chaco in Argentina and southern Paraguay became a land of great ranches (estancias) raising criollo cattle, and numerous, small, independent camps (obrajes) of woodcutters exploited the abundant hardwoods of the Chaco forests for lumber and firewood. Cattle grazing has continued to be the most extensive use of the land, with few substantial changes from pioneer days. One of the key problems in improving the cattle industry has been the apparent endemic nature of many serious cattle diseases and pests against which criollo cattle have developed some immunity, whereas purebred cattle have remained fully susceptible.

In the eastern Chaco, vast, highly capitalized industrial ventures established large plants to process the great quantities of tannin found in the various quebracho species. Unlike the obrajes of the woodcutters, these operations were large, centralized mills adjacent to rivers or rail lines, from which the selective cutting of quebracho has proceeded at a systematic pace. The slow growth habits of the quebracho trees, however, pose a threat to the tannin industry, as the pace of the harvest easily can exceed reforestation efforts. The relatively untouched Bolivian Chaco contains stands of quebracho timber, but most of these are in remote areas and have not been exploited while production has continued in the more accessible areas. Quebracho tannin has remained one of the economic mainstays of the Chaco, but it has faced competition from other sources of tannin, both natural and synthetic. Other forest products include lumber and heavy timbers from a variety of other species, firewood, and palo santo oil from the wood of Bulnesia sarmientii, a tree found in the more arid portions of the Chaco.