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A major question in the archaeology of the southwestern region of the United States is why so many impressive settlements, and even entire regions, were abandoned in prehistoric times. Archaeologist Tim Kohler has suggested that the nature of human-environmental interaction was an important reason in the case of the Anasazi people. The actual case study that Kohler relies on is from the Dolores River basin of southwest Colorado, where the Anasazi seem to have moved in about A.D. 600. Over the following couple of centuries, the population increased, and they aggregated (or gathered) into villages, but by about A.D. 900 the area began to be abandoned. Other archaeologists have identified the immediate cause of this abandonment to be a series of short growing seasons that would have put pressure on corn production at that high an altitude. Kohler, however, assets that a growing population led to human-environmental interactions that caused people to live in villages, intensify agrarian food production, deforest the region, deplete the local soils, and ultimately abandon the area. Kohler uses several kinds of evidence to show that human effects, not solely climatic factors, were important factors in the abandonment of settlements. One key indicator of change in the environment surrounding these prehistoric settlements is the wood that was used there. Archaeological study of wood charcoal found in hearths dating to the various episodes of occupation indicated that the species use changed in a patterned way. Over time there was a decline in the use of juniper and pinon (native, slow-growing species of trees) and an increase in woody shrubs and fast-growing cottonwood. The species of wood used in the construction of buildings also changed. Fewer pinon were being used, and those that were used seem to be from increasingly old trees, while juniper continued to be from young trees. The implication is that the forest that did remain was changing to relatively more junipers, a tree that is more fire resistant, better able to reproduce in open settings, and less desirable for construction than pinon. Kohler argues that pinon was disappearing from the locale of settlements and that this put an additional nutritional strain on the population, which used nuts from the tree as well as its wood. The relative proportion of different species of animals hunted by people in the region also changed progressively. A final source of evidence was the seeds found in the archaeological deposits, which had blown or been brought to the settlement. As time went on, there was a substantial increase in seeds from pioneer plants, attesting both to agricultural intensification and to an increasingly disturbed local environment.   This evidence has convinced Kohler of the importance of human impact in degrading the local environment. His interpretation of the situation is that by about A.D. 840, people had aggregated into villages in favorable settings because of their competitive organizational advantages over smaller units in the face of growing population and depletion of local wild resources. Hence, the very nature of the initial slash-and-burn agriculture encouraged a further dependence on agriculture and the aggregation of people into denser settlements. However, there are costs to aggregation, such as the increasing distance to usable fields, the heavier pressure on local soils, and the accompanying increase in agricultural risk. The Anasazi responded to this by further intensification, such as water-control mechanisms, to feed the increasing population. Such a trajectory is fraught with risks, but it is also pushed forward by advantages it bestows on its participants who organize and cooperate. Advantages might include sharing food across groups in a village, investment in facilities to improve the processing and storage of food, and cooperative labor pools and social groupings larger than villages, which would enable organized long-distance hunts and participation in trading networks. Larger and larger villages became possible, but this also made the system vulnerable to collapse. A reliance on the management of resources through cooperative action reduced their flexibility of action, so that when poor seasons occurred, people were seriously hurt. Thus an expectable aberration in the climatic regime may have been enough to cause the collapse of the village system in the Dolores area.