Mesa Verde from Escalante Pueblo
The southwest has always been something of a marginal environment for agriculture, and many theories that seek to explain the complicated history of human settlement in the prehistoric southwest appeal to environmental changes and fluctuations of various sorts as causal factors in many of the large-scale processes seen in the archaeological record. The concept of carrying capacity looms large in many of these theories; while a slippery concept to define rigorously, it basically refers to the number of people that could be supported by a given environment at a given technological level. Under this interpretation, major changes are often the result of population density reaching or exceeding the carrying capacity for a given area, which forces the people in question to make some hard choices. The options available are typically to adopt new subsistence technology, move into a less populous or more fertile area, or stay and fight other groups over the diminishing resources in the area.
This is a pretty attractive framework for explaining many aspects of southwestern prehistory, and it is no surprise that many archaeologists have adopted it as part of their theories. Among these is Steven LeBlanc in his book on warfare, in which he presents a theory of the origins of prehistoric warfare based on carrying-capacity constraints. Warfare is particularly well-suited to arguments based on carrying capacity, since the connection between diminishing resources and violence is intuitive and straightforward. Also amenable to this sort of explanation is abandonment, particularly the remarkably rapid and complete regional abandonments that swept the Colorado Plateau and other areas around AD 1300.
As Carla Van West points out in chapter 15 of The Prehistoric Pueblo World, however, it’s important to look carefully at the available evidence before applying these theories too broadly. She presents the results of an elaborate computer simulation of carrying capacity in the Mesa Verde region from AD 900 to 1300. While it is always possible to quibble with the choices made in constructing these simulations, Van West seems to have done an impressive job of at least trying to account for all the variables that are likely to have affected the carrying capacity of this region. The Mesa Verde region is both one of the most fertile areas of the southwest and one of the most complicated in its prehistoric population patterns, culminating in its apparently total depopulation at the end of the Pueblo III period. Van West’s simulation is intended in part to evaluate the relationship between environmental conditions and demographic trends, and it offers an excellent opportunity to evaluate the role of carrying capacity in processes such as regional abandonment.
Perins Peak from Durango, Colorado
The results are somewhat surprising. While the actual population figures the simulation produces vary depending on a number of factors both natural and social, in all cases the figures are quite high, indicating that the prehistoric population of the Mesa Verde region never came close to maxing out the carrying capacity of the land. The abandonment of the area, therefore, must be due to other factors, although the possibility of more localized constraints on carrying capacity affecting individual communities remains open.
This result sounds a cautionary note for theories that veer too far in the direction of environmental determinism, but in some respects it’s not very surprising. This region is one of the few areas of the southwest that is today primarily used for intensive (and quite productive) agriculture, so its high capacity for supporting agricultural communities might be both expected and idiosyncratic. That is, if any part of the southwest could support large populations, surely it’s this part. This, in turn, implies that the results of this simulation may not be that relevant to other, more marginal areas such as the southern San Juan Basin.
On the other hand, the mere fact that one of the most productive agricultural regions was also one of the most affected by regional abandonment is meaningful in and of itself, especially since some areas that weren’t abandoned, and even gained population during this time, were and are much more marginal for farming. The Hopi area, for example, is much less reliable and productive than Mesa Verde, and it surely had a much lower carrying capacity, and yet it gained population when other regions were being totally depopulated.
The main lesson here, perhaps, is that we need more research like this. Careful analysis of the data available can be very valuable, and innovative techniques such as simulation can be particularly so, but to truly be capable of answering the most important questions we need to have more of these models covering more areas. A detailed simulation like this is useful but limited in its relevance to a rather small area, and there would certainly be no way to apply it directly to other regions. With the increasing availability of advanced computing technologies, it should become easier to do this on a wider scale, and that can only improve our understanding of the past.
McPhee Reservoir, Dolores, Colorado
Gambler's House 
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