Leaving Mesa Verde: Peril and Change in the Thirteenth-Century Southwest

5. A New Paleoproductivity Reconstruction for Southwestern Colorado, and Its Implications for Understanding Thirteenth-Century Depopulation

A New Paleoproductivity Reconstruction for Southwestern Colorado, and Its Implications for Understanding Thirteenth-Century Depopulation Timothy A. Kohler 5 In complex systems, we accept that processes that occur simultaneously on different scales or levels are important, and the intricate behaviour of the whole system depends on its units in a nontrivial way. —Vicsek 2002:131 With its well-known and high-resolution archaeological record, the northern San Juan region is a place where we can hope to make relatively firm statements about the causes of human behavior. And as Varien recounts in the first chapter of this volume, no event or process in this region has attracted more popular and scientific attention than its thirteenth-century depopulation. This would then appear to be one region—and one process—for which the data are so strong that archaeologists would long ago have agreed on the causes of the thirteenthcentury population collapse. This volume is simultaneously evidence that this has not been the case, and an argument that new data and approaches are putting that old goal firmly in our sights. In this chapter, I take what might be called a complex adaptivesystems perspective on the thirteenth-century depopulation. By this I mean several things. First, social and natural processes governed by evolutionary logics are expected to interact, and neither is given unexamined explanatory preference. Second, there is a commitment to examining these processes in as disaggregated a way as possible, with the expectation that systems-level novelty frequently arises from the interactions of constituent entities. Here, I use agent-based modeling to leverage our insight on human environmental impacts. Third, we expect that even though human societies and their dynamics will have some special properties because of their greater use of symbolic systems A New Paleoproductivity Reconstruction 103 and the Lamarckian nature of culture change, nevertheless their organizational and dynamic commonalities with other living systems remain significant. I would also include in this approach much of what Bintliff (2007) has recently characterized as the key tendencies of a “chaoscomplexity ” approach to understanding change and stability in human societies. I begin by reviewing results of the Village Ecodynamics Project (VEP), focusing on our estimates for maize production from AD 600 to 1300 (Kohler et al. 2007), a regional reconstruction of low-frequency climate change (Wright 2006), and local estimates of momentary human population through time (Ortman, Varien, and Gripp 2007; Varien et al. 2007). These data can be complemented by estimates of the human impact on forest and game resources as modeled by the VEP agent-based simulations (see Kohler et al. 2007). After this review, I integrate the most relevant data from the thirteenth century into a longer-term perspective. I then add a more general discussion of inferred changes in social organization in the late Pueblo II and Pueblo III (PII, PIII) periods that seem to me to have been leading toward a society whose organization was less robust and more brittle (as these terms are defined below). A complete understanding of these large-scale population movements would ideally begin from four time-series of information for both the population source and the sink areas. These are (1) paleoproductivity data that are sensitive to high-frequency changes in both precipitation and temperature, (2) proxy climatic series that can help us put that high-frequency variability into a low-frequency perspective, (3) temporally sensitive estimates of local prehistoric population size, and (4) models that specify how population size relates to access to critical environmental resources such as fuelwood and animal protein (Johnson 2006; Johnson et al. 2005). Any credible explanation will need to consider whether differential per-capita subsistence opportunities were influential in decisions to migrate, rather than simply making assumptions about what was important. Unfortunately, southwestern archaeologists and paleoclimatologists are still a long way from being able to provide all of these series (though Wright, this volume, reviews the available climatic data in the central Mesa Verde region as well as in possible regions receiving population from the central Mesa Verde in the AD 1200s). We can, however, fulfill 104 Timothy A. Kohler these requirements reasonably well for the portion of the central Mesa Verde (CMV) region studied by the VEP (figs. 1.1, 1.2), as we’ll see below. In the absence of comparable data from other areas, however, we cannot perform the comparative evaluations of different areas that presumably informed the movements of Pueblo peoples. One approach to doing the best we can while waiting for these...