Maintaining the social-ecological sustainability of regions is a challenge in the present as it has been in the past. For at least 4000 years, indigenous agriculturalists in the U.S. Southwest and Mexican Northwest (SW/NW; Fig. 1) developed diverse lifeways and a repertoire of social and environmental dryland strategies (Ortiz 1979, Ingram and Hunt 2015, Cordell and McBrinn 2012). This multi-millennial trajectory of variable population growth ended during the early 1300s CE, and by the late 1400s, well before Spanish contact, population levels in the region declined by about one-half (Dean et al. 1994, Doelle 2000, Hill et al. 2004, Robinson et al. 2021). In some places within the region, the proximate causes of the decline are fairly well documented, such as environmental challenges to food security (Gumerman 1988) and changing climate patterns (Dean 1996). In the region as a whole, however, differences in social organization, histories, lifeways, subsistence strategies, population densities, and climatic and environmental conditions were substantial. Such diversity likely resulted in an uneven landscape of human vulnerability to social perturbations and environmental hazards such that impacts would not have been uniform across the region. Why, then, did population levels substantially decline, relatively quickly, throughout this large and diverse region?
The aim of this research is to identify social and environmental conditions that contributed to the depopulation of the SW/NW during the 13th through 15th centuries and use this knowledge to inform present and future sustainability efforts. Millions of smallholder farmers worldwide continue to practice subsistence agriculture and engage in limited market economies (Lowder et al. 2016) similar to those in the past. The depopulation of the SW/NW includes declining population levels, substantial settlement relocation, and accelerated out-migration. It is an indicator of a loss of regional-scale sustainability and an example of the problem modern sustainability efforts seek to avoid. For the purposes of this study of the past, sustainability is defined as the persistence of peoples in places. Archaeological documentation of human-environmental interactions over centuries provides an opportunity to observe social, environmental, and economic variables interacting in many contexts (van der Leeuw and Redman 2002). Outcomes of these interactions are a reservoir of insights on what worked, what did not, and why (Nelson et al. 2012, Kintigh et al. 2014, Redman 2014, Clark et al. 2019, Rick and Sandweiss 2020).
We apply the United Nations Development Programme’s (UNDP) human security framework to investigate this loss of sustainability in the past and the influence of human securities on sustainability and depopulation. Improving human securities is necessary, according to U.N. leadership, for achieving Sustainable Development Goals established by the 2030 Agenda for Sustainable Development (Risse 2019). The human securities approach was developed to reduce global human insecurity by working toward freedom from fear, freedom from want, and freedom to live in dignity (UNDP 1994, UN Human Security Unit 2009, 2016, Gomez and Gasper 2013, O'Brien and Barnett 2013). This people-centered approach addresses the totality of conditions (e.g., economic, food, health, etc.) that impact human beings and recognizes that multiple interconnected and mutually reinforcing factors generate complex social phenomena such as depopulation and other humanitarian crises. The root causes of some insecurities can lead to declining population levels through famine, changes in fertility and mortality, and violence. Declining insecurities can also influence human decisions to migrate away from untenable social, environmental, and economic conditions (Vietti and Scribner 2013), the key dimensions of sustainable development. Improving human securities is expected to minimize forced migration by easing some of the pressures that influence human decisions to move or remain in place. We document human insecurities throughout the SW/NW region during the roughly 100-year period before depopulation in each sub-region. Human securities are measured and understood in the present with sets of evolving indicators. Archaeologists are well versed in identifying indicators for past human behaviors and processes and share this reliance on indicators with the sustainability policy community.
The human security framework provides a meta-language for the sustainability community and archaeologists to communicate about problems of shared interest. Investigating and contributing insights useful for lessening human suffering associated with declining securities, improving sustainability, and decreasing forced migration is not the exclusive domain of specific disciplines or actors; it is a shared disciplinary and human responsibility. From an archaeological perspective, most societal configurations eventually fail but some persist through learning and transformation. Studies of long-term (multi-generational) human-environmental interactions can inform this learning by revealing policy-relevant information. For example, complex social-ecological system behavior, including thresholds and emergent phenomena, is not evident in studies of shorter duration (van der Leeuw and Redman 2002). Current sustainability planning is already informed by institutional learning from past successes and failures. We advocate here for an expanded temporal scale that can generate insights to better understand and address modern sustainability problems. Aspiring to policies built for the present and future but informed by the past accepts our common humanity.
Identifying research questions in the present, adopting global-scale approaches and language for investigating these questions, and testing assumptions embedded in modern policies is a transdisciplinary pathway forward for archaeology. Archaeologists are increasingly communicating insights from the past to address modern concerns (Altschul et al. 2017, 2020), especially those associated with a changing climate (Cooper and Sheets 2012, Nelson et al. 2016, Rick and Sandweiss 2020, Rockman and Hritz 2020). Archaeologists also have much to learn from collaborations with the sustainability community. Recognizing past depopulations as an analog of modern sustainability concerns and analyzing these events with a current framework will generate fresh insights about the past.
Population levels began to decline throughout the SW/NW around 1300 CE (Dean et al. 1994, Doelle 2000, Hill et al. 2004, Robinson et al. 2021). Prior to the decline, population levels in the U.S. Southwest rose for at least a millennium, peaking between 100,000 and 160,000 people around 1000 CE (Dean et al. 1994, Doelle 2000, Hill et al. 2004). By the late 1400s, population levels throughout the SW had declined ~50–60% (Fig. 2 in Hill et al. 2004). Population estimates in Northwest Mexico are less well developed, but based on the available evidence, population levels there appear to have similarly declined. Although absolute population levels are notoriously difficult to estimate from archaeological data, there is a strong consensus that a millennium of population growth slowed and reversed itself around 1300 CE. There is also consensus that depopulation resulted in the archaeological invisibility of some peoples and traditions because of low population densities and relatively high mobility. Although we focus here on depopulation, people persisted in multiple areas through the depopulation, as discussed further below.
Archaeological studies of the causes of the depopulation have focused primarily on specific areas, such as the Mesa Verde region of southwestern Colorado (Glowacki 2015, Van West and Dean 2000, Kohler et al. 2010) and the Lower Salt River Valley of modern day Phoenix, Arizona (Graybill et al. 2006, Hill et al. 2015), rather than on the greater SW/NW region. Most studies have moved beyond early single cause depopulation explanations such as droughts and floods and increasingly identify complex interactions between multiple social and environmental variables, such as the relationship between rising population levels, the increasing use of lands marginal for farming, and social conflict (Schwindt et al. 2016). Migration studies have emphasized that population movements are most likely to occur when there are “push” factors at the population origin and “pull” factors (attractions) at the population destination (Anthony 1990, Cameron 1995). Several studies of large-scale population movements in the SW have identified push-pull environmental and social factors to explain sub-regional depopulations (Ahlstrom et al. 1995, Lipe 1995).
Environmental, climatic, and demographic explanations of local to near regional-scale depopulation have always been a focus in the Southwest given relatively low and variable rainfall conditions and the dependence on maize agriculture. Dean (1996) and colleagues (Gumerman 1988) documented an unprecedented convergence of conditions unfavorable for farming in the northern SW during the 1250 to 1450 period: falling alluvial water tables, floodplain erosion, drought (1275 to 1300), low temporal and spatial climatic variability, the breakdown of the bimodal summer-winter precipitation pattern in the northwest sub-region, and population levels in the northern Southwest that reached their peak around 1000 CE followed by increasing settlement aggregation. They argue that the result was major stresses from population-resource imbalances, breaches in local carrying capacities, and large-scale migrations into the climatically more stable southeastern SW sub-region. Phillips et al. (2018) argue that settlement aggregation throughout the region could have triggered emerging infectious diseases responsible for the depopulation. Cooling temperatures, due to the effect of the Little Ice Age (LeBlanc 1999) and/or explosive volcanism (Salzer 2000), have been linked to declining agricultural productivity and the resulting effect of rising violence and depopulation in the northern SW. Megadroughts affecting large portions of the western U.S. have also been argued to have caused depopulation (Benson and Berry 2009, Cook et al. 2010, 2016).
There is no consensus among SW/NW archaeologists regarding the causes of the depopulation. This study combines data from many sub-regional studies to document the rise of human insecurities and examine whether these can explain the broad regional depopulation in the 14th and 15th centuries. Sub-regional studies do not capture conditions throughout the SW/NW social-ecological system at the spatial scale at which depopulation occurred.
Documenting human securities throughout the SW/NW region and assessing the relationship between insecurities, sustainability, and migration consists of four steps: (1) defining the spatial, analytical units (cases) for the study, (2) identifying archaeological indicators for each security, (3) coding the insecurities in eight culture areas (Fig. 1), and (4) assessing the relationship between the insecurities and the speed (duration) of depopulation in each area. We describe the procedures here, and Appendix 1 contains the insecurity coding definitions and additional information on the methods. Appendix 2 provides thorough documentation of the evidence used to assess each insecurity. Appendix 2 offers researchers the opportunity to compare results in their areas of study to regional-scale patterns and is intended to be a foundational resource for future and similar studies. Our procedures follow similar efforts developed by Hegmon (2016), Hegmon et al. (2018), and others in a special issue of the journal, Archaeological Papers of the American Anthropological Association (Hegmon 2016).
The archaeological culture areas for comparison are the archaeologically and geographically identified peoples of the region: Ancestral Puebloan, Fremont, Patayan, Sinagua, Hohokam, Mogollon, Trincheras, Rio Sonora, and Casas Grandes (Fig. 1). Archaeological cultures are “abstract units of analysis defined by comparative sets of material traits” (Dongoske et al. 1997:600) and should not be equated with modern cultural, ethnic, and tribal affiliations (Dongoske et al. 1997). The culture area names and the geographical extent of the polygons are commonly used by archaeologists of the SW/NW to identify “similarities and differences in the remains people left behind and that have been preserved--for example, the houses they built, the stone tools and pottery they made, the sizes and forms of their settlements, and the foods they ate” (Cordell and McBrinn 2012:35). Similarities likely represent shared lifeways and histories of interaction. Within culture area differences in language, sociopolitical organization, and histories are expected to have been substantial, based on differences among Native peoples of the SW/NW today. The timing and duration of depopulation varied among culture areas (Table 1, Appendix 3) and the contours of the polygons (Appendix 4) began to change by 1300, due to migration. The culture area polygons in Figure 1 represent the period prior to the initiation of the depopulation, about 1100 CE.
There are many threats to human security, but most can be considered under seven categories: economic, food, health, environmental, personal, community, and political security (UNDP 1994). We selected archaeological indicators for each category to assess the extent of insecurity in each culture area. Many of the indicators are threats that have been advanced in the scholarly literature of the SW/NW and elsewhere as social, demographic, and environmental causes of depopulation (e.g., Adler et al. 1996, Nelson and Schachner 2002, Hegmon et al. 2008, 2018, Railey and Reycraft 2008). Thus, each indicator has a plausible causal relationship with the sustainability of places, human decisions to migrate, and declining population levels (Table 2). There is no standard set of insecurity indicators in the present or archaeologically; indicators are selected based on available data, assessor judgments, and assessment objectives (U.N. Human Security Unit 2016). Indicator selection in this study was driven by the need for comparable data throughout the region and limited by the types of data and archaeological proxies available. We expect future archaeological studies will improve and rely on an expanded range of indicators. Collaborations with the human securities and sustainability planning communities could also improve past and present security indicators. We extend previous depopulation studies spatially by documenting securities throughout the SW/NW region to produce a “comprehensive-mapping report” that assesses all human securities rather than an in-depth report on a specific threat (Gomez and Gasper 2013, Owen 2013, U.N. Human Security Unit 2016).
Qualitative and quantitative indicators are used to determine the extent of insecurities in each culture area (Table 2). Qualitative indicators rely on the published judgment of scholars active in each culture area, an approach applied in synthetic studies of broad geographic scope where comparable data do not exist (Hegmon et al. 2008, 2018, Nelson et al. 2016). For example, published and data-informed archaeological syntheses of conflict and violence are used to assess personal insecurities in each culture area. Within the UN human securities framework, the “root causes” of personal insecurity are “physical violence in all its forms, human trafficking, child labor” (Human Security Unit 2016:7). Quantitative indicators assess insecurities that could be measured with comparable datasets across the region. For example, we assess the extent and trajectory of immigration (the indicator of community insecurity) with compound annual population growth rates by comparing two, 50-year intervals prior to the start of depopulation in those areas with available settlement data (Hill et al. 2012). Within culture area population growth rates in excess of what can be expected from changes in fertility and mortality (Cowgill 1975) contribute to assessing the extent of immigration in those areas. Some plausible relationships between the archaeological indicators, sustainability, decisions to migrate, and declining population levels are described in Table 2.
The indicators capture the “initial conditions” of depopulation, the lived experience of people in culture areas that informed their decisions to migrate or resulted in declining population levels. We assumed that changes in these conditions affected human perceptions of security. Thus, our focus was on the extent and trajectory of insecurities in each culture area relative to previous conditions in that area. We documented conditions present during the ~50- to 100-year period prior to each culture area depopulation (Table 1, Appendices 2 and 3). The periods varied in length because of variation in the methods used to date the conditions we were documenting. In areas where tree-ring dating of structures is possible and more precise dates are available, we were able to identify conditions closer in time to depopulation than in areas where radiocarbon or ceramic seriation are the only dating methods available.
We reviewed hundreds of published archaeological studies to identify the extent and trajectory of insecurities in each culture area and coded these insecurities as low, moderate, high, or unknown. Our procedures followed standard coding procedures developed by Ember and Ember (2009) for cross-cultural studies using the Human Relations Area Files (https://hraf.yale.edu/). Within each culture area, the insecurities are the independent variables and the speed (duration in years) of depopulation in each area are the dependent variables. To avoid bias, coders did not know values for the dependent variables or the hypothesis: as insecurities increased, the rate of depopulation increased. The authors developed both after the insecurities were coded. Each coder conducted a separate literature search to maximize the discovery and breadth of the studies considered. Coders were, however, provided with some synthetic literature for each case so that expert foundational studies were included in the coding. Specific questions and code definitions were developed to guide the coding effort (Appendix 1). At least two coders coded each insecurity, and most insecurities were coded during two distinct coding periods by different coders. As an additional step the authors obtained comments on the coding from archaeological culture area experts for most areas.
As an example of the coding effort, we coded personal insecurity as High in the Ancestral Puebloan area during the ~50-year period prior to depopulation. Consistent with archaeological and anthropological understanding of the influence of conflict and violence on human decision making and mortality, we expected that as conflict and violence increased, mortality increased, personal insecurity increased, and movement away from dangerous conditions increased (e.g., LeBlanc 1999). The material indicators (proxies) of High conflict and violence include strong and widespread evidence of skeletal trauma, unburied bodies, and fortified/defensive structures. Data-informed and synthetic interpretations of the extent of conflict and violence by leading scholars of the culture area were especially influential. Our coding efforts identified at least 12 studies that found evidence of these indicators and interpreted high and increasing conflict and violence within the Ancestral Puebloan area prior to depopulation. We cite these studies to build a case for our coding decision in Appendix 2.
We acknowledge the internal diversity of conditions within the Ancestral Puebloan area (and elsewhere) and address this analytical challenge in our study. For example, in the Kayenta sub-region (NE Arizona) of the Ancestral Puebloan area, Dean (2010) argues that conflict and violence was not extensive or widespread. Haas and Creamer (1996), however, argue that warfare was increasing and endemic in the Kayenta area and throughout the Ancestral Puebloan area preceding and during depopulation. We cannot resolve these interpretive differences and we address such intra-culture area variation for all insecurities in three ways. First, we focus our coding on characterizing insecurities in areas with the highest population density within each culture area (population levels in the Kayenta area were about one-third of population levels in the central Mesa Verde area [Dean 2010], a focus of most of our coding). These “demographic centers of gravity” (see maps in Hill et al. 2010:36-44) are also usually the most well-documented areas. Thus, we have documented the dominant trends in the insecurities within culture areas and look forward to intra-culture area studies, as advocated below. Second, in addition to investigating multiple sources to document conditions, we rely on published syntheses of conditions and, in some cases, expert assessments offered to us by prominent culture area scholars. Their qualitative assessments provide a holistic understanding of conditions that inspection of published studies sometimes cannot. Third, we code and document our uncertainties in Appendix 2, including those caused by intra-culture area diversity and variation, for each assigned insecurity.
To identify and compare differences in the spatial distribution, intensity, and variation in insecurities among culture areas we assign an ordinal scale code to low (1), moderate (2), and high (3) insecurities. When these codes are summed, aggregate insecurity is identified with higher values representing relatively higher insecurity and lower values representing relatively lower insecurity. In the Patayan culture area, only environmental insecurities could be assessed. Patayan is the least studied and documented culture area in the region.
To identify the relationship, if any, between human insecurities and sustainability, we investigate the correlation between aggregate (summed) insecurities and the speed (duration) of depopulation in each culture area. The speed of depopulation is a measure of the sustainability of places, with faster depopulation associated with less sustainable conditions and slower depopulation associated with more sustainable conditions. People had persisted in each culture area for at least a millennium prior to depopulation. Although depopulation occurred throughout the region, variation in depopulation speed provides a reasonable but imperfect window into the sustainability of each culture area, declining population levels, and conditions that were influencing human decisions to migrate. Five of the nine culture areas of the SW/NW region, containing most of the people of the region, are included in this analysis. The Trincheras and Casas Grandes areas are excluded in this analysis because there is no data, evidence, or published argument to confidently estimate the speed of depopulation in those areas. [However, we speculate that depopulation was relatively short in these areas based on a lack of evidence of prolonged decline.] The Rio Sonora and Patayan culture areas are excluded because there is no evidence of population decline in those areas. We ranked the speed of depopulation based on the number of years of population decline from slowest (1) to fastest (5). We ranked the summed insecurity codes from lowest insecurity (1) to highest insecurity (5). If insecurities influenced the speed of depopulation, then we expect as insecurities accumulated, the speed of depopulation increased. If insecurities did not influence the speed of depopulation, we expect the speed of depopulation to be unrelated to the extent of accumulated insecurities.
This effort is the first systematic comparative study of social and environmental conditions associated with the 13th through 15th century depopulation at the scale of the entire SW/NW region. We hope our efforts enable increasingly refined spatial documentation of insecurities as well as further big history efforts that can be understood beyond the small community of SW/NW archaeologists.
The spatial distribution, intensity (low, moderate, high), and variation of human insecurities prior to depopulation in each culture area are displayed in Figures 2 through 8. The insecurities mapped in each culture area occur at different times, earlier in the north in the Fremont and Ancestral Puebloan areas and later in the south in the Mogollon, Hohokam, Sinagua/Central Arizona, Trincheras, Rio Sonora, and Casas Grandes areas (see Table 1 for depopulation date ranges). Most of the seven dimensions of human security show moderate to high insecurity in each area. Although many social and environmental conditions indicating insecurities have been documented within some sub-regions, the distribution of these conditions and their contribution to insecurities throughout the Southwest/Northwest region was previously unrecognized. Sub-regional/culture area and case studies dominate the archaeological literature. These regional-scale results allow researchers to compare results in their areas of study to regional-scale conditions and begin systematic investigations of the causes of similarities and differences.
The “weight” of aggregate insecurity (sum of all insecurities, columns; Table 3) was highest in the Ancestral Puebloan (20) and Hohokam culture areas (19), more moderate in the Mogollon (16), Trincheras (15), and Sinagua/Central Arizona (14), and lowest in the Fremont (13), Casas Grandes (12), and Rio Sonora & Serrana (11) areas (Table 3). Data does not yet exist to document population densities across the region systematically. However, based on common assumptions and existing demographic information, this pattern suggests that culture areas with high population densities (Ancestral Puebloan and Hohokam) are associated with higher aggregate insecurity and areas with low population densities are associated with lower insecurities (Fremont and Mogollon). When insecurities are considered individually (sum of individual insecurities, rows), health (19), personal (19), political (19), and environmental (18) insecurities were highest and increasing almost everywhere across the region. Economic (16) and community (16) insecurity were more moderate and food insecurity (13) was lowest.
If only insecurities coded as high across the region are considered, then environmental (5 high codes), economic, health, personal, community, and political insecurities (each with 3 high codes) may have been more influential on decisions to migrate and conditions leading to declining population levels than food insecurity (1 high code). In other words (using only the indicators), the extent of climatic dryness, trade/exchange, settlement aggregation, conflict/violence, immigration, and social stratification may have been more influential on decisions to migrate and conditions leading to declining population levels than the extent of resource depletion/degradation. Comparing these results to those identified in modern security case studies may reveal combinations of insecurities most and least likely to result in large-scale migrations.
As human insecurities accumulated and the weight of these insecurities increased, the speed of depopulation increased for most people living in the SW/NW (Fig. 9). For depopulation due to out-migration, people living in areas with the highest aggregate insecurity decided to migrate relatively quicker than people living in areas with the lowest aggregate insecurity. For depopulation due to other factors contributing to population decline (e.g., violence, decreases in fertility/increases in mortality), population levels in areas with the highest aggregate insecurity were declining relatively quicker than population levels in areas with the lowest aggregate insecurity. The positive direction and strength of the relationship between aggregated insecurities and the speed of depopulation (Spearman’s rho = 0.7, p = 0.19) affirms the influence of insecurities on depopulation and the declining sustainability of places. Additional support for this relationship is provided by the Rio Sonora and Serrana culture area. The area was not depopulated and insecurities there were the lowest in the region. These results support the important role of the UN’s holistic efforts to assess and decrease all insecurities to achieve sustainable development goals. This policy assumes “the advancement of human security results in greater resilience, peace and sustainable development” (U.N. Human Security Unit 2016:17). For archaeologists of the SW/NW, the positive relationship between accumulating insecurities and the increasing speed of depopulation validates the progress they have made toward identifying variables that influenced depopulation; the archaeological indicators of individual insecurities often are identified in arguments that seek to explain depopulation in the SW/NW.
Through these results and the approach demonstrated here we aim to stimulate collaborations between archaeologists and others in service of modern sustainability planning. The results presented here are not intended to be the last word on human securities and their influence on SW/NW depopulation. Rather, we expect interpretations to evolve as data continue to accumulate. The global urgency of challenges to the sustainability of social-ecological systems has motivated us to intentionally push the boundaries of what has been previously attempted and is possible with the available data. For archaeologists working on depopulation issues in other regions of the world, we recommend applying the approach presented here to expand the range of cases that investigate the human securities-depopulation relationship. For archaeologists of the SW/NW, we recommend investigating the relationship between human securities and depopulation within culture areas at smaller spatial and temporal scales than we have attempted here.
We also recommend identifying conditions in the SW/NW (and elsewhere) that promoted the sustainability of places, rather than depopulation. These conditions can be identified by comparing those areas and conditions where people persisted and securities improved to those where substantial depopulation occurred. Such comparative studies of key variables and outcomes in the past (Diamond and Robinson 2010, Smith 2012) can be a rich source of insights for sustainability planning. Studies of conditions in the present can more accurately measure insecurities, identify the interaction effects of insecurities, and identify chains of causality. Whether or not securities will improve and communities persist, however, is unknown. Studies of conditions in the past have access to less specific data but the outcomes, whether or not securities improved or deteriorated and if depopulation occurred, are known. Other outcomes can also be investigated (e.g., large scale vs. small scale migrations).
Depopulation across the region follows a north to south pattern over time (Table 1, Fig. 1; Hill et al. 2010). Depopulation began in the mid-1100s in the Fremont area (Allison 2019) and in the mid-1200s in Ancestral Puebloan area (Varien 2010), where aggregated insecurities were the highest in the region (Table 3). Depopulation in the densely populated Mesa Verde region (SW Colorado) of the Ancestral Puebloan culture area was comprised mostly of migration south into Arizona and southeast into New Mexico (Clark et al. 2019) rather than population loss (Ortman 2012). Some research has documented greater security in one migration destination area along the northern Rio Grande River in central New Mexico (Ortman 2016). Although conditions creating insecurity can decrease when people move away from insecure places, new insecurities can result simply from being a migrant (Amnesty International 2010).
The insight that emerges from the human security framework and the results here is that perceived insecurities may have been a mechanism for increasing insecurities in destination areas. In other words, a mechanism for the progressive regional scale rather than limited northern sub-regional depopulation is a contagion of insecurity. As many as 10,000 people were moving from the northern SW into the southern SW (Clark et al. 2019) and possibly NW Mexico during the late 1200s. Even if insecurities were not untenable in destination areas, the knowledge of unprecedented numbers of people on the move likely would have generated insecurities in potential destination areas. If the sources of insecurities were primarily perceived rather than actual, this might explain, for example, why direct evidence of conflict and violence in the Phoenix basin (Hohokam culture area) does not match the substantial change in residential and community architecture to more defensible spaces (e.g., walled residential compounds with limited access). Similarly, evidence for violence in some parts of the Kayenta area (NE Arizona) of the Ancestral Puebloan region was apparently minimal (Dean 2010). People from this area, however, were not able to avoid the contagion of insecurity generated by their migrating neighbors from the north. They also later chose to migrate.
Sustainability and its depopulation indicator are multi-causal phenomena. Single-cause explanations for either likely will not be found in the past or future. Use of the human security approach in this study has clarified and documented prevalent conditions throughout the SW/NW region. The regional-scale loss of sustainability can be explained, at least in part, by the proximate cause of accumulating and interacting human insecurities. Ultimate causes of individual insecurities were not the focus here, but it seems clear that beginning in the 13th century, existing social and environmental configurations and social units (e.g., families, villages) were losing their ability to provide for the basic human needs of freedom from want, freedom from fear, and freedom to live in dignity. Many people chose to move away from the places where insecurities had taken root. This movement created a contagion of insecurity across the region resulting in human population loss, substantial settlement relocation, and accelerated migration—the loss of sustainability of place.
The focus of this study is depopulation and a loss of sustainability, but the history of the SW/NW is also one of persistence and continuity. The indigenous peoples of the SW/NW did not vanish during this critical period in the history of the region. There are 47 federally recognized tribes in the U.S. SW and five tribes with traditional territories in the Mexican NW (Colwell-Chanthaphonh 2010). Many are the descendants of those that experienced this decline and persisted. In some cases, the archaeological invisibility of persistent peoples and places limits current understanding among archaeologists of the extent and distribution of people during the late 1400s until the arrival of the Spanish in the SW, ~1540. Archaeological challenges do not, however, influence the knowledge Native peoples have of their own histories. Memories of challenging periods, such as the focus of this study, are sometimes shared and interpretations offered (Teague 1993, Bahr et al. 1994, Hayes-Gilpin and Lomatewama 2013).
People continued to live in at least 10 areas in the SW/NW through the period of depopulation, a conclusion based on archaeological and historical evidence (oral and written). Places with continuity include Hopi (NE Arizona), Zuni (NW New Mexico), Acoma (west central New Mexico), along the northern Rio Grande River in central New Mexico (Hill et al. 2010), along the Colorado River separating Arizona and Nevada (Rogers 1945), the Middle Gila River in Arizona (Loendorf and Lewis 2017), and possibly in the San Pedro Valley (Hill et al. 2015). Populations also remained in many areas in the Mexican NW such as the Rio Sonora (Pailes 2017), Casas Grandes (Whalen and Minnis 2017), and Trincheras culture areas (Gallaga and Newell 2004), although many of the densely populated central places such as Paquimé and Cerros de Trincheras were depopulated. The identities of people that persisted in NW Mexico are, however, unclear because of in-migration.
The depopulation of the SW/NW is an indicator of a large-scale decline in the sustainability of a region and an analog for the type of problem modern efforts seek to avoid. This historical study of initial conditions and consequences allows us to reason by analogy and generate insights for a more complete understanding of the problem. What has been learned for the present and future? In response to this question, we offer some insights from this study for policy makers, sustainability scientists, and the public and assimilate more that have emerged from the work of others on depopulation in the SW/NW. These insights are meant to stimulate a deeper understanding of human behavior and historical processes and generate useful questions for sustainability policy planning.
1. Holistic efforts by the United Nations to improve human securities to enhance the sustainability of regions and decrease migration are supported by the results of this study. This analysis of long-term historic cases revealed that as insecurities accumulated, the speed of depopulation and associated migration increased. Depopulation is an ex post facto indicator of the loss of sustainability of places. We hope this finding is helpful and contributes to the accumulating case study evidence supporting the human security framework and the link between human securities, sustainable development, and migration. For the modern world where population levels are much higher it is, however, likely that if increasing security involves substantial resource investments and unlimited economic growth through consumption, a negative impact on sustainability will eventually occur.
2. The extent of migration out of sub-regions (culture areas), specifically, the proportion of people deciding to leave, need not be directly related to the extent or category of insecurities. For example, archaeological studies of sub-regional depopulations have revealed that when environmental conditions (e.g., drought) negatively impacted food security, areas of productivity remained that could have supported reduced population levels (Van West 1996, Schwindt et al. 2016). However, entire communities across large areas moved and people aggregated into new social and settlement configurations leaving vast areas unoccupied. Similarly, to the extent that conflict contributes to depopulation, SW/NW archaeologists do not observe the “winners” remaining in newly acquired territories. In other words, although we identified a moderately strong, positive linear relationship between the extent of insecurities and the speed of depopulation, the result was substantial population loss, reorganization, and out-migration throughout the region, regardless of the extent of insecurities. This suggests that social responses to widespread and increasing insecurities were not linear. Thus, sustainability research and planning efforts that identify and expect social thresholds, tipping points, and emergent behavior—the characteristics of complex adaptive systems (Carmichael and Hadžikadić 2019)—will likely be most effective.
3. Once sub-regional out-migration occurs and securities improve, people may not return. For example, a severe drought and associated environmental insecurities in the late 1200s has long been considered as one of many factors that contributed to people’s decisions to move out of the northern Southwest, especially the Mesa Verde region of SW Colorado (Douglass 1929). After the drought ended (~1299) a period of unprecedented wet conditions throughout the SW began in the early 1300s (Ingram 2014). These favorable conditions for agricultural success, the opportunity to re-settle in their ancestral homelands (apparently unoccupied), and the relatively short distances back to these homelands did not induce people to return. We do not know why people did not return; however, this outcome does suggest the need for a better understanding of human perceptions and decision making associated with newly unsettled areas.
4. A loss of sustainability in one place can affect sustainability in distant places. Thousands of people migrated from the northern SW and we suspect the migrants created insecurities throughout their destination areas in the southern SW and NW Mexico. Although it might be convenient to focus on the sustainability of our own community, if insecurities arise in one place, responses can start to impinge on others and a contagion of insecurity can develop. If this speculation is correct, then broad-scale responses to emerging insecurities will be critical for maintaining regional-scale sustainability.
5. Simplistic, single cause explanations (e.g., drought, resource exploitation, over-population) of complex sustainability challenges such as the depopulation of the SW/NW should be replaced with an acceptance of multi-causality. This study (as well as countless other archaeological studies of the past, see for example Varien 2010) demonstrates how depopulation can only be explained by multiple and interacting factors, which themselves have their own complex causal chains. Similarly, it is important for modern planning efforts to distinguish between proximate (nearest in time and space) and ultimate causes (essential for initiating the chain of causality). The long-term, multi-generational duration of the loss of the sustainability of existing social configurations in the SW/NW region as well as the archaeological consensus (and lack thereof) on the causes demonstrates that simple explanations and solutions are unwarranted. As we work and aspire toward future sustainability, approaches that pursue an “all of the above” strategy are likely the most productive. We should question political actors and policies that offer overly simple explanations and solutions to complex phenomena.
6. Loss of sustainability is a multi-generational phenomenon; it is a process, not an event. The slowest sub-regional depopulation occurred over six generations, about 150 years (Fremont culture area). The fastest depopulation occurred over about two generations (in the northern portion of the Ancestral Puebloan culture area). It will be easy for humans to defer decision making on slow-moving problems, but insecurities can continue to accumulate without action.
7. Sustainability is not just an environmental concern. If sustainability is only considered as an environmental issue, the depopulation of the SW/NW is a strong counter argument. We did not find strong evidence of extensive resource degradation (the food insecurity indicator) throughout the region, although local examples exist (Kohler and Matthews 1988, Hill et al. 2015). Systematic synthesis of available and comparable environmental data is necessary to evaluate this finding further. Drought impacts on sub-regional depopulations should, however, remain a focus of future investigations—not as ultimate causes (they were not often temporally coincident with the initiation of depopulations) but as proximate and exacerbating conditions (see the environmental insecurity indicator). The two major pulses of depopulation in the SW/NW (the late 1200s and mid-1400s) temporally coincide with “megadroughts” at 1271 to 1297 and 1435 to 1478 (Cook et al. 2016). Drought is endemic in the SW/NW and we suspect accumulating insecurities lessened the effectiveness of existing strategies which had long-been effective, and increased vulnerability to drought.
8. Migration is a resilient strategy for maintaining the sustainability of human groups (e.g., Spielmann et al. 2011). The indigenous peoples of the SW/NW region persist, some in their ancestral homelands. Migration was and remains a way of life in the region (Naranjo 1995, Nelson and Schachner 2002, Kuwanwisiwma et al. 2018, Duwe and Preucel 2019). This view is consistent with modern sustainability approaches that recognize that migration is not always a problem to be solved but rather a process that can benefit people and host countries. This study also demonstrates that migration can be an indicator of insecurities in the area from which people migrate. Although we have used extensive out-migration as an indicator of a loss of regional-scale sustainability, migration at the scale of households and communities is a human adaptive strategy that has sustained our species and indigenous peoples of the SW/NW for millennia. Policies that unnecessarily prevent migration diminish this shared human strategy.
The work presented has multiple objectives and seeks to communicate with several audiences. First, we aspired to contribute insights toward social-ecological sustainability planning by evaluating existing policy assumptions about the relationship between human securities, sustainability, and migration. The asserted relationships were affirmed. Second, we intended to advance understanding of the 13th through 15th century SW/NW depopulation. A new, more spatially comprehensive assessment of conditions that likely influenced human decisions to migrate and were responsible for substantial population loss was identified. We provided a comprehensive source (Appendix 2) for future and improved efforts. Finally, we were inspired to respond to the questions posed by the “Grand challenges for archaeology” (Kintigh et al. 2014:880): “Can we characterize social collapse or decline in a way that is applicable across cultures?” and “What are the relationships among environment, population dynamics, settlement structure, and human mobility?” We have presented some partial answers to these questions and hope to have demonstrated rather than asserted the relevance of archaeology to modern sustainability efforts.
Both authors contributed equally to the study.
We thank the 13 scholars of the archaeology of the U.S. Southwest and Mexican Northwest who reviewed and commented on the paper and/or the evidence of human insecurities presented in the Appendices. We also thank the coders, named in Appendix 2, who participated in this research. All remaining errors are our own.
Data are presented in the tables and included appendices.
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