Crystal H. McMichael

Sparse Pre-Columbian Human Habitation in Western Amazonia

Population Limits Extensive pre-Columbian populations inhabited the central and eastern Amazon basin, as evidenced by the clearing and modification of forests. McMichael et al. (p. 1429) examined how far inland such activities may have extended by sampling soils across western Amazonia, including river bluffs, which were heavily occupied downstream. Little evidence of human disturbance across a wide region was found by looking for charcoal layers (which would suggest use of fire) and phytoliths, which trace local plants and would indicate the presence of crops. Furthermore, no ceramics or tools were found. Thus, pre-Columbian human populations seem to have been sparse in western Amazonia. Analysis of soils in western Amazonia finds little evidence for pre-Columbian human occupations there. Locally extensive pre-Columbian human occupation and modification occurred in the forests of the central and eastern Amazon Basin, but whether comparable impacts extend westward and into the vast terra firme (interfluvial) zones, remains unclear. We analyzed soils from 55 sites across central and western Amazonia to assess the history of human occupation. Sparse occurrences of charcoal and the lack of phytoliths from agricultural and disturbance species in the soils during pre-Columbian times indicated that human impacts on interfluvial forests were small, infrequent, and highly localized. No human artifacts or modified soils were found at any site surveyed. Riverine bluff areas also appeared less heavily occupied and disturbed than similar settings elsewhere. Our data indicate that human impacts on Amazonian forests were heterogeneous across this vast landscape.

Fire, climate change and biodiversity in Amazonia: a Late-Holocene perspective

Fire is an important and arguably unnatural component of many wet Amazonian and Andean forest systems. Soil charcoal has been used to infer widespread human use of landscapes prior to European Conquest. An analysis of Amazonian soil carbon records reveals that the records have distinct spatial and temporal patterns, suggesting that either fires were only set in moderately seasonal areas of Amazonia or that strongly seasonal and aseasonal areas are undersampled. Synthesizing data from 300 charcoal records, an age–frequency diagram reveals peaks of fire apparently coinciding with some periods of very strong El Niño activity. However, the El Niño record does not always provide an accurate prediction of fire timing, and a better match is found in the record of insolation minima. After the time of European contact, fires became much scarcer within Amazonia. In both the Amazonia and the Andes, modern fire pattern is strongly allied to human activity. On the flank of the Andes, forests that have never burned are being eroded by fire spreading downslope from grasslands. Species of these same forests are being forced to migrate upslope due to warming and will encounter a firm artificial fire boundary of human activity.

Spatial and temporal scales of pre-Columbian disturbance associated with western Amazonian lakes

The history of landscape alteration in Amazonia by humans prior to the arrival of Europeans remains poorly understood. Estimates of human population size at the time of European contact vary by several million, and the trajectories of cultural and agricultural development are equally uncertain. The extent to which human populations altered Amazonian ecosystems is a function of population size, land management style, radius of influence of each subpopulation, and the temporal development of settlements. Here we report evidence that relates to the temporal and spatial scales of human disturbance in western Amazonia. Three lake sediment records and 94 terrestrial soil cores located 0–12 km from the adjacent lake were used to examine scales of pre-Columbian human impacts at two sites in western Amazonian forests. Lake sediment and soil charcoal records indicate discontinuous and localized burning around the study areas. Terrestrial soil phytolith and nutrient analyses reveal a range in disturbance intensity between the two sites but contain no evidence of large-scale forest clearing or anthropogenic soil enrichment. Our data suggest that while all of the settings examined were occupied or used, the halo of influence around each was limited. It should not be assumed that intensive landscape transformations by prehistoric human populations occurred throughout Amazonia or that Amazonian forests were resilient in the face of heavy historical disturbance.

Amazonia and the Anthropocene: What was the spatial extent and intensity of human landscape modification in the Amazon Basin at the end of prehistory?

The nature and spatial scale of prehistoric human landscape modifications in Amazonia are enduring questions. Original conceptions of the issues by archaeologists published more than 40 years ago posited little human influence because of putative environmental constraints. Empirical data accumulated more recently demonstrated dense, permanent settlements along major watercourses of the central and southern Amazon, and profound landscape alterations in seasonally flooded savanna regions of Bolivia. These results led some investigators to propose that most to all of Amazonia was heavily populated and modified before European arrival, and that prehistoric fires and forest clearing were of such a massive scale that post-Columbian reforestation was a significant contributor to decreasing atmospheric CO2 levels and the onset of the ‘Little Ice Age’. Recent data generated from investigations of soils sampled from underneath standing terra firme forests in parts of the western Amazon indicate ephemeral ancient human occupation and little vegetation disturbance there. These issues are central to ongoing discussions surrounding the classification and timing of the Anthropocene, because a geological epoch proposed on the basis of profound human alteration of environments and considerations of when during the Holocene it should begin, require robust paleo-environmental and archaeological data from major, critically important zones such as the Amazon Basin. This paper reviews debates and existing information on prehistoric human influences in Amazonia, provides new data from the western Amazon, and attempts to arrive at reasonable conclusions based on the available empirical data.

Phytolith assemblages along a gradient of ancient human disturbance in western Amazonia

The ecological status of prehistoric Amazonian forests remains widely debated. The concept of ancient Amazonia as a pristine wilderness is largely discredited, but the alternative hypothesis of extensive anthropogenic landscape remains untested in many regions. We assessed the degree of ancient human impacts across western Amazonia based on archaeological and paleoecological data using methodologies that would allow inter-regional comparisons. We also aimed to establish baselines for estimating the legacies of ancient disturbances on modern vegetation. We analyzed charcoal and phytolith assemblages from soil samples from an archaeological site, sites in close proximity to archaeological sites, sites from riverine and interfluvial forests, and a biological research station believed to contain some of the least disturbed forests within Amazonia. We then quantitatively compared phytolith assemblages within and between the surveyed regions. Palm enrichment was evident at the archaeological site, and the biological station survey contained little to no evidence of ancient human activity. The other sites exhibited a gradient of ancient disturbance across the landscape. The phytolith assemblages showed statistically significant between-region variations that indicated our metrics were sufficiently sensitive to detecting ancient disturbance. Our data highlight the spatial heterogeneity of ancient human disturbances in Amazonian forests. The quantification of these disturbances provides empirical data and a more concrete link between the composition of the modern forest and ancient disturbance regimes. Accounting for ancient disturbances will allow a deeper understanding of the landscape heterogeneity observed in the modern forests.

Ancient human disturbances may be skewing our understanding of Amazonian forests

Significance The Amazon harbors thousands of species and plays a vital role in the Earth’s climate and carbon cycles. Much of what we know about the Amazon is based on censuses of only a small number of forest inventory plots, an even smaller number of which are censused repeatedly and used to study forest dynamics and carbon fluxes. The effects of ancient human impacts have never been properly assessed or accounted for in studies of Amazonian plots. New spatial analyses show that plots significantly oversample areas with high abundances of archaeological evidence of past human activities. This suggests that our interpretations of the Amazon’s structure, composition, and function are based disproportionately on forests still reflecting the legacies of past human disturbances. Although the Amazon rainforest houses much of Earth’s biodiversity and plays a major role in the global carbon budget, estimates of tree biodiversity originate from fewer than 1,000 forest inventory plots, and estimates of carbon dynamics are derived from fewer than 200 recensus plots. It is well documented that the pre-European inhabitants of Amazonia actively transformed and modified the forest in many regions before their population collapse around 1491 AD; however, the impacts of these ancient disturbances remain entirely unaccounted for in the many highly influential studies using Amazonian forest plots. Here we examine whether Amazonian forest inventory plot locations are spatially biased toward areas with high probability of ancient human impacts. Our analyses reveal that forest inventory plots, and especially forest recensus plots, in all regions of Amazonia are located disproportionately near archaeological evidence and in areas likely to have ancient human impacts. Furthermore, regions of the Amazon that are relatively oversampled with inventory plots also contain the highest values of predicted ancient human impacts. Given the long lifespan of Amazonian trees, many forest inventory and recensus sites may still be recovering from past disturbances, potentially skewing our interpretations of forest dynamics and our understanding of how these forests are responding to global change. Empirical data on the human history of forest inventory sites are crucial for determining how past disturbances affect modern patterns of forest composition and carbon flux in Amazonian forests.

Geospatial modeling approach to monument construction using Michigan from A.D. 1000–1600 as a case study

Significance Monumental construction was one way that past societies reconfigured their landscapes in response to social and environmental factors. Given that these factors changed through space and time, societies often constructed multiple types of monuments. We develop a maximum entropy modeling (MaxEnt) framework for identifying the spatioenvironmental factors that mattered most in the placement of different monument types. We turned to MaxEnt, given its ability to model spatial distributions from presence-only data. With modern development continuing to destroy archaeological sites across the globe, archaeological datasets, including monument locations, are evermore restricted to being presence-only data. Our framework shows how archaeologists can harness MaxEnt to develop robust models of past cultural processes, including monumentality, even when faced with limited, presence-only data. Building monuments was one way that past societies reconfigured their landscapes in response to shifting social and ecological factors. Understanding the connections between those factors and monument construction is critical, especially when multiple types of monuments were constructed across the same landscape. Geospatial technologies enable past cultural activities and environmental variables to be examined together at large scales. Many geospatial modeling approaches, however, are not designed for presence-only (occurrence) data, which can be limiting given that many archaeological site records are presence only. We use maximum entropy modeling (MaxEnt), which works with presence-only data, to predict the distribution of monuments across large landscapes, and we analyze MaxEnt output to quantify the contributions of spatioenvironmental variables to predicted distributions. We apply our approach to co-occurring Late Precontact (ca. A.D. 1000–1600) monuments in Michigan: (i) mounds and (ii) earthwork enclosures. Many of these features have been destroyed by modern development, and therefore, we conducted archival research to develop our monument occurrence database. We modeled each monument type separately using the same input variables. Analyzing variable contribution to MaxEnt output, we show that mound and enclosure landscape suitability was driven by contrasting variables. Proximity to inland lakes was key to mound placement, and proximity to rivers was key to sacred enclosures. This juxtaposition suggests that mounds met local needs for resource procurement success, whereas enclosures filled broader regional needs for intergroup exchange and shared ritual. Our study shows how MaxEnt can be used to develop sophisticated models of past cultural processes, including monument building, with imperfect, limited, presence-only data.

Further evidence for localized, short-term anthropogenic forest alterations across pre-Columbian Amazonia

By generating soil phytolith and charcoal data from a region of geoglyph construction in southwest Amazonia, Watling et al. (1) provide important evidence on a current debate over the scale and intensity of pre-Columbian modification of Amazonia (e.g., refs. 2⇓⇓⇓–6). The clear evidence for human activity in their study region from approximately 2000–600 B.P. makes it all the more interesting that the authors found “no evidence that sizeable clearings were created for any significant length of time (i.e., over multidecadal to centennial timescales) for geoglyph construction and use,” concluding the geoglyphs were used on a sporadic rather than continual basis (1). However, Watling et al. misstate … [↵][1]1To whom correspondence should be addressed. Email: pipernod{at}si.edu. [1]: #xref-corresp-1-1

Sporormiella as a tool for detecting the presence of large herbivores in the Neotropics

The reliability of using the abundance of Sporormiella spores as a proxy for the presence and abundance of megaherbivores was tested in southern Brazil. Mud-water interface samples from nine lakes, in which cattle-use was categorized as high, medium, or low, were assayed for Sporormiella representation. The sampling design allowed an analysis of both the influence of the number of animals using the shoreline and the distance of the sampling site from the nearest shoreline. Sporormiella was found to be a reliable proxy for the presence of large livestock. The concentration and abundance of spores declined from the edge of the lake toward the center, with the strongest response being in sites with high livestock use. Consistent with prior studies in temperate regions, we find that Sporormiella spores are a useful proxy to study the extinction of Pleistocene megafauna or the arrival of European livestock in Neotropical landscapes.

Comment on “Persistent effects of pre-Columbian plant domestication on Amazonian forest composition”

Levis et al. (Research Articles, 3 March 2017, p. 925) concluded that pre-Columbian tree domestication has shaped present-day Amazonian forest composition. The study, however, downplays five centuries of human influence following European arrival to the Americas. We show that the effects of post-Columbian activities in Amazonia are likely to have played a larger role than pre-Columbian ones in shaping the observed floristic patterns.