Edited by: Simon Haberle, Australian National University, Australia
Reviewed by: Encarni Montoya, The Open University, UK; Juanma Rubiales, Universidad Politécnica de Madrid, Spain
*Correspondence: Crystal H. McMichael
This article was submitted to Paleoecology, a section of the journal Frontiers in Ecology and Evolution
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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 landscapes remains untested in many regions. We assessed the degree of ancient human impacts across western Amazonia based on archeological 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 archeological site, sites in close proximity to archeological 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 archeological 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.
Tropical rainforests are a test bed for ecological research linking species diversity and distributions with the ecological niche (Elton,
Humans have been present as hunter-gatherers in Amazonia throughout the Holocene (Roosevelt et al.,
This evidence refutes the idea that Amazonia was a pristine forest prior to European discovery because of environmental limitations, particularly poor soils, which prevented ancient people from advanced societal development (Meggers,
Here, we took an interdisciplinary approach (e.g., Mayle and Iriarte,
All sites are located in lowland Amazonian forests (Figure
Teotônio | 1 | 11 | 1 | 2 | yes |
Barcelos |
3 | 11 | 0 | 31 | yes |
Tefe |
10 | 39 | 0 | 40 | yes |
Ayauchi |
11 | 58 | 1 | 15 | no |
Gentry |
10 | 78 | 1 | 79 | no |
Porto Velho-Manaus |
16 | 50 | 1 | 52 | no |
Rio Branco |
2 | 18 | 0 | 10 | no |
Los Amigos |
3 | 13 | 0 | 74 | no |
Iquitos |
3 | 7 | 0 | 40 | no |
Cocha Cashu |
1 | 9 | 0 | 37 | no |
“
The Cocha Cashu Biological Research Station (−11.9, −71.37) is located on lowland floodplain forests on rich alluvial soils and terra firme forests within Manu National Park in Peru. These forests receive 2100 mm of rain per year, most of which falls in October to May. Tree plot data from Cocha Cashu contain over 1215 tree species, documenting it as one of the most diverse and believed to be most pristine locations within Amazonia (Foster,
The Teotônio archeological site (−8.86, −64.06) is located on a river bluff on the Upper Madeira River in Brazil (Almeida and Neves,
Lake Ayauchi lies at 330 m above sea level on the eastern flank of the Andes, and is located above a floodplain that is within 2 km of Rio Santiago. The region receives 2000–3000 mm year−1 of rainfall with 0–2 months of dry season that supports dense tropical rain forest. Members of the Shuar nation currently inhabit the area, and they cultivate manioc, plantains, and papayas near the lake. The lake was cored in 1983 with a modified Livingstone piston-corer (Bush et al.,
Barcelos and Tefe are located in the central Amazon in Brazil. Both areas contain
Lakes Gentry and Parker (hereafter Gentry) are located at elevations of 200–300 m, lie >200 km from the Andes, and are located >20 km away from the Madre de Dios River in southeastern Peru. A precipitation gradient extends across the Madre de Dios watershed, ranging from ca. 2000 mm/year−1 in the north to ca. 1700 mm/year−1 in the south. The duration of the dry season is typically 2–4 months. Lakes Gentry and Parker were cored in 1998 and 2001 (respectively) with a Colinvaux–Vohnout coring rig (Colinvaux et al.,
Iquitos lies in the Peruvian state of Loreto, and typically receives 3400 mm/year with basically no dry season (1999–2005 data; Morrison et al.,
Los Amigos Biological Research Station is located at the confluence of the Madre de Dios and Los Amigos Rivers in southeastern Peru over 100 km away from Lakes Gentry and Parker. The area is also wetter, and receives 2700–3000 mm precipitation per year with a 3–5 month dry season (2000–2006 data; Pitman,
Porto Velho is located in central Amazonia in Brazil, and has the same climatic conditions as described for Teotônio.
Rio Branco is located in the Brazilian state of Acre, and the BR-164 traverses the state from Rio Branco in the east to Cruzeiro do Sul in the west. Annual rainfall near Rio Branco is ca. 1900 mm/year−1. The dry season lasts from May to September and is most severe (i.e., < 60 mm per month) during June, July, and August (Fonseca Duarte,
The methodology of soil charcoal analysis followed standard techniques (McMichael et al.,
Soil charcoal abundances, however, do not directly reflect fire parameters, but instead are a proxy for the presence or absence of fire (e.g., Clark and Patterson,
Phytoliths were extracted from ca. 20 g of soil using standard laboratory procedures (Piperno,
Charcoal presence was determined for each sample in each core for all of the regions, including the new data presented from Cocha Cashu and Teotônio. If any depth interval within a given core contained charcoal, the core was assigned as have charcoal presence. We then calculated the percentage of cores containing present charcoal for each region. Though the Teotônio site only contains one soil profile that was examined for charcoal, it is well established that macroscopic charcoal is frequently found in lowland Amazonian archeological sites, and is a required component for the formation of
We quantified percentages of arboreal, herb, and palm phytoliths for the Teotônio site. We then took those percentages and the previously published data from the other regions, and derived metrics of past vegetation change that could be compared within and between regions. Within-region variability was examined using boxplots that showed medians and quartiles of phytolith percentages of all samples within a region. To compare the regional values to the baseline sites of Teotônio and Cocha Cashu, which have only one core of phytolith data each (Table
Though age-depth relationships cannot be assumed in soils, several generalizations can be made regarding the temporal context captured by the soil charcoal and phytolith data. The first is that while reversals in dated material indeed occur in Amazonian soils, the majority of dated charcoal fragments within single cores or profiles have maintained a general stratigraphic integrity. A second consistent pattern is that the first 80–100 cm of soil typically captures the last 3000–5000 years of fire history (Sanford et al.,
A “human impact score” was derived for each region based on the following lines of evidence contained within soil cores: (1) the presence or absence of a known archeological site (i.e., Teotônio), which includes sites containing terras pretas, earthworks, or pottery; (2) the presence or absence of agricultural pollen or phytoliths; (3) the number of cores per region that contained charcoal; (4) the magnitude of change of early-successional and grass taxa in a given core; (5) the magnitude of change of palm taxa within a given core; and (6) the total magnitude of change of arboreal taxa in a given core. The first two provide direct evidence of human activity, and were given a weighting of five (5). The third line of evidence is likely also a direct result of human activity and was given a weight of four (4). The last three provide indirect evidence (i.e., changes in vegetation that may or may not reflect human activity) and were given lower weights. We then used the weighted sum of all proxies as the “human impact score.”
The Teotônio site had large and frequent charcoal fragments in every sample analyzed from 0 to 120 cm depth in each core. The survey at Cocha Cashu was conducted over an area of 8 km2, and only six of the 37 cores (16%) contained macroscopic charcoal (Figure
The percentage of cores containing charcoal from other regions ranged from 23% (Iquitos) to 90% (Barcelos). Barcelos and Tefe, which contain terras pretas within 10 km of the sampled locations (McMichael et al.,
The phytolith assemblages from the Cocha Cashu were consistently dominated by non-palm arboreal morphotypes (Figure
A mixture of arboreal and palm phytoliths dominated the phytolith assemblage at Teotônio (Figure
The cores exhibited variability within cores, but the median arboreal phytolith percentages were above 80% in all regions except at the Teotônio archeological site, where percentages were markedly lower (Figure
The one-sample
PVM | arboreal | 1.22 | 14 | 0.24 | 0.48 | 24.50 | 14 | 7E-13 | |
Tefe | arboreal | 0.21 | 9 | 0.84 | 1.00 | 11.15 | 9 | 1E-06 | |
Iquitos | arboreal | 8.00 | 2 | 0.02 | 55.00 | 2 | 3E-04 | ||
Barcelos | arboreal | 5.20 | 2 | 0.04 | 0.07 | 45.90 | 2 | 5E-04 | |
Ayauchi | arboreal | −6.51 | 10 | 0.00 | 0.00 | −25.44 | 10 | 2E-10 | |
Gentry | arboreal | −0.25 | 9 | 0.81 | 1.00 | −8.68 | 9 | 1E-05 | |
PVM | palm | −1.08 | 14 | 0.30 | 0.59 | −21.85 | 14 | 3E-12 | |
Tefe | palm | 0.27 | 9 | 0.79 | 1.00 | −9.68 | 9 | 5E-06 | |
Iquitos | palm | −6.43 | 2 | 0.02 | −48.38 | 2 | 4E-04 | ||
Barcelos | palm | −3.46 | 2 | 0.07 | 0.15 | −35.51 | 2 | 8E-04 | |
Ayauchi | palm | −6.51 | 10 | 0.00 | < |
−25.44 | 10 | 2E-10 | |
Gentry | palm | −0.25 | 9 | 0.81 | 1.00 | −8.68 | 9 | 1E-05 | |
PVM | herb | −0.58 | 14 | 0.57 | 1.00 | −9.33 | 14 | 2E-07 | |
Tefe | herb | −1.14 | 9 | 0.28 | 0.57 | −6.51 | 9 | 1E-04 | |
Iquitos | herb | −7.00 | 2 | 0.02 | −31.00 | 2 | 1E-03 | ||
Barcelos | herb | N/A | N/A | ||||||
Ayauchi | herb | −1.15 | 10 | 0.28 | 0.56 | −6.45 | 10 | 7E-05 | |
Gentry | herb | −4.64 | 9 | 0.00 | < |
−8.46 | 9 | 1E-05 |
When the delta values from the Cocha Cashu core were compared with the other regions, several differences emerged (Table
PVM | Arboreal | −3.24 | 14 | 0.01 | −27.81 | 14 | 1E-13 | ||
Tefe | Arboreal | 0.23 | 9 | 0.82 | 1.00 | −15.34 | 9 | 9E-08 | |
Iquitos | Arboreal | −15.50 | 2 | 0.00 | −68.79 | 2 | 2E-04 | ||
Barcelos | Arboreal | −6.43 | 2 | 0.02 | −33.07 | 2 | 9E-04 | ||
Ayauchi | Arboreal | −1.62 | 10 | 0.14 | 0.27 | −16.15 | 10 | 2E-08 | |
Gentry | Arboreal | 0.16 | 9 | 0.88 | 1.00 | −13.57 | 9 | 3E-07 | |
PVM | Palm | −4.83 | 14 | 0.00 | < |
−31.61 | 14 | 2E-14 | |
Tefe | Palm | −1.49 | 9 | 0.17 | 0.34 | −16.04 | 9 | 6E-08 | |
Iquitos | Palm | −16.63 | 2 | 0.00 | −66.52 | 2 | 2E-04 | ||
Barcelos | Palm | −7.86 | 2 | 0.02 | −36.66 | 2 | 7E-04 | ||
Ayauchi | Palm | −1.62 | 10 | 0.14 | 0.27 | −16.15 | 10 | 2E-08 | |
Gentry | Palm | 0.16 | 9 | 0.88 | 1.00 | −13.57 | 9 | 3E-07 | |
PVM | Herb | 0.09 | 14 | 0.93 | 1.00 | −8.95 | 14 | 4E-07 | |
Tefe | Herb | 3.72 | 9 | 0.00 | −6.36 | 9 | 1E-04 | ||
Iquitos | Herb | −8.00 | 2 | 0.02 | −47.00 | 2 | 5E-04 | ||
Barcelos | Herb | −2.00 | 2 | 0.18 | 0.37 | −15.00 | 2 | 4E-03 | |
Ayauchi | Herb | 4.40 | 10 | 0.00 | < |
−3.37 | 10 | 7E-03 | |
Gentry | Herb | 3.26 | 9 | 0.01 | −2.32 | 9 | 5E-02 |
The one-sample
Based on the collective archeological and palaeoecological evidence, the weighted human impact scores ranged from 635 (Teotônio) to 52 (Iquitos), with lower scores meaning less evidence of human impact and vegetation change (Table
Teotonio | Teo | 1 | 1 | 3 | 100 | 3 | 100 | 2 | 16 | 1 | 60 | 1 | 56 | 1 | 635 |
Ayauchi | Ay | 11 | 0 | 3 | 55 | 3 | 67 | 2 | 20 | 1 | 36 | 1 | 16 | 1 | 370 |
Gentry | G | 10 | 0 | 3 | 10 | 3 | 63 | 2 | 27 | 1 | 36 | 1 | 28 | 1 | 248 |
Tefe | T | 10 | 1 | 3 | 0 | 3 | 75 | 2 | 15 | 1 | 32 | 1 | 34 | 1 | 234 |
Porto Velho–Manaus | PV-M | 16 | 0 | 3 | 6 | 3 | 62 | 2 | 20 | 1 | 20 | 1 | 25 | 1 | 206 |
Barcelos | B | 3 | 1 | 3 | 0 | 3 | 90 | 2 | 3 | 1 | 7 | 1 | 5 | 1 | 199 |
Rio Branco | RB | 2 | 0 | 3 | 0 | 3 | 50 | 2 | 3 | 1 | 38 | 1 | 27 | 1 | 168 |
Los Amigos | LA | 3 | 0 | 3 | 0 | 3 | 47 | 2 | 4 | 1 | 5 | 1 | 3 | 1 | 107 |
Iquitos | Iq | 3 | 0 | 3 | 0 | 3 | 23 | 2 | 1 | 1 | 3 | 1 | 3 | 1 | 52 |
Cocha Cashu | CC | 1 | 0 | 3 | 0 | 3 | 16 | 2 | 3 | 1 | 16 | 1 | 9 | 1 | 60 |
Most scholars loosely agree on the heterogeneity of the ancient human footprint in Amazonia (Heckenberger and Neves,
The seasonal regions of the Amazon, here considered those containing more than 3 months per year with < 100 mm precipitation, have a higher density of known settlement sites (McMichael et al.,
The impact scores were in agreement with these models, except for the lake sites in the aseasonal portions of western Amazonia, which contained higher impact scores than most of the regions surveyed (Table
The predicted and observed distribution of these ancient impacts (Figure
Further, our data support the view that the hyper-diverse forests of Cocha Cashu are some of the most undisturbed forests remaining in the Amazon (Terborgh,
The Los Amigos and Cocha Cashu research stations provide an excellent opportunity for comparing tree plot data with modern and prehistoric phytolith signatures to examine the successional trajectories of tropical forests under different ancient disturbance regimes. Most ecological study sites (i.e., tree plots) are located in areas that are not known to contain archeological features or modified
A suite of 227 species, termed hyperdominant species, accounted for approximately half of all the individual trees inventoried across 1170 Amazonian tree plots (ter Steege et al.,
The phytolith analyses, including the metrics used to assess within-region and between-region variability, were sufficiently sensitive to detect changes in forest composition, both in the presence and likely absence of human activity (Figures
The data presented here also identify regions that can be considered baselines from which to measure ancient disturbance, which recent reviews claiming that the majority of Amazonia was domesticated failed to do (Clement et al.,
It must be noted, however, that an observed high density of Brazil nut, or Arecaceae (palms) does not necessarily imply ancient human manipulation. The Cocha Cashu phytolith assemblages establish a first measurement of the variability of palms through time where ancient human impact appears to be much lower than in the other regions surveyed (Table
All other regions that were sampled exhibited a lesser human signature than Teotônio, but more evidence of human activity than Cocha Cashu and Iquitos. These lightly- to moderately-impacted regions contained evidence of past fires and/or evidence of localized agriculture. A clear and expected trend in the vegetation response was the increase of herb phytoliths in areas containing more evidence of fire (Figures
Soil data from around Lakes Ayauchi (Ecuador) and Gentry (Peru) contained trends toward more forested systems in modern times (i.e., signals of localized ancient forest clearing), and localized increases in herb and palm phytoliths (Figures
Signatures of human activity in paleoecological records, including the presence or absence of charcoal, presence or absence of agricultural microfossils, presence or absence of ceramics, lithics, and other cultural artifacts, and presence or absence of terra preta soils, can all be assessed alongside phytolith assemblages to provide a robust reconstruction of effects of ancient humans on the vegetation. Here we have provided a method to quantify those effects across regions and with dealing with unequal sample numbers. Multi-proxy approaches are required to continue unraveling the effects of both ancient and modern humans on Amazonian landscapes.
CM conceived the idea, collected the soil cores, contributed to the phytolith analysis, and wrote the manuscript. DP conceived the idea, and performed the majority of the phytolith analyses. EN conceived the idea and performed the archeological surveys MB conceived the idea. FA and GM performed the archeological excavations. ME performed the charcoal analyses.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We would like to thank NSF DEB 0742301. This is publication #129 from the Climate Change Institute at Florida Institute of Technology.