PALEOECOLOGY PALEOECOLOGY PALEOECOLOGY
Amazonian exploitation revisited: ecological
asymmetry and the policy pendulum
Mark B Bush1* and Miles R Silman2
The influence of pre-Columbian human populations on Amazonian ecosystems is being actively debated. The longstanding view that Amazonia was only minimally impacted by human actions has been challenged, and a new paradigm of Amazonia as a “manufactured landscape” is emerging. If such disturbance was the norm until just 500
years ago, Amazonian ecosystems could be far more ecologically resilient to disturbance than previously supposed.
Alternatively, if the “manufactured landscape” label is an overstatement, then policy that assumes such resilience
may cause substantial and long-lasting ecological damage. We present paleoecological data suggesting a middle
path, in which some areas were heavily modified, but most of Amazonia was minimally impacted. Bluffs adjacent
to main river channels and highly seasonal areas appear to have been the most extensively settled locations. Away
from areas where humans lived, their influence on ecosystems was very local. Consequently, we see no evidence suggesting that large areas at a distance from rivers or in the less seasonal parts of Amazonia were substantially altered
by human activity. Extrapolating from sites of known human occupation to infer Amazon-wide landscape disturbance may therefore potentially lead to unrealistic projections of human impact and misguided policy.
Front Ecol Environ 2007; 5(9): 457–465, doi:10.1890/070018
T
he first scientists to enter Amazonia encountered a
wonderland of undescribed organisms living in what
appeared to be unoccupied and untouched forest.
Adjectives they used to describe the forest included “virgin”, “pristine”, and “timeless”, a vision which became
incorporated into scientific thinking. Explanations of
high Amazonian diversity invoked the stability and the
museum-like quality of unchanging environments that
accumulated species and minimized extinctions (Stebbins
1974). Coupled with this view was the romantic idyll of
hunter–gatherers living in harmony with nature.
Such views have been altered by excavations of archaeological sites from the mouth of the Amazon to the High
Andes, which reveal a long record of human occupation,
ceramics manufacture, and agriculture (eg Roosevelt 1991;
Roosevelt et al. 1991). For at least some groups, a trajectory
In a nutshell:
• Pre-Columbian human influence on Amazonia was spatially
heterogeneous, with some sites intensively altered
• The large majority of Amazonia was probably barely influenced
by human activity
• Fossil pollen and charcoal data point to localized influence
around widely scattered occupation sites rather than a uniform
influence
• Arguments that the Amazon forest is a manufactured landscape, and hence resilient to human activity, are only locally
true and should not be used to set regional management policy
1
Department of Biological Sciences, Florida Institute of Technology,
Melbourne, FL 32901 *(mbush@fit.edu); 2Department of Biology,
Wake Forest University, Winston Salem, NC 27104
© The Ecological Society of America
of increasing populations and greater reliance on agriculture is evident for the past several thousand years. As
anthropological and paleoecological knowledge of these
systems has deepened, the view of Amazonia as untrammeled and changeless has disappeared (Clark 1996).
The realization that human populations throughout the
Americas declined sharply following European contact
altered expectations of the level of human disturbance
and modification of systems prior to 1492. Indeed, a series
of articles (eg Clark 1996; Erickson 2000; Heckenberger
et al. 2003; Erickson 2006) and a recent book (Mann
2005) suggest that the pendulum of scientific opinion has
swung from the extreme view of the Amazon as “virgin”,
has passed a midpoint of “disturbance localized around
main waterways”, and is now headed toward the other
extreme of “widespread and pervasive human disturbance”. The title of Heckenberger et al.’s (2003) article,
Amazonia 1492: pristine forest or cultural parkland?, was
deliberately provocative, but if taken literally depicts a
simple dichotomy. We suggest the possibility of a middle
path. Here, we provide an alternative interpretation of
the existing data, and caution that uncritical acceptance
of Amazonia as a manufactured landscape may be misguided and could lead to unsound policy.
In Amazonia, there has been a commendable use of science by governments, particularly the Brazilian
Government, to set conservation policy. Refuge theory
was widely accepted in the mid-1980s and was used at the
time, together with maps of diversity, to prioritize areas
for conservation (Dinerstein et al. 1995). Although this
theory is now largely discredited (Colinvaux et al. 2001),
the basic biogeographic patterns of high local endemicity
and diversity used to identify “refugia” also formed effecwww.frontiersinecology.or g
457
Exploitation of the Amazon revisited
human population size, cultural development, and the geographic extent of their
influence. The initial European descriptions of Amazonian populations, their size,
High
wealth, and societal sophistication, are at
1532 Arrival
(8–10 million)
10
of Pizarro in
best incomplete and at worst misleading.
Ecuador
1524–25
Friar Gaspar de Carvajal, who accompaAndean smallpox
nied Francisco de Orellana on the first
epidemic
1743
European voyage down the Amazon River
Amazonian
in 1541, was writing to impress the King of
Medium
account of
Spain. While he describes extensive settle5
(3–5 million)
de la
ments stretching for tens of kilometers
Condamine
along the river, and encounters with thouLow
sands of warriors, his credibility is under(1 million)
mined by fanciful descriptions of such
curiosities as single-breasted, warrior
0
women. Although the river was named for
1000
1500
2000
these “Amazons”, they were never encounAD
tered in subsequent exploration. In stark
Figur e 1. Annotated schematic diagram of human population in Amazonia. contrast to Carvajal’s account, Charles
Low, medium, and high estimates are shown.
Marie de la Condamine, who traveled the
Amazon in 1743, described a very different
tive guides for the establishment of protected areas. Land- setting, in which isolated bands of people struggled for
use policy must be founded on scientific knowledge, but existence in the most basic circumstances (Smith 1990).
scientific knowledge is subject to continual revision and Many subsequent visitors validated this portrayal of physmodification. Consequently, when science appears to ical and cultural poverty, so different from the agrarian
support policies that could irreversibly damage ecosys- idyll described by Carvajal. However, in a similar debate
tems, the science deserves additional scrutiny.
over indigenous influence on the vegetation of New
England, Foster and Motzkin (2003) caution against
accepting eyewitness accounts that post-dated European
Population
size
and
human
impact
arrival by > 100 years as proper reflections of conditions
Estimates of human population in Amazonia at the time immediately prior to first contact. For many years,
of European arrival range from one to 11 million, with a Carvajal’s account was seen to be essentially unreliable
tendency toward higher values in more recent estimates (Smith 1990). However, there was renewed interest in
(for overviews see Denevan [1996] and Mann [2005]). the possibility of disease decimating this society, promptThe population collapse that occurred between ca 1520 ing Mann (2005) and others to suggest that both
and 1600 AD may have reduced Amazonian populations accounts were essentially true. Carvajal did witness dense
by an astounding, yet plausible 95% (Denevan 1976).
populations, whereas Condamine was viewing the shatDiseases brought to Central America by the first tered remnants of a society in which 90–99% of the popEuropeans spread into South America via trade among ulation had succumbed to smallpox and other European
native peoples. It is probable that the Incan emperor diseases (Smith 1990). If there had been large, settled
Huayna Capac died when a deadly epidemic (probably populations in Amazonia for a thousand or more years
smallpox) swept through the Andes in 1524–1525 prior to contact, this raises the question: “To what extent
(Figure 1), 6 years before the arrival of Pizarro, the first did human activities prior to the population collapse
conquistador to enter Peru (Hemming 1970). Smallpox, influence Amazonian ecosystems?”
measles, diptheria, and influenza ravaged the Andean
Drawing from the archeological literature, Mann
populations over the next 50 years (Denevan 2003). (2005) elaborates a vision of Amazonia in which the area
While Spanish accounts describe scenes of bodies piled supported upwards of 10 million people, with great urban
up and abandoned in Andean villages, nothing is known centers (eg Marajó Island, Santarém), and many other,
about epidemic diseases in the lowlands. However, the smaller cities with well-developed infrastructure (eg
same genetic susceptibility to European diseases (Black those adjacent to the Xingu River and Lake
1992), known trade contact with Andean communities Manacapuru). The banks of the main Amazon channel
(Hemming 1970), and densely packed populations along and tributaries such as the Rio Negro, Madeira, Xingu,
major rivers (Smith 1990), make it likely that and Tapajós, teemed with villages (Denevan 1996).
Amazonian populations were similarly devastated.
Great transforming public works, such as the raised vilAt the heart of the discussion over pre-Columbian lages, causeways, and fish weirs of the Beni region of
influence in Amazonia are contradictory estimates of southern Amazonia, are attributed to large workforces
1492 Columbus
arrives in
New World
1525 Death of
Huayna Capac
1541 Orellana
navigates the Amazon
Millions of inhabitants
458
MB Bush and MR Silman
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© The Ecological Society of America
Exploitation of the Amazon revisited
Courtesy of B Glaser
and complex societal hierarchies (Balée (a)
(b)
1989; Erickson 2000).
A further legacy of human endeavor is
the distinctive soil layering, up to 2 m
deep, of black or brown soils with elevated nutrient availability, known as terra
preta del Indio (Figure 2). These nutrientrich black soils often contain broken
ceramics and appear to have been altered
by the addition of ash, green manure, or,
in some cases, fish meal (Lehmann et al.
2003). Such soil modification by humans
may have held the key to sustainable
exploitation of the land.
Some Amazonian locations were
occupied for several thousand years.
Shellfish and fish were undoubtedly
important dietary components, but so
too were maize and manioc. On most
Amazonian soils, five harvests of maize
in a span of 2–3 years exhausts the soil Figure 2. Natural and modified Amazonian soils. (a) An oxisol and (b) a terra preta
of essential nutrients. A fallow period of soil. Oxisols are the most common dry land soils in Amazonia and when modified with
about 20–30 years is needed before the the addition of ash and organic waste are stained black to form terra preta.
land can again support crops of maize
(Kellman and Tackberry 1997). A prevailing view is that heartwood of an old tree killed by the fire, or dating longthese villages remained occupied year after year at popu- dead but undecomposed wood lying on the soil surface. As
lation densities too high to allow such long fallowing most tropical forest fires char bark but not heartwood
between periods of cultivation. The elevated nutrient (sensu Cochrane 2003), it is unlikely that heartwood from
availability (particularly phosphorus) of terra preta soils this source is represented in the charcoal. Also, because
may have allowed near-permanent cultivation of these rapid rates of decay in the tropics lessen the fuel load of
soils, potentially supporting a much larger human popula- undecomposed heartwood lying on the forest floor comtion density than the native soils (Glaser et al. 2001). pared to temperate or boreal settings, the probability of
Indeed, the terra preta model may offer insight into how charcoal being composed of heartwood is similarly low.
Amazonia could withstand intensified agricultural The charcoal ages in tropical forest soils are therefore
likely to be reasonably representative of the actual fire
exploitation in the future (Glaser et al. 2001).
Based on a wide array of evidence, Balée (1989) esti- date (taking into account the errors of 14C dating). When
mated that 11% of Amazonian vegetation in pre- 304 ages for Amazonian soil charcoal are plotted against
Columbian Amazonia was extensively used and altered time (Figure 3 a,b), the data strongly suggest an expansion
by human activity. That use ranged from slash-and-burn of agricultural activity at ca 250 AD, then a series of peaks
agriculture to enrichment with fruit-bearing trees (eg of fire activity between ca 700 and 1550 AD, followed by
Bactris gasipaes [peach palm], Bertholettia excelsa [Brazil a sudden collapse at ca 1600 AD. One of the more surprisnut], Annona spp [includes guanabana or sweet soursop], ing results of this analysis is that the highest peak of apparand Mauritia flexuosa [aguaje, moriche, or nontoca; a ent fire activity is not at ca 1550 AD, but at ca 700–800
AD. Nevertheless, the trajectory of implied disturbance
palm whose fruit is used to make a drink]).
fits well with archaeological estimates of village expansion
(eg Roosevelt 1980; Heckenberger et al. 1999).
Inferring past human disturbance
Hammond et al. (2006) conducted the first systematic
Fire is the oldest human tool for both small- and large- survey for soil charcoal in a 60 000-ha area of Guyana.
scale manipulation of the landscape, and one of the major Their analysis of > 280 soil profiles revealed charcoal in all
sources of evidence for past human disturbance of tropical of them. The authors considered it unlikely that these were
forest systems comes from fire histories. Charcoal is pro- natural fires – although this section of Guyana is subject to
duced by forest fires and is gradually buried within a soil El Niño-related droughts – and concluded that there had
profile. Charcoal layers from soil pits can be radiocarbon been extensive human influence in upland eastern
dated, yielding an age estimate for fire events (generally Amazonian forests within the past millennia.
± 50–100 years). If old wood is burned, the resulting 14C
Pollen, charcoal, and phytolith data from Amazonian
dating of charcoal overestimates the time since the fire lakes also provide evidence for widespread human occupa(Gavin 2001). Sources of such old wood could be the tion prior to European contact (Figure 4), and there is vir© The Ecological Society of America
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459
Courtesy of B Glaser
MB Bush and MR Silman
Exploitation of the Amazon revisited
(a)
Known terra preta
Soil charcoal dated
Atlantic
Ocean
Pacific
Ocean
18
16
14
12
10
8
6
4
2
0
0
500 1000
Kilometers
(b)
50
15
0
25
0
35
0
45
0
55
0
65
0
75
0
85
0
95
10 0
5
11 0
5
12 0
5
13 0
5
14 0
5
15 0
5
16 0
5
17 0
5
18 0
5
19 0
50
Number of records
460
MB Bush and MR Silman
Year AD
Figure 3. Soil carbon data from Amazonia. (a) Sketch map
showing major Amazonian rivers, locations of radiocarbon-dated
soil charcoal, and terra preta. (b) Ages of 228 carbonized layers in
Amazonian soils. Data are derived from published accounts of soil
charcoal in Amazonia (Sanford et al. 1985; Saldarriaga and West
1986; Desjardins et al. 1996; Piperno and Becker 1996; Pessenda
et al. 1998; Tardy 1998; Santos et al. 2000; Walker 2000;
Francis and Knowles 2001; Neves et al. 2004; Pessenda et al.
2004; Hammond et al. 2006). All ages are expressed in calibrated
calendar-years before present (Stuiver and Reimer 1993).
tually no sign of human presence at almost every site for
the period 1500–1900 AD. However, these data do not
portray the intensity or scale of land use at a local level.
Within the archaeological community, a full spectrum
of opinion can be found, from those suggesting minimal
pre-contact disturbance (Meggers 1954, 2003) to those
citing extensive local impacts (Denevan 1996), to those
who believe in regional, even basin-wide, habitat modification (Erickson 2000). Some anthropologists are now
referring to the forests of Amazonia as “a cultural parkland”, “human created”, or “built landscapes” (Erickson
2000; Heckenberger et al. 2003; Mann 2005). Indeed,
Charles Clement, an anthropologist based at INPA
(National Institute for Amazonian Research) in Brazil is
quoted in Mann (2005) as stating, “I basically think
[Amazonian lowland forest is] all human created”. Clark
Erickson is quoted in Mann (2005) as saying that
“…built environment applies to almost [all], if not all,
Neotropical landscapes”. As these ideas are repeated
they tend to gain credence, and may subsequently influence public perceptions and policy. But how robust are
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the data underlying the assertions of widespread alteration of Amazonia?
The hypothesis of widespread Amazonian landscape
management is based on analyses of archaeological sites and
the assumption that there was a large pre-contact
Amazonian population (> 10 million people). A caveat
must be applied to these data, and indeed all of the data
that we have to date about human disturbance in the
Amazon, which is that they are derived from just a few locations, and do not represent either a systematic or a randomized sampling design. There is no ecological component
predicting which forest was most likely to be occupied. Was
disturbance spread evenly across all of Amazonia or concentrated near human habitation? Is it safe to extrapolate
results from sites where we know human habitation occured
to the rest of Amazonia?
Ecologists are familiar with problems of scale. Indeed,
“the problem of relating phenomena across scales is the
central problem in biology and in all of science” (Levin
1992). Some general observations from ecology raise at
least three concerns regarding how we may extrapolate data
from the kind of dot map shown in Figures 3 and 4. First,
landscapes are heterogeneous, and the resulting variability
in environmental factors produces characteristic patterns of
distribution and habitat use in nearly all species. Second,
although species distributions can be widespread, their
occurrence may be very local. For example, a dot map of the
painted turtle (Chrysemys picta) would show that it is distributed across much of the continental US, but its actual
occurrence is highly localized within a landscape. Third, a
species can be widespread and also have a very substantial
impact on the system where it is found, but this is not the
same as saying that a species has a widespread, intense
impact, or makes wholesale changes to the habitat in which
it lives. Thus, extrapolating observations from dot maps can
be dangerous, especially when the dots represent discrete
activities of limited spatial extent (eg terra preta formation).
A first step in making the conjectures that are so critical
to ecosystem management in Amazonia is recognizing the
potential sources of bias that affect what we infer from data.
The data in Figure 4 have two major potential sources of
bias. First, lakes used to study paleoecology may not accurately represent the landscape as a whole, because they are
attractive places for human settlement – humans like to live
by water now and, apparently, always have (eg Bush et al.
2007b). Also, while maps of terra preta sites are available,
we have no data on the distribution of soil pits that did not
show modified soils. It is a statistical certainty that when
extrapolations about land use are made from known archaeological centers and exclude other samples, the analyses
will exaggerate human impacts. So how do we go about
painting a more accurate picture of past human impacts?
Investigating occupation: the scale of impact
Fossil pollen and charcoal records (Figure 5 a,b) derived
from the analysis of lake sediments can provide detailed
© The Ecological Society of America
MB Bush and MR Silman
Exploitation of the Amazon revisited
insights into Amazonian history. Pollen
and spores released from plants and
1
blown or washed into lakes become
4
incorporated into the sediment. Year
2
after year, the sediment accumulates,
burying and preserving these plant
microfossils in anoxic mud. Lake sediments are retrieved using coring rigs supported by inflatable boats. The vertical
3
columns of mud and the buried layers of
5
microfossils they contain record the
composition of vegetation that grew
around the lake. Neotropical pollen and
spores can often be identified to genus,
and sometimes even to species. The
radius of land represented in the microfossil record – the vegetation directly
represented in the record – depends on Figure 4. Evidence of widespread pre-Columbian occupation in Amazonia. Map
lake size. In the moderate-sized lakes we shows the known distribution of modified (terra preta) soils, lakes with
discuss here, the great majority of the paleoecological records with and without evidence of human occupation, the major
pollen would have been derived within a archaeological settings of (1) Marajó Island, (2) Santarém, (3) the upper Xingu,
(4) Manacapuru, and (5) the Beni. The region that Mann (2005) maps as having a
few kilometers of the lake.
During a forest fire, charcoal particles high probability of dense settlement is shown. Terra preta data from Lehmann et al.
are created in both the smoke and the (2003) with additional data from A Zimmerman. Pollen data from numerous
charred plant remains. This charcoal authors summarized in Bush et al. (2004; see also Bush et al. 2007a).
falls or is washed into nearby lakes. The
finest fraction of charcoal reflects regional fire histories, one or more lakes in a cluster show human activity, then
whereas relatively large particles (ie those > 160 ␮m in the district is circled. At the broadest scale – whether a
length) indicate past fire in the adjacent watershed region shows human impacts or not – it appears that
(Clark 1988). Today, fire is rare in most natural Amazon Amazonia was broadly, and possibly universally, influsystems, and many sediments from undisturbed settings enced by people.
contain no charcoal. Finding evidence of regular burns
Looking at the lake data on a finer scale, however,
may therefore indicate past hunting activity, where fire is shows the dangers of extrapolation. Three lake districts in
used to drive game or improve its habitat, as a mechanism which multiple lakes have been analyzed provide similar
to clear forest for semi-permanent dwellings, or to spatial extents of human activity in widely separated
increase populations of those light-demanding plants pre- Amazonian landscapes (Athens and Ward 1999; Weng et
ferred by hunter–gatherers. Finding both charcoal and al. 2002; Bush et al. 2007 b,c). In two settings, Prainha
pollen from crops such as corn (Zea mays) or manioc and Maldonado (Figure 6 a,b), one lake has an unambigu(Manihot esculenta) is a clear indicator of past agriculture. ous record of occupation and agricultural use from about
Zea mays is not native to Amazonia, and its pollen is dis- 4000 years ago until ca 1600 AD. The third location, a
tinctive, due to its surface pattern and large size (com- swamp named Maxus 5, near Yasuni in Ecuador (Figure
monly 80–110 ␮m). Maize pollen is particularly poorly 6c), lies in wet forest that has very little seasonality. This
dispersed, making it a good marker of local crop use.
site has a long record of charcoal in its sediment, while
Given these two proxies for human activity – microfos- two neighboring wetlands have none. Human occupation
sils and charcoal – we can start to look at regional com- in this landscape is more tentative than in the other setparisons of where humans have altered the landscape tings, as no crop pollen or artifacts are associated with
(Figure 4). Of the 22 pollen and charcoal records from Maxus 5. Nevertheless, the discovery of charcoal at just
lakes shown as yellow squares, 13 contained evidence of one of three settings is a likely indicator of human activpre-Columbian occupation. A minority of these sites lay ity. The forest at Yasuni receives ~ 2800 mm of precipitawithin the area predicted by Mann (2005) to be heavily tion each year, with no distinct dry season, making natoccupied, but this pattern may reflect the relative abun- ural fires exceptionally rare, if not totally absent. That
dance of lakes suitable for paleoecological study as much only one of the three sites contains charcoal, and that the
as it does human distribution. Most of these lakes were charcoal was found in multiple samples, adds strength to
chosen for the purpose of looking at the vegetation his- the suggestion by Athens and Ward (1999) that the chartory of the particular area, not as a random or even repre- coal was produced by local human activity.
sentative sample of Amazonia. At the scale of Figure 4, it
In the Maldonado lake district, the paleoecological
is not always possible to show individual lakes, and so if record of human activity at Lake Gentry is supported by
© The Ecological Society of America
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461
Exploitation of the Amazon revisited
462
MB Bush and MR Silman
widespread those effects were in the vast
area of the Amazon basin, particularly those
far from water, requires further study.
Specifically, it requires a research program
that integrates existing data with new samples taken in ways that allow inferences to
be made about broader Amazonia.
Two other observations emerge from
analysis of the charcoal data from
Amazonian soils. The peak of fire frequency
was observed not when human populations
were presumably at their largest (ie immeFigur e 5. Markers of human occupation from Amazonian lake sediment. (a) A diately prior to European contact), but at ca
pollen grain of corn (Zea mays) and (b) microscopic charcoal fragments.
700–800 AD (Figure 3b). This period was
thought to be one of peak El Niño activity
the presence of stone tools and a midden adjacent to the (Thompson 2000), when droughts beset Amazonia and
lake. The paleoecological markers of human occupation forests were more highly flammable. It is highly probable
at this lake, corn and manioc pollen and charcoal, indi- that the great majority of these fires were humancate that agricultural activity in the region began around induced, but that during periods of intense drought, large
4400 years ago, with as much as 3000 years of burning areas were burned (making them more likely to be
prior to that time. Similarly, at Lake Geral in the Prainha detected today) when small-scale fires escaped to become
group, corn pollen was found regularly between 4000 and wildfire. In this way, the spatial scale of human distur400 years ago (Bush et al. 2000, 2007c), and regularly bance in Amazonia as a whole does not necessarily track
occurring charcoal appeared between 8000 and 400 years the intensifying history of local land use in the most popago. However, in both the Maldonado and Prainha ulous areas. The capacity of humans to disturb the system
groups, other lakes showed either a small increase in appears to have been strongly influenced by climatic concharcoal abundance or no charcoal whatsoever, and none ditions, not simply a growing human presence. In addicontained pollen from corn or manioc. These records sug- tion, the time for post-disturbance forest recovery in
gest that human occupation and land conversion were these settings is not the 500 years implied by the disease
local in nature. Sites within 3–5 km of an occupation model, but > 1000 years.
center appeared to be quite heavily used but, beyond this,
One model of human settlement in Amazonia does
human influence declined markedly, so that at distances incorporate known and inferred densities at the landof 50 km there was no evidence of human activity. This scape level, albeit through observations of the preferences
pattern is very similar to that described by modern of modern indigenous populations. Denevan (1996) proanthropologists, where hunting and land management is posed a bluff model of human occupation, in which
concentrated in a 1–3 km radius around the center of indigenous peoples preferentially settled sandy bluffs
habitation (Glanz 1991; Apaza et al. 2002).
alongside rivers and where there were views over wetThe absence of humans in these records is also striking lands. Denevan estimates that as many as 10 million peoin another way. In both the Prainha and Maldonado lake ple lived in these densely populated riverbank settings.
districts, human population collapses followed European With just 400 000 scattered through all the rest of
contact, as both crops and charcoal disappear from the Amazonia, the impact of people outside the high-density
records in the past 400 years. Notably, the apparent col- locations may have been very limited. The paleoecologilapse is evident even at sites that were never visited by cal data are entirely consistent with the long-term aggreEuropeans, providing unambiguous support for the gation of people around key landscape features implied by
anthropological and historical hypotheses of widespread Denevan’s settlement model, and the inference that the
epidemics.
rest of Amazonia was lightly settled.
An overall picture is clear: the paleoecological data confirm that Amazonia was exploited by indigenous peoples Seasonality and location
who practiced agriculture and developed urban centers
(Roosevelt 1980, 1991; Roosevelt et al. 1991; Hecken- Although our knowledge of past human occupation in
berger et al. 2007), and the populations collapsed shortly Amazonia is still in its infancy, the case for extensive occuafter European contact. However, the data do not support pation and use of the landscape by pre-Columbian human
the extrapolation of observations from occupied sites to activity in Mesoamerica is clear. We need to develop
infer uniformly widespread impacts and landscape transfor- hypotheses that predict where humans would have existed
mation. There is no doubt that humans are important in Amazonia at densities sufficient to alter landscapes. For
transformers of Amazonian landscapes and have been so example, one initial prediction might be based on seasonthroughout much of the Holocene. Determining how ality. Civilizations such as the Olmec, Maya, and Aztec
(a)
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(b)
© The Ecological Society of America
MB Bush and MR Silman
Exploitation of the Amazon revisited
50˚
(a)
50˚
(b)
0˚
Santa Maria:
lightly used,
not farmed
(c)
Saracuri:
used, but not
farmed
N
20
km
Maxus 1,
not used
Werth: not
used
Geral:
ocupied 7000
years, farmed
4000 years,
abandoned
c 1600 AD
0
0˚
Gentry: 4000 years
farming, abandoned
c 1600 AD
Rio Tipitini
Parker:
used, but
not farmed
Vargas:
little use,
not farmed
N
Maxus 5,
charcoal and
occupation
50˚
0˚
Maxus 4,
not used
R. Madre de Dios
human activity (Balée 1989), but it is
at odds with assertions regarding the
km
Amazon River
km
generality of disturbance in AmaFigure 6. A spatial context for pre-Columbian disturbance within three Amazonian zonia, particularly broad-scale, intenlake districts. (a) Lakes near Prainha, Brazil (1˚41’23.20” S, 53˚33’44.41” W); (b) sive disturbances. More importantly,
lakes near Puerto Maldonado, Peru (12˚9’51.51” S, 69˚5’59.74” W); (c) lakes near our admittedly preliminary analysis
Yasuni, Ecuador (0˚53’53.02” S, 76˚10’24.58” W).
provides a set of predictions and a
means for testing them, so that we
flourished in the highly seasonal settings of the Yucatan can update our model of past human disturbance in the
and central Mexico. In landscapes with stronger gradients Amazon, and of the effects of the different kinds and
between ever-wet and seasonal forests, such as in Panama intensities of disturbances on Amazonian forests.
and Costa Rica, the dense populations may have deforested the seasonal areas (Piperno et al. 1991), whereas Asymmetry of policy
impacts on the dense, wet forests of the Caribbean slope
Some years ago, Bush and Colinvaux (1994) showed that
were never as great (Dickau et al. in press).
A similar pattern may also have prevailed in Amazonia, two lake records from the Darien region of Panama
with highly favorable areas supporting large populations. demonstrated surprisingly long histories of human disturFigure 7 shows the major archaeological sites that have bance and abandonment. Since then, logging companies
been described. The main Amazon channel is clearly have attempted to use that paper to justify timbering
important as a trade route, and also as an excellent source some of these remote forests, arguing that, if the region
of fish, shellfish, and game. Within this setting, the con- supported cornfields 500 years ago, the forest ecosystems
fluences of rivers became centers of settlement and popu- must be young and resilient, and that, therefore, logging
lation hubs, as did the mouth of the Amazon. Away from now would not lead to loss of biodiversity. The problem
this ribbon of occupation, signs of human habitation can with this argument is that the real scale of disturbance is
be found scattered across Amazonia. Of the areas further ignored. The extended-family, slash-and-burn farming
away from the major rivers, the most complex cultural style of pre-Columbian times would have had a very limdevelopment appears to have occurred in the seasonal ited impact on these systems. Certainly, there may have
(defined as having the greatest intra-annual monthly been much larger impacts around major population cenvariability in rainfall) locations of southern Amazonia ters, but the case has yet to be made that the halo of
(eg Heckenberger et al. 2007). This incorporation of sea- human influence was anything other than local for the
sonality as a factor paralleling geographic location may great majority of settlement sites.
There is no question that more effort needs to be
form a powerful predictive model for where major occuinvested in obtaining a clearer picture of the extent of
pation sites are likely to be found.
Our contention that pre-Columbian transformation of pre-Columbian settlement and alteration of Amazonian
Amazonian landscapes was essentially local and spatially habitats. In the meantime, it would be prudent for ecolopredictable does not contradict the development of com- gists to dig a soil pit on their research site and analyze the
plex societies (Heckenberger et al. 2003; Heckenberger et soil for charcoal (in tropical soils, manganese and titaal. 2007) or that substantial areas were influenced by nium also form black accretions that are strikingly similar
0
5
© The Ecological Society of America
0
10
N
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463
Exploitation of the Amazon revisited
MB Bush and MR Silman
that needs to be conserved to retain functioning ecosystems.
It is not hyperbole to state that, together, these factors
could precipitate an Amazonian extinction crisis. Thus,
although the pendulum of scientific opinion can swing
through an arc from pristine wilderness to cultural parkland, the implications of the different resulting policies
could be ecologically asymmetric. Determining the true
scale of pre-Columbian influence on Amazonian ecosystems will undoubtedly lead to a more sophisticated view
of these landscapes, in which their ecological, climatic,
and cultural heterogeneity are recognized.
464
Acknowledgements
Figure 7. Seasonality and major settlement sites. If, as in
Central America, colonization of seasonal sites was more
extensive than colonization of ever-wet sites, predictions of
human population densities might be improved by inclusion of
seasonality as a factor. Seasonality is shown here on a relative
scale of intra-annual monthly variability, with a score of 0 being
aseasonal data from Tropical Rainfall Monitoring Mission
satellite 1998–2004. Legend: (1) Guyana charcoal study area,
(2) Marajó Island, (3) Santarém, (4) Manacapuru, (5) the
upper Xingu, and (6) the Beni (Silman 2007).
in appearance to charcoal) to determine the probability
that they are working on an impacted site. However, it
would be sensible for policy makers to assume minimal
past impacts at sites away from major rivers and known
archaeological centers.
If policy makers err on the side of underestimating past
human impacts and the forests are truly more resilient
than we believe, the forest may be overprotected. At
worst, this policy conserves a resource for later exploitation and buffers against disruptions of ecological interactions, such as trophic cascades, which may be vitally
important to the structure of tropical forests (Silman et al.
2003; Terborgh et al. 2006). If, on the other hand, the
error is in overestimating past exploitation and assumed
forest resilience, then forests are likely to be used unsustainably, and in ways that may have broad repercussions
in the face of climate change. A particular worry is that
increasing fragmentation will lead to more frequent fires
in the future (Nepstad et al. 2001), and will also impede
plant migration in the face of climate change (Ibáñez et
al. 2006). Worse still, Amazonian forests are much more
than just trees, and the plant–animal interactions crucial
to the maintenance of diversity, and regeneration after
disturbance, may also be disrupted by such disturbance
(Peres 2005). Trophic cascades and other strong ecological interactions may greatly increase the amount of forest
www.fr ontiersinecology.or g
Thanks to our collaborators and field companions, M de
Toledo, P Colinvaux, C Weng, and C Listopad, and to D
Piperno, C Peres, and J Terborgh for many formative conversations. This research was funded by the National
Science Foundation grants DEB 9732951 and 0237682 to
MBB and BSR 9007019 to P Colinvaux, DEB-0237684
and a Wake Forest University research grant to MRS, and
a CNPq grant to M de Toledo. The manuscript was
strengthened by input from S Jackson.
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