amazonian exploitation revisited ecological asymand the policy pendulum mark b. bush and miles r....

8/11/2019 Amazonian Exploitation Revisited Ecological Asymand the Policy Pendulum Mark B. Bush and Miles R. Silman - Un

1/10

Amazonian Exploitation Revisited: Ecological Asymmetry and the Policy Pendulum

Author(s): Mark B. Bush and Miles R. SilmanSource: Frontiers in Ecology and the Environment, Vol. 5, No. 9, Paleocology: Using the Pastas a Key to the Future (Nov., 2007), pp. 457-465Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/20440741.

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2/10

PALEOECOLOGY PALEOECOLOGY PAL

L

_ECOL_______

Amazonian exploitation revisited:

ecological

asymmetry and the

policy pendulum

Mark

B

Bushl andMiles

R

Silman2

The influence of pre-Columbianhuman

populations

on Amazonian ecosystems

is

being actively

debated.

The

long

standing view thatAmazonia was only minimally impacted

by human

actions

has been

challenged,

and

a

new par

adigm

of

Amazonia

as a manufactured landscape is emerging.

If

such disturbance

was the

norm until

just 500

years ago,Amazonian ecosystems

could be farmore 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 areaswere

heavily modified,

but most of Amazonia was

minimally impacted.

Bluffs

adjacent

tomain riverchannels and

highly

seasonal areas appear

to have been themost extensively settled locations.Away

from areaswhere humans lived, their influence on ecosystems

was very local.Consequently,we seeno evidence sug

gesting

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 inferAmazon-wide landscapedistur

bancemay

therefore

potentially

lead to

unrealistic projections

of human impact andmisguided

policy.

Front

Ecol Environ

2007;

5(9): 457-465, doi:10.1890/070018

The

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 vir

gin , 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 andminimized extinctions (Stebbins

1974). Coupled with this view was the romantic idyll of

hunter-gatherers living in harmonywith nature.

Such

views have been altered by excavations of archaeo

logical sites from the mouth of the Amazon to the

High

Andes, which reveal a long recordof human

occupation,

ceramicsmanufacture, and agriculture (eg

Roosevelt 1991;

Roosevelt et al. 1991). For at least some groups,a

trajectory

of increasing

populations

and

greater

reliance on

agricul

ture is evident for the past several thousand years. As

anthropological

and

paleoecological

knowledge

of these

systems

has

deepened,

the view of Amazonia as untram

meled 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

andmodification of systemsprior

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 theAmazon 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 distur

bance . 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 amiddle

path. Here,

we

provide

an alternative

interpretation

of

the existing data,

and caution that uncritical

acceptance

of Amazonia as a manufactured landscapemay be mis

guided

and could lead to unsound policy.

In

Amazonia, there

has

been

a commendable use of sci

ence

by governments, particularly

the Brazilian

Government,

to

set conservation

policy. Refuge theory

was

widely accepted

in themid-1980s and was used at the

time, together

with

maps

of

diversity,

to

prioritize

areas

for conservation

(Dinerstein

et al.

1995). Although

this

theory

isnow

largelydiscredited (Colinvaux

et al.

2001),

the basic

biogeographic

patterns

of

high

local

endemicity

and

diversity

used to

identify

refugia

lso

formed

effec

ww.fwrontiersinecology.org

In

a

nutshell:

*

Pre-Columbian

human influence on Amazonia was spatially

heterogeneous, with some sites intensively altered

*

The largemajority 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 land

scape,

and hence

resilient to human activity, are

only locally

true

and should not be used to set regionalmanagement

policy

'Department

of Biological Sciences,

Florida Institute of

Technology,

Melbourne,

FL

32901

([email protected]); 2Department

of Biology,

Wake

Forest

University, Winston

Salem,

NC

27104

C)The Ecological Society ofAmerica

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3/10

Exploitation of theAmazon revisited MB Bush andMR

Silman

1492

Columbus

1525

Death of 1541 Orell

a149ves

in

Huayna Capac

navigates

New World

\\/

High

3 1532 Arrval

(8-10

million)

of Pizarro

in

1

52 5 / \

Ecuador

Andean

smallpox

epidemic

|1

743

Amazonian

Medium |

accountof

(3-5million)

_

4

dela

/

womommom

Condamine

Low| \

(I

million)\

1000 1500

AD

Figure 1. Annotated schematic iagram f humanpopulation

Low,

medium, andhigh stimates re

shown.

tive

guides

for the establishment of

protected

areas.

Land

use

policy

must be founded on scientific

knowledge,

but

scientific

knowledge

is

subject

to

continual

revision

and

modification.

Consequently,

when

science

appears

to

support policies

that could

irreversiblydamage ecosys

tems,

the science deserves additional

scrutiny.

*

Population

size and human

impact

Estimates of human

population

in

Amazonia at the time

of European arrival range from one to 11million, with a

tendency

toward

higher

values inmore

recent estimates

(for

overviews

see Denevan

[1996]

and

Mann

[2005]).

The

population collapse

that occurred between ca 1520

and 1600 AD

may

have

reduced

Amazonian

populations

by

an

astounding, yet plausible

95%

(Denevan

1976).

Diseases

brought

to Central America

by

the

first

Europeans spread

into

South

America via trade

among

native

peoples. It

is

probable that the Incan

emperor

Huayna Capac

died when a

deadly

epidemic (probably

smallpox) swept

through

the

Andes

in

1524-1525

(Figure

1),

6

years before the

arrival of

Pizarro,

the first

conquistador

to enter Peru

(Hemming 1970). Smallpox,

measles,

diptheria,

and

influenza

ravaged the Andean

populations over the next 50 years (Denevan

2003).

While

Spanish

accounts

describe scenes

of

bodies

piled

up

and abandoned

in

Andean

villages,

nothing

isknown

about

epidemic

diseases

in

the lowlands.

However,

the

same

genetic

susceptibility

to

European

diseases

(Black

1992),

known

trade

contact

with Andean

communities

(Hemming

1970),

and

densely packed populations along

major

rivers

(Smith

1990),

make

it

likely

that

Amazonian populations

were similarlydevastated.

At the

heart of the discussion over

pre-Columbian

influence

in Amazonia are contradictory

estimates of

ina

human

population

size,

cultural

develop

theAmazon ment, and the geographic extent of their

influence. The initial European descrip

tions

of

Amazonian

populations,

their

size,

wealth, and societal sophistication, are at

/

10 best incomplete and at worst misleading.

/ en

Friar

Gaspar

de

Carvajal,

who

accompa

c

nied Francisco de Orellana

on

the first

Q

European

voyage

down the Amazon River

X

in

1541, was writing to impress the King of

5 o Spain.While he describes extensive settle

enments

stretching or tens of kilometers

.

along

the

river,

and

encounters

with thou

2

sands of warriors, his credibility is

under

mined by fanciful descriptions of

such

0

curiosities

as

single-breasted,

warrior

2010

women.Although the riverwasnamed

or

000

~

these Amazons ,heywerenever ncoun

tered in

subsequent exploration.

In

stark

n Amazonia. contrast to Carvajal's account, Charles

Marie de IaCondamine, who traveled the

Amazon in

1743, described

a

very different

setting, in which isolated bands of people struggled for

existence in themost basic circumstances (Smith 1990).

Many subsequent

visitors

validated this portrayal

of

phys

ical and cultural poverty, so different from the

agrarian

idyll described by Carvajal. However, in a similar

debate

over

indigenous

influence on the

vegetation

of New

England, Foster and Motzkin (2003) caution

against

accepting eyewitness accounts that post-dated European

arrival

by

>

100

years

as

proper reflections

of conditions

immediately prior to first contact. For many

years,

Carvajal's account was seen to be essentially unreliable

(Smith 1990).

However,

there

was

renewed

interest

in

the possibility of disease decimating

this society, prompt

ing Mann (2005) and others to suggest that

both

accounts were

essentially

true.

Carvajal

didwitness dense

populations, whereas Condamine was viewing the shat

tered remnants of a society inwhich 90-99% of the pop

ulation had succumbed

to

smallpox

and other

European

diseases

(Smith 1990). If there had been large, settled

populations in Amazonia for a thousand or more years

prior to contact, this raises the question: Towhat extent

did human activities prior to the population collapse

influenceAmazonian ecosystems?

Drawing from the archeological literature,

Mann

(2005) elaborates a vision of Amazonia inwhich the area

supported upwardsof 10million people, with great

urban

centers

(eg

Marajo

Island, Santarem),

and

many

other,

smaller cities with well-developed infrastructure (eg

those adjacent to the Xingu River and Lake

Manacapuru).

The banks of the

main

Amazon channel

and tributaries such as the Rio Negro, Madeira, Xingu,

and

Tapaj6s,

teemed with villages (Denevan

1996).

Great transformingpublic works,

such

as

the

raised vil

lages,

causeways,

and fish

weirs of the Beni region of

southern Amazonia, are attributed to large workforces

www.frontiersinecology.org

i

The Ecological

Society of America



4/10

MB Bush andMR Silman Exploitation

of theAmazon revisited

and complex societal hierarchies

(Balee

1989; Erickson 2000).

A further

legacy

of human endeavor is

the distinctive

soil

layering, up

to 2m

deep,

of black or

brown soils with

ele

vated nutrient availability,known as terra

pretadel Indio(Figure2).

These nutrient

rich black soils often

contain broken

ceramics

and appear

to have been

altered

by

the addition of

ash, greenmanure, or,

in

some cases, fishmeal

(Lehmann et

al.

2003). Such soilmodification 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

Fiure

2. Natural

of

essential nutrients.

A

fallow

period

of soil.Oxisols are

thf

about 20-30

years

is

needed

before the the ddition

f

asht

land

can

again support

crops

of maize

(Kellman and Tackberry

1997).

A

prevailing

view is that

these

villages

remained

occupied year

after

year

at

popu

lation densities too high

to allow such long fallowing

between

periods

of

cultivation. The

elevated

nutrient

availability (particularly

phosphorus)

of terra

preta

soils

may

have allowed

near-permanent

cultivation

of

these

soils, potentially supporting

a

much

larger

uman

popula

tion

density

than the native soils

(Glaser

et al.

2001).

Indeed, the terrapretamodel may offer insight into how

Amazonia could withstand

intensified agricultural

exploitation

in

the future

(Glaser

et al.

2001).

Based on a

wide array of evidence, Balee (1989) esti

mated

that

11%

of Amazonian

vegetation

in

pre

Columbian Amazonia was

extensively

used and altered

by

human

activity.

That

use

ranged

from slash-and-bum

agriculture

to

enrichment

with

fruit-bearing

trees

(eg

Bactris

gasipaes [peach

palm],

Bertholettia excelsa

[Brazil

nut], Annona spp [includes

guanabana

or

sweet soursop],

and Mauritia

flexuosa

[aguaje,moriche,

or

nontoca;

a

palm

whose

fruit

is

used tomake a

drink]).

Inferringpast human disturbance

Fire is the

oldest human

tool for both small-

and large

scale

manipulation

of

the landscape,

and one of the

major

sources

of

evidence for

past

human

disturbance of

tropical

forest

systems

comes from fire

histories. Charcoal

is

pro

duced

by

forest fires

and is

gradually

buried

within

a

soil

profile.

Charcoal

layers

from

soil pits

can

be radiocarbon

dated, yielding

an

age

estimate

for fire

events

(generally

+ 50-100 years). If old

wood

is burned, the resulting

14C

dating

f

charcoal

verestimateshe timesince the

fire

(Gavin2001). Sources f such

old wood couldbe the

C,~~~~~~~~~~~~~~~~~~~~~~~~~~~~C

andmodified mazonian

soils. (a)

An

oxisoland (b)

a terra

reta

most common ry land oils

n

Amazoniaandwhenmodifiedwith

rndrganic aste are stained lack o orn terra reta.

heartwood of an old

tree killed by the fire,

or

dating long

dead but

undecomposed

wood

lying

on

the

soil surface.As

most tropical forest fires char bark but not heartwood

(sensuCochrane 2003), it isunlikely that

heartwood from

this source is represented

in

the charcoal.Also,

because

rapid rates of decay in the tropics lessen the

fuel load of

undecomposed heartwood lying on the

forest floor com

pared

to

temperate or boreal settings, the

probability of

charcoal being composed of heartwood is similarly low.

The charcoal ages in tropical forest soils

are therefore

likely to be reasonably

representative of the actual fire

date

(taking

into account

the

errors

of

14C

dating).

When

304 ages

forAmazonian

soil charcoal

are

plotted

against

time

(Figure

3

a,b),

the data strongly suggest

an

expansion

of

agricultural

activity

at ca 250

AD, then

a series

of

peaks

of

fire activity

between ca 700 and 1550 AD, followed by

a

sudden collapse

at ca 1600 AD. One of the

more surpris

ing resultsof this analysis is that the highest peak of appar

ent fire

activity

is not at ca 1550

AD,

but at

ca 700-800

AD.

Nevertheless, the trajectoryof implied disturbance

fitswell with archaeological estimates

of

village expansion

(eg

Roosevelt

1980;Heckenberger

et

al.

1999).

Hammond et al. (2006) conducted the first systematic

survey

for soil charcoal

in

a 60000-ha

area of

Guyana.

Their analysisof

>

280 soil profiles revealed

charcoal in all

of them.The authors

considered

it

unlikely

that

these

were

natural fires although

this

section

of

Guyana

is subject to

El

Nifio-related

droughts

-

and concluded that there had

been

extensive

human influence

in

upland

eastem

Amazonian

forestswithin

the

past

millennia.

Pollen, charcoal, and phytolith data fromAmazonian

lakes lsoprovide

vidence

orwidespreaduman ccupa

tionprior oEuropeanontact Figure), and there svir

C)

he cological

ociety

f

America




5/10

Exploitation of the

Amazon revisited

MB

Bush andMR Silman

(a)

Known terra

preta

* Soil charcoal dated

Atlantic

Ocean

Pacific

_-ooWIooo0

Ocean

Kilometers

18

(b)

E

16

14

E

12

'-10.

0

6.

0'

Year

AD

Figure

3.

Soil carbondata

from

Amazonia. (a)

Sketchmap

shouingmajor

Amazonian rivers, ocations

f radicarbon-dated

soilcharcoal, nd terra reta.

(b)Ages of 228 carbonizedayers n

Amazonian

oils.Data are

derivedrompublished

ccounts

f soil

charcoal

n

Amazonia(Sanford

t al. 1985; Saldarriagand

West

1986;Desjardins

t al. 1996; Piperno

ndBecker

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

t

al. 2006).

All ages

are

expressed

n

calibrated

calendar-yearsefore resent

Stuiver

nd

Reimer 993).

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

modi

fication

(Erickson

2000).

Some

anthropologists

are now

referring

to the forestsof Amazonia as a cultural

park

land , human created , or built landscapes (Erickson

2000;

Heckenberger

et al.

2003;

Mann 2005). Indeed,

Charles

Clement,

an

anthropologist based

at

INPA

(National

Institute forAmazonian 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 influ

ence public perceptions

and policy. But how robust are

the

data

underlying

the assertions

of

widespread

alter

ation ofAmazonia?

The

hypothesis of widespread

Amazonian landscape

management

is

based

on

analyses

of archaeologicalsites and

the assumption

that there was a large pre-contact

Amazonian

population (>

10million 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 loca

tions, and do not represent

ither a systematicor

a random

ized sampling design.

There is no ecological component

predictingwhich

forestwas most

likely to be occupied.

Was

disturbance spread

evenly

across all of Amazonia

or con

centrated near

human habitation?

Is it safe

to

extrapolate

results romsites

where we know

human habitation

occured

to

the

rest

of

Amazonia?

Ecologists

are familiarwith

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

concems

regarding

ow we

may extrapolate

data

from the

kind of dot map

shown

in

Figures 3 and 4. First,

landscapes

are heterogeneous,

and the resultingvariability

in

environmental factorsproducescharacteristic

patternsof

distribution

and habitat

use

in

nearly

all

species. Second,

although

species distributions

can

be widespread,

their

occurrence

may be

very

local.

For

example,

a

dotmap

of

the

painted

turtle

(Chrysemys icta)

would show that

it

is

dis

tributedacross

much of the

continental

US,

but

its actual

occurrence is

highly

localizedwithin

a

landscape.

Third,

a

species

can be

widespread

and also

have

a

very

substantial

impact

on

the

system

where

it is

found,

but this isnot the

same as

saying

that a species has

a

widespread,

intense

impact,

or

makes wholesale changes

to

the habitat

inwhich

it lives.

Thus, extrapolating

observations from

dot maps

can

be

dangerous,

especially

when the

dots

represent

discrete

activities

of

limited

spatial

extent

(eg terra retaformation).

A

first

step

in

making

the

conjectures

that

are so

critical

to

ecosystemmanagement

inAmazonia is

recognizing

the

potential

sources

of

bias that affectwhat

we

infer

rom

data.

The

data

in

Figure 4

have two

major potential sources

of

bias.

First,

lakes

used to

study paleoecology

may

not accu

rately represent

the

landscape

as awhole,

because

they

are

attractive

places

forhuman

settlement

-

humans

like

to live

by

water

now

and, apparently,

always

have

(eg

Bush et al.

2007b).

Also, while maps of terra reta

sites are

available,

we have no dataon thedistributionof soil pits that didnot

showmodified

soils. It is a

statistical certainty

thatwhen

extrapolations

about

land

use are

made

fromknown

archae

ological

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

Fossilpollenandcharcoal ecordsFigure a,b)derived

from he analysis f lake

edimentsan provide

etailed

wwwAmk

ersinecology.org

C

The

Ecological

Society

of America



6/10

MB

Bush and

MR

Silman

Exploitation

of

the

Amazon revisited

insights into Amazonian history. Pollen

and spores

released from

plants

and

blown or washed

into

lakes

become

incorporated into the sediment. Year

after year, the sediment accumulates,

burying and

preserving

these

plant

microfossils

in

anoxic mud. Lake sedi

ments are retrieved using coring rigs sup

ported by

inflatable boats.

The

vertical

columns of mud and the buried

layers

of

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

radiusof land represented

in

the micro

fossil

record

-

the vegetation directly

represented

in

the record

-

depends

on

lake

size. In

the moderate-sized lakes

we

discuss here, the great majority of the

pollen would have been derivedwithin a

few kilometers of the lake.

During

a

forest fire, charcoal particles

are

created

in

both the smoke and the

charred

plant

remains.

This charcoal

fallsor iswashed into

nearby lakes.

The

finest fraction

of

charcoal reflects regional fire histories,

whereas relatively largeparticles (ie those

>

160 ,um

in

length)

indicate

past

fire

in

the adjacent

watershed

(Clark 1988). Today,

fire is rare

in

most naturalAmazon

systems, and many

sediments from undisturbed

settings

contain no

charcoal.

Finding evidence

of

regular bums

may therefore indicatepast hunting activity,where fire is

used to drive game or

improve itshabitat, as amechanism

to

clear

forest for

semi-permanent dwellings,

or to

increasepopulations of those light-demanding plants pre

ferred by hunter-gatherers. Finding both charcoal

and

pollen from crops such

as com (Zea mays) or manioc

(Manihotesculenta) is a clear indicatorof past agriculture.

Zea

mays

isnot

native toAmazonia, and its pollen is dis

tinctive, due

to

its surface pattem

and

large

size

(com

monly 80-110 ,um).Maize pollen is particularly poorly

dispersed,making

it a

good

marker of local

crop

use.

Given these two

proxies

forhuman

activity

-

microfos

sils and charcoal

-

we can

start to

look

at

regional

com

parisons of where humans have altered the

landscape

(Figure 4). Of the 22 pollen and charcoal records from

lakes shown as

yellow

squares,

13

contained evidence

of

pre-Columbian occupation.

A

minority

of

these sites lay

within the area predicted byMann (2005) to be heavily

occupied, but this

pattern may reflect the relative abun

dance

of

lakes suitable for

paleoecological study

asmuch

as it

does

human distribution.

Most of

these lakes

were

chosen

for

the

purpose of

looking

at

the

vegetation

his

tory

of the

particulararea,

not as a

random

or even

repre

sentative sample

of

Amazonia. At the scale of Figure 4, it

is

not

always possible to

show

individual lakes, and

so if

.1

* Known

terra

reta

Major

archaeologicalsite

Fossil pollen

data

0

Human

impact/pollen

*

Suggested

high

pre-Columbian

human

population

density

Fgre

4.

Evidence

of

widespread

pre-Columbian occupation

in

Amazonia.

Map

shows the known distribution

of

modified (terra

preta)

soils,

lakes

with

paleoecologicalecords ith andwithout evidence f humanoccupation,themajor

archaeological

ettings

f (1) Marajo

Island,

(2) Santare'm,

3)

the

upper

Xingu,

(4)

Manacapuru,

and

(5)

the

Beni.

The

region

that

Mann

(2005)

maps

as

having

a

high

robability

f

dense settlement

s

shown.

Terra

preta

data

from

Lehmann

t

al.

(2003)

with additional data

from

A Zimmerman.

Pollen data

from

numerous

authors

summarized

in

Bush

et al.

(2004;

see

also Bush

et

al.

2007a).

one or more lakes in a cluster show human activity, then

the district iscircled.

At the

broadest scale

-

whether

a

region shows

human impacts or not

-

it appears

that

Amazonia was broadly, and possibly universally, influ

enced by people.

Looking at the lake data on

a finer scale, however,

shows the dangers of extrapolation.Three lakedistricts in

which multiple lakes have been

analyzed provide similar

spatial

extents of

human

activity

in

widely separated

Amazonian landscapes (Athens and

Ward 1999;Weng et

al. 2002; Bush et al. 2007 b,c).

In

two settings, Prainha

andMaldonado (Figure6 a,b),

one lakehas

an

unambigu

ous

recordof occupation and agricultural

use fromabout

4000 years ago

until

ca 1600

AD.

The third

location,

a

swamp named Maxus 5, near

Yasuni in Ecuador

(Figure

6c),

lies

in

wet

forest that

has

very

little

seasonality.

This

site

has

a

long

record

of

charcoal

in its

sediment,

while

two

neighboringwetlands

have none. Human

occupation

in

this landscape

ismore tentative

than in the other set

tings, as

no

crop pollen

or

artifacts

are

associated with

Maxus 5.Nevertheless, the discovery of charcoal at just

one of

three

settings is a likely

indicator

of human activ

ity.

The forest at Yasuni receives

-

2800

mm of

precipita

tion each

year,with

no distinct

dry season,making

nat

ural fires

exceptionally rare,

if not

totally

absent.

That

only

one of the three

sites contains

charcoal, and that

the

charcoal

was

found

in

multiple samples,

adds

strength

to

the

suggestion

by

Athens

and

Ward

(1999)

that the char

coal

was

produced by

local human

activity.

In

the

Maldonado lake

district, the paleoecological

record of human activity

at Lake

Gentry is supported by

C) he cological

ocietyfAmerica www.frontierwlnecolog.org



7/10

Exploitation

of the

Amazon

revisited

MB Bush andMR

Silman

(a)~ ~~~~~b

v

10 m

Figure

5. Markers of

humanoccupationromAmazonian

lake

ediment. a)

A

pollen

grainof

corn

(Zeamays)

and (b)

microscopic harcoal

ragments.

the presence

of

stone

tools

and a midden adjacent

to

the

lake.

The paleoecological

markers of human occupation

at this lake, corn and

manioc

pollen

and

charcoal,

indi

cate

that agricultural

activity

in

the

region

began

around

4400 years ago,with as much as 3000 years of burning

prior to that time.

Similarly, at

LakeGeral

in

the

Prainha

group, com pollen was found regularly

between 4000

and

400 years ago

(Bush

et

al.

2000, 2007c),

and

regularly

occurring charcoal

appearedbetween

8000 and 400 years

ago. However,

in

both the

Maldonado and

Prainha

groups, other

lakes showed either

a small increase

in

charcoal abundance or no charcoal

whatsoever,

and

none

contained pollen

from

com

ormanioc.

These

records

sug

gest

that human occupation and land

conversion

were

local

in

nature.

Sites

within 3-5

km

of

an occupation

center

appeared

to be quite heavily

used but, beyond this,

human influence declined markedly,

so

that at distances

of 50 km therewas no evidence of human activity.This

pattem

is

very

similar to

that described

by modern

anthropologists,

where hunting

and landmanagement

is

concentrated

in

a

1-3

km radius around

the center

of

habitation (Glanz 1991;

Apaza

et

al. 2002).

The absence

of humans

in

these records is also striking

in

another

way. In both

the Prainha and

Maldonado lake

districts, human population

collapses

followed European

contact, as both crops

and charcoal disappear

from the

records

in

the

past 400 years.Notably, the apparent

col

lapse

is

evident even at

sites

that were never

visited by

Europeans, providing

unambiguous support for

the

anthropological and

historical hypotheses

of widespread

epidemics.

An overall picture isclear: the paleoecological data con

firm that

Amazonia was

exploited by

indigenous peoples

who practiced agriculture and

developed urban centers

(Roosevelt

1980, 1991;

Roosevelt et

al. 1991;

Hecken

berger

et

al.

2007),

and the

populations

collapsed shortly

after

European

contact.

However,

the data

do

not

support

the

extrapolation

of

observations

from

occupied

sites to

infer

uniformly

widespread impacts

and landscape

transfor

mation. There is no doubt that humans

are

important

transformers

f Amazonian

landscapes

and have

been so

throughout

much

of the Holocene.

Determining

how

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 datawith new sam

ples

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 imme

diately prior

to

European

contact),

but at

ca

700-800

AD

(Figure

3b).

This

period

was

(Thompson

2000), when droughts

beset Amazonia

and

forestswere

more highly

flammable.

It

is

highly

probable

that

the

great majority

of these fires were human

induced,

but that during

periods

of intense

drought, large

areas were burned (making them more likely to be

detected today)when

small-scale fires escaped

to

become

wildfire. In this

way, the spatial

scale of human distur

bance

inAmazonia as awhole does

not necessarily

track

the intensifying

history

of

local land use

in

the

most

pop

ulous areas.

The

capacity of

humans todisturb the system

appears

to have been

strongly

influenced

by climatic

con

ditions,

not

simply

a

growing

human

presence. In

addi

tion,

the time

for

post-disturbance

forest recovery

in

these settings

is not

the

500

years implied

by the disease

model, but

>

1000 years.

One model of human

settlement

in Amazonia does

incorporate

known and inferred

densities at the land

scape level, albeit throughobservationsof thepreferences

of

modern indigenous

populations.

Denevan (1996) pro

posed

a

bluff

model

of human

occupation,

in which

indigenous

peoples

preferentially

settled sandy bluffs

alongside

rivers and where there

were views

over wet

lands.Denevan

estimates that

asmany as

10million peo

ple

lived

in

these densely populated

riverbank settings.

With just 400

000

scattered

through all

the rest

of

Amazonia,

the

impact

of

people

outside the

high-density

locations

may have been

very limited.

The paleoecologi

cal data

are

entirely

consistent with the long-term

aggre

gation

of

people

around key

landscape

features implied by

Denevan's

settlement model,

and

the inference that the

restofAmazonia

was

lightly

settled.

Seasonality

and location

Although

our

knowledge

of

past

human

occupation

in

Amazonia

is

still

in its infancy, the

case

for extensive

occu

pation

and

use

of

the landscapeby pre-Columbian

human

activity

in Mesoamerica

is

clear.

We need to develop

hypotheses

that

predict

where

humans would

have

existed

inAmazonia at

densities

sufficient to

alter landscapes.

For

example,

one

initial

prediction

might

be

based

on

season

ality.

Civilizations such

as the

Olmec,

Maya,

and

Aztec


C

The

Ecologicalociety

fAmerica



8/10

MB

Bush and

MR

Silman

Exploitation

of theAmazon

revisited

. 0

Figure

6. A

spatial

context

for

pre-Columbian

isturbance ithin three

Amazonian

lake

districts.

a) Lakes

near

Prainha,

Brazil (1 41'23.20

S,

5333'44.41

W);

(b)

lakes ear

Puerto

Maldonado,

Peru

(12?9'51.51

S,

69?5'59.74 W);

(c)

lakes

ear

Yasuni,

Ecuador

0053'53.02 S,

76?10'24.58

W).

flourished in the

highly seasonal

settings of the Yucatan

and

centralMexico.

In

landscapeswith

strongergradients

between ever-wet and

seasonal

forests,

such as in

Panama

and

Costa

Rica, the

dense populations

may have

defor

ested the

seasonal

areas (Pipemo

et

al.

1991), whereas

impactson the dense, wet forestsof theCaribbean slope

were never as great (Dickau et al. in

press).

A

similarpattem

may also have

prevailed in

Amazonia,

with highly favorable areas

supporting large

populations.

Figure 7 shows the

major

archaeological sites that have

been

described.

The main

Amazon

channel

is

clearly

important

as a trade route,

and also as an excellent source

of

fish, shellfish,

and game.Within

this setting, the

con

fluences

of

rivers

became centers of

settlement and

popu

lation

hubs,

as

did

the

mouth

of

theAmazon.

Away

from

this

ribbon

of

occupation, signs of human

habitation can

be

found

scattered across

Amazonia. Of the

areas

further

away

from

the

major

rivers,

the

most

complex

cultural

development

appears to have occurred

in

the seasonal

(defined as having the greatest intra-annualmonthly

variability

in

rainfall)

locations of southern Amazonia

(eg

Heckenberger et al.

2007).

This

incorporation

of sea

sonality

as

a factor

paralleling

geographic locationmay

form a

powerful

predictive model

for

where

major

occu

pation

sites are

likely

to

be found.

Our

contention

that

pre-Columbian

transformation

of

Amazonian

landscapeswas

essentially local and

spatially

predictable

does not

contradict the

development

of

com

plex

societies

(Heckenberger

et al.

2003; Heckenberger

et

al.

2007)

or that

substantial

areas were

influenced

by

human activity (Balee 1989), but it is

at odds with assertions regarding the

generality of disturbance

in

Ama

zonia, particularly broad-scale, inten

sive disturbances. More importantly,

our admittedly preliminary

analysis

provides a set of predictions and a

can

update

our model of

past human disturbance

in

the

Amazon, and of the effects of the

different kinds and

intensities of disturbances on Amazonian

forests.

U

Asymmetry of

policy

Some

years ago,

Bush and Colinvaux

(1994)

showed

that

two lake

records from

the Darien

region

of Panama

demonstrated surprisingly ong histories

of

human

distur

bance and abandonment. Since

then, logging companies

have

attempted

to

use that paper to justify timbering

some

of

these remote forests,

arguing that,

if

the region

supported cornfields 500

years ago,

the forest

ecosystems

must be young and

resilient, and that, therefore,

logging

now

would not lead to loss

of

biodiversity. The problem

with this argument is

that the real scale of

disturbance

is

ignored. The extended-family, slash-and-bum

farming

style

of

pre-Columbian

times

would have had

a

very

lim

ited impact on these

systems.Certainly, theremay

have

been much largerimpactsaroundmajor population cen

ters,

but the case

has

yet

to be

made that

the

halo of

human

influence was anything other than

local for the

greatmajority

of settlement

sites.

There

is no

question that more effort

needs to be

invested

in

obtaining

a

clearer picture

of

the

extent of

pre-Columbian settlement and alteration

of

Amazonian

habitats.

In

the

meantime,

itwould

be

prudent

for ecolo

gists to

dig

a soil

pit

on

their

researchsite and analyze the

soil for

charcoal

(in tropical soils,

manganese

and

tita

nium also form black

accretions that

are

strikingly

similar

?

The Ecological Society of America




9/10

Exploitation

of

theAmazon revisited MB

Bush

andMR

Silman

'3~~~~

Relative scale of seasonality

0.3

04

=0.5

_06

0.7

0.8 0.9

M10

Frgae

7.

Seasonalityand major settlement

ites.

If,

as

in

Central America, colonization

of seasonal

sites was more

extensive

than

colonizationof

ever-wet

sites,

predictions f

human

populationdensitiesmight be improved

y

inclusion

f

seasonality

as a

factor.

Seasonality

is

shown here

on a

relative

scale

of intra-annual

onthlyvariability, ith

a score

of

0

being

aseasonal data

from Tropical RainfallMonitoring

Mission

satellite

998-2004. Legend:

(1)

Guyana charcoal

tudyarea,

(2) Marajo

Island, (3) Santarem, (4) Manacapuru,

(5)

the

upperXingu, and

(6)

the

Beni

(Silman 007).

in

appearance to

charcoal)

to

determine

the

probability

that they areworking on an impacted site. However, it

would be sensible for policy makers to assumeminimal

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

forestmay be

overprotected.

At

worst,

this

policy conserves a resource for later

exploita

tion

and buffers

against

disruptions

of

ecological

interac

tions,

such as

trophic cascades,

which

may

be

vitally

important

to the structureof

tropicalforests

(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

unsus

tainably,and inways thatmay have broad repercussions

in

the

face

of

climate

change.

A

particularworry

is that

increasing fragmentation

will

lead tomore

frequent fires

in

the future

(Nepstad

et al.

2001),

and

will

also

impede

plant migration

in

the face of climate

change (Ibaiinez

t

al.

2006).

Worse

still,

Amazonian forests aremuch 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

ecologi

cal interactionsmay greatly

increase the amount of forest

that needs to be conserved to retain

functioning ecosys

tems.

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 park

land, the implications of the

different resulting policies

could be

ecologically asymmetric.Determining the true

scale of

pre-Columbian

influence on

Amazonian ecosys

tems

will

undoubtedly lead

to a more

sophisticated

view

of

these

landscapes,

in

which their

ecological, climatic,

and cultural

heterogeneity

are

recognized.

*

Acknowledgements

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

con

versations. This research was

funded by the

National

Science Foundation grantsDEB 9732951 and 0237682 to

MBB

and BSR

9007019

to P

Colinvaux, DEB-0237684

and aWake 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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