RETHINKING EASTER ISLAND'S ECOLOGICAL CATASTROPHE

Journal of Archaeological Science, Article in Press, Corrected Proof 
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Rethinking Easter Island's ecological catastrophe 

Terry L. Hunt a,  

A Department of Anthropology, University of Hawai'i-Manoa, 2424 Maile
Way Honolulu, HI 96822, USA 


Easter Island (Rapa Nui) has become the paragon for prehistoric human
induced ecological catastrophe and cultural collapse. Today a popular
narrative recounts an obsession with monumental statuary - a mania for
moai - that led to the island's ecological devastation and the collapse
of the ancient civilization. Scholars offer this story as a parable for
our own reckless destruction of the global environment.

In this paper, I critically examine the historical and popular narrative
of human-induced environmental change, its causes and consequences, for
Rapa Nui. I review new and emerging Rapa Nui evidence, compare
ecological and palaeo-environmental data from the Hawaiian and other
Pacific Islands, and offer some perspectives for the island's
prehistoric ecological transformation and its consequences. I argue here
that a revised, later chronology for Rapa Nui calls into question
aspects of the current model for the island's ecological history. A
critical examination of the paleo-environmental and archaeological
records also reveals a more complex historical ecology for the island;
one best explained by a synergy of impacts, rather than simply the
reckless over-exploitation by prehistoric Polynesians. While my focus is
on the palaeo-environmental record, it is essential to disentangle the
related notion of prehistoric "ecocide" with the demographic collapse
(i.e., post-contact genocide) that would come centuries later with
European disease, slave-trading, and the other abuses heaped upon the
Rapanui people. Contrary to the now popular narratives (e.g., Diamond,
1995 and Diamond, 2005), prehistoric deforestation did not cause
population collapse, nor was it associated with it. Such an argument can
be based only on facile assumptions and an uncritical faith in
contradictory accounts from the island's oral histories; but this is a
critical subject worthy of detailed, continued examination (see Metraux,
1957, Peiser, 2005 and Rainbird, 2002).

[...]

5.1. Rethinking Rapa Nui's ecological catastrophe

By early historic times the deforestation of Rapa Nui was complete, or
nearly complete. A dense forest of palm trees and more than 20 other
woody tree and shrub species had mostly disappeared. Perhaps as many as
six land birds, several seabirds, and an unknown number of other native
fauna were lost to extinction (Steadman et al., 1994). It seems likely
that most of the biotic loss occurred before the final disastrous
effects of thousands of grazing sheep in historic times (Fischer, 2005
and Rainbird, 2002). Certainly from an ecological and biodiversity
perspective, Rapa Nui experienced an environmental catastrophe. Flenley
et al. (1991) suggested that the Pacific rat might have played a
significant role. Nevertheless, many accounts and their popular
derivations have gravitated to a narrative of human recklessness,
over-exploitation, and over-population leading to demographic and
cultural collapse (e.g., Diamond, 2005). These scholars speculate about
the human motives in cutting down the last tree. However, the ecological
impact-indeed the ultimate ecological catastrophe-undoubtedly was not as
simple as the popular story implies.

Once rats arrived on Rapa Nui, probably in the first canoe(s) of
Polynesian colonization, their numbers could irrupt and reach a
population well over a million within just a few years. At this historic
instance, rat predation of palm nuts, other seeds, bark, seedlings,
etc., effectively halted the regeneration of Rapa Nui's forest. The
Jubaea palm and other forest plants would have comprised the primary
food for rats. Moreover, rat predation of Jubaea nuts probably
effectively removed this resource from human consumption,5 suggesting
that the palms never provided a primary food source to prehistoric
people.

As shown in Table 1, the majority of extinct plants from Rapa Nui were
highly vulnerable to impacts by rat predation. Most yielded relatively
large edible fruits/seeds, lacked toxicity, and faced destruction rather
than dispersal by rodent predation. Perhaps significantly, Sophora
toromiro was one of the few native plants that survived into historic
times. Field studies from New Zealand show that rats do not appear to
affect juvenile recruitment of a member of the same genus, Sophora
microphylla. Indeed, through damage to the seed casings, rats may aid in
the germination of these seeds (Campbell and Atkinson, 2002, p. 29).
Does the survival of S. toromiro on Rapa Nui into historic times reflect
its tolerance or even symbiotic relationship with rats? Research in
field ecology may help to resolve this question.

Researchers who have adhered to a longer chronology for Rapa Nui must
assume a remarkably slow human population growth rate (e.g., 1.0%) over
several centuries in which Polynesian colonists (and their introduced
rats) had virtually no impact on the island's ecology, assuring that
they would remain archaeologically and ecologically invisible. A shorter
chronology notwithstanding (Hunt and Lipo, 2006), a scenario of slow
growth is unlikely among populations that likely succeeded as
"supertramps" expanding their numbers over the vast Pacific in
remarkably short bursts of time (Clark and Terrell, 1978, Diamond and
Keegan, 1984 and Kennett et al., 2006b). Birdsell's (1957) analysis of
historically documented cases of small human colonizing events to
remote, unoccupied islands has shown that growth rates exceeding 3.0%
can occur for short periods of time. Indeed, rapid population growth
would be critical to successful human colonization of remote islands.
With even small numbers of initial colonists at a 3.0% growth rate,
populations would rise dramatically and reach more than 2000 (a density
of over 10 people per km2 on Rapa Nui) in just over a century. Rapid
population growth is consistent with late colonization as well as the
timing of deforestation. Direct human impacts from felling and fire, as
population grew, would lag behind the initial, but continuing, impact of
rats. With the usual precision of palaeo-environmental research, the
relative chronological impacts of rats and humans might appear to be
one-and-the-same.

In cases such as the 'Ewa Plain, the records for decline of the forest
and advent of local fires show a separation of centuries, reflecting the
work of rats in advance of people settling this part of the island
(Athens et al., 2002). In other sequences, the discernible demise of the
forest and the rise of local fire are not separated by centuries, but
perhaps only decades. Rapa Nui is likely a case of the latter, where
early impacts may be separated by a short period of time, but not by
proximity. The effects of rats could have persisted as human populations
grew and brought increasing impacts with felling and the use of fire.
Testing the role of rats as a major factor in the process of ecological
change on Rapa Nui will require extraordinarily well documented
palaeo-environmental records to distinguish timing, causes, and
consequences. Such demanding research may reveal a more complex, synergy
of forces in the ecological transformations of the island.

If rats could have played a major role in Rapa Nui's deforestation, how
about other Pacific islands? The palaeo-environmental record from the
Hawaiian Islands reveals that vast lowland areas were largely deforested
over a short duration, in some cases (e.g., 'Ewa, O'ahu Island) prior to
the direct human impacts of fire and felling. Rolett and Diamond (2004)
have recently considered environmental correlates with pre-European
deforestation on Pacific Islands. For Rapa Nui Rolett and Diamond (2004,
p. 445) correctly point to "extreme environmental fragility" as a major
factor in the island's deforestation. Rapa Nui's extreme fragility is in
part explained by the environmental factors they consider, including
latitude, size, elevation, and precipitation. However, island
biogeography may better explain the pattern Rolett and Diamond (2004)
interpret for the Pacific by way of comparison. The persistence of
forests west of the Andesite line, dividing continental and oceanic
islands, may be less a function of dust fall or other environmental
factors they propose. Instead, lower rates of deforestation in the
southwestern Pacific may be explained by (1) generally lower human
population densities controlled by the density-dependent nature of
malaria infection over much of Melanesia; (2) on average larger islands
with higher elevations provide forest refugia; and perhaps especially
(3) a complex co-evolutionary history of diverse plants and animals,
including native rodents and their predators. The co-evolution of plants
and rodents, among other relationships, and the presence of predators in
complex ecosystems, imply a minimal impact would result from the human
dispersal of Rattus exulans to the continental islands of the western
Pacific, or even the comparative biotic complexity of oceanic high
islands of the central Pacific (e.g., Samoa, Tahiti). Biogeography also
explains how island elevation, area, and isolation (distance) are
auto-correlated, as continental islands are on average larger, higher,
and less remote. Islands generally decrease in size (and elevation), as
distances increase moving from west to east, from continental islands to
the truly oceanic islands of the Pacific Plate. Remote, predominantly
small, oceanic islands have biota dictated by biogeographic factors with
lower taxonomic diversity at the family and generic levels, but commonly
with higher rates of speciation. As a consequence, biogeography
anticipates the environmental fragility of Rapa Nui and other small,
remote oceanic islands, particularly to the impacts of an invasive
species such as the rat.

The available palaeo-environmental record for Rapa Nui shows that
deforestation occurred over about 400 years (ca. 1250-1650 AD), with
remnants of forest lasting into historic times. This means that the
human population increased even as forest resources declined with direct
and indirect ecological impacts over 400-500 years. A maximum
population, perhaps 3000-4000, could have been reached by about
1350-1370 AD (Fig. 14), following an early growth rate of 3.0%
(Birdsell, 1957) and assuming growth stabilized at a relatively low
number given the unpredictable and limited environmental carrying
capacity of Rapa Nui. A maximum population would fluctuate slightly, but
probably remained in close balance with the limitations of environmental
uncertainty, particularly given the effects of rainfall shortages and
damaging salt-laden winds on Polynesian cultigens (Hunt and Lipo, 2001).


Erosion of primary soils-although perhaps never very fertile (Ladefoged
et al., 2005 and Louwagie et al., 2006)-may have brought problems for
cultivation (see Mieth and Bork, 2004 and Mieth and Bork, 2005). The
effects of drought, wind, and soil erosion could have intensified as
forest cover declined over 400-500 years. Perhaps as a consequence, even
early Rapa Nui agriculture comprised managed soils and field systems
using stone mulch and enclosures (manavai) (e.g., Ayala-Bradford et al.,
2005, Bork et al., 2004, Stevenson et al., 1999 and Wozniak, 1999). As
Stevenson et al. (1999, p. 811) hypothesize; stone mulch agricultural
systems may date to as early as the 14th century.

Contrary to common claims, there exists no reliable evidence that the
Rapa Nui population ever grew to a large, unsustainable maximum such as
15,000 or more (Diamond, 2005, p. 91), and then crashed as a consequence
of deforestation (Peiser, 2005 and Rainbird, 2002). Certainly, many
dimensions of the island's supposedly turbulent prehistory rest on
little, if any, reliable evidence. The large population numbers (e.g.,
6000-30,000) (Diamond, 2005, p. 90) often cited for prehistoric Rapa Nui
are little more than speculation. Nonetheless, arguing a large
pre-contact population (e.g., 15,000) in reference to reduced historic
numbers (e.g., in 1864), some 200 years earlier, is critical to notions
of "ecocide" (Diamond, 2005).

Despite repeated claims, Rapa Nui does not appear to represent a case of
"ecocide." The documented population collapse for Rapa Nui occurred as a
consequence of European contacts, with Old World diseases and
slave-trading (Peiser, 2005 and Rainbird, 2002). As VanTilburg (1994, p.
164, emphasis added) noted, the scary parables and metaphors for
disaster represent "a projection of Western values which emphasizes the
self-destruction of the Rapa Nui culture over the actual,
near-annihilation of it by contact with the West." Indeed, early
ethnographer Alfred Metraux described the historic genocide as "one of
the most hideous atrocities committed by white men in the South Seas"
and as "the catastrophe that wiped out Easter Island's civilization"
(Metraux, 1957, p. 38). Today the idea of "ecocide" enjoys popular
acceptance, but an actual genocide decimated the native Rapa Nui
population and its culture (Peiser, 2005 and Rainbird, 2002).
Unfortunately, the victims of cultural and physical extermination have
been turned into the perpetrators of their own demise.

The model of "ecocide" was constructed in part on the foundations of
faith in a long chronology, speculation about prehistoric population
size, and a remarkable, but still somewhat coarse-grained
palaeo-environmental record for the island. Recent field research,
including comparative case studies in places such as the Hawaiian
Islands, have changed some perspectives and allowed us to raise
questions about Rapa Nui's historical ecology. In this review I have
examined archaeological, palaeo-environmental, and contemporary
ecological evidence to suggest that the Pacific rat may have played a
major role in Rapa Nui's ecological catastrophe. The fact that rats
alone are capable of widespread forest destruction compels us to
evaluate their contribution to the transformation of Rapa Nui, as well
as in other island ecosystems. While I argue that the role of rats has
often been underestimated, direct human actions of felling and use of
fire likely have played a significant role as well. Additional research
will be essential to disentangle the contributing factors. The
environmental catastrophe of Rapa Nui is likely a complex history, one
that has been masked by speculations about the intentions of people
cutting down the last tree. Indeed, the "last tree" may simply have
died. Rats may have simply eaten the last seeds. Perhaps the lessons of
Rapa Nui tell of the effects of invasive species, invasional meltdowns,
and the synergy of effects that ensue as people and their portmanteau
biota reach evolutionary isolates in the remote islands of the Pacific.

As biological invasions and extinctions continue at unprecedented rates,
deciphering ecological histories and the consequences of biological
invasions has gained urgent significance. It is increasingly evident
that invasions and extinctions predate and precondition modern
ecosystems. Research in archaeology and palaeo-ecology is providing
evidence of ecological impacts that may have few, if any, modern
analogs. Lacking modern analogs, the dramatic ecological transformation
of Rapa Nui is that much more difficult to imagine, but all the more
important to fully comprehend.

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doi:10.1016/j.jas.2006.10.003       
Copyright (c) 2006 Elsevier Ltd All rights reserved.