Drought, vegetation change, and human history on Rapa Nui (Isla de Pascua, Easter Island)

Abstract

Stratigraphic records from lake sediment cores and slope deposits on Rapa Nui document prehistoric human impacts and natural environmental changes. A hiatus in sedimentation in Rano Raraku suggests that this lake basin dried out sometime after 4090–4410 cal yr BP and refilled only decades to centuries before AD 1180–1290. Widespread ecosystem changes caused by forest clearance by Polynesian farmers began shortly after the end of this drought. Terrestrial sections show a chronology of burning and soil erosion similar to the lake cores. Although changing sediment types and shifts in the pollen rain suggest that droughts occurred earlier in the Holocene, as yet there is no evidence for droughts occurring after AD 1180–1290. The timing of the agricultural colonization of Rapa Nui now seems well established at ca. AD 1200 and it was accompanied by rapid deforestation that was probably exacerbated by the island's small size, its droughty climate, and the rarity of primeval fires. Detailed records of a large interval of Rapa Nui's ecological history remain elusive due to the drought hiatus in the Rano Raraku sediment record. We find no evidence for a "rat outbreak impact" on Rapa Nui's vegetation preceding anthropogenic forest clearance.

Analytical pyrolysis and stable isotope analyses reveal past environmental changes in coralloid speleothems from Easter Island (Chile)

“…However, to fully understand the internal banded structure, composition and origin of these coralloids, more comprehensive biogeochemical analyses based on isotopic signatures and molecular compositions have been incorporated. Moreover, these data have been related to past environmental changes previously documented in peat sequences and sediments from Easter Island [42,46,49,50].…”

“…This suggests that during the formation of the coralloid speleothems, a drastic reduction in plant diversity occurred during the final stage (Opal layer) of the coralloid formation. Mann et al [49] stated that currently Easter Island is a treeless landscape and grass species dominate the vegetation, mainly due to deforestation by Polynesian farmers. Before their arrival around AD 1200 to Easter Island, the prehistoric land vegetation was a mesophytic forest dominated by giant palms [69].…”

Paleoecology of Easter Island: Evidence and uncertainties

“…Depending on the authors, the deforestation took place either around AD 1000-1200 (Flenley and Bahn, 2007b;Mann et al, 2008) or some 600 years later (Hunt, 2006;Hunt and Lipo, 2007). The more popular hypothesis is that indigenous Rapa Nui people were responsible for the forest demise, as they used the wood for a variety of purposes, including the transportation of their characteristic stone statues or moais (Gurley and Liller, 1997;Grau, 2001).…”

“…According to this author, humans (Polynesians) arrived on the island during a "climatic optimum", from a human point of view, occurring between about 1200 and 600 BP, at which time the island was already grass-covered with only a few trees and shrubs growing on protected sites. Recently, Mann et al (2008) and Sáez et al (2009), using pollen analysis and lake level reconstructions, respectively, proposed the existence of a mid-Holocene dry period characterized by a sedimentary hiatus. According to Mann et al (2008), this period was followed by forest clearance by humans.…”

“…These issues are at the core of the problem as they deal with the evidence itself and its interpretation, which is the starting point for any further discussion. The magnitude of ecological and cultural inferences about Easter Island's history contrasts with the limited number of cores studied, their fragmentary nature and the low resolution of the analyses, the few proxies used (mostly pollen), and the existence of a sedimentary hiatus within which important information 4 about the environmental shift is still hidden (Mann et al, 2008). At present, studies about the paleoecology of the island seem to have spring up again and new cores have been obtained and are being analyzed (Gossen, 2007;Azizi and Flenley, 2008;Mann et al, 2008;Sáez et al, 2009).…”