Proc. Natl. Acad. Sci. USAVol. 93, pp. 5296-5300, May 1996Anthropo ogy

Late Holocene human-induced modifications to a centralPolynesian island ecosystem

(Pacific islands/archaeology/palynology/global change/island biogeography)

PATRICK V. KIRCHDepartment of Anthropology, University of California, Berkeley, CA 94720

Contributed by Patrick V Kirch, January 23, 1996

ABSTRACT A 7000-year-long sequence of environmentalchange during the Holocene has been reconstructed for acentral Pacific island (Mangaia, Cook Islands). The researchdesign used geomorphological and palynological methods toreconstruct vegetation history, fire regime, and erosion anddepositional rates, whereas archaeological methods were usedto determine prehistoric Polynesian land use and resourceexploitation. Certain mid-Holocene environmental changesare putatively linked with natural phenomena such as eustaticsea-level rise and periodic El Nifio-Southern Oscillationevents. However, the most significant changes were initiatedbetween 2500 and 1800 years and were directly or indirectlyassociated with colonization by seafaring Polynesian peoples.These human-induced effects included major forest clearance,increased erosion of volcanic hillsides and alluvial depositionin valley bottoms, significant increases in charcoal influx,extinctions of endemic terrestrial species, and the introduc-tion of exotic species.

The assumptions that human (cultural) inputs to global changeare primarily a modern phenomenon (i.e., -0.3 kyr; 1 kyr =thousand years) and stem from industrialization (1-3) aresubject to test by archaeological investigation of preindustrialhuman/environment interactions over significantly longertime spans. The islands of Remote Oceania (4) are especiallysuited to such research because their ecosystems had generallyevolved to steady-state equilibria, in isolation from continentalbiota (5-7). Humans (and other nonvolant vertebrates) did notreach Remote Oceania until 3.6-1.0 kyr, when seafaring,Austronesian-speaking peoples dispersed rapidly out of islandSoutheast Asia (8). Possessing a horticultural economy basedon intensive root/tuber/tree crops augmented by marineexploitation, Austronesians were skilled at interisland trans-port of crop plants and domestic animals (9). Their technologywas limited to stone (basalt), shell, bone, wood, and fibermaterials; fire was a major tool for agricultural clearance. Thisreport summarizes results from an interdisciplinary projectthat aimed to reconstruct a Holocene paleoenvironmentalrecord for Mangaia Island and to assess the impact of colo-nizing Austronesians. By analyzing stratigraphically controlledproxy measures for erosion, vegetation, burning, and terres-trial biota, we may compare the prehuman ecosystem withchanges that occurred after human colonization. Specific casestudies have been published by project members (10-18); thisarticle provides a synthesis.

THE STUDY SITE: MANGAIA ISLANDMangaia Island (1570 55' E, 210 55' S) lies along the Cook-Austral quasilinear volcanic chain, a magma plume trace ("hotspot" lineament) extending 2500 km from Macdonald Sea-mount to Palmerston Island (19). K/Ar dates on basalt indicate

TAVA tENGA

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TM7VT6 TIR- I 0 1 2km

FIG. 1. Map of Mangaia, showing concentric geological structure,radial drainage pattern, and depositional basins (stippled). Corelocations are indicated by black dots. Reprinted with permission ofAntiquity Publications, Ltd.

an age of 16.6-18.9 myr (1 myr = million years) for Mangaia'svolcanic core, which consists of ankaramite lavas (20). Theseare deeply weathered to laterite, with fresh rock exposed onlyin a few dike structures. Subcircular in outline (area 52 kM2),the island has a concentric geomorphological structure (Fig.1), with a deeply weathered volcanic cone (maximum eleva-tion, 169 m) surrounded by a ring (0.7-2 km wide; maximumelevation, 70 m) of elevated, coralgal limestones of reef origin,called makatea (21). Fossil corals from the makatea cliff faceyielded 230Th/234U ages of 90-110 kyr and U-series ages of-115 kyr (22, 23). Rainfall runoff (1967 mm xi annual; ref. 24)from the central volcanic cone has incised a radial drainagepattern, creating a solution escarpment along the makatea-volcanic interface (21). Sediments derived from the volcaniccone are deposited in basins at this interface (some surfacewater escapes through subterranean solution caverns in themakatea, issuing below sea level along the coast; ref. 11). Thepotential for a deep Holocene stratigraphic record from thesedepositional basins was our main reason for selecting Mangaia.The makatea escarpment also provided overhang rocksheltersfrequented by the Polynesian inhabitants of Mangaia, provid-ing a rich archaeological record of land use and resourceexploitation (16).

Vegetation cover is concentrically zonated as follows: zone1, a littoral forest dominated by Pandanus, Guettarda, Bar-ringtonia, and Hernandia trees; zone 2, where sufficient soil is

Abbreviation: kyr, thousand years.

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present, a makatea forest dominated by Hernandia and Elaeo-carpus, but strongly modified through cultivation of gardensand tree crops (e.g., Cocos, Artocarpus, Aleurites); zone 3,where soil is absent, Pandanus scrub on pinnacle-karstmakatea; zone 4, in the valley bottoms, a zone of intensivecultivation including tree crops (e.g., Inocarpus, Cocos) andirrigated pondfields for taro (Colocasia esculenta); and zone 5,blanketing the laterized volcanic cone, a pyrophytic associa-tion of fern (Dicranopteris linearis), scrub Pandanus, andisolated Casuarina trees (25, 26). Zone 5 was described as earlyas A.D. 1777 (27) and appears to be a successional communityof anthropogenic origin. The modern terrestrial vertebratefauna of Mangaia is depauperate, with only five landbirdspecies and six seabird species (18), a single, endangered fruitbat species (Pteropus tonganus), a few species of geckos andskinks (introduced both by Polynesians and Europeans), andthe Polynesian-introduced rat (Rattus exulans), chicken (Gal-lus gallus), and pig (Sus scrofa).

Polynesians (a subgroup of the Austronesians) settled theTonga and Samoa archipelagoes west of Mangaia by 3 kyr (28);palynological evidence suggests that Mangaia was colonized byPolynesians between 2.5 and 1.6 kyr, whereas archaeologicalmaterials directly document human habitation by 1 kyr. ThePolynesian population was -3,000 in 1822, although Europe-an-introduced diseases may already have reduced the popu-lation significantly. Mangaian society was a variant of classicPolynesian hierarchical chiefship in which warfare and com-petition for limited irrigated lands were intense (29).

MATERIALS AND METHODSStratigraphy and Palynology. Holocene stratigraphic records

were obtained from 25 cores, sampling each of the seven maindepositional basins (Fig. 1); maximum core depth was 15 m (inTIR-1, Veitatei Valley). Taken manually with Livingstone andD-section samplers, cores were x-rayed for microstratigraphy;peat samples were selected for 14C dating. Core TIR-1 wasanalyzed for bulk chemical composition by S. Dawson (Universityof Hull, England) using x-ray fluorescence. An x-ray diffractom-eter was used for mineralogical analysis; dithionite-extractableiron was determined to measure free-iron content as a signal ofweathering. Cores TIR-1, TM-7, and VT-6 were analyzed forloss-on-ignition (at 550°C) and sampled for pollen analysis fol-lowing standard methods (30). Other aspects of geochemical andpalynological analyses are presented elsewhere (10, 13, 15).

Archaeology. Archaeological excavations were conducted inseven rockshelter sites situated along the inner makatea es-carpment. Excavations followed chronostratigraphic units, andfine-sieving of sediments (through nested 12.8, 6.4, 3.2, and 1.6mm mesh) assured recovery of diminutive faunal and floralspecimens. The rockshelters produced a cultural sequencefrom 1 to 0.2 kyr. Of particular importance to this study are the35,157 bones obtained from site MAN-44 (16), providing a richrecord of human resource exploitation, particularly of birdsand fish. Steadman and colleagues analyzed avifaunal remains(16-18); fish bones were studied by V. Butler (Portland StateUniversity). The MAN-44 deposits also yielded invertebrateremains (marine mollusks and echinoderms) and carbonizedplant parts and wood charcoal (identified by J. Hather, Uni-versity College, London).

Radiometric Dating. Accurate chronology was obtainedthrough extensive 14C dating. The stratigraphic cores weredated with 26 peat samples using conventional gas-proportional counting methods performed by Beta Analytic(Coral Gables, FL) (10, 14), whereas the archaeological siteswere similarly dated with 40 samples of charcoal or otherorganic materials (16). In addition, 23 samples of bone (in-cluding extinct bird species) from rockshelter site MAN-44were dated using the accelerator mass spectrometry facility atLawrence Livermore National Laboratory (Berkeley). These

89 samples provide one of the most comprehensive suites ofradiometric dates for Holocene contexts on a central Pacificisland.

RESULTSHolocene Stratigraphy. The 25 cores exhibit consistent

stratigraphic sequences throughout the main drainage basins.The four facies groups present reveal significant changes indepositional environments within the basins over the past -7kyr. (i) In cores TM4, TM7, IV1, KA3, and KA4, a pedogenicmud was penetrated at 6-13 m below the modern land surface.This basal facies represents an early Holocene land surface,which began to be buried between 7260 ± 80 and 6450 ± 80B.P. based on five 14C ages. (ii) After -7 kyr, this earlyHolocene paleosol was buried by thick deposits of black(Munsell color 10 YR 2-3/1-2) lake peat (gyttja), indicatingdrowning of valley floors due to rapid post-Pleistocene sealevel rise (31). Solution caverns in the makatea were open tothe sea at this time, providing channels through which seawater could flood valley bottoms, creating brackish water lakes(11). This phase is marked by high spore concentrations of thebrackish-water tolerant fern Acrostichum aureum (15). (iii) Inmost cores, lake peat deposition changed to red/brown (7.5YR 3/4) reed peat in the upper levels, signaling the cessationof Holocene sea-level rise; in TM4, the transition dates to 4000± 70 B.P. A mid-Holocene sea level stand of - + 1.1 m is alsoindicated by elevated solution notches in the makatea escarp-ment of five drainage basins (10), matched by additionalcoastal evidence (32). (iv) The uppermost facies in all drainagebasins is a reduced gray clay (5 Y 3/1) derived from thevolcanic cone (mineralogical analysis indicates the presence ofsmectite, kaolinite, illite, and chlorite). The peat/clay transi-tion varies temporally between basins, but 14C ages of 1930 ±60 (IV1), 1830 ± 80 (VT5), and 1640 ± 80 B.P. (TM7) suggestthe onset of a major phase of erosion on the central volcaniccone at -1.8 kyr. For reasons indicated below, this onset ofclay deposition correlates with human colonization and hor-ticultural land use. The clay facies vary in depth within eachdrainage basin, reaching up to 6 m in Veitatei Valley.

In sum, the consistent stratigraphy indicates an island-widesequence commencing with a stable (late Pleistocene?) landsurface that was inundated at lower elevations -7 kyr, due torapid post-Pleistocene sea level rise. As brackish lakes filledvalley bottoms, peat deposition dominated until the sea levelreached a mid-Holocene maximum (-+1.1 m above modern)at -4 kyr. At 1.8 kyr, a rapid change in depositional environ-ment ensued, with clay in-filling of the valley bottoms.Charcoal Influx. Microscopic charcoal particles were

counted in samples from cores VT6 and TM7 (10). Micro-scopic charcoal is absent below samples dated to 2570 ± 90 and2480 ± 60 B.P., whereas above these levels charcoal particlesrise to peaks of 3 x 105 grains per cm3. The absence ofmicroscopic charcoal in older samples indicates that naturalfires were rare or absent in prehuman times. The onset ofmicroscopic charcoal after -2.5 kyr can be correlated with thearrival of humans practicing slash-and-bum horticulture (13).Core Sediment Geochemistry. Chemical analysis of the deep

TIR-1 core from Lake Tiriara revealed changes in the weath-ering regime of the central volcanic cone, as revealed in thesediment influx to the Veitatei Valley depositional basin (15);selected geochemical data for the upper 5 m of core TIR-1 aregraphed in Fig. 2. Significant changes include increases in sio2and A1203, along with free iron, and a decrease in P205. Thesetrends correlate with the pollen evidence indicating removal ofclimax forest vegetation from the central volcanic cone, ero-sion of the thin organic soil horizon, and exposure of the deeplyweathered laterite. Organic content of all cores (determinedby loss-on-ignition) also drops steeply in the uppermost levels,consistent with the above interpretation.

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35

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Rates of Erosion and Deposition. As indicated by corestratigraphy and geochemical analysis, erosion rates withinMangaia's radial drainage basins have changed significantlyduring the last 7 kyr. From 7 to 2.5 kyr, deposition was largelyorganic (peat) with only thin, intermittent clay bands derivedfrom short-term disturbances of the forest, probably by cy-clones and/or El Ninio-Southern Oscillation events (33). Such claybands occur in cores IV1 and TM7 at 6500 ± 80 and 6480 ± 100B.P. After -1.8 kyr, however, large influxes of weathered clayfrom the exposed volcanic slopes began to supplant peat depo-sition; in some valleys, clay deposits reached depths of 6 m.

Pollen Spectra. Pollen spectra were analyzed for three coresfrom Veitatei and Tamarua (10, 15); the TM7 core is dia-grammed in Fig. 3. They have a consistent sequence with amajor transition occurring between 2.5 and 1.6 kyr. Below thistransition, the pollen spectra are dominated by indigenousforest taxa, such as Sophora tomentosa, Erythrina sp., Wein-mannia samoensis, Ficus sp., and Cyathea treeferns. During thetransition, these and other forest taxa decline or disappearentirely, while three new taxa dominate: the monocot treePandanus tectorius and ferns Dicranopteris linearis and Cy-closorus interruptus. Dicranopteris ferns dominate the laterizedvolcanic cone today, and their dominance in the pollen spectrasince 1.6 kyr signals the removal of indigenous forest vegetation

dpcb

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3270 + 70 3-4-

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7240 + 1000 50 100

and its replacement with successional femland. That this forestremoval and replacement with a pyrophytic fernland/scrub Pan-danus association resulted from burning is indicated by the highcharcoal particle influx for this same period (see above).Wood Charcoal. An independent data set signaling vegeta-

tion patterns within the last 1 kyr is provided by carbonizedwood fragments from site MAN-44. Of 30 identified woodytaxa, 29 occur in the basal occupation deposits (zones 2 and 3);later deposits show a decrease in species diversity. There is amarked decline in indigenous forest trees, such as Canthiumbarbatum, Fagraea berteriana, Geniostoma sykesii, Homaliumacuminatum, Myoporum sandwicense, and Sophora tomentosa.The absence of these taxa in higher strata signals reductions inindigenous forest. The upper deposits (zones 6-17) are markedby high frequencies of a few economically-important trees(Aleurites moluccana, Cocos nucifera, Inocarpus fagiferus) andof Hibiscus tiliaceus, which dominates modern valley floors.The wood charcoal record thus provides a proxy measure ofthe conversion of native forest cover to an anthropogenicvegetation dominated by economically important taxa.

Avifaunal Extinctions. An avifaunal record for Mangaia isprovided by 795 identified bird bones from the main excavationblock in site MAN-44 (16-18). Basal (culturally cryptic) zones1A and 1B represent a mid-Holocene period 14C (AMS) datedon 21 bone samples between -7.5 and 1 kyr. These basalsediments contain six species of seabirds (in the generaNesofregetta, Phaethon, Anous, Gygis) and 14 species of nativeland birds, including rails (Gallirallus, Porzana), pigeons anddoves (Gallicolumba, Ptilinopus, Ducula), and parrots (Vini).Species diversity remains high in zones 2-4 (initial cultural useof the rockshelter) then plunges in zones 5-17 (the lateprehistoric period, -0.5-0.3 kyr). Of the 17 native land birdspecies represented in MAN-44, 13 are now extinct or extir-pated on Mangaia (17). Of 13 seabird species represented inthe site, 3 are extirpated, while all but two of the extant speciesare currently endangered (< 100 breeding pairs). This dramaticdecline in avifaunal diversity during the period of humanoccupation of Mangaia (Fig. 4A) probably reflects both theimpact of direct predation (for food, feathers, and bone) aswell as habitat destruction, particularly forest clearance. Athird likely factor was predation by introduced rats.

XcP

0 102 30 100 200 300 XI10 _-mm

I. .* a 30 20 40 X103Concentration Scale in Grains/cm2

FIG. 3. Simplified pollen diagram for core TM7, showing absolute concentrations of selected taxa, charcoal particle influx, and organic matter(%). Reprinted with permission of Antiquity Publications, Ltd.

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FIG. 4. (A) Frequencies of landbird bones (NISP = number ofidentified specimens) and species by chronostratigraphic zone in siteMAN-44. (B) Frequencies of pig (Sus) and chicken (Gallus) bones insite MAN-44.

Human impact on the Mangaian avifauna is matched bysharp declines in the frequency of another indigenous, volantspecies, the fruit bat (Pteropus tonganus). Pteropus bones arecommon in the early levels of MAN-44 but become extremelyrare in the upper deposits (18); the species persists but isendangered on Mangaia today.

Biotic Introductions. The MAN-44 sediments provide arecord of biotic introductions, including economic plants,domestic animals, and synanthropic (commensal) species in-troduced inadvertently. Introduced crop plants evidenced by

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carbonized stem, leaf, tuber or other parts (including woodcharcoal) from basal cultural zone 2 (-1-0.8 kyr) includearoids (Colocasia esculenta, Cyrtosperma chamissonis), banana(Musa), breadfruit (Artocarpus altilis), Tahitian chestnut (Ino-carpus fagiferus), sugarcane (Saccharum officinarum), ti (Cor-dyline terminalis), and sweet potato (Ipomoea batatas); thelatter is of South American origin (12, 16). Wood charcoaldemonstrates the early presence of tree crops, including Ino-carpus fagiferus, Artocarpus altilis, Cocos nucifera, and Syzy-gium malaccense. Other introduced economic species includebamboo (Schizostachyum glaucophyllum) and candlenut (Aleu-rites moluccana). Pig (Sus scrofa), dog (Canis domesticus), andchicken (Gallus gallus) were all introduced as food items (Fig.4B), although pigs were eliminated from the island in lateprehistory. This set of adventive, economic species dominatesthe island's modern biota (25, 26).

Polynesians also introduced the Pacific rat (Rattus exulans),a commensal species widely distributed throughout Oceania(34,35). R. exulans bones occur in zones lA-lB at site MAN-44and throughout the upper, cultural deposits. Predation by R.exulans on ground nests may have been an important factor in thedecline of some flightless, native bird species. Another synan-thropic species present in MAN-44 is the aquatic snail Thiara sp.,which thrives in irrigated pondfields, and may have been intro-duced with planting stocks of taro (Colocasia esculenta).Aquatic and Marine Faunal Changes. Although most of our

data pertain to the island's terrestrial environment, somematerials from the MAN-44 deposits indicate marine andaquatic changes over the past 1 kyr. Freshwater eels (Anguillasp.) and fish (Eleotrididae) were heavily exploited by theprehistoric Mangaians. High frequencies ofAnguilla in the earlydeposits may correlate with more extensive lakes (and higher lakelevels) before 1 kyr. The impact of human gathering of mollusks(for food and artifact manufacture) is exhibited for the reefgastropod Turbo setosus, the mean shell size of which declinessignificantly from early to later deposits (16).

DISCUSSIONOur interdisciplinary study of Mangaia enables a paleoenvi-ronmental reconstruction spanning the past 7 kyr, including

4NGAIA

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FIG. 5. Summary diagram of major changes in forest resource, soil erosion, charcoal influx, and human population over the past 5 kyr.

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significant changes initiated through colonization and occu-pation of the island by Polynesians, between 2.5 and 1.8 kyr.Fig. 5 summarizes some major trends over the past 5 kyr.Before the arrival of humans, the island was heavily forestedand supported a diverse terrestrial biota including at least 30species of land and sea birds. Natural fires were insignificantand erosion was limited to short-term periodic episodes asso-ciated with cyclones and/or El Ninlo-Southern Oscillationevents. Major environmental changes occurring during theearly-to-mid Holocene were lake formation and peat deposi-tion over Pleistocene land surfaces in the valley bottoms,resulting from rapid sea level rise. Polynesian arrival precipi-tated the following interrelated changes: (i) forest clearanceand conversion of central volcanic cone to pyrophytic Dicran-opteris fernland; (ii) burning, with significant increases incharcoal influx; (iii) increased rates of soil erosion, and alluvialinfilling of valley bottoms; (iv) reductions in populations ofnative birds and fruit bats, leading to extinction and extirpationof many species; and (v) introduction of many economic plantsand domestic animals, along with certain synanthropic species.The Holocene paleoenvironmental sequence reconstructed

for Mangaia demonstrates that preindustrial, nonmetal usinghorticulturally based human populations were capable ofmajor, irreversible transformations to Pacific island ecosys-tems. The Mangaian sequence is not unique, and is matchedby evidence from such islands as Easter, Marquesas, Tikopia,and Hawaii (17, 36-43). Human impacts on terrestrial envi-ronments clearly have a long history preceding the industrialera (44). It will behoove those interested in understanding the"human dimensions of global change" to take account ofarchaeological and paleoenvironmental records spanning atleast the full Holocene period.

I thank D. W. Steadman for critically commenting on a draft of thispaper. I thank my project collaborators D. W. Steadman, J. Ellison, J.Flenley, V. Butler, J. Hather, F. Lamont, and S. Dawson for sharingtheir data and results. The Mangaia Project was supported by NationalGeographic Society Grant 4001-89 and by National Science Founda-tion Grant BNS-9020750; laboratory research on faunal materials wassupported by National Science Foundation Grant BSR-8607535 toD. W. Steadman. The Cook Islands Government granted a researchpermit and the Mangaia Island Council facilitated our fieldwork.

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