Avifaunal Extinctions, Vegetation Change, and Polynesian Impacts in Prehistoric Hawai`i

Article excerpt

Abstract

Pre-contact avifaunal extinctions in Hawai`i generally have been attributed to human predation and/or landscape alteration by colonizing Polynesians. However, until recently there have been insufficient data for evaluating most of the important variables involved in this issue. This situation has changed with recent archaeological, paleontological, and wetland coring research conducted on O`ahu's `Ewa Plain, a hot, dry emerged limestone reef characterized by numerous sinkholes. The main evidence obtained from this research includes (1) wetland coring data that stratigraphically demonstrate forest decline before any burning, (2) radiocarbon dating of bones of rats and extinct birds that provides a time frame for their occurrence unavailable from stratified deposits, and (3) the radiocarbon-based history of human settlement of the `Ewa Plain.

Based on this evidence the argument is made that (1) at least some major avian extinctions occurred within the period immediately following Polynesian colonization, (2) these extinctions were due primarily to the rapid decline of their native lowland forest habitat, (3) human settlement of the `Ewa Plain occurred after native forest collapse, not coincident with it, and (4) the main source of destruction of the native forests was the introduced Polynesian rat, Rattus exulans, not Hawaiian agricultural clearing and burning. This model also explains the absence of large quantities of bird bone in early sites (in contrast to other places in Polynesia and Micronesia), and the absence in early middens of many plants (notably Kanaloa kahoolawensis) that were common in the native forest.

Introduction

The cause of Holocene avifaunal extinctions in Hawai`i has been a topic of intense interest since the first detailed investigation of subfossil paleontological remains almost two decades ago demonstrated that Hawai`i once had a much richer species inventory of birds than previously suspected (Olson and James, 1982, 1991; James and Olson, 1991). Recent interdisciplinary investigations on the `Ewa Plain of southwest O`ahu (Fig. 1) provide new insights into the process of extinction and environmental change in the Hawaiian lowlands (Athens et al., 1999). The new data 1) document the unexpected rapidity of vegetation change, 2) provide detailed information on the chronology of human settlement on the `Ewa Plain, and 3) provide detailed paleontological evidence, particularly as concerns the chronology of avian remains and vegetation change. These data indicate that avian extinctions and extirpations were the result of habitat destruction in the form of lowland forest decline. However, there is strong evidence that forest decline followed Polynesian colonization of Hawai`i but preceded dispersal, suggesting that direct human activity such as burning or gardening had nothing to do with the general decline. We propose that the main cause of forest decline was the Polynesian rat, Rattus exulans, brought by early colonists.

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Problem of avifaunal extinctions and extirpations

Some 35 to 57 extinct avian species have been identified from subfossil remains in Hawai`i, the number depending upon classification (James, 1995). Considering that historically known avifauna numbered from 40 to 55 endemic species, then about 100 avian species were present in Hawai`i prior to the archipelago's settlement by Polynesians (James and Olson, 1991), which occurred between about AD 700-800. (1) Chronological data from a stratified assemblage of avian subfossils on Maui (James et al., 1987) indicate a strong association of a major extinction event with the period of Polynesian settlement. Other studies of subfossil birds in the tropical Pacific follow a similar pattern, indicating that long periods passed with few extinctions despite major climatic oscillations with the last ice age (Steadman, 1995). …