Paleoenvironmental and archaeological investigations from the ’Ewa Plain of O’ahu provide insight into the problem of understanding lowland native forest loss in Hawai’i. Data from pollen analysis of a pond core record, avian paleontology, and archeology, document a precipitous decline of the native forest starting before Polynesian settlement on the ’Ewa Plain but after Polynesian colonization of O’ahu. It is hypothesized that rats, introduced by Polynesian colonizers, increased exponentially in the absence of significant predators or competitors, feeding on a largely endemic vegetation that had evolved in the absence of mammalian predators. Rats radiated ahead of human colonizers on O’ahu, eating their way through the vegetation, perhaps before the colonizers had encountered much of the pristine lowland forest into which the rats had radiated. This hypothesis is supported by several observations, including the almost complete absence of extinct or extirpated avian faunal remains in archaeological deposits, the present distribution of endemic vegetation in Hawai’i, rat ecology, population biology, and other evidence.
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Subsequent O’ahu cores did reveal a more consistent record of anthropogenic charcoal particles as seen in the Ordy Pond pollen diagram, Fig. 2b. These always display a pattern of gradually increasing concentrations from early to late during prehistoric times, which is an antithetical implication of the slash-and-burn model, which would be expected to show more charcoal early on, and then diminution once the forest was entirely removed.
A subsequent project raised two 17.4 m cores from Ordy Pond. These cores displayed the accumulation of finely laminated sediments beginning about 8150 cal. BP (13.3 m below the top of the core), with late Pleistocene shell hash in a non-aquatic carbonate matrix below (see Uchikawa 2006).
Some investigators might suggest an alternative explanation for rapid catastrophic forest decline in the lowlands, which is that it was due to a drop in the water table in response to the mid-Holocene highstand drawdown. However, highstand data and models suggest that drawdown occurred in Hawai’i much earlier than forest collapse (Fletcher and Jones 1996, p. 639; Grossman and Fletcher 1998, pp. 365–366), dating to about 2000 cal. BP. Interestingly, a 7500 year pollen sequence from a nearby NASBP core at an evaporite pan (Athens et al. 1999), while not having the high resolution of the Ordy Pond core, fails to provide any evidence of the drawdown having an impact on the vegetation ca. 2000 cal. BP. In fact, this core’s pollen data demonstrate that the ’Ewa Plain was forested since initiation of its record in the early Holocene when sea level certainly was well below its modern level (e.g., Fairbanks 1989). It appears that whatever influence, if any, sea level may have had on the coastal lowland water table, the water table had little or no impact on the near-coastal vegetation (i.e., the ’Ewa Plain was entirely forested even during the early Holocene when sea level was well below its modern level). Other O’ahu pollen cores also fail to support the water table hypothesis, showing, like Ordy Pond, a sharp decline in the native forest starting about 1000 cal. BP, but in areas where even if there had been a drop in the water table, rainfall would have been sufficient to support a forest (e.g., Athens and Ward 2000; Athens et al. 1992, 1995).
The earliest date for Polynesian colonization of Hawai’i has been a thorny issue in Hawaiian archeology. However, with research conducted in the past decade or so, there is a growing consensus that colonization was late, occurring sometime during the second half of the first millennium AD. Given the data, many would now even restrict the time of colonization to the middle or latter part of the second half of the first millennium AD (e.g., Tuggle and Spriggs 2001; Kirch and McCoy 2007). While generally sharing this perspective, this author has come to believe that a date of about 900–1,000 BP is most parsimonious with the evidence, especially the paleo coring data. While it is true that one paleo core did suggest a slightly earlier time frame (Athens 1997), there is a remarkable consistency in the cores that have this crucial time period represented in them. The appearance of charcoal particles, vegetation changes, and Polynesian plant introductions all become evident around 900–1,000 cal. BP (see also Burney and Kikuchi 2006 for Kaua’i). In view of the apparent sensitivity of Hawaiian vegetation to human colonization, as argued in this paper (i.e., as a result of the introduction of the rat), there could not have been an earlier colonization unless the earlier colonizing group arrived without rats, which were then introduced by a later colonizing group around 900–1,000 cal. BP. But lacking evidence for such a hypothetical non-rat earlier colonization event, vegetation change was for all practical purposes coincident with first colonization, which included rats. There is no credible archaeological evidence for archaeological sites dating earlier than about AD 1000 to 1100, surely an arguable point for some investigators, but one that presently cannot survive the test of chronometric hygiene.
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Financial support for these investigations came from the Department of the Navy, Pacific Division, Naval Facilities Engineering Command, Pacific, through planning and environmental compliance contracts with Belt Collins Hawaii (BCH). International Archaeological Research Institute, Inc., served as the BCH subconsultant for archaeological investigations at NASBP. The author is extremely grateful for the support of these institutions and their staffs, and also the very substantive collaboration he enjoyed with his colleagues and their contributions to the results presented herein: H. David Tuggle, Jerome V. Ward, and David J. Welch. The author also thanks the many other people who worked on this project in what was a truly interdisciplinary effort. Finally, he wishes to thank Terry Hunt and Don Drake for their interest in this work and giving him an opportunity to present it at the conference they organized, Rats, Humans and Their Impact on Islands, held at the University of Hawai’i, Honolulu, March 27–31, 2007.
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Athens, J.S. Rattus exulans and the catastrophic disappearance of Hawai’i’s native lowland forest. Biol Invasions 11, 1489 (2009). https://doi.org/10.1007/s10530-008-9402-3
- Prehistoric rats
- Endemic vegetation
- Forest loss
- Avian extinctions