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Article Excerpt
Abstract
In the tropical Pacific, climate change has been implicated as a causal variable in the development of a variety of social processes, including resource scarcity, cultural diversification, changes in spatial organization, and conflict. Hypotheses concerning the effects of climatic variability on cultural change can be better evaluated once links between environmental processes and subsistence patterns are established. Here we present data on approximately 1500 years of shellfish exploitation at the Fatu-ma-Futi site, Tutuila Island, American Samoa. We generate an Agent Based Model to test hypotheses regarding resource exploitation and the effects of climate change on near-shore marine fauna. To date, little archaeological data regarding prehistoric marine resource use in Samoa is available, demonstrating the need for more field research. Integrating models generated from foraging theory and agent based computer simulations provides a new technique for modeling social and ecological processes in complex environments.
Keywords: climate change, coral bleaching, foraging theory, agent based modeling, Samoa
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The potential impacts of prehistoric human populations and natural climatic phenomena on island ecosystems and human cultural change have long been recognized by archaeologists working in Oceania (e.g. Athens and Ward 1993, 1997; Kirch 1983, 1997; Kirch and Hunt 1997; Steadman 1995; Anderson et al. 2006; Field 2004; Nunn 2000a, 2000b). Understanding the effects of environmental variability on Pacific islands requires constructing models that distinguish between the empirical result of human impacts and those related to natural climate variability. Furthermore, environmental variability occurs at scales varying from global phenomena to more localized processes that must be assessed in each context. The situation is made even more complex with recognition that prehistoric habitat alteration is likely influenced by numerous processes occurring simultaneously (Rietz 2004:65-66; Wolverton 2001).
Documentation of climate change in the Southern Hemisphere, including the islands of the tropical South Pacific, has been hindered by a lack of instrumental and proxy climate records, with twice as many reconstructions available from the Northern Hemisphere (Jones et al. 2001:663). Nevertheless, increasing numbers of multiproxy records for the tropical Pacific are being constructed (e.g. Cobb et al. 2003; Jones et al. 1998; Mann et al. 1999; Linsley et al. 2000; Hendy et al. 2002) and offer the potential for a better understanding of past local climate variability in the central Pacific (Allen 2006). In order to critically assess the relationship between cultural changes, including variability in spatial organization, competition, and subsistence, formal links between climate and the empirical expectations of the archaeological record must be established. Consequently, a detailed understanding of marine-resource use in the prehistoric Pacific is necessary.
Recent research in the West Polynesia region, including the islands of Fiji and Samoa (Figure 1), examines the relationship between climate and environmental variability and changes in settlement (Pearl 2004), competition (Field 2004), landscape alteration (Pearl 2006), and ceramic diversity (Cochrane 2004; Cochrane and Neff 2006). Exploring the mechanistic relationship between climate change and marine resources is essential for providing the necessary link between environmental variability and cultural changes (Allen 2006:531).
In Samoa, given the relative paucity of archaeological research in the archipelago over the last 20 years and the difficulty associated with locating well preserved faunal remains, the use of marine resources (and subsistence in general) are not well documented (but see Nagaoka 1993 for an exception). However, recent excavations at Fatu-ma-Futi, on Tutuila Island recovered substantial mollusc and fish assemblages in a stratified sequence covering the last ca. 1500 years (Addison 2006; Addison and Asaua 2006; Morrison 2006).
This paper begins by briefly outlining the evidence for climate change in the tropical central Pacific over the last millennium. Next, we discuss the integration of foraging models and Agent Based Modeling (ABM) to better distinguish human impacts to prey populations from natural changes induced by climatic phenomena. We use the Fatu-ma-Futi shellfish assemblage as a case study to assess the usefulness of the models presented below.
Background to the problem
One of the greatest challenges facing archaeologists researching the effects of climate and environmental variability is distinguishing the results of natural perturbations from the effects of human induced changes. Nunn (1991, 1999) has also expressed this dilemma when discussing the history of Pacific coastal landscape alterations during the period of human occupation. Indeed, zooarchaeologists have long recognized the potential of environmental changes to affect prey populations in complex ways that must be disentangled from the effects of human predation. For example, studies of prehistoric fish use on Mangaia (Butler 2001), and Rotuma (Allen et al. 2001) address the potential effects of habitat alteration on the abundance of archaeofauna. Relevant environmental variables discussed by these authors include hillside erosion (Butler 2001:96) and changes in siltation and salinity in the near-shore environment (Allen et al. 2001:67).
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Foraging theory models (Stephens and Krebs 1986) have proven robust for documenting declines in foraging efficiency and changes in habitat use in a variety of ecological contexts (Allen 1992, 2002, 2003; Cannon 2003; Morrison and Hunt 2007; Nagaoka 2001, 2002; Wolverton 2001). These models provide a theoretical basis for examining temporal shifts in resource exploitation as a result of human resource depression, environmentally induced decreases in prey abundance, and changes in harvesting technology.
Foraging Theory Models
Although space limits a detailed discussion of the application of foraging theory to archaeological research (for details see Allen 2002, 2003; Broughton 1994; Butler 2001; Morrison and Hunt 2007; Nagaoka 2001, 2002), resource depression is generally indicated in the archaeological record by: 1) a decreased amount of large-bodied prey relative to smaller prey; 2) an increase in the use of less profitable habitats; 3) an increase in taxonomic diversity; and 4) a decrease in the average age and size of exploited taxa.
Relevant studies applying foraging theory to shellfish populations include, Anderson (1981), Raab (1992), and Morrison and Hunt (2007). These authors, among others have demonstrated instances of resource depression through the application of foraging theory models (but see Bird and Bliege Bird 1997; Thomas 2002 for important theoretical considerations). Numerous other studies have also measured human impacts to mollusc populations through changes in species composition, and decreased prey size, and age (e.g. Swadling 1976, 1986; Jerardino 1997; Mannino and Thomas 2001; Milner et al. in press). While some of these researchers do not operate from an explicit foraging-theory framework, many of the applications are indeed compatible with the logic of foraging theory.
Understanding the complex relationship between human predators and their prey requires properly modeling the environmental contexts of these interactions. Without taking the local ecological history into consideration, archaeologists may incorrectly assign resource shifts to the direct result of human predation when in fact the distribution of taxa is actually a consequence of an environmental circumstance or a combination of foraging pressure and environmental change. As a result, researchers should assess the role climate change and other natural processes play in the archaeological distribution of species. Paleo-climatic reconstructions from the central Pacific suggest periods of climate variability over the past 1000 years that may have affected marine resource use (and subsistence in general) on Pacific Islands.
Climate change in the Tropical Pacific over the last 1000 years
Working with a combination of proxy reconstructions, Nunn (2000a; 2000b) and Nunn and Britton (2001) hypothesize that a drop in temperature and an increase in precipitation and El Nino Southern Oscillation (ENSO) frequency during the transition between the Little Climatic Optimum (LOC) and the Little Ice Age (LIA) caused marine ecosystem stress with consequences for human subsistence and settlement (Nunn 2000a, 2000b). Additionally, Nunn has also argued for a regional sea-level fall of up to 75 cm at 680-625 cal BP (Nunn 1998; Nunn 2000b).
Reviewing multiple proxy reconstructions from coral cores in the central Pacific, Allen (2006) suggests a more locally variable relationship exists between global climate change and region-specific trends. In particular, the climatic patterns documented during the LCO and LIA in the temperate Northern Hemisphere data do not correlate well for the tropical central Pacific (Allen 2006: 521; Cobb et al. 2003:275; Jones et al. 1998: 462; 2001:664). The following section focuses only on climate proxy records from reconstructions in the tropical Pacific (Figure 1). These records suggest the possibility of increased environmental variability including changes in sea surface temperature (SST) and increased ENSO frequency and magnitude after...
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