Ancient actions predict modern consequences: Prehistoric lessons in marine shellfish exploitation
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2012 The Arizona Board of Regents. All rights reserved. Human population growth and coastal development the world over are threats to marine biodiversity. Modern conservation efforts focus on elucidating the phenomena that mediate maintenance, loss, and restoration of biodiversity. The high degree of interdependence between these phenomena requires conservation efforts to focus on the community and ecosystem levels. However, the magnitude of complexity at these levels makes efforts difficult (Shaffer 1981). Marine ecosystems often take decades or centuries to fully respond to disturbance; yet most ecological studies examine ecosystem dynamics over much shorter time frames. Collaborative research between conservation biologists and archaeologists has the potential to greatly enhance understanding of these dynamics (Briggs et al. 2006). The archaeological record reflects millennia of human exploitation of marine resources and provides an evolutionary time scale to marine ecosystems. Zooarchaeological analysis of faunal assemblages reveals ecological characteristics of exploited species within a variety of environmental and anthropogenic contexts. The archaeological record represents the results of a series of long-term natural experiments that provide researchers with an unprecedented ability to identify and anticipate vulnerability within marine communities (Lyman 1996; Lyman and Cannon 2004). Conservation biologists have long recognized that small populations exhibit increased susceptibility to extinction; yet a strong predictive understanding of the relationship between population size and the probability of extinction continues to elude researchers (Brook et al. 2006; Reed and Frankman 2003; Shaffer 1981). It is difficult to predict future extinctions on the basis of current population fitness as species vary in sensitivity to environmental and anthropogenic conditions. As a result, populations may begin to decline long before ecologists detect degradation within the marine ecosystem (Brook et al. 2006; Reed and Bryant 2000). The need to characterize the long-term viability of small populations has driven conservation biologists to focus on minimum viable populations. To date, the majority of efforts to determine the minimum viable size for a population have been theoretical (e.g., references in Soule 1987). Theoretical expectations for a minimum viable population vary based on the postulated effects of demographic, genetic, and environmental variation (Reed and Frankham 2003). Modern conservation efforts require empirical evidence to parallel these theoretical predictions and establish relationships between population size, fitness, and extinction (Reed and Bryant 2000). A minimum viable population must be of sufficient size to endure both environmental and anthropogenic perturbations within its particular biogeographic context. Further, survival of a population must be measured relative to particular ecological contexts under particular predation regimes (Shaffer 1981). Shaffer (1981) indicates that the most direct approach to assessing minimum viable population size for a given species is to create isolated populations under specific environmental and predation regimes and to monitor the persistence of each population. However, long-term empirical studies are beyond the scope of most experiments and neglect the immediacy of current resource demands (Lehmkuhl 1984; Reed and Bryant 2000). Issues that plague the determination of minimum viable population size are particularly acute for metapopulations, or sets of local populations. Shaffer (1981) indicates that the net effect of all types of perturbations on a local population's prospects for survival depends to a great extent on its geographic relationship to other local populations. Further, any factor (anthropogenic or other) depressing the size or growth of a local molluscan population may be mitigated or exacerbated by the relative health of nearby populations (Hastings and Harrison 1994). Most marine invertebrate communities consist of isolated populations of relatively sessile adults; isolated local populations interact through dispersing larvae. Metapopulations are vulnerable to collapse should local popula tions become fewer or more isolated (Hanski et al. 1996; Kritzer and Sale 2003; Wilcox and Murphy 1985). The situation may be exacerbated by strong predispersal Allee effects due to depressed fertilization success at low population densities (Gascoigne and Lipcius 2004a). Dispersal breeders require dense stable adult populations to ensure successful fertilization of gametes released into the open water. There is a strong exponential relationship between local population density or nearest neighbor and fertilization efficiency among marine invertebrates (Gasciogne and Lipcius 2004b; Gascoigne et al. 2009). Conservation biologists concerned with the regional persistence of marine invertebrates must focus on both the minimum viable population size and the minimum viable metapopulation size (Soule 1986) as human population growth and coastal development accelerate habitat fragmentation along the California coastline (Dugan et al. 2000). But modern ecologists struggle with basic determination of metapopulation structure in marine environments (Smedbol et al. 2002). Further, a multidimensional evaluation of temporal and spatial metapopulation distribution is necessary for empirical evaluation of metapopulation presence and minimum viable metapopulation size. However, modern ecological studies cannot approach the evolutionary time scale at which assessments of metapopulation persistence can be made. The zooarchaeological record can contribute significantly to the determination of key variables. Although zooarchaeological remains cannot be used as a reliable source of census data for a prehistoric population (Lyman 2008), they can provide certain kinds of demographic data for a taxon. In conjunction with knowledge of life history p
