Resource Depression on the Northwest Coast of North America
Butler, Virginia L., Antiquity
A variety of evidence is accumulating from various parts of the world that suggests past human foragers greatly affected the animal and plant populations they were exploiting (Botkin 1980; Broughton 1997; Holdaway & Jacomb 2000; Nagaoka 1998; Steadman 1995; Stiner et al. 1999). Besides identifying prey response to human harvesting pressure, such studies also track ways human predators adjusted to reduced prey abundance, pointing out the dynamic nature of predator-prey interactions. Many studies have been conducted in a theoretical framework derived from evolutionary ecology, particularly foraging theory, and have demonstrated enormous explanatory power in accounting for subsistence change in human economies (e.g. Christenson 1980; Grayson & Cannon 1999; Gremillion 1996; O'Connell & Hawkes 1981).
With some exceptions (Cohen 1981; Croes 1995) researchers reviewing economies in Pacific Northwest North America have not considered whether human foragers altered prey populations. Some workers (e.g. Matson 1992) have expressed the view that key resources, like salmonids, were difficult for humans to `overuse'. Moreover, the Northwest Coast region has long been viewed as a `Garden of Eden', where the richness of the resource base was in some measure responsible for the `exceptional' hunter-gatherers that lived there, known for their relatively high population density, social organization which included slavery, elaborate art style and sedentary or semi-sedentary settlement patterns -- attributes generally linked to agriculture-based societies.
Several conditions suggest it would be useful to re-examine these views. Abundant archaeological evidence shows that human populations were growing and becoming increasingly sedentary, circumscribed and territorial during the late Holocene (Ames & Maschner 1999; Matson & Coupland 1995). Certainly in light of results from other parts of the world, it is reasonable to hypothesize that growing populations of relatively sedentary foragers depleted local food resources. Furthermore, human population size was drastically reduced with the introduction of disease at European contact. Such a decline should result in reduced foraging pressure; prey populations would have the opportunity to rebound and, in turn, human foragers could take advantage of increased prey abundance and shift resource selection.
This paper examines questions of late Holocene subsistence change, particularly the case for human-induced resource depression in the Pacific Northwest. I draw on the prey choice model from foraging theory to derive expectations about resource selection and subsistence change that would result from changes in foraging pressure. I then test this model using the mammal and fish faunal record from several sites on the Lower Columbia River dating to the last 2200 years. Results show an increased use of low-ranked resources before European contact and greater use of high-ranked resources after contact, which are predicted from changes in human demography. The results are provocative, suggesting that historic descriptions of exceptional resource abundance in part may reflect drastically reduced Native population size and that ethnographic records of subsistence practices may not reflect long-term trends.
The prey choice model provides a set of predictions on which prey items a predator should choose when an array of potential choices are encountered (Stephens & Krebs 1986). The model holds that a predator's most efficient strategy is to take the highest-ranked prey when it is encountered. Whether lower-ranked prey is taken depends on the encounter rate with the higher-ranked resources. Encounter rates are based mainly on prey densities; low-ranked prey will enter the diet more frequently as the abundance of higher-ranked prey declines. If mobility is limited or if the predator population increases, or if both occur, then resource depression of the high-ranked prey is likely to occur. …