Declines in Mammalian Foraging Efficiency during the Late Holocene, San Francisco Bay, California

Resource intensification models that have been posited for prehistoric California predict decreases in foraging efficiency during the late Holocene, Using implications of the fine-grained prey model of optimal foraging theory, I derive an index of the efficiency of vertebrate prey choice from the relative abundances of large- and small-sized prey items. I then test the intensification models with late Holocene mammalian faunas from San Francisco Bay shellmounds. Dramatic linear decreases in the relalive frequency of artiodactyls compared to the smaller sea otters (Enhydra lutris) throughout the occupational histories of particular localities strongly support the resource intensification models. The declines in artiodactyl abundances are not correlated with late Holocene climatic indices developed for this region, with changes in the seasonal use of shellmounds, or with technological innovations. An intra- and interregionally consistent pattern in declining abundances of large mammals in environmentally distinct regions throughout California suggests that resource depression driven by human predators may be the single most important cause of the declines. These patterns have far-reaching implications concerning the long-term human role in structuring prehistoric ecosystems.