ISOTOPIC EVIDENCE FOR LIFE HISTORY CHANGES IN ESTUARINE MOLLUSKS IN RESPONSE TO HUMAN PREDATION IN THE SOCONUSCO, CHIAPAS, MEXICO (Invited Presentation)

Archaeologists often observe that the remains of animals with indeterminate growth patterns (e.g., fishes, pelecypods, gastropods) found in ancient middens are much smaller than their counterparts in contemporary, unexploited ecosystems. Human predation pressure is inferred to have led to smaller average body size in a population over time, the implicit assumption being that frequent harvest prevents prey from growing to their full potential size.

Stable oxygen isotope data (δ¹⁸O) in marsh clams (Polymesoda radiata) from multiple archaeological shellmound sites in the Soconusco region of Mexico demonstrate that reductions in body size result not only from a shortened lifespan but from the selective advantage conferred on clams with slower growth rates under heavy harvest pressure. As forager-farmer groups intensified P. radiata harvesting through the Archaic Period (7500-4200 cal BP), slower average growth rates and hence smaller body size at sexual maturity are indicated by compressed seasonal isotope oscillations recorded in the shells. During the Formative Period (after ca. 3800 cal BP), as increasing reliance on maize agriculture reduced pressure on estuarine resources, the isotopic signals became attenuated, indicating faster growth rates and larger size at sexual maturity.

We argue that in addition to short-term (annual to decadal scale) body size reduction in available prey – a phenotypic effect – persistent long-term (centennial to millennial scale) human predation selects for slower growth in these animals – a genotypic effect – indicating a life-history adaptation in Polymesoda that allows it to persist in the anthropogenic ecosystem. Further, the use of incremental isotope sampling and high-resolution ¹⁴C dating indicates that a population may fairly rapidly re-establish a previous life history pattern after the predation pressure is removed. We discuss concepts of sustainability and human impacts in contemporary aquatic settings in light of the finding that foragers without elaborate harvesting technology have induced life history adaptations in certain prey groups simply through intensive foraging practices.