ALTERED BIOTIC INTERACTIONS IN THE ANTHROPOCENE: A 3.5 MYR BASELINE OF GASTROPOD DRILLING PREDATION FROM THE EASTERN GULF OF MEXICO

The Pleistocene fossil record of biotic interactions is increasingly being used as a natural baseline for understanding anthropogenic change. Here, we extend an existing 3.5 myr baseline on predator-prey interactions between the venerid bivalve Chione and its predators, infaunal naticid and epifaunal muricid drilling gastropods, into the Anthropocene. Fossil samples included 30,325 bulk-collected Chione spp. from the Tamiami (late Pliocene), Caloosahatchee (early Pleistocene), Bermont (middle Pleistocene), and Ft. Thompson (late Pleistocene) formations of Florida. Modern samples included 11,382 bulk-collected Chione elevata from the full range of modern habitats for this species in west and south Florida, including seagrass beds in bays and open coasts to depths of 15 m. Data were collected on drilling frequency (d.f.), incomplete d.f., edge d.f., and predator-prey size selectivity. These measure predation intensity, direct and indirect competition, and optimal foraging, respectively.

Our results show that infaunal naticids decreased their d.f. on Chione prey (11 to 4%) at the Plio-Pleistocene boundary; naticid d.f. declined to a low of 1.5% in the late Pleistocene before more than tripling to >5% in the Recent. Epifaunal muricids exhibit a similar early decline in d.f. (33 to 24%), followed by a lower Pleistocene baseline around 20% and a precipitous decline to 7% in the Recent. Predator-prey size selectivity varied throughout the study interval, but incomplete d.f. and edge d.f. show early declines, stable low baselines throughout the Pleistocene, and dramatic increases in the Recent for both predatory types. The intensity of biotic interactions among these benthic invertebrates today is unprecedented over the last 2.5 million years, possibly indicating broader changes in species composition and community structure. Moreover, the Anthropocene shift is comparable in magnitude to that observed at the Pliocene-Pleistocene boundary, a period of ecological adjustment preceding a regional mass extinction event roughly 2 Ma. The Anthropocene may be a similar period of adjustment preceding extinction.