TESTING ABUNDANCE CENTER HYPOTHESES IN THE FOSSIL RECORD OF NORTH AMERICAN INVERTEBRATE SPECIES

The Abundant Center Hypothesis (ACH) describes an ecological relationship between the local abundance of a species and the distance between that population and the center of that species’ fundamental environmental niche (Brown, 1984). Because geographic variability within a species’ range may impose dispersal boundaries or rapid change in environmental variables – such as a continental shelf drop-off imposing a sharp temperature and depth gradient – the ACH is more likely to be measurable in environmental space (e-space) vs. geographic space (g-space; e.g., Sagarin & Gaines, 2002; Martínez-Meyer et al., 2013). Modern tests of this hypothesis have yielded mixed results (e.g., Osorio-Olvera et al., 2019; Dallas et al., 2020), but the ACH has never been tested in the fossil record, where a cumulative dataset over a timespan on par with the expected lifetime of a species may provide a comprehensive look at any emergent niche distance - abundance relationships. This study aims to test the ACH in shallow marine bivalve species of the Atlantic/Gulf and Pacific Coastal Plains, using extant species with roots in the Pleistocene, to capture a wide variety of coastal depositional environments and climatic extremes across the Pleistocene – Holocene and meaningfully compare results from the fossil record of occurrences with similar modern analyses. Preliminary results support prior findings of no apparent center-abundance relationship in geographic space; testing this hypothesis in e-space and comparing time-series results will provide additional clues into the dynamics of intraspecies distributions over space and time. Quantifying if and how niche distance - abundance relationships change over the course of significant climatic shifts across the Pleistocene-Holocene may help improve predictions of species distribution shifts in response to rapid climate change, as a step toward measuring climate change resilience in marine invertebrates.