Reconstructing oyster paleocommunity structure over the last 3.6 million years: A southern California case study

Abstract

We culled abundance record data from the NSF-funded TCN, Eastern Pacific Invertebrate Communities of the Cenozoic (EPICC), including all southern California localities that recorded the presence of oysters from the last 3.6 million years to document how oyster communities change through time. In total, over 120,000 specimens from 78 localities throughout southern California (i.e., Los Angeles, Orange, and San Diego counties) were examined. The data were broken down into four-time bins: late Pliocene, middle Pleistocene, late Pleistocene, and Holocene. Using multivariate statistics, several statistically coherent groups based on occurrences and abundances through time were indentified. Results indicate that the late Pliocene coherent groups possessed a loose, facultative, individualistic community structure that allowed taxa to shift their latitudinal gradients as they tracked shifting environments. The dominant oyster—Dendrostrea vespertina—as well as other taxa, became extinct at the Plio-Pleistocene boundary. Afterwards, community structure changed, as did the dominant oyster. We suspect that the onset of northern hemisphere glaciation at the Plio-Pleistocene boundary changed both the magnitude and rate of sea surface temperatures such that local extinction occurred causing changes in dominance within marine communities. During the middle Pleistocene, Ostrea conchaphila (lurida) appeared and remained dominant throughout the Holocene. In addition, distinct spatial groups existed causing reduced migration along the coast of southern California. Perhaps southern California marine communities responded to the water-mass differences associated with the mid-Pleistocene transition from a mild, 41 ka glacial-interglacial cycle to the more variable ~100 ka glacial-interglacial cycle reducing migration along the coast of southern California. The loose, individualistic community structure seen in the late Pliocene returned during the late Pleistocene and continued through the Holocene allowing marine communities the flexibility to track shifting environments.