THE POSEIDON ADVENTURE: RICHNESS VERSUS COMMONNESS

Common species shape the world around us, and changes in their commonness signify large-scale shifts in ecosystem structure and function. However, studies of biotic response to global change in the geological past typically target taxonomic richness and turnover, which has inspired new techniques to correct for sampling biases. Here we use a very simple measure of relative changes in global occurrences of the most common and widespread species, and apply it to Cenozoic deep-sea records of calcifying plankton (coccolithophores and foraminifera). Model experiments with Poseidon, a synthetic database of planktonic microfossil occurrences, show that our simple approach is highly robust to variability in spatial sampling and variability in the species abundance distribution. In contrast, popular sampling-standardization methods are highly sensitive to changes in the shape of the abundance distribution. We find that subsampled estimates of richness can be reproduced by simply combining the raw richness and raw evenness signals, and this relationship holds in both Cenozoic plankton records and across all Poseidon experiments. Coccolithophores and foraminifera show markedly different changes in the commonness of common species through the Cenozoic, possibly linked to different components of long-term climate change across one of Earth's major climate transitions. Our findings suggest that commonness provides a focal point for understanding the role of an evolving biota in Earth's dynamic history.