COMPARATIVE INTENSITY AND SELECTIVITY OF THE EMERGING SIXTH EXTINCTION IN THE OCEAN

The episode of biodiversity loss during the late Pleistocene and Anthropocene is widely referred to as the ‘Sixth Extinction’ in reference to the five major mass extinctions first identified in the fossil record of marine animals. However, the intensity and selectivity of the Sixth Extinction in the oceans have not yet been assessed directly at the genus level, the conventional taxonomic unit in the study of extinction in the fossil record. Here, we review 2399 genera of fossil and living chordates and mollusks and categorize each according to its body size, ecological mode, stratigraphic range, and International Union for Conservation of Nature (IUCN) threat status (modern only). We use these assignments to compare directly the intensity and selectivity of the Sixth Extinction with stage-level data from the fossil record spanning the late Cretaceous through the present day. We find that genus-level extinction intensity for the Sixth Extinction will fall in the upper range of background Cenozoic values if all threatened genera go extinct but all genera classified as at least risk or lacking IUCN assessments survive the Sixth Extinction. If the prevalence of extinction risk among non-IUCN assessed genera is instead comparable to that of their assessed counterparts, then the Sixth Extinction could approach the intensity of the end-Cretaceous extinction if conservation measures prove ineffective. In addition, the ecological selectivity of extinction risk in the modern oceans is far different from background intervals spanning the past 65 million years, particularly due to strong selectivity against larger-bodied genera. This distinctive selectivity will be independent of the ultimate magnitude of the Sixth Extinction. The end-Cretaceous mass extinction is the most recent event in the fossil record to exhibit a comparably distinctive selectivity fingerprint. The importance of mass extinction derives as much or more from the extinction of ecological function as from outright rates of taxonomic extinction. Consequently, these observations suggest that, left unimpeded, the current biodiversity crisis may permanently alter marine ecosystems in a fashion that is quantitatively similar to the era-bounding extinctions of the Permian and Cretaceous periods.