Graptolite community responses to global climate change and the Late Ordovician mass extinction
Significance Climate change during Late Ordovician mass extinction drove an ecologically selective decline in the abundance of deep water macrozooplankton, accompanied by a shift to simpler, less even communities. These results indicate that the species abundance structure of planktic communities may be a leading indicator of the effects of climate change on biodiversity and a more sensitive gauge of those effects than taxonomic diversity change alone. Additionally, we present a Bayesian likelihood method of habitat inference that may be applicable to other similar cases. Mass extinctions disrupt ecological communities. Although climate changes produce stress in ecological communities, few paleobiological studies have systematically addressed the impact of global climate changes on the fine details of community structure with a view to understanding how changes in community structure presage, or even cause, biodiversity decline during mass extinctions. Based on a novel Bayesian approach to biotope assessment, we present a study of changes in species abundance distribution patterns of macroplanktonic graptolite faunas ~447 444 Ma) leading into the Late Ordovician mass extinction. Communities at two contrasting sites exhibit significant decreases in complexity and evenness as a consequence of the preferential decline in abundance of dysaerobic zone specialist species. The observed changes in community complexity and evenness commenced well before the dramatic population depletions that mark the tipping point of the extinction event. Initially, community changes tracked changes in the oceanic water masses, but these relations broke down during the onset of mass extinction. Environmental isotope and biomarker data suggest that sea surface temperature and nutrient cycling in the paleotropical oceans changed sharply during the latest Katian time, with consequent changes in the extent of the oxygen minimum zone and phytoplankton community composition. Although many impacted species persisted in ephemeral populations, increased extinction risk selectively depleted the diversity of paleotropical graptolite species during the latest Katian and early Hirnantian. The effects of long term climate change on habitats can thus degrade populations in ways that cascade through communities, with effects that culminate in mass extinction.
Proceedings of the National Academy of Sciences