title: Graptolite community responses to global climate change and the Late Ordovician mass extinction.
abstract: 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.
authors.affiliation: Department of Physics, Canisius College, Buffalo, NY 14208; sheets@canisius.edu.
authors.forename: H David
authors.initials: HD
authors.lastname: Sheets
authors.affiliation: Department of Geology, University at Buffalo, The State University of New York, Buffalo, NY 14260;
authors.forename: Charles E
authors.initials: CE
authors.lastname: Mitchell
authors.affiliation: Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS, Canada B2G 2W5;
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Melchin
authors.affiliation: Department of Earth Sciences, Dalhousie University, Halifax, NS, Canada B3H 4R2;
authors.forename: Jason
authors.initials: J
authors.lastname: Loxton
authors.affiliation: Institute of Geology, The Czech Academy of Sciences, 165 00 Prague 6, Czech Republic.
authors.forename: Petr
authors.initials: P
authors.lastname: Štorch
authors.affiliation: Department of Geology, University at Buffalo, The State University of New York, Buffalo, NY 14260;
authors.forename: Kristi L
authors.initials: KL
authors.lastname: Carlucci
authors.affiliation: Department of Geology, University at Buffalo, The State University of New York, Buffalo, NY 14260;
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Hawkins
mesh_terms: D000818:Animals; D059001:Aquatic Organisms / Q000145:classification / Q000254:growth & development; D001499:Bayes Theorem; D044822:Biodiversity; D057231:Climate Change; D053476:Extinction, Biological; D005580:Fossils; D019015:Geologic Sediments; D007448:Invertebrates / Q000145:classification / Q000254:growth & development*; D008954:Models, Biological; D009792:Oceans and Seas; D013997:Time Factors
keywords: abundance; climate change; extinction; macroevolution; selection