GRAPTOLITE COMMUNITY RESPONSES TO LATE ORDOVICIAN CLIMATE CHANGE, THRESHOLD EFFECTS AND THE HIRNANTIAN MASS EXTINCTION

Mass extinctions disrupt ecological communities. As large numbers of species go extinct, complex inter-related communities are replaced by simpler communities comprising mainly generalist and opportunist species. Although climate changes clearly are a source of 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 community structure changes presage, or even cause, the dramatic losses in biodiversity in mass extinctions. We present here a detailed study of changes in the within-community species abundance distribution patterns of macro-planktonic graptolite fauna in the great Hirnantian Mass Extinction at the end of the Ordovician Period. Structural changes within graptolite communities at two contrasting sites exhibit significant decreases in community complexity and evenness as a consequence of the preferential decline in abundance of deep-water specialist species. At both sites, the decline of deep-water specialists took place during an interval of eustatic sea level rise. The observed changes in community complexity and evenness commenced well before the dramatic loss in species diversity and population depletions that mark the tipping point of the extinction event. Environmental isotope and biomarker data suggest that the extent of the oxygen minimum zone in the paleotropical oceans, upon which these species relied, decreased sharply during the latest Katian time, with a consequent change in phytoplankton community composition. Most deep-water species became rare as populations were depleted in step with this habitat loss and extinction risks rose correspondingly. Although many of the affected species persisted in ephemeral populations for hundreds of thousands of years, the toll of enhanced extinction risk depleted the diversity of paleotropical (diplograptine) graptolite species during the latest Katian and early Hirnantian. These results indicate that the effects of long-term climate change on habitats can degrade populations in ways that cascade through communities, with effects that persist for geologically significant intervals of time and culminate in mass extinction.