PRESS/PULSE: A GENERAL THEORY OF MASS EXTINCTION?

Previous discussions of mass extinction mechanisms focused on events unique to the extinction they explain. To propose and test a general mechanism of mass extinction, we borrow a pair of concepts from community ecology: Press disturbances alter community composition by placing multigenerational stress on ecosystems; pulse disturbances are sudden, catastrophic, and can alter communities by causing extensive mortality. We hypothesize that the coincidence of press and pulse events is required to produce the greatest episodes of dying in Phanerozoic history. To test this hypothesis, we compiled generic extinction rates for each age of the Phanerozoic based on data from the Compendium of Fossil Marine Animal Genera (Sepkoski, 2002). Cratering events served as a proxy for pulse disturbances as the effects of such impacts would be instantaneous and potentially catastrophic. Episodes of continental flood volcanism producing large igneous provinces stood in for press disturbances; these events are geologically long-lasting and have been linked with extensively discussed extinction mechanisms such as climate change. Average extinction rates were similar during geologic ages in which either press or pulse events occurred alone. Extinction rates during these times were statistically indistinguishable from rates associated with ages when neither impacts nor flood volcanism occurred. In contrast, when press and pulse events occurred together, higher average extinction rates were recorded. Interestingly, the size of the associated flood basalt or crater was poorly correlated with extinction rate. Thus, it is the combination of press and pulse events—a geologic one-two punch—rather than the magnitude of single events that explains Earth's greatest episodes of extinction, including, perhaps, the modern biodiversity crisis.