PRESS/PULSE: AN ECOLOGICAL MODEL FOR SELECTIVE EXTINCTIONS

Single-cause mass extinction scenarios require extreme conditions to generate sufficiently strong kill mechanisms. Such dire effects typically do not jibe with the taxonomic selectivity that characterizes most extinction events. In contrast, some have proposed that a combination of factors produce elevated extinction. To refine and test a multi-cause model of mass extinction, we borrow a pair of concepts from community ecology: Press disturbances alter community composition by placing multigenerational stress on populations; pulse disturbances are sudden, catastrophic, and cause extensive mortality. We hypothesize that the coincidence of press and pulse events is required to elevate extinction rates. To test this hypothesis we considered the press effects of continental flood basalt volcanism and the pulse effects of bolide impact. We compared generic extinction percentages for 73 stages or substages of the Mesozoic and Cenozoic, when the record of volcanism and impact are most complete. We found that the highest frequency of intervals with elevated extinction occurred when continental flood basalt volcanism and bolide impact co-occurred. In contrast, neither volcanism nor impact alone yielded statistically elevated extinction frequencies. Although the magnitude of extinction was uncorrelated with the size of the associated flood basalt or impact structure, crater diameter did correlate with extinction percentage when volcanism and impact coincided.

This combined mechanism more effectively explains selectivity in terrestrial vertebrate extinctions at the Cretaceous-Paleogene boundary than do single-mechanisms scenarios. The global effects of flood volcanism and other latest Cretaceous trends would have exerted stress—press disturbance—on some elements of the fauna, while favoring others. This press stress may have rendered some lineages endangered and vulnerable to extinction in the face of the terminal-Cretaceous bolide impact(s)—pulse disturbance.