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Disturbance to biotic systems is usually studied on two very different scales. Ecologists by necessity limit their studies to short-lived, localized perturbations such as the pounding of sessile intertidal communities by drifting logs and rolling rocks, the burning of forests, the felling of trees by the wind, or the scraping of encrusting organisms by grazers (see 92, 112). Paleontolog-ists, on the other hand, are mainly concerned with long-term, spatially widespread, usually irreversible events, such as extinctions of species or groups of species (see 24, 81), biotic effects of global climatic changes (113), faunal upheavals created by invasions between continents (115), or range contractions and expansions due to sea-level fluctuations (76). The large gap between ecological and paleontological temporal and spatial scales usually remains unstudied because ecologists are unable to cause global effects experimentally and …