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of postextinction biotic recovery as the refilling of empty ecospace fail to capture the dynamics of this diversity increase.

Extinction is the inevitable fate of organisms, although there is considerable variance in both rates of extinction through time and the duration of particular species or clades. By some estimates, extant multicellular biodiversity is but 1–2% of all multicellular species that have existed over the past 600 Ma (Valentine, 1970; Raup, 1991). Paleontologists have long recognized that the relatively regular overturn of species is occasionally punctuated by more severe biotic crises, including at least five events recognized as mass extinctions. Some have claimed that rates of current species loss exceed those of past mass extinctions. Perhaps the most valuable contribution that paleontologists can make to understanding the current biodiversity crisis is to identify the relationship between attributes of the loss of past evolutionary history and both the depth of past crises and the speed and structure of subsequent biotic recovery. Given that conservation biologists increasingly face a problem of triage, where not all species can be saved, can paleontological data provide any insights into the species, communities, or structures that should have the highest priority for support? Paleontological data are unlikely to be decisive in such decisions, but the unique perspective provided by the fossil record may provide a useful input.

Here, I discuss a range of potential metrics for the impact of extinction on the loss of evolutionary history and provide a preliminary application of them to the five canonical mass extinctions (see also Alroy, Chapter 11, this volume). There are, however, relatively few applications of these metrics to understanding the processes of postextinction biotic recovery.

METRICS FOR THE LOSS OF EVOLUTIONARY HISTORY

The traditional accounting method for the loss of evolutionary history is taxa: populations and species for biologists, often genera or families for paleontologists because the vagaries of preservation and correlation make species-level compilations impractical. Conservation biologists have long focused on species, an approach enshrined in the U.S. Endangered Species Act. This reliance on taxa tends to assume, implicitly, that taxonomic entities are a reliable metric to the impact of extinction on ecosystem structure and function, morphological variability, behavior complexity, and developmental processes. This assumption is often far from true. Consequently, conservation biologists have proposed other metrics for identifying critical targets for conservation (Purvis and Hector, 2000), including biogeographic centers of endemic taxa, or hotspots (Meyers et al., 2000),



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