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the fact that a considerable ecological network needs to be constructed, in appropriate environmental settings, before such architectural diversity can succeed.

Although there are a growing number of case studies of biotic recoveries after mass extinctions and some smaller biotic crises, our theoretical understanding of increases in taxic diversity remains lacking, as does our knowledge of the response of some of the other diversity metrics described here and the factors underlying them. One hesitates to suggest that there is a considerable empty niche here for future research.

CONCLUSION

This survey of mass extinction episodes illustrates that different metrics capture different dimensions of the loss of evolutionary history. Although these extinction events have been defined by loss of taxic diversity, this metric often captures only one perspective on the loss of biodiversity and evolutionary history. Indeed, debates continue among paleontologists about whether some of these episodes (particularly the Late Devonian and end-Triassic) actually constitute mass extinctions on the scale of the end-Permian and end-Cretaceous events. Fully appreciating the extent of the loss of evolutionary history during any biodiversity crisis requires a more complete accounting of other dimensions of biodiversity, a task that is in its infancy for some of the metrics discussed here.

The metrics of past loss of evolutionary history may provide some insights into more recent events. Although this survey illustrates that the available data on these various metrics are often meager, enough information is available to suggest that the loss of different aspects of evolutionary history may portend very different outcomes for recovery. For example, if architectural diversity is lost early in a biodiversity crisis one might expect greater loss of other aspects of diversity than if architectural diversity remains high. Empirical investigations of such effects will require very high-resolution studies, but may be possible in the Cenozoic. This is clearly an area where well-designed modeling studies may prove useful.

ACKNOWLEDGMENTS

I appreciate the invitation of John Avise, Francisco Ayala, and Steve Hubbel to contribute to the symposium, discussions with David Krakauer and David Jablonski, and the editorial comments of the organizers and two anonymous reviewers. This research was funded by the Smithsonian Institution, the National Aeronautics and Space Administration Astrobiology Institute, and grants to the Santa Fe Institute.



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