Restructuring the Hystory of Life: The Role of 
Mass Extinction, Recoveries and Radiation
        Douglas H. Erwin
        Department of Paleobiology, 
        National Museum of Natural History 
        Washington, DC 20560 
        erwin.doug@nmnh.si.edu

Patterns in the history of life over tens to hundreds of millions of years reflect a combination of long-term adaptive trends, filtered by the perturbations imposed by mass extinctions. How are ecosystems restructured following extinctions?  Biological intuition, reflected in many models and simulations, suggests that diversification occurs through re-occupation and diversification into niches. A variety of lines of evidence suggest this
view is too simplistic.  Rather, niches are created during recoveries (and some other biotic radiations), driving further diversification.  How these niches are created has important implications for the long-term effect of mass extinctions and subsequent recoveries. 
        Some mass extinctions appear to have had little immediate ecological effect, or long-term evolutionary effect (end-Ordovician, end-Triassic).  Other mass extinctions have dramatically shifted the course of evolution (end-Permian, for marine invertebrates, some tetrapods, and perhaps insects; end-Cretaceous, for tetrapods).  Detailed studies of post-extinction recoveries have revealed that many of these changes reflect the pattern of
the recovery at least as much, and perhaps more than the pattern of extinction.  Thus, mass extinctions may remove entire clades and even ecological guilds, depending on the pattern of selectivity.  The pattern of rebuilding ecosystems during post-extinction recoveries appears to be driven by a different dynamic, however.  The immediate post-extinction survival interval is often dominated by opportunists, but these clades do not necessarily dominate ecosystems when diversification increases during the recovery interval.  During this interval ecological relationships may be paramount in determining the long-term success of clades. The major features of gastropod evolution during the Paleozoic were driven by the need to accommodate, water currents through the gill.  During the course of the Paleozoic, gastropods appear to have repeatedly discovered the same six solutions to this adaptive problem.  The appearance of shell-crushing predators in the aftermath of the end-Permian mass extinction changed this system.  No longer was adaptation to water currents the primary adaptive problem;  this had been replaced by resistance to predation.  Consequently, as these new ecosystems developed in the Middle Triassic, several groups
which suffered relatively little during the mass extinction disappeared during the recovery because the trajectory taken during the recovery transformed the ecosystem.  Detailed studies make it clear that this was a product of the pattern of recovery, not simply a result of the extinction. Recent work by Kircher and Weil reveals an approximately 10 myr lag
following biotic crises at a broad range of scales, suggesting the creation of ecological roles is an ongoing component of recoveries. Models of recovery and radiations need to include a positive feedback component which dynamically changes the dimensions of the ecospace.

Back