THE DYNAMICS OF BIOTIC COLLAPSE DURING DIVERSITY CRISES

Are the disappearances of most species during mass extinctions due to the direct, abiotic causes of mass extinction (blast effects, rapid climatic change, anoxia, loss of habitat, etc), or are they secondary extinctions caused by the unraveling of ecological interactions as an extinction event progresses? Most studies of mass extinctions have compared the pre-extinction biota with survivors to infer patterns of selectivity, largely because we lack the temporal resolution to interrogate the internal dynamics of the great mass extinctions or other biodiversity crises documented by the Phanerozoic fossil record. But as improved geochronology, statistical analysis and correlation methods increase the resolution of extinctions intervals paleontologists have greater opportunities to probe extinction dynamics. Recent high-resolution radiometric dating of the Permo-Triassic, Triassic-Jurassic, Cretaceous-Paleogene has revealed that these events were far more rapid than previously appreciated (maximum durations of 61 ± 48 ky, <5 ky and 5 ± 27 ky, respectively); some smaller crises may have been similarly brief. Given the evidence for different triggers for these events, this suggests that mass extinctions, independent of their physical drivers, invoke similar patterns of biotic collapse. Application of concepts from network dynamics may help discriminate between the contributions of biotic and abiotic contributions, as alternative distributions of loss of taxa across a network of ecological interactions imply very different patterns of biotic collapse.