Ecological ranking of Phanerozoic biodiversity crises: ecological and taxonomic severities are decoupled

The past two decades have seen extensive analyses of the taxonomic severity of major biodiversity crises in geologic time. In contrast, we propose here an alternative analysis of the ecological severity of biodiversity crises. It is clear that the ecological impacts of the five Phanerozoic biodiversity crises were not all the same. Ranking the five Phanerozoic biodiversity crises by ecological severity reveals that the taxonomic and ecological severities of the events are decoupled. The most striking example of the decoupling is the end-Cretaceous biodiversity crisis, which is the least severe in terms of taxonomic diversity loss yet is ecologically the second most severe event in the entire Phanerozoic. A second striking example is the end-Ordovician biodiversity crisis: the environmental degradation produced by the end-Ordovician glaciations precipitated a major loss of marine diversity, yet the extinction failed to eliminate any key taxa or evolutionary traits, and was of minimal ecological impact. We suggest that the decoupled severities indicates that the ecological importance of component species in an ecosystem is at least as important as species diversity in maintaining the integrity of the ecosystem and that this ecological phenomenon operates on geological timescales. The selective elimination of dominant and/or keystone taxa that occurs in the ecologically most devastating biodiversity crises indicates that a strategy emphasizing the preservation of taxa with high ecological value is necessary to mitigate the ecological effects of the current ongoing loss of global biodiversity. (C) 2004 Elsevier B.V. All rights reserved.