Ranking the five Phanerozoic biodiversity crises by ecological severity reveals that taxonomic and ecological severities of the events are decoupled. The most striking example of the decoupling is the end-Cretaceous biodiversity crisis, which was the least severe in terms of taxonomic diversity loss yet was ecologically the second most severe event in the entire Phanerozoic. A second striking example is the end-Ordovician biodiversity crisis in which environmental degradation produced by a glaciation precipitated the second largest loss of marine diversity in Earth history. However, the extinction failed to eliminate key taxa or evolutionary traits, resulting in only minimal ecological impact. The decoupled severities indicates that the ecological value 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.