SELECTIVITY OF EXTINCTION VERSUS RECOVERY: REINFORCEMENT, REVERSAL, AND RESHAPING OF BIOTIC CHANGE

The selectivity of a mass extinction is useful in evaluating the plausibility of proposed kill mechanisms. It can also help explain the long-term history of life because selective extinctions alter the relative diversity of clades or functional groups: clades with high extinction magnitude will decrease in relative richness, while those with low extinction will increase. However, the long-term effects of a mass extinction on biotic composition also depend on the dynamics of the subsequent recovery, during which diversity increases. Origination during recovery can also be selective, and the relationship between selectivity of extinction and selectivity of origination is an important component in determining the trajectory of relative diversity change. We introduce several parameters that describe the interaction between selectivity of extinction and origination and their effects on biotic composition. Gamma (γ) describes the degree to which extinction and origination have similar vs. different selectivity. The reinforcement index (R_in) describes the extent to which changes in biotic composition driven by extinction are reinforced vs. reversed during the recovery, and the reshaping index (R_sh) describes the extent to which the recovery drives changes in composition that are uncorrelated with the changes driven by mass extinction. If clades with relatively high extinction magnitude also have relatively high origination magnitude during the recovery, then γ and R_in will be low, and the effects of the mass extinction on biotic composition will be at least partially reversed. If, however, clades with relatively high extinction magnitude have relatively low origination magnitude during the recovery, then γ and R_in will be high, the effects of extinction will be reinforced, and the extinction/recovery couplet will have a much larger effect on the history of life.

For marine animals at the phylum level, there is a tendency towards mild reversal during recoveries, although there is a lot of variation. This tendency can be explained by the correlation between extinction and origination magnitude among taxa. Reinforcement occurs during some minor mass extinctions, which means they have a greater impact on biotic composition than would be obvious from an analysis of extinction alone.