ADJUSTING EXTINCTION RATES FOR TAXONOMIC SUSCEPTIBILITY

Extinction levels in Phanerozoic intervals reflect the influences of many factors, including the severity of the causal mechanism (e.g., impact, sea level change, volcanism), as well as taxonomic susceptibility, ecosystem stability, and other characteristics specific to an interval.

Here we isolate the effect of taxonomic susceptibility — the degree of susceptibility among classes extant in an interval — on extinction levels in the marine realm. Our approach is similar in spirit to that used in demography to adjust mortality rates for differences in age distributions. We use logistic regression to model extinction rates as a function of taxonomic susceptibility and other interval-specific characteristics. We then use these results to (1) estimate a Phanerozoic susceptibility curve, and (2) adjust observed extinction rates to account for taxonomic susceptibility.

Using our results, we compare adjusted mass extinction rates to adjusted background extinction rates in order to investigate whether selectivity in mass extinctions is qualitatively different from that in background intervals. We also explore whether mass extinctions tend to occur in intervals of high or low susceptibility, and whether recovery intervals show patterns in susceptibility. Finally, we explore the relationship between susceptibility and evenness at the class level.