Previous Studies

Put together, these results combine to make a plausible and straightforward scenario. First, a major drop in diversity for any reason will be followed by a significant recovery due to the near-absence of extinction. Extinction rates may continue to be low for a second 11.0-Myr interval after the immediate recovery. Second, if the drop was due to a large extinction pulse and not merely an unfavorable balance of origination and extinction, then speciation rates will increase dramatically.

Earlier researchers have argued for entirely different models. For example, autocorrelation in Sepkoski's origination rates (Kirchner and Weil, 2000a), apparently greater variation in extinction than origination (Kirchner, 2002), and, most importantly, a lagged correlation between extinction and subsequent origination in Sepkoski's family- and genuslevel data (Kirchner and Weil, 2000b) have been used to construct a theory that diversity is slow to rebound from extinction because time is required to reconstruct ecological niches that might be filled.

There are numerous problems with Sepkoski's data that call these results into question: the lack of any sampling standardization, the unavoidable backward smearing of extinction rates and forward smearing of origination events that results from using simple range data (Foote, 2003), and the oversplitting of the timescale into 106 intervals (as opposed to 48 in this study). Not surprisingly, the first two hypothesized patterns do not exist in the current dataset: Origination rates lack any significant autocorrelation and are not much less variable than extinction rates.

An extinction-origination correlation is indeed present, but the pattern is different from expected under the hypothesis that niches need to be reconstructed. First, very high origination rates come immediately after what are clearly rapid mass extinctions (Fig. 11.3B), not a full temporal bin later (Kirchner and Weil, 2000b). Second, only the largest extinctions seem to boost immediately subsequent origination. If Kirchner and Weil (Kirchner and Weil, 2000a,b; Kirchner, 2002) are right that niches need to be reconstructed, then recoveries should be slower and not faster after a large extinction, because the relevant ecological interactions are more highly disrupted. Thus, if niches are relevant, then speciation in the wake of mass extinctions is most likely fostered by the lack of competition for existing niches instead of being delayed by the elimination of old niches that need to be reconstructed.

Meanwhile, some support for a diversity/extinction relationship has been found in Sepkoski's datasets (Sepkoski, 1984; Foote, 2000b). However, these results did not involve lagging, and Sepkoski's genus-level data provide mixed support for density dependence in both kinds of rates instead of just extinction rates (Foote, 2000b). The consistency of the diversity-extinction relationship through the Phanerozoic (Fig. 11.3A) also is at odds with earlier analyses suggesting fundamental changes across the Permo-Triassic in ecology (Erwin, 2001; Wagner et al., 2006) and especially diversity dynamics (Foote, 2000b). The latter study found correlations between changes in diversity and changes in rates. The motivation for differencing the rates was to avoid biases and autocorrelation (Foote, 2000b), but the current dataset uses rates with low bias that lack autocorrelation, so differencing is moot. Again, biases related to sampling, counting, and rate equations are likely to be a problem in all of the earlier studies.

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