The tests reported here show that diversity is constrained and provide direct evidence of the major mechanism. When diversity is high for any reason, extinction rates rise both in the same interval and the immediately following ones. When diversity is low because of a preceding major mass extinction, origination rates rise. Because the relationships are temporally offset, the direction of each causal relationship is clear. Extinction seems to be forced by some aspect of rising biodiversity, but the reverse is not

222 / John Alroy true. Meanwhile, origination seems to be spurred by the emptying of many niches at once but origination bursts do not cause those niches to be emptied rapidly later on.

The only clear-cut processes that could create such responses are ecological interactions, such as competition and predation, that affect the overall probability of speciation and extinction. Presumably, these interactions influence attributes of species, such as population size, population density, and geographic range size, that are not properties of individual organisms regardless of whether one wants to call them emergent or aggregate (Jablonski, 2000). Density dependence therefore may intensify differential reproduction of species based on traits that are likely to be heritable between species, again, regardless of whether one wishes to call this process species selection or sorting.

None of these results imply that density dependence is the sole governing mechanism of diversification. Quite the contrary, there is much variation left in both the diversity data and the turnover rates that might be explained by other factors, such as short-term perturbations [e.g., bolide impacts (Raup, 1992)] and longer-term changes in the environment [e.g., sea level (Peters and Foote, 2001; Peters, 2006)]. The analyses also only pertain to genera common and widespread enough to be recovered in the fossil record, and the diversity of rare genera may be modulated by different mechanisms. Finally, a finer timescale might make it possible to demonstrate more complicated patterns, such as periodicity (Raup and Sepkoski, 1984). However, the current number of data points should have been enough to demonstrate any biologically important patterns of this kind, and indeed it is sufficient to demonstrate the relationships leading to a dynamic equilibrium.

There are also dangers in misinterpreting the fact that diversity is regulated. On the one hand, the relevant correlations weaken considerably if the diversity data are not detrended. Thus, not only is the equilibrium dynamic, but the underlying equilibrium point evolves through time. On the other hand, the fact that a recovery in sheer taxonomic diversity will occur does not give cause for optimism about the current crisis. Any recovery will be unimaginably long on a human timescale and substantially protracted on a geological timescale. It also is clear that major mass extinctions in the past have led to enormous changes in taxonomic composition, trophic diversity, and body mass distributions that have effects for not merely tens of millions but hundreds of millions of years (Sepkoski, 1981, 1996; Jablonski, 1986a; Alroy, 2000; Todd et al., 2002; Payne, 2005). Finally, the numerous anthropogenic causes of today's mass extinction are deeply unrelated to the known causes of earlier ones, so we may never be able to predict much more about the next geological era beyond the general pace of recovery in numerical terms.

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