Constant extinction, constrained diversification, and uncoordinated stasis in North American mammals

Abstract

Abstract The coordinated stasis model has far-reaching implications. Among them are three important predictions concerning diversity dynamics that I test here against the Cenozoic fossil record of terrestrial North American mammals. First, origination and extinction rates should be correlated; second, turnover should be a composite function of very low background rates and occasional, dramatic turnover pulses; and finally, stasis should result from ecological (niche) incumbency, with the domains of incumbent species being defined by ecological similarity, which in the case of mammals corresponds closely with taxonomic affinity. The data used to test these hypotheses are standing diversity levels and counts of originations and extinctions for 1193 genera and 3161 species. Instead of relying on a traditional time scale comprised of “ages” having uneven and unpredictable durations, the diversity curve is computed directly from a multivariate ordination of 3870 faunal lists, and then sectioned into 1.0 m.y. intervals. The lists span the late Cretaceous through late Pleistocene interval, exclusive of the Wisconsinan, and are taxonomically standardized to remove junior synonyms, out-dated combinations, and nomina dubia. Because Cretaceous and Paleocene diversity dynamics are idiosyncratic, only the last 55 intervals (Eocene-Pleistocene: 55-0.01 Ma) are analyzed. The test of origination and extinction rates shows that an apparent correlation between them is a statistical artifact related to the necessary coincidence of first and last appearances for taxa known from just one interval. The test of variation in turnover shows that most of the observed extinction rates could be generated by a single, invariant underlying rate, wheras origination rates show many well-defined pulses. Furthermore, origination pulses within particular orders are not fully coincident. The very largest pulses of origination therefore seem to be mediated by key adaptations within particular groups, not by the general opportunity to fill niches opened up by extinction. Both of these tests argue against the idea of sweeping “reorganization” intervals bounding placid “stasis” intervals, and against Vrba's turnover pulse hypothesis. Finally, tests for niche incumbency, based on plots of per-taxon turnover rates against standing diversity, show that incumbency is widespread and mediated by the suppression of origination at high diversity levels in all groups. Extinction is a far less important controlling factor. Because orders are ecologically distinct, but random subsamples of the entire data set actually show stronger controls than groupings based on ordinal affinity, it appears that niche space has little or no important ecological substructuring. Therefore, mammalian diversity seems to be integrated at the highest possible taxonomic level, in opposition to the coordinated stasis concept of static guilds. On balance, the results indicate that although the data are robust and provide strong support for the niche incumbency model and the idea of diversity equilibrium, they generally disconfirm the unique predictions of coordinated stasis.