A HIERARCHICAL VIEW OF THE SHAPE OF SURVIVORSHIP CURVES

The shape of survivorship curves for genera or other higher taxa is dependent on the origination and extinction rates of subordinate taxa. Survivorship curves of higher taxa have been found to exhibit either a log-linear or a concave-up shape. Log-linearity is indicative of a single rate of extinction, independent of the age. Concavity is a result of a lower extinction rate for older taxa than for younger taxa. Three models of the relationship between species-level origination and extinction rates and survivorship of genera have been proposed. First, the Red Queen's hypothesis predicts log-linear survivorship of genera. If species go extinct independently, then each genus will have distinct species-level origination and extinction rate. In the second model, if species go extinct independently and all share a common origination and extinction rate, then the genus-level survivorship curve will be of a particular concave-up shape. Third, in a new hierarchical extinction model, if there is selective extinction at the genus-level, species ranges will be truncated during times of generic extinction. If species-level rates are constant, generic survivorship will be either concave-up or log-linear depending on the genus extinction rate. A maximum likelihood framework is used to simultaneously evaluate the three models for empirical genus-level survivorship. In addition, the predictions each model makes for species-level survivorship are compared to observed species-level survivorship.