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In the Light of Evolution: Volume II—Biodiversity and Extinction
cal diversity is strongly depleted. The fossil record can provide valuable insights, and their application to present-day issues will be enhanced by partitioning past and present-day extinctions by driving mechanism rather than emphasizing intensity.
The inventory of life on Earth has always been determined at the most basic level by the difference between origination and extinction. This fundamental macroevolutionary equation, richness = origination − extinction, has been formally applied in many ways and with many elaborations, but takes on special consequence when attempting to evaluate the processes shaping present-day biodiversity, where neither term in the right side of the equation can be observed directly. Some progress has been made in modeling these parameters, but most approaches involve strong assumptions, require very large datasets, and carry large uncertainties [e.g., Paradis (2004), Ricklefs (2007a,b)]. The spatially explicit form of this equation, where richness is a local or regional pool of species or higher taxa, and immigration and emigration terms enter the right side of the equation (Jablonski et al., 2006), is important in many situations, from biotic responses to Pleistocene climate cycles and ongoing climate changes to recoveries from mass extinctions. However, this form is even more difficult to apply rigorously without historical data, and my emphasis here will be on the fossil record. Few would argue against the idea that the spatial fabric of extinction has shaped, and will continue to shape, the biosphere in profound ways, but spatial effects have been neglected relative to temporal patterns (partly because documentation is so challenging). I will argue that the insights beginning to emerge from spatially explicit approaches to ancient extinctions have significant implications for the dynamics of diversity of the past and in the future.
This discussion will encompass a range of extinction intensities and focus on marine systems, where the fossil record is richest: application of these generalizations to terrestrial realms requires more study. Opinions are divided on whether the handful of mass extinctions of the geologic past are a separate class of intensities from the “background” extinction that constitutes the great bulk of geologic time [and the bulk of total extinction; Raup (1994)], but this is a secondary issue that can probably be resolved by factoring out the well-known secular decline in background extinction rates (Bambach et al., 2004; Jablonski, 2005; Stanley, 2007). As discussed below, extinction selectivities evidently shift between episodes of low and high extinction rates, and this selectivity is the more important issue for understanding the role of extinction in shaping past and future biotas. I will corroborate previous evidence for a strong spatial component to survivorship during major extinction events, present a multifactorial analysis of the end-Cretaceous (K-T) mass extinction in which geographic