Some Extinctions Are Spatially Complex

The K-T extinction is remarkably homogenous on a global scale, except perhaps for greater intensity in tropical carbonate settings (Raup and Jablonski, 1993; Jablonski, 2005). However, other extinction events, particularly those that are less severe on a global scale, tend to show more spatial structure. For example, the mid-Cretaceous (end-Cenomanian) marine extinction appears to have been concentrated in northern Europe and the Western Interior Seaway of North America. The smaller events in the geologic record must be interpreted critically, because at least some of them may represent, or at least be heavily overprinted by, sampling variations [e.g., Foote (2003) and Smith and McGowan (2007)]. However, a seemingly genuine extinction pulse or regional series of pulses occurs in the oceans near the start of the Pleistocene. These regional extinctions are generally taken to represent a culling of taxa unable to cope with the onset of rapid, high-amplitude climate swings and oceanographic shifts that typify the Pleistocene. They vary in intensity and occur at slightly different times among regions [e.g., Kitamura et al. (2000), Monegatti and Raffi (2001), Todd et al. (2002), Smith and Roy (2006), Rivadeneira and Marquet (2007)], perhaps owing to regional variations in the timing of oceanographic transitions toward a glacial state [e.g., Ravelo et al. (2004)]; the many subsequent glacial-interglacial cycles evidently drove few extinctions in marine or terrestrial settings [e.g., Huntley (2005)].

This spatial structuring of Plio-Pleistocene extinctions is interesting from many perspectives, but perhaps the most urgent need is to understand the dynamical consequences of these extinctions, which bear directly on the path of future biodiversity. Do these events reflect the setting of a new regional diversity level, such that taxa capable of weathering the volatile Pleistocene climate regime are more generalized and thus structure a biota capable of accommodating fewer species and clades? [See Valentine (1983) and Valentine et al. (2008) for a view of diversity-dependent factors that would favor this explanation.] Or do some regions incur a ''diversification debt,'' a more positive analog to the extinction debt sometimes inferred for modern biotas squeezed into refugia too small to accommodate their present richness? The rapid recovery of diversity in the Caribbean, which evidently suffered more severely than the tropical eastern Pacific just on the other side of the Panama Isthmus (Todd et al., 2002), suggests that at least some Plio-Pleistocene extinctions involve diversification debts rather than diversity resettings. [By this argument, the anomalously low diversity of the Southeast Pacific molluscan fauna (e.g., Rivadeneira and Marquet, 2007) is a transient effect rather than a permanent biogeographic feature, attributable perhaps to the lower rates of diversity accumulation in extratropical regions; cf. Jablonski et al. (2006).] However, the spatially explicit form of the fundamental macroevolutionary equation shows that regional diversity can accrue either by in situ origination or invasion (immigration). The high present-day rate of anthropogenic introductions in marine systems will likely outstrip regional evolutionary recovery by an order of magnitude or more, exacerbated of course by other anthropogenic stresses [e.g., Ruiz et al. (2000), Jackson et al. (2001), Lotze et al. (2006)]. This paleontological perspective on regional marine biodiversity adds another element to the urgency of slowing anthropogenic homogenization of marine biotas, if the diversification debt of some regions indeed makes them more susceptible to invasions (and see below).

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