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Jablonski et al., 2006; Valentine et al., 2006), the tropical values must be underestimates of their true origination rates, and the extratropical values must be overestimates. Further, most of the genera that first appeared in the tropics over the past 11 Myr have since spread to higher latitudes. This dynamic accounts for the striking inverse relation between diversity in a latitudinal bin and the median age of the genera in that bin: most of the geologically old genera at high latitudes also occur in the tropics, but the young genera are concentrated at low latitudes, decreasing the low-latitude median value significantly.

The number of taxa that expand out of the tropics is impressive, particularly given that these clades are invading new climate zones, traversing a gradient of increasing physical challenges for most taxa. Further, these extratropical expansions occurred in the face of progressive global refrigeration, culminating in the full-blown glacial cycles of the Pleistocene. However, while clades regularly left the tropics, few, if any, of the analyzed cohort have expanded above ≈50° north or south latitude. If the relative invasibility of the temperate and polar zones over the past 11 Myr was underlain by regional variation in background extinction intensity (E), we would expect, not the usual two-bin model, tropical E < extratropical E, or low-latitude E < polar E (Goldberg et al., 2005; Jablonski et al., 2006; Roy and Goldberg, 2007), or the monotonic latitudinal trend in extinction rates assumed by many others, but a hump-shaped pattern with an extinction maximum at midlatitudes.

A preliminary test of these alternatives did find a humped extinction pattern with latitude for Northern Hemisphere bivalve genera in the latest Cenozoic, with global plus regional extinction totaling ≈9% in the tropics, ≈20% in the temperate zone, and ≈12% in the Arctic (Valentine et al., 2008). This result suggests that the temperate zones are invasible on geological timescales because they suffer the highest extinction rates, at least in global climate states approaching our own. Thus, even if climate does not directly set standing diversity, its fluctuations, which are greatest both in temperate latitudes today and during Pleistocene climate swings [e.g., Jansson and Dynesius (2002), Ravelo et al. (2004), and Lyle et al. (2008)], may set the pattern of extinction intensities. The data are not yet sufficient to study these dynamics in detail, but the relation between midlatitude thermal variability (which coincides with fluctuations in many additional factors) and extinction patterns clearly deserve further scrutiny. The poles are doubtless demanding places to live, but taxa evolve to cope with the challenges; Valentine and colleagues (Valentine, 1983; Valentine et al., 2008) suggest they do this by becoming highly generalized trophically and argue that these broad niche dimensions are what tend to block invasions and allow them to weather glacial episodes subtidally, as they avoid seasonal extremes today. In any case, invasion resistance is apparently not a func-

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