Extinctions apparently promote not only invasion but evolutionary diversification in the fossil record, the classic case being the impressive radiation of mammals after the demise of the (nonavian) dinosaurs and other marine and terrestrial vertebrates at the end of the Cretaceous [e.g., Alroy (1999, 2000) and Cifelli and Gordon (2007); for general discussions, see Erwin (2001) and Jablonski (2001, 2005, 2008)]. These macroevolu-tionary observations are often seen as two sides of the same coin, as they intersect nicely with ecological work on the potential for incumbency or priority effects to resist extinction or damp diversification (Jablonski, 2007, 2008). The three most prevalent explanations for both invasions and diversifications today and in the geologic past are (i) extinction or at least suppression of incumbents, already mentioned, (ii) superior competitive ability of the invaders, not least owing to their escape from their own competitors, predators, and pathogens when they enter a new area (Sax et al., 2007) (although this may be a transient effect and therefore less likely to play a macroevolutionary role), and (iii) changing climatic and other environmental conditions, such as those that drove the extensive invasions and reshuffling of Pleistocene biotas.
One of the most pervasive spatial patterns of invasions, seemingly independent of mass extinction events, underlies the marine latitudinal diversity gradient, wherein morphologies, species, and higher taxa are richest in the tropics and decline toward the poles. Although the gradient has been known for a long time and is documented for many groups and regions, the processes underlying this pervasive biodiversity pattern remain poorly understood (Hillebrand, 2004; Mittelbach et al., 2007). The ''out of the tropics'' model for the marine gradient has taxa preferentially originating in the tropics, and then expanding their latitudinal ranges over time without actually abandoning their tropical cradle (Jablonski et al., 2006). The tropics are thus a diversity source, containing both young and old taxa, which accumulate to high richness. The poles are a diversity sink, mainly containing older taxa that have moved in from lower latitudes, and the temperate zones have intermediate richness and taxon ages, at least for the marine invertebrates where direct fossil evidence is available. If this model is generally true, then invasion is a basic factor in the latitudinal deployment of life on Earth.
The out-of-the-tropics model is strongly supported in the marine bivalve fossil record. For each of three time slices (Pleistocene, Pliocene, and late Miocene), roughly twice as many bivalve genera first occur in the tropics than in higher latitudes (Jablonski et al., 2006). Because the extratropical fossil record is far better sampled than that of the tropics (Allison and Briggs, 1993; Jackson and Johnson, 2001; Bush and Bambach, 2004;
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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 tion of standing diversity alone, but of regional extinction rates, suggesting a significant role for incumbency [see also Vermeij (2005)]. The polar mollusks may ultimately have come out of the tropics as well, but if so this must have occurred before the 11-Myr window presently available (which would be consistent with the much older genus ages seen at the poles). The interplay of extinction, origination, and immigration is complex, and of course it need not be in a steady state.
Terrestrial animals may well follow a different dynamic (Hawkins et al., 2007b; Weir and Schluter, 2007; Wiens, 2007a). Marine organisms can move down the continental shelf when ice forms at the surface, but terrestrial animals, plants, and fungi do not have that luxury when confronted with a kilometer-thick ice sheet. High-latitude extinction and recoloniza-tion are thus almost certainly more important factors on land. Whatever the spatial dynamic near the poles, however, the tropics appear to be a crucial reservoir for biodiversity, with a subset of low-latitude clades expanding out of the tropics over geological timescales. Although this pattern is most readily detected in the shallowest part of the geologic record, thus falling entirely within times away from the major mass extinctions, some evidence suggests that postextinction recoveries are also fueled by the tropics, on land (Kerp et al., 2006) and in the oceans (Jablonski, 1998; Brayard et al., 2006; Krug and Patzkowsky, 2007). The tropics thus appear to be key to the generation and maintenance of global biodiversity across a wide range of boundary conditions.
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