Integrating Paleontological And Neontological Perspectives

I have touched on four spatial aspects of ancient extinctions that should be integrated with theoretical and applied approaches to the present-day biota. The fossil record amply demonstrates that the spatial fabric of extinction has profoundly shaped the biosphere. First, broad geographic range probably always buffers clades from extinction, but it becomes most important and clear-cut as the suite of other factors that enhance species and genus survival during normal times become ineffective. It is not yet clear whether the selectivity regime changes steadily with increasing extinction intensity or as a step function (Jablonski, 2005). More intense extinctions may tend to be less selective, which might explain the failure of intrinsic factors to predict extinction risk in the some of the most heavily stressed elements of the modern biota, such as freshwater fishes, amphibians, and Australian marsupials (Duncan and Lockwood, 2001; Fisher et al., 2003).

This shift to a strong spatial component in survivorship during major extinction events greatly increases the likelihood of hitchhiking effects.

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Organismic traits can rise or fall according to the strength of their linkage to broad geographic range or other factors promoting survivorship through those bottlenecks, lending a highly stochastic element to the expansion or demise of individual adaptations or clades. Thinking about the present day, these linkages are unlikely to promote factors beneficial to, or even desirable for, humans or the ecosystems they hope to conserve (see Jackson, Chapter 1, this volume). Given that narrow-ranging genera cannot have wide-ranging species, the net effect must be to deplete specialists in favor of weedy generalists, but this pattern can be ameliorated by survival of clades whose broad ranges arise from the far-flung deployment of individually localized species.

Second, the fossil record is rich in regional extinction events of intermediate intensities, and these can provide insights into present-day biodiversity issues. For example, the Cenozoic history of today's biodiversity hotspots and coldspots (relative to expectations for their latitudes, for example) may help to predict the potential of these regions to accommodate further diversification, or alternatively to be subject to biotic invasions.

Third, the rules of successful recovery are poorly known, but are important for our understanding of both the larger outlines of the history of life and the future of modern diversity. The inordinate production of evolutionary novelties during recoveries suggests that postextinction dynamics do not simply involve an immediate return to business as usual. At the same time the spatial heterogeneity of recoveries, with significant invasions driving some of the regional patterns, requires a more careful look at the dynamics if we want to avoid biotic homogenization even after the reduction of the pressures on the modern biota. This could be another highly fruitful area at the intersection of paleontology and conservation biology.

Fourth, invasion has always been an evolutionary fact of life (Vermeij, 2005), even across biogeographic barriers and against climate gradients. The out-of-the-tropics model suggests an evolutionary approach to modeling biotic responses to future climate changes and attests to the evolutionary consequences of the stresses on tropical biotas today. If the tropics are the engine of global biodiversity, then driving tropical populations into extinction will have a global effect, by cutting off the primary source of new taxa for all latitudes. Further, if invasibility is more closely tied to extinction than to diversity per se, then there is the possibility of a reversal of the diversity flow, increasing the influx of invaders from higher latitudes. A tropical diversity crisis, now or in the geologic past, has profound long-term evolutionary consequences at a truly global scale.

Simply comparing the magnitude of the extinction occurring today, which is undoubtedly severe, with ancient intensities is not the most fruit ful way to draw on the insights of the fossil record, or catalyze integrative research. A better approach might be to recognize that present-day extinctions have many drivers, and then to test for common patterns of selectivity on that basis: partitioning present-day extinction mechanisms should permit a clearer application of insights from the fossil record. Extinction selectivity probably does vary with driving mechanism to some extent. In birds, for example, habitat loss preferentially removes specialized and small-bodied taxa (but does not select on generation time), whereas exploitation and introduced predators preferentially remove large-bodied and long-lived taxa (Owens and Bennett, 2000). Such systematic variations in selectivity help explain the apparent contradictions in and among analyses of present-day extinction risk [e.g., Purvis et al. (2000b) and Freville et al. (2007)], and similar arguments can be made for differences among ancient extinctions as well. As several authors have noted, extinction drivers have probably compounded over human history, with exploitation perhaps the most important in early phases, species invasions rising in frequency with the era of European exploration, and finally habitat alteration on a global scale accelerating with increased human population pressure, pollution, and climate change [e.g., Purvis et al. (2000b) and Jackson et al. (2001)].

Many paleontological perturbations are probably most analogous to present-day habitat loss and could be explored in comparative fashion on that basis. Others will more closely correspond to the introduction of enemies, as when provinces collide or novel predation mechanisms evolve (see Barnosky, Chapter 12, this volume). The particular combination of pressures seen today may be unique, just as they may have been for the K-T or end-Permian events. For example, today the ordinary biotic response to climate change (range translocation) is disallowed or at least severely curtailed over much of the planet owing to occupation or conversion of suitable habitat or migration corridors by humans and their artifacts. The unique combination of forces behind each major extinction puts a premium on focusing on first principles rather than extinction-specific patterns, underscoring the need for integrative research. It also underscores the need to take a hard look at the roles of incumbency and hitchhiking effects, to separate large-scale artifacts or byproducts from the underlying drivers. Extinction thresholds presumably exist for today's biota, beyond which whole systems collapse and most selectivity factors drop out, as seen for major events of the geologic past. Identifying such thresholds among environments, clades, and regions using fossil data, as another basis for avoiding them in the future, would be a valuable undertaking.

More generally, spatially explicit approaches to the fossil record have great potential for new insights into diversity dynamics, not just in the geologic past, but in the present day as well. The integration of paleon-

206 / David Jablonski tological and neontological insights takes on special urgency with the acceleration of extinction rates in the modern world, and the incorporation of the spatial dimension offers a powerful vehicle for that integration.

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