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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 Fréville 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-



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