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In the Light of Evolution: Volume II—Biodiversity and Extinction
Part III
TRENDS AND PROCESSES IN THE PALEONTOLOGICAL PAST
Extinction has always been a part of life on Earth and is the ultimate fate of all species. Rates of extinction have varied across time, from standard or “background” rates to occasional mass events. The chapters in this section place the current biodiversity crisis in temporal perspective by scrutinizing the fossil record for patterns and processes of extinction in the distant and near past.
The fossil record traditionally has been interpreted to register five episodes of wholesale biotic change so severe as to qualify as mass extinctions: at the end of the Ordovician (ca. 444 Mya), Devonian (360 Mya), Permian (252 Mya), Triassic (200 Mya), and Cretaceous (66 Mya). Each was characterized (indeed identified) by a substantial loss of then-extant taxa. In Chapter 9, Douglas Erwin reexamines these five mass extinction events in terms of the respective impacts on each of seven metrics of biodiversity—taxonomic diversity, phylogenetic diversity, morphologic disparity, functional diversity, architectural diversity, behavioral complexity, and developmental diversity—which potentially capture different aspects of the loss of evolutionary history. Erwin reports that the canonical mass extinctions differed with respect to their impacts on these various metrics. For example, the end-Permian extinction had major consequences for essentially all dimensions of global biodiversity whereas the end-Ordovician extinction heavily impacted morphologic disparity but had low or medium effects on several other biodiversity measures. The biodiversity fallout from mass extinction events can vary both quantitatively
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In the Light of Evolution: Volume II—Biodiversity and Extinction
and qualitatively, and the nature of each extinction influences the rate and pattern of evolutionary recovery from the catastrophe.
David Jablonski develops a somewhat similar theme in Chapter 10 by emphasizing the selectivity of mass extinctions with respect to potential risk factors such as body size, species richness, and geographic range. From a consideration of the fossil record for marine organisms (especially bivalve mollusks), the author concludes that every mass extinction event seems to show some degree of selectivity, but also that disproportionately high clade survivorship during mass extinction episodes is consistently associated with the size of the geographic range of genus-level clades. From this and other evidence, the author’s take-home message is that spatial considerations are fundamental to understanding the evolutionary dynamics of biodiversity, including a clade’s susceptibility to extinction and its potential for recovery and expansion following a mass extinction event. These findings have ramifications for the current biodiversity crisis because human activities are altering the geographic distributions of many taxa around the world.
In Chapter 11, John Alroy uses information from a recent web-based “Paleobiology Database” to revisit classical questions about the marine fossil record, such as: Do biotic turnovers occur in pulses that coincide with the boundaries between geological intervals? Did extinction rates decline during the Phanerozoic? Are biotic extinction rates more volatile than origination rates? Do large-scale extinctions exhibit a 26 Myr periodicity as some have claimed? Were the “Big Five” mass extinction events qualitatively distinct from lesser extinction episodes? Alroy’s provisional answers to some of these questions are unorthodox. For example, he suggests that the Big Five are merely the upper end of a continuous spectrum of extinction intensities, such that it is “a matter of taste whether to speak of the Big Five, the Big Three, or just the Big One….” The analyses yield empirical estimates of typical recovery times from mass extinctions. Alroy concludes that the rebound from the ongoing mass extinction will probably take between 15 and 30 million years, if past mass extinction events are any guide.
Moving closer to the present time, late-Quaternary extinctions heavily impacted large mammals especially. The last 50,000 years were witness to the extinction of approximately two-thirds of all genera and one-half of all species of mammal weighing more than 44 kg (about the size of a sheep). Causal factors for this megafaunal extinction have been much debated, with a leading hypothesis being human hunting (overkill) arguably augmented by habitat alteration and climate change. In Chapter 12, Anthony Barnosky examines the situation from the fresh perspective of historical tradeoffs in biomass. An inverse relationship between human biomass and nonhuman megafaunal biomass indicates that before the mass extinction,
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In the Light of Evolution: Volume II—Biodiversity and Extinction
the energy needed to construct large animals was divided among many species, whereas after the extinction much more of the planet’s total supply of energy became concentrated in one species (Homo sapiens) and its domesticates. Based on the historical chronologies of biomass transitions in various parts of the world, Barnosky draws several biological implications, including how the current depletion of fossil fuels as an energy source may translate into near-future challenges for global biodiversity.
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