In the Light of Evolution Volume II: Biodiversity and Extinction (2008) / Chapter Skim
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9 Extinction as the Loss of Evolutionary History--DOUGLAS H. ERWIN
9 Extinction as the Loss of Evolutionary History--DOUGLAS H. ERWIN
Pages 171-188
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From page 171... ...
Other measures of lost diversity include: functional diversity, architectural components, behavioral and social repertoires, and developmental strategies. The canonical five mass extinctions of the Phanerozoic reveal the loss of differ ent, albeit sometimes overlapping, aspects of loss of evolutionary history.
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From page 172... ...
Some have claimed that rates of current species loss exceed those of past mass extinctions. Perhaps the most valuable contribution that paleontologists can make to understanding the current biodiversity crisis is to identify the relationship between attributes of the loss of past evolutionary history and both the depth of past crises and the speed and structure of subsequent biotic recovery.
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From page 173... ...
. Although paleontologists are aware of the diversity of effects on evolutionary history caused by past extinctions, particularly mass extinctions, we have been slow to develop and apply comparative metrics beyond taxic compilations and estimates of geographic range.
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From page 174... ...
Both onycophorans and tuataras are far more evolutionarily distinct than any two members of their sister clade, a fact not captured by a simple taxic approach. A simple exercise illustrates that identical levels of species loss can conceal very different effects on evolutionary history (Fig.
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From page 175... ...
zpq9990837700001.g.tif clades, each of which represents unique units with long evolutionary history. This simple example demonstrates how knowledge of the phylogenetic structure is essential to evaluating the amount of evolutionary history lost or at risk, and not surprisingly, conservation biologists have proposed several different metrics for measuring phylogenetic diversity (Vane-Wright et al., 1991; Faith, 1992a; Faith and Baker, 2006)
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From page 176... ...
No studies have explicitly addressed the impact of mass extinctions on phylogenetic diversity to my knowledge. Could one develop a metric of the severity of past extinction crises based on the extent of phylogenetic diversity lost?
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From page 177... ...
Such categories can often readily be identified in fossils and can be consistent across larger taxonomic groups. Paleontologists have long discussed the selective impact of mass extinctions on trophic groups, such as the pervasive extinction of epifaunal, suspension-feeding marine taxa during the end-Permian mass extinction (Erwin, 1993)
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From page 178... ...
Our results demonstrate that ancient food webs can be reliably reconstructed, opening up the potential to study changes in the network properties of ecosystems across mass extinctions (Dunne et al., 2008)
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From page 179... ...
Architecturally similar structures, at least at a gross scale, have been built by microbial communities, sponges and archaeocyathids, tabulate and rugose corals, stomatoporoids, bryozoans, brachiopods, and rudist bivalves. Reefs are a specific example of the provisioning of architectural diversity, which can provide a positive feedback on biodiversity.
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From page 180... ...
From the Middle Ordovician radiation through the Late Devonian mass extinctions, stromatoporoids (coralline sponges) and corals were the primary reef builders, with important contributions from other sponges in the early part of the interval.
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From page 181... ...
Studies of other arthropods, coupled with detailed studies of the patterns of morphologic evolution of trilobites (including developmental information retrieved from fossilized representatives of larval stages) , have demonstrated that information on developmental patterning can be recovered (Hughes, 2007)
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From page 182... ...
APPLICATION TO PAST BIOTIC CRISES Applying some of these different aspects of diversity to past mass extinctions is difficult because of both lack of data and difficulties in establishing appropriate criteria and reproducible metrics, but identifying these different measures of diversity is the first step toward building a more robust and quantifiable approach. Table 9.1 provides a preliminary, somewhat impressionistic, application of these metrics for marine animals across the five classic mass extinction intervals.
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From page 183... ...
. Estimates of loss of phylogenetic diversity are based on the loss of major clades, as documented by phylogenetic analyses; morphologic disparity is assessed within particular clades, and the loss of major clades; functional diversity is assessed based on published paleoecological studies.
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From page 184... ...
Although there are few studies of paleoecological patterns across this boundary, much less studies of food web structure, there is little evidence for major disruptions of functional diversity except among reefs, where a major drop in sea level triggered a substantial decline in reef volume
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From page 185... ...
Indeed by some metrics, particularly morphologic disparity and developmental diversity, these events may have winnowed a greater degree of evolutionary history than any of the subsequent biodiversity crises of the Phanerozoic.
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From page 186... ...
Thus understanding the growth of taxic diversity after mass extinctions requires understanding the ecological relationships that build these networks, including both the positive feedbacks (such as niche construction and environmental engineering) and the more commonly invoked negative feedbacks such as competition.
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From page 187... ...
actually constitute mass extinctions on the scale of the end-Permian and end-Cretaceous events. Fully appreciating the extent of the loss of evolutionary history during any biodiversity crisis requires a more complete accounting of other dimensions of biodiversity, a task that is in its infancy for some of the metrics discussed here.
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