Perhaps one of the most interesting aspects of this analysis of mass extinctions using our approach of paleoecological levels is the phenomenon that we term "taxonomic and ecological decoupling," where the relative level of ecological degradation is not as great as the degree of taxonomic degradation during a mass extinction event (Droser et al. 2000). This ecological decoupling appears to occur at the second paleoecological level. For example, as discussed, the Late Ordovician mass extinction, the second-largest extinction in the history of metazoan life, only has third and fourth level paleoecological changes, but the Late Devonian mass extinction has second, third, and fourth level changes. In essence, the ecological difference between these two mass extinctions is that the Late Devonian mass extinction had (1) changes in ecological dominants of higher taxa; (2) loss of metazoan reefs; and (3) loss of Bambachian megaguilds.
These differences imply that although taxonomically the two mass extinctions were relatively similar in size, the organisms lost during each mass extinction had different relative ecological importance. Such differences emphasize the "apples and oranges" relative value of taxa within an ecological context. It appears that in terms of retaining ecological structure after a mass extinction, some taxa are much more important (Droser et al. 2000).
In modern ecological studies, the importance of the differing relative ecological values of taxa is well recognized. For example, in a community, a keystone species may exist and without its presence, the whole ecological structure would collapse. This concept was introduced by Paine (1969) and has become a central principle in ecological studies. Examples include starfish in rocky intertidal communities (Paine 1969), kangaroo rats in desert shrub habitats (Brown and Heske 1990), snow geese in areas adjacent to Hudson's Bay (Kerbes, Kotanen, and Jeffries 1990) and sapsuckers in subalpine ecosystems (Daily et al. 1993).
Keystone species are typically not the most abundant species in a community (Power et al. 1996). Other types of species in a community that have relatively great ecological value are also recognized and include dominant species. Dominant species are the most abundant species in a community and play a major role in controlling the direction and rates of community processes, as well as commonly providing the major energy flow and three-dimensional structure that supports a community (Power et al. 1996).
Therefore, if a community loses 50% of its species, because each species has a different relative ecological value, it really depends on which species are lost as to whether very much damage has been done to the ecological structure. If that 50% includes the widespread loss of dominant or keystone groups, then this biotic crisis could result in major ecological changes that would be manifested at our second level. If that 50% only includes the rare taxa that do not have keystone properties, or if its effect upon dominant species is only on a regional level, then possibly very little of the ecological structure would be lost, and it may be relatively easy to rebuild the community after the crisis, with similar ecological structures as before.
Thus, second level changes can be caused primarily by the loss of ecological dominants but also possibly through the loss of keystone species in a variety of communities. Such changes in ecological dominants are exemplified by the change from dominance of brachiopods in late Paleozoic benthic level-bottom settings before the end-Permian mass extinction, to dominance by bivalves in the Mesozoic after the mass extinction. Similarly, our categorization of the loss of metazoan reefs as a second level phenomenon is similar because so much of the history of reefs is one of change from one dominant taxonomic group to another during the course of major events in life's history (e.g., Fagerstrom 1987). For example, reef change caused by the Cretaceous-Tertiary mass extinction is that of Cretaceous reefs dominated by rudist bivalves to Tertiary reefs dominated by scleractinian corals.
Ecologists and conservation biologists are struggling to develop ways to measure modern ecological degradation and to evaluate ecological changes. We have postulated herein that the most ecologically devastating mass extinc tions are those where taxa of relatively high ecological value underwent selective extinction. When dealing with modern settings, identification of keystone taxa is problematical (e.g., Power et al. 1996). However, studies on modern keystones can be extended back into the relatively recent geological past, using an approach of taxonomic uniformitarianism, as done by Owen-Smith (1987) for the Pleistocene terrestrial mass extinctions. However, when viewed from 500, 350, or even 100 million years ago, we not only have a scale problem, but the actual identification of keystone taxa from the truly ancient fossil record is at best difficult. In contrast, as discussed in our analysis of various examples, the existence of dominant taxa in paleoenvironments is readily recognizable, so we can presently document patterns of large-scale second level paleoeco-logical changes through the course of major events in the Phanerozoic.
The ultimate question to ask of this decoupling phenomenon is why one mass extinction preferentially concentrates on taxa of relatively high ecological value, such as dominant taxa, while another does not. Most likely it is just chance as to whether a particular mass extinction mechanism preferentially affects taxa of high ecological value. Thus, different mass extinction causes may lead one cause to concentrate on dominant taxa, while another mass extinction cause of equal taxonomic effect may just eliminate taxa in an ecologically nonpreferential way.
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