Extinctions and biotic recovery generalities

From the conservation biologist's point of view, the extinction of even a single species is a catastrophe. From a paleontologists view, species extinction is the normal case and more than 99% of all the species that ever inhabited earth are now extinct (Raup 1991b). Yet there is even a further, more important dimension to this: during the history of life, mass extinctions provided also huge opportunities for taxa that hitherto played only minor roles, by removing or marignalizing incumbents (Jablonski 2001).

The same databases used in diversity studies can be used to construct plots of extinction (and also origination) intensities over time. Two major points emerge from such an analysis: peaks of high extinction intensities are separated from each other by times of lesser extinction, and overall extinction intensities decline over the phanerozoic (Sepkoski 1996; MacLeod 2003; O Figure 16.4). According to Raup and Sepkoski (1982), there were 5 major and at least 18 lesser mass extinctions in the Phanerozoic, and the "big five'' major extinctions (O Table 16.1) are also recognized in newer studies (Hallam and Wignall 1997; MacLeod 2003; Bambach et al. 2004; Taylor 2004). A "mass extinction'' is an event that was (1) nearly global, (2) removed a significant proportion of the existing species (perhaps more than 30%), (3) affected species from abroad range of ecologies, and (4) happened within a (geologically spoken) short time.

For the decline in background extinction intensitiy, widely disparate explanations were proposed. The change might reflect the general decrease in "volatility" between the three evolutionary faunas (Sepkoski 1981) or secular changes in the geochemistry and nutrition levels in the seas (Martin 1996). Yet it might also just be a taxonomic artifact: through the Phanerozoic, there is a trend toward more species per family/genus. It, therefore, needs more species to become extinct so that the entire family/genus becomes extinct (Taylor 2004).

An extinction event can be either abrupt, or stepped, or gradual. An abrupt or "pulse''-extinction evidently leaves a species no time to adapt or migrate, whereas this would be possible during a gradual or "press" extinction (Erwin 1996b, 2001b). Yet it has proven difficult to establish the exact disappearance of taxa, especially the rare ones. As a consequence of the imperfect fossil record, the observed last appearance of fossil taxa is always "smeared back'' in time through a time interval before their actual extinction (Hallam and Wignall 1997; Taylor 2004). This "Signor-Lipps effect'' (Signor and Lipps 1982) will lead to the perception of a gradual extinction pattern even if it was abrupt. The "zombie'' lineage, that is the unsampled portion of a taxon's range occurring after the final appearance of the taxon in the fossil record prior to its actual extinction (Lane et al. 2005), can be inferred at some level of probability with statistical methods (Marshall 1990).

Not even the largest mass extinctions acted in a completely random manner. Extinction selectivity can be geographical (e.g., tropical versus nontropical, terrestrial versus marine), taxonomic (different extinction rates among higher taxa, e.g., dinosaurs versus mammals, plants versus animals), or linked to trait (e.g., body size, trophic level; McKinney 1997, 2001). The selectivity patterns seen during a major extinction interval can be the same as those acting during

O Figure 16.4

Extinction intensities in the Phanerozoic. Mass extinctions clearly stand out against background intensities, which decreased during the Phanerozoic. After Mac Leod 2003

O Table 16.1

Observed (families, genera) and calculated (species) extinction intensities at the five major Phanerozoic mass extinctions

O Table 16.1

Observed (families, genera) and calculated (species) extinction intensities at the five major Phanerozoic mass extinctions

Mass extinction

Families extinct (%)

Genera extinct (%)

Species extinct (%)

End-Ordovician

Was this article helpful?

0 0

Post a comment