The Effect of Mass Extinctions

Are mass extinctions that stop short of complete sterilization necessarily detrimental to planetary diversity? Perhaps it can be argued that instead of being deleterious to diversity, they are actually forces that increase diversity. For example, it can be argued that the various Paleozoic extinctions caused archaic reef communities to be reassembled with more modern types of corals. Mass extinctions paved the way for a takeover of bottom communities previously dominated by brachiopods (archaic shellfish) by the more modern (and more diverse) mollusks. In another case, the extinction of the dinosaurs paved the way for the evolution of many new types of mammals, and it appears that there are more types of mammal species than there were dinosaur species. If these mass extinctions had not occurred, would planetary diversity (the number of extant species) be higher or lower than it is today, other variables (the history of continental drift, for instance) remaining the same?

We can illustrate the enigma of mass extinctions and their effect on global biodiversity as follows: The Cambrian Explosion results in a sudden rise in diversity, followed by an approximately steady state during the Paleozoic. The mass extinctions during the Ordovician and Devonian cause short-term drops in diversity, but these are soon compensated for by evolution of new forms. The great mass extinction that ended the Permian creates a longer-term deficit in diversity, but eventually, in the Mesozoic era, it also is compensated for. In fact, after every mass extinction that occurred on Earth over the past 500 million years, biodiversity has not just returned to its former value but has exceeded that value. Today, in our world, biodiversity is

Figure 8.2 Two models of how mass extinction affects diversity. (A) Increasing number of mass extinction events leads to ever lower diversity. (B) There is some critical number of mass extinction events that causes lower diversity.

higher than it has been at any time in the past 500 million years. If there had been twice the number of mass extinctions, would there be an even higher level of diversity than there is on Earth now? Perhaps mass extinctions exert a positive effect, creating new opportunities and fostering evolutionary innovation by weeding out decadent or poorly adapted but entrenched and resource-hogging species. On the other hand, perhaps just the opposite is true: If the mass extinctions had not occurred, biodiversity would be higher than it is today (see Figure 8.2). How do we choose?

Interesting as this question is, it has not yet been tested in any way. The fossil record, however, does yield some clues that mass extinctions must be entered on the deleterious rather than the positive side of the biodiversity

Figure 8.3 Phases of a mass extinction crisis. (After Kaufman, 1986; based on information in Donovan, 1989.)

ledger. Perhaps the best clue comes from the comparative history of reef ecosystems following mass extinction. Reefs are the most diverse of all marine habitats; they are the rainforests of the ocean. Because they contain so many organisms with hard skeletons (in contrast to a rainforest, which bears very few creatures with any fossilization potential), we have an excellent record of reefs through time. Reef environments have been severely and adversely affected by all mass extinctions. They suffered a higher proportion of extinction than any other marine ecosystem during each of the six major extinction episodes of the last 500 million years. Reefs disappear from the planet after each mass extinction, and it usually takes tens of millions of years for them to be reestablished. For instance, there were no reefs whatsoever after the Cambrian, Ordovician, Devonian, Permian, Triassic, and Cretaceous mass extinctions. When they do come back, they do so very gradually. It appears that complex ecosystems take a long time to build and to rebuild (see Figure 8.3). When reef systems eventually reappear, they are composed of entirely new suites of creatures. The implication is that mass extinctions, at least for reefs, are highly deleterious and yield net deficits of biodiversity.

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