The typical sequence of events in a mass extinction begins with the extinction phase, when biotic diversity falls rapidly. During this time, the extinction rate (the number or percentage of taxa going extinct in any time interval) far exceeds the "origination rate" (the number of new taxa evolving through speciation). After some period of time, the extinction phase ends and is succeeded by a second phase, often called the survival phase. This is a time of minimal diversity, but no or few further extinctions. During this interval the number of species on Earth levels out, neither increasing or decreasing. The third phase, called the rebound phase, is when taxonomic diversity slowly begins to increase. The final phase is the expansion phase, and it is characterized by a rapid increase in diversity due to the evolution of new species. The latter three phases are grouped together into what is known as a "recovery interval," which is followed by a long period of environmental stability (until the next mass extinction). The rate of the recovery is usually proportional to the intensity of the extinction that triggered it: the more intense the mass extinction, the more rapid the rate of new species formation.
Three types of taxa are generally found immediately after the mass extinction: survivors, or holdover taxa; progenitor taxa, the evolutionary seeds of the ensuing recovery; and disaster taxa, species that proliferate immediately after the end of the mass extinction. All three types of taxa are generally forms that can not only tolerate, but thrive in, the harsh ecological conditions following the mass extinction event. They are generally small, simple forms capable of living and surviving in a wide variety of environments. We have another term for such organisms: weeds.
The recovery interval is marked by a rise in diversity. This sudden surge in evolution is generally due to the many vacant niches found within the various ecosystems following the mass extinction. Because so many species are lost in a mass extinction, it creates new opportunities for speciation. Darwin once likened the spe-ciation process to a wedge: the modern world has so many species in it that for a new species to survive and compete, it must act like a wedge, pushing out some other already entrenched species. But after a mass extinction no wedging is necessary.
Early on, virtually any new design will do. Many new species appear with morphologies or designs seemingly rather poorly adapted to their environment and inferior to those of species existing prior to the extinction. Rather quickly, however, a winnowing process takes place through natural selection, and new, increasing efficient suites of species rapidly evolve.
The great mass extinction ending the Permian created a long-term deficit in diversity, but eventually, in the Mesozoic Era, that deficit was made up. In fact, after every mass extinction that has occurred on Earth over "the past 500 million years, biodiversity has not only returned to its former value, but exceeded it. Sometime during the last 100,000 years, biodiversity appears to have been 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?
Interesting as this question is, it has not yet been tested in any way. The fossil record, however, does yield some evidence that mass extinctions belong on the deleterious rather than the positive side of the biodiversity ledger. Perhaps the best such clue comes from the comparative history of reef ecosystems. 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 past mass extinctions. They suffered a higher proportion of extinctions than any other marine ecosystem during each of the six major extinction episodes of the last 500 million years. After each mass extinction reefs disappear from the planet, and usually take tens of million of years to become reestablished. When they do come back, they do so only very gradually. The implication is that mass extinctions, at least for reefs, are highly deleterious and create net deficits of biodiversity. And whether we are talking about reefs, rainforests, or any other ecosystem, the reality is that for millions of years following a mass extinction the biodiversity of the planet is impoverished.
So, while there are many who would argue that since mass extinctions are sources of innovation, a modern one would not be such a bad thing, as it would be the source, ultimately, of a new age and even greater biodiversity, I will argue in the following pages that this is simply not the case.
This arsinotherium, a distant relative of the rhinoceros, contemplates its geological past.
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