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THE Earth may need 10 million years to recover from the significant dent in
species diversity caused by humans, regardless of the scale of the extinction,
claim two American researchers.

鈥淣one of my descendants that I would recognise as being in the same genus are
going to live to see the recovery,鈥� says Anne Weil of Duke University in North
Carolina. She came to this conclusion after carrying out the first comprehensive
analysis of extinctions and recoveries over the past 530 million years. 鈥淲e
couldn鈥檛 decide if it was the most exciting result we鈥檇 ever seen or the most
depressing,鈥� Weil told New Scientist.

The Earth took around 6 million years to recover its diversity after the most
severe mass extinction to date, which occurred at the end of the Permian period,
248 million years ago. But studies on smaller scales implied that ecosystems
should recover faster after lesser events.

The new study analysed a database of the fluctuating fortunes of hard-shelled
marine animals, which leave an abundant fossil record. James Kirchner from the
University of California at Berkeley borrowed a mathematical technique from
astrophysics to cross-correlate the rates of extinction and subsequent
re-emergence of these animals鈥攏o easy job since the data cover irregularly
spaced intervals.

He and Weil found no correlation between the rates of extinction and rates at
which new species evolved at the same time. But there was a very clear
correlation between species dying out and the origin of new hard-shelled species
about 10 million years later. The time lag was not related to the size of the
extinction, remaining the same even after the researchers removed the five
largest extinctions from the sample data. No matter how many extinctions we
cause, says Kirchner, 鈥渙ur biological impact on the biosphere is going to be a
very long-lasting one.鈥�

Weil believes that species take this long to recover because new ones cannot
simply evolve to fill empty niches. Instead, completely new niches must develop,
and that takes time. 鈥淓xtinction is more as if a car disappears because the
parking space disappears,鈥� she says. The long delay is a measure of the time it
takes to recreate a diverse ecosystem with a wide variety of niches.

Doug Erwin of the National Museum of Natural History in Washington DC says
the delay could be less than 10 million years, but not dramatically less. But
even though he could find no flaws in their analysis, he confesses: 鈥淭here鈥檚 a
part of me that鈥檚 suspicious.鈥� He is not convinced by equilibrium models that
predict rapid recoveries, but wonders why none of the studies covering smaller
areas and intervals has shown long delays in species recovery. But he says that
Weil and Kirchner worked on a global scale, which might explain the
disparity.