The exact causes of a number of the major extinctions that have taken place over the past 500 million years on Earth are still open to debate. Some seem to have occurred slowly, over the course of hundreds of thousands of years, whereas others seem to have happened relatively suddenly, after a cataclysmic event such as an asteroid impact. Now a new study published in the current issue of Nature suggests that regardless of how quickly an extinction takes place, speed limits set by evolutionary processes restrict the rate of recovery and the subsequent diversification of life.

James Kirchner of the University of California at Berkeley arrived at this conclusion after analyzing a database cataloguing families and genera of marine animal fossils from the past 530 million years. By examining patterns in the rates at which new organisms appeared and disappeared, he determined that over long periods of time, extinction rates and diversification rates vary in a similar way. Over short periods, however, diversification rates show much less variability¿they do not speed up in response to a rapid burst of extinction. "There seem to be biological mechanisms that limit diversification of new organisms and control which ones become successful enough to persist," he notes. "Biodiversity is slow to recover after an extinction."

Kirchner proposes that these speed limits arise because extinctions eliminate not only species but also their ecological niches¿the roles they play in an ecosystem. "Extinction is not like knocking chess pieces off a chessboard, with the empty squares ready for you to plunk down new pieces," he explains. "Extinction is more like knocking down a house of cards. You only have places to put new cards as you rebuild the structure of the house." Though the study shows that occupying a niche in a recovering ecosystem is crucial to the success of an organism, Kirchner cautions that the factors controlling the rate of this process are still unclear. He asserts, however, that "if we substantially diminish biodiversity on Earth, we can't expect the biosphere to just bounce back. It doesn't do that."

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