For any mass extinction, the flora and fauna that appear afterwards comprise three major groups: 1) newly evolved species, 2) local survivors (living in the region prior to the extinction) and 3) invading survivors (living elsewhere prior to the extinction). Previous studies have found that the numbers of invaders coming into a region is usually correlated with the intensity of the extinction, but Jablonski's results don't show that pattern. In North America, there was a significantly higher proportion of invaders than in the other regions he studied.
One explanation for the differences found in the Gulf Coast community is its proximity to the Chicxulub impact site, where a massive meteorite slammed into Earth 65 million years ago. But that explanation does not account for the similarity in extinction intensities found in all four regions. Studies of the recovery rates in northern South America could determine whether proximity to the impact site was a determining factor.
"These results open up all sorts of questions," said Jablonski. "Maybe there is a threshold of extinction intensity above which all bets are off, and you can't predict invasion intensity. Or possibly the data are wrong-maybe invasions happen so quickly that the conceal the magnitude of the extinction. Or maybe not all extinctions are created equal: it's not how much you lose but who.
"Perhaps mass extinctions can remove species that are important in the biota-just by chance the Gulf Coast of North America lost the critical ones."
Jablonski's findings may have implications for present-day ecosystems. One of
the biggest problems facing biodiversity today is human-mediated invasions. For
example, the marine communities of San Francisco Bay are dominated by an
invading clam from Asia, brought into the bay in the ballast water of ships. It
has been thought that the degree of i
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Contact: Diana Steele
d-steele@uchicago.edu
773-702-6241
University of Chicago Medical Center
27-Feb-1998