A study by University of Chicago paleontologist David Jablonski shows that recoveries from mass extinctions differ widely from one geographical region to another, even though the extinction intensities and patterns are more or less the same everywhere. His results are published in the Friday, February 27, issue of the journal Science.
"These are completely unexpected results," said Jablonski, Professor in Geophysical Sciences at the University of Chicago. He found that regions differed greatly from each other not only in terms of which species diversified and how quickly, but also in the ratio of surviving local species to foreign invaders.
"Maybe not all extinctions are created equal: it's not only how much you lose, but who," he said. Jablonski's findings may have implications for modern conservation strategies.
Jablonski's is the first study that has looked at the geographical variation of recovery patterns on a region-by-region basis. Focusing on the mass extinction event that killed the dinosaurs, at the Cretaceous/Tertiary (K/T) boundary 65 million years ago, Jablonski found that recovery rates in North America and Europe were very different even though they are at roughly the same latitude.
"In North America, there was a large pulse of 'bloom' taxa, a great diversification of a few groups," said Jablonski. "This phenomenon is analogous to a plankton bloom or weed growth after some ecological disturbance." After seeing the pattern of bloom taxa in North America-first studied in 1988 by Thor Hansen from Western Washington University-Jablonski expected to find the same pattern in Europe.
"I was astounded when I found startling differences between Europe and North America," he said. "Europe lacks any kind of rapid expansion of 'bloom' taxa." Spurred by his findings, Jablonski burrowed into museum collections and investigated the collections of molluscan fossils for northern Africa and India.
Jablonski found that the Gulf Coast of North America differed from all three regions, not only in respect to bloom taxa but in other aspects, as well.
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 invasion of a region depends on how heavily disturbed that region has been, ecologically.
"My results show that it's not that simple. After the K/T extinction, regions with what appeared to be similar extinction intensities had very different invasion susceptibilities. Understanding recovery dynamics under extreme conditions such as the K/T extinction helps us better understand how these communities work under stress," said Jablonski.
"If it really is important not how many species you lose, but which ones, then that can help us understand different responses and even help ameliorate invasion intensities in different regions."
A commentary written by Douglas Erwin of the Smithsonian Institution accompanies Jablonski's paper in Science.