After mass extinctionResearch reveals surprising variety among recoveries
By Diana Steele
A study by 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 Feb. 27 issue of the journal Science.
"These are completely unexpected results," said Jablonski, Professor in Geophysical Sciences. 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 many species you lose, but which ones," he said. Jablonski's findings may have implications for modern conservation strategies.
Jablonski's is the first study to show the geographical variation of recoveries 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 recoveries in the Gulf Coast of North America and in northern Europe were vastly different, even though they were at roughly the same latitude at the time.
"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 delved into museum collections and investigated the molluscan fossils from two other regions, northern Africa and Pakistan/northern India.
Jablonski found that the Gulf Coast of North America differed from the other 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 afterward comprise three major groups: newly evolved species, local survivors and invading survivors. Previous studies have shown that the numbers of invaders coming into a region usually correlates 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 rates found in all four regions. Studies of the recoveries in the northern area of South America could determine whether proximity to the impact site was a determining factor.
"These results open up all sorts of questions," Jablonski said. "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 they 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, and 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 invasions caused by human activity. For example, the marine communities of San Francisco Bay are now dominated by an invading clam from Asia, which was brought into the bay in the ballast water of ships. It has been thought that the degree of such invasions is related to the degree of ecological disturbance in a region. In the case of San Francisco Bay, it has been thought that the high degree of invasion reflects the heavy ecological disturbance there.
"My results show that it's not always that simple," Jablonski said. "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.
"If the important thing is not just how many species you lose, but which ones, then that can help us understand different responses and even help improve invasion intensities in different regions."