Ancient marine invasion sheds light on diversity

Fossils from the sea floor illuminate the relationship between local and global diversity, and these relationships may help us understand the effects of global climate change on species diversity.

“Looking at fossils can tell you something about the controls on global diversity, but so much of the investigation of the fossil record has looked only at global compilations of fossil species,” says Dr. Mark E. Patzkowsky, associate professor of geosciences. “Recently, a few studies have shown that if you look at diversity locally, it is not necessarily mimicking global diversity at that time.”

Researchers look at three types of diversity: local or alpha diversity, which is diversity at a specific location; regional or gamma diversity, which is diversity in a large area up to continent size; and beta diversity which is the diversity gained as one moves from one area to another.

Patzkowsky and Steven M. Holland, professor of geology, University of Georgia, looked at fossils similar to modern corals, clams, snails and brachiopods from 450 million-year-old fossil beds in the area around Cincinnati, Ohio. These Ordovician fossils show great diversity of sea floor invertebrates.

They made 700 individual collections and counted about 41,000 individual animal fossils. A typical collection area was a half meter piece of exposed ancient sea floor. Fossils were collected from areas that were originally deep sea floor, deep subtidal sea floor, shallow subtidal sea floor and protected lagoon sea floor. The fossils represent six time slices, each of about a million years, however, for this study, they only looked at fossils from the four time slices that contained fossils from deep and shallow subtidal areas. “We already knew that there was an invasion of species during one of the time slices,” says Patzkowsky. “We wanted to see how that invasion affected diversity.”

The researchers found that alpha, or local, diversity changed only slightly, but on a regional or gamma diversity basis, diversity increase was much more by about 26 percent. “We realized that the large change in gamma diversity was not coming from the meager change in alpha diversity,” said Patzkowsky. “If not alpha diversity than it had to come from the beta diversity, but how?”

Patzkowsky and Holland suggest two ways beta diversity could increase gamma diversity. One way to increase beta diversity is for the habitat distribution of populations to become much narrower so that populations are using smaller amounts of resources. The other way is that the habitat becomes much more diverse providing different habitat niches.

The researchers want to test if the habitat distribution narrowed or if there was greater heterogeneity of habitat. They also want to figure out which species invaded, where they came from and if they replace species or simply add to the mix. “We are fairly certain that the invasion occurred because of a climate change,” says Patzkowsky. “We would like to know if the invasion caused extinctions.”

Carbonate rocks created in cool waters exist before the invasion, but during the invasion, the rocks were formed in warmer waters. During the Ordovician, the central United States was situated on the equator. The rock deposits indicate that the temperatures were increasing on a regional scale, just prior to a significant interval of global cooling. “We want to see how changes in the environment and temperature can affect the distribution of species,” says Patzkowsky. “We are only one of a series of studies trying to understand the factors that control global diversity.”