Corals, big mammals and many tropical species could all go extinct in the not too distant future, predict scientists who are attempting to forecast the fate of today's animals by studying what happened to those in the distant past.
A complication is that no prior mass extinction event on the planet was driven by a single species. In a period of more than a half-billion years, only three such extinction events appear to have been as devastating as the present one, which is being caused by humans.
"We're 100 percent responsible for it," John Alroy, a researcher in the Department of Biological Sciences at Macquarie University, told Discovery News.
"There is no precedent at all for what we're doing," he added. "All well-understood extinctions in the deep fossil record are tied to environmental changes that were not triggered by the behavior of individual species, such as the asteroid impact 65 million years ago that wiped out the terrestrial (non-avian) dinosaurs."
Alroy used the Paleobiology Database, which compiles data from nearly 100,000 fossil collections worldwide, to track the fate of major groups of animals during Earth's most massive extinction event 250 million years ago: the Permian-Triassic Extinction Event, also known as the "Great Dying."
Alroy, whose findings are published in the latest issue of the journal Science, focused on marine animals, since the fossil record includes many such species.
He determined that two of the most important and plentiful groups of marine animals 250 million years ago were corals and brachiopods, also called lamp shells. After the Great Dying, corals were almost wiped out.
"There are almost no early Triassic coral fossils in the world," explained Alroy, who added that corals "eventually recovered all of their lost diversity."
The lamp shells, on the other hand, never recovered. While they're still in existence, they exhibit little diversity and not many of them are around compared to other animal populations.
He said these are just a few examples from the past that demonstrate how a species-rich animal group may not necessarily fare well after a major extinction event. The rules governing their, and other animals', diversity change over time, and really go off the chart during and after mass extinction events.
Species-rich animal groups "could happen to be very vulnerable to the particular mechanism that creates a particular mass extinction," he said. They could also lose all of their subspecies, or "during the scramble to fill empty niches after a mass extinction, rival groups may get there first, making it difficult for a group to get back where it was."
Alroy is particularly worried about today's corals.
"They don't seem to do well when there's a big environmental change," he explained. "It's possible that future reef builders won't be corals at all. At different times in the past, reefs have been built by such organisms as sponges and clams."
Mammals with big body sizes, highly endemic tropical species, and certain plants may also die out before this latest extinction event concludes, Charles Marshall told Discovery News. Marshall is a professor in the Department of Integrative Biology at the University of California at Berkeley, where he also directs the university's Museum of Paleontology. He wrote an accompanying "Perspectives" article in the latest Science.
Marshall agrees with Alroy that studying past extinctions and diversity patterns can help us to learn what makes different groups of animals more or less prone to dying out.
In terms of humanity's impact on the planet, Marshall also agrees that "we have no evidence of a single species causing such havoc."
"However," he added, "if you are willing to broaden the taxonomic scope a little, when cyanobacteria started producing oxygen in abundance, they basically poisoned the world, converting it from one that was primarily anoxic (without oxygen) to one that was oxic."