Scientists studying an ancient group of marine plankton found that dramatic changes in climate and ocean currents can speed up the rate of extinctions and influence which species are more likely to die out.
Graptolites have been extinct for 400 million years. During their heydays, however, these intricate, tiny marine animals were so abundant throughout the world’s oceans that their signature in the geological record has been used to determine the boundaries between geological periods. Now, they have also revealed surprising information about extinctions.
The group as a whole thrived for 70 million years during a period when Earth moved from a warm ‘greenhouse’ world into an ‘icehouse’. Within this period, individual species of graptolite came and went, and their fossilised skeletons now act as geological signposts that help palaeontologists to date ancient rocks.
Because graptolites are so useful as a dating tool, they have been studied globally, with a focus on exactly when species appear in the geological record and then disappear again.
Roger Cooper, a palaeontologist at GNS Science, and James Crampton, also at GNS Science and Victoria University, have worked alongside American colleagues to collate a database of more than 2000 graptolite species and to develop sophisticated computational programmes to track their extinction rates. They have also matched the database against known changes in the climate – and they found that extinctions happen in short burst, separated by longer settled periods, and that they are influenced by large-scale environmental changes.
“The background pattern of extinction prevails right through the lifespan of this group,” says Roger Cooper. “But it is punctuated by climatic events that occurred during the Silurian, including a number of severe cold spikes during which the extinction rate shot up radically, many times the background value.”
One particularly extreme cold peak, at the end of the Ordovician period, is credited with causing a mass extinction and killing off 75 per cent of all species. The graptolites themselves were nearly wiped out.
It was also during these periods of dramatic change that different species became more prone to extinction.
James Crampton says one of the widespread, but disputed, ideas in biology is known as the Red Queen theory, which suggests that species have an equal probability to go extinct, no matter how long-lived they are. But the graptolites tell a very different story.
“This new data shows that most of the time, it’s the newly evolved species that are far more vulnerable to extinction. Nature is throwing out new species all the time, new things are evolving, new species are appearing, but actually they don’t cut the mustard."
“The species that have been around for a long time have already established themselves in the environment. They have a bigger geographic range, they have a bigger ecological footprint. It’s what’s called incumbency, they are the residents on the block, and they can outcompete the newly evolved species most of the time.”
Except when the climate changes rapidly, pushing well adapted, long-lived species out of their window of tolerance.
“Species will tolerate a certain level of climate variability. Within certain limits, the older species have the advantage, but once the environmental change passes a certain threshold, the whole extinction regime changes.”
He says the findings are relevant today and will add to the broader debate about the relative roles of environmental factors and species competition as drivers of biological diversity and extinctions.
“Plankton species are major players in the global ecosystem. They form the basis of the marine food chain and play an important role in sequestering carbon, drawing carbon dioxide from the atmosphere and locking it up. It’s important to understand how extinction works on a geological time scale.”
He says the findings suggest that “we can expect to see that different groups will become prone to extinction" as the climate and marine environment changes.
While it's too early to draw general conclusions, James Crampton says if he had to go out on a limb, he would say that extinction is more influenced by environmental factors while speciation is more driven by competition between species.