Ancient plant fossils recovered in Greenland by a UCD scientist can tell us something about climate change today and its possible impact on plants, writes CLAIRE O'CONNELL.
HERE’S A cautionary tale from the fossil record: plant biodiversity plummeted abruptly 200 million years ago under conditions of environmental change that are not too far off today’s scale.
Researchers made the surprise discovery by analysing fossils laid down just before a mass extinction event between the Triassic and Jurassic periods, a flashpoint that ranks among the top five catastrophic extinctions in the earth’s history.
To date, little has been known about the rate of biodiversity loss in the run up to the Triassic-Jurassic event, explains Dr Jennifer McElwain, a lecturer in paleobotany at University College Dublin and first author on a study published in the current issue of Science.
“All mass extinctions are of huge magnitude, but the pace of extinction has always been unknown,” she says. “So we have targeted these extreme events in earth’s history, such as the T-J boundary, times which saw major changes in climate and biodiversity, atmospheric composition, geological events and biological events – and we are trying to understand what caused what and at what pace it happened.”
Working on fossil samples she collected in east Greenland in 2002, while employed by the Field Museum in Chicago, McElwain analysed plant fossils from sediment layers approaching the T-J boundary.
“At that time, the environment there would have been a cross between the Florida Everglades and New Zealand broadleaf conifer forests,” she says.
Initially, her analysis of the rock-bound specimens had suggested a gradual decline as the massive extinction loomed. But the fossil record does not provide a complete ledger of plants that lived at the time, she notes.
So McElwain and colleagues revisited the samples with a mathematical approach that looks at the bigger picture: rather than just counting samples they calculated the relative abundance of plants to see whether certain ones were becoming dominant.
“It really lets you assess how ecosystems as a whole are changing and it’s the first time anyone has really explicitly tested the pace of extinction,” she says of the study, which was funded by a Marie Curie excellence grant. “And what’s really exciting is we have been able to say the extinction of plants was sudden.”
In geological terms “sudden” means on the order of thousands of years, but nonetheless the abruptness of the biodoversity loss was a surprise finding.
“It’s very unexpected. And there were definitely signs of instability and collapse in the whole ecosystem before we even see species go extinct,” says McElwain.
The massive changes happened in a period of global warming and increased atmospheric carbon dioxide that compares with today.
“I would have expected the biodiversity loss to start changing when CO2 levels were much higher, and it’s worrying that the big biodiversity loss was occurring with very little CO2 change,” she says.
“To put it in context, we are seeing changes at about 900 parts per million CO2 , and that would be a worst-case scenario for the year 2100. It’s not too crazy at all in terms of where we are going.”
However, she notes that other factors could have contributed to the T-J boundary event too, like toxic sulphur dioxide emitted through volcanic activity. “We are leaving open the possibility of SO2 being involved in biodiversity loss,” she says. “We have no way of detecting SO2 in the fossil record at the moment, but we are working on it.”
As the bigger picture emerges from the run-up to the T-J extinction, McElwain suggests we pay attention to what is happening right now.
“We know from looking at the past systems that you get early ecological warning signs well before complete collapse and extinction. And we have loads of warning signs coming out from ecosystems, things like species reign shifts, migration, changes in the timing of biological events, like flowering,” she says.
“The big message from this paper is that we have to take heed of the ecological signs, because it won’t be long before ecosystems will reach this kind of tipping point and then they just collapse very suddenly.”