Climate change, over-fishing and other human impacts have pushed the oceans to the brink of a mass extinction that could take tens of millions of years to recover from, an Australian scientist says.
Dr John Alroy from Macquarie University in Sydney has used the fossil record of the ocean, dating back more than 500 million years, to study how major changes in marine animal groups take place.
His work is published today in the journal Science.
In the course of the past 540 million years, marine animals have undergone several mass extinctions that saw dominant life forms suddenly replaced by others, he says.
For example, about 250 million years ago, species of animals known as lamp shells, which had dominated sea-beds, were suddenly replaced by clams and snails.
"The lamp shells were all over the place and diverse for a quarter of a billion years, then the biggest mass extinction in the history of life on earth happened - the Permian-Triassic extinction - and they went from being all over the place, to being rare and not very diverse."
Resetting the rules
Until recently, scientists had thought these extinction events were governed by the slow unwinding of predictable evolutionary "rules" that operated over hundreds of millions of years, says Alroy.
"What my paper shows is that that story is fundamentally wrong, in that it doesn't take into account the way a big evolutionary innovation or mass extinction can overturn the rules."
"The change in the balance of groups is not random. It's not that some groups have good luck and some have bad luck. There has actually been a resetting of the rules of evolution."
Human activities such as over-fishing, the acidification of the oceans and the introduction of imported species are threatening to trigger another such event, Alroy warns.
"It's not just a mass extinction, but a massive reshuffling of species across the globe. We're simultaneously ruining the environment and selectively wiping out certain groups."
This combination of stresses threatens to leave ocean biodiversity devastated, he says. "Things are so bad right now in so many different ways it's very hard to imagine that you wouldn't have a big long-term overturn in the balance of groups."
The fossil record shows that the consequences of this kind of mass extinction can last for many millions of years.
"It will take tens of millions of years before there is a full recovery with respect to the number of species in the ocean and the balance of groups," Alroy says. "It will establish a new order that will persist for a very long time."