Here's who would go first in the next mass extinction

Reuters/Lucas Jackson

Researchers have discovered an unprecedented quirk of the sixth mass extinction event that the world appears to be entering.

According to a new study, for the first time in the history of mass extinctions, the largest animals in the world's oceans are expected to die off first. The culprit? You guessed it: that old chestnut, human activity.

"We've found that extinction threat in the modern oceans is very strongly associated with larger body size," says paleobiologist Jonathan Payne from Stanford University. "This is most likely due to people targeting larger species for consumption first."

To better predict how the emerging biodiversity crisis in our oceans might play out, Payne and his team examined fossil evidence of previous mass extinctions dating back as far as 445 million years ago, and compared it to what's happening now.

For the two major groups of marine animals that the team was looking at – mollusks and vertebrates – there's a big difference between then and now.

All previous mass extinctions had a greater impact on smaller sea animals or were non-selective when it comes to body size – meaning marine species were threatened equally, irrespective of how big they were.

Not so this time around, which is looking like a case of 'the bigger they are, the harder they fall'.

"What our analysis shows is that for every factor of 10 increase in body mass, the odds of being threatened by extinction go up by a factor of 13 or so," says Payne. "The bigger you are, the more likely you are to be facing extinction."

Nobody knows for sure what the ultimate impact on other ocean life would be if all the bigger animals start disappearing – because this has never happened before – but it certainly doesn't look good.

"The preferential threat to large-bodied marine animals poses a danger to ecosystems disproportionate to the percentage of threatened species," the authors explain in their paper.

"Large-bodied animals are critical to ecosystem function because of their preferential position at the top of food webs and importance to nutrient cycling and bioturbation of sediments."

In other words, the removal of larger species that often occupy higher slots in the marine food chain doesn't actually bring about positives for the smaller sea creatures further down the ladder (sorry, little guys, you probably thought this was your moment).

While an analysis of why humans are responsible for the threats currently facing marine species was beyond the scope of the study, the researchers aren't holding onto any doubts on that front, either.

"It is consistent with the tendency for fisheries to first exploit larger species and subsequently move down the food web and target smaller species," says one of the team, Matthew Knope.

While humans have been implicated in previous extinction events on land, this is the first time in history that our damaging sphere of influence has extended so far out into the deep blue sea.

"We see this over and over again. Humans enter into a new ecosystem, and the largest animals are killed off first," says researcher Noel Heim. "Marine systems have been spared up to now, because until relatively recently, humans were restricted to coastal areas and didn't have the technology to fish in the deep ocean on an industrial scale."

That's progress for you, folks. But there's good news too. While the threat to ocean life due to rising water temperatures and increasing acidification could take decades or centuries to reverse, our environmental impact due to over-fishing is something we can act upon now – if we grasp this crucial opportunity.

"We can't do much to quickly reverse the trends of ocean warming or ocean acidification, which are both real threats that must be addressed," says Payne.

"But we can change treaties related to how we hunt and fish. Fish populations also have the potential to recover much more quickly than climate or ocean chemistry."

Of course, there's no time like the present. With the right actions, the researchers say this gloomy vision for the future of the oceans – and the very prospect of the sixth mass extinction itself – could be swept away.

"We have the opportunity to totally avert this, if we make the right decisions," Payne told Maddie Stone at Gizmodo. "To claim we're in a sixth mass extinction is something very enormous. It is a possibility. It is not the reality yet."

The paper has been published in Science.

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