The world has experienced mass extinctions before, but a comparison between the current decline in marine animals and recent extinction events reveals a big difference. This time, size really does matter, with large species most at risk.
A team led by Stanford's Dr Jonathan Payne used a database of 2,497 genera of marine vertebrates and mollusks to compare the proportion lost in past extinctions with numbers in recent years.
When an asteroid ended the Cretaceous Period about 65 million years ago, almost half the genera of marine vertebrates, and nearly 40 percent of mollusks, disappeared from the fossil record. Some genera contain many species, of which only a few survived, so the proportion of lost species was much higher.
Things have been more placid since, but the world has experienced several more minor extinction events. During the late Eocene, around 15 percent of mollusk genera vanished within a relatively short amount of time, although the toll on vertebrates was lighter. Anthropocene extinctions have been rarer still, but the next century is expected to see that change, and Payne's work may identify the most vulnerable types of animals.
The findings are dramatic. “The odds of being threatened with extinction increase by a factor of 13 for each order of magnitude increase in body length,” Payne and his co-authors write in Science.
Such studies can never be completely reliable – species thought to have gone extinct subsequently turn up somewhere unexpected – and there can be selection bias in the genera studied. However, the authors report that they processed the data using several different methods, and being big always came out as a hazard. "This is most likely due to people targeting larger species for consumption first," Payne said in a statement.
Familiar examples of extinct species, such as non-avian dinosaurs and Ice Age megafauna, might lead us to think that mass extinctions always hit the biggest the hardest. However, this isn't true, at least in the oceans. In some bouts of extinction over the last 66 million years size made no difference. In others, being larger was actually associated with slightly improved survival prospects.
The authors also compared animals that inhabit the open oceans with those living on the sea floor and contrasted predators with prey. On these tests the current extinction far more closely resembles previous ones, with roughly equal risk across categories. Nevertheless, free swimming species are doing somewhat worse than those anchored to one spot, something not seen in previous extinctions.
Ecosystems seldom cope well with losing numerous species at once, but some are more necessary than others. Large animals play oversized roles in nutrient recycling and their influence can ripple down the food chain. Payne argued there is still time to act, however: "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. 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."