Unlike in every other mass die-off of animal species, these days bigger marine animals are more likely to die out than smaller species, a new study suggests. Why is that? People, probably.

We may be entering a sixth mass extinction

Over the course of Earth’s history, there have been five mass extinction events. The largest killed off nearly 90 percent of marine species on Earth about 250 million years ago — and the most well-known event wiped out the dinosaurs 66 million years ago. But we may be entering a sixth mass extinction as people fracture habitats, spread invasive species and diseases, hunt, poach, and drive global temperatures up.

No past extinction event has shown this trend toward large animal die-offs, according to a study published today in Science. Size didn’t really matter in previous extinctions caused by environmental catastrophes. That points the finger of blame at humans, who have a tendency to think that bigger is better. In fact, humankind’s inclination to kill larger creatures is so strong that it can even drive evolution.

"Humans are unbelievably good at being predators," says Brian Swartz, a professor of global ecology at Moorpark College who was not involved in the study. All previous mass extinction events were caused by large scale, environmental catastrophes. "Now, for the first time in history, you have a living group of organisms in a sense changing the way rules work on this planet."

"Recovery from a mass extinction takes millions of years."

A team of scientists led by Jonathan Payne, a paleobiologist at Stanford University, and his colleagues set out to understand out how this sixth mass extinction might differ from previous ones. They identified which marine animals are currently at risk of dying out by checking the list of threatened species compiled by the International Union for the Conservation of Nature. And they dug into the fossil record to see which sea creatures died out in past extinctions. To make sure it was a fair comparison, they only included current species whose relatives they could find as fossils.

They discovered that today, a species is more likely to die out as its size increases. That means that whales are in a lot more danger than guppies. And that’s bad news because big creatures are frequently at the top of the food chain. Losing them can have cascading effects in their ecosystem. Plus, large animals like whales keep nutrients moving through their environments as they feed and poop.

"You’re essentially losing the equivalent the elephants in the sea. You’re losing the sharks and the whales that might live through multiple El Niño events, multiple bad years," says Pincelli Hull, an ocean ecologist at Yale University who was not involved in the research. What’s more, Hull’s research suggests that species that we currently think of as threatened might already be extinct — at least, according to the fossil record. There are so few left that future humans might never find their bones.

"In a broad sense, we know that recovery from a mass extinction takes millions of years, and that should be pretty sobering and cautioning in terms of thinking about how important it is to manage diversity today," Payne says.

"We’re not screwed."

There’s hope, though — as long as we act. "We’re not screwed," says Boris Worm, a marine conservation biologist at Dalhousie University. In fact, many ocean species are still around — although their populations might be shrinking. For instance, there are fewer than 100 individuals of a species of porpoise called the vaquita left in the world. By identifying that we’re driving larger creatures extinct, we know that we need to work especially hard at protecting them.

"We’re really at a point where we need to make a decision," Worm says. Whether we let this destructive pattern continue, or whether we reverse it. "And we have the power to reverse it," he says

That means being smart about which species we allow people to fish, and where we allow boats to travel. It means rethinking fishing methods to prevent big animals like whales and dolphins from becoming entangled in nets. And it means curbing our impulse to kill things bigger and badder than ourselves.