We mostly can’t see it around us, and too few of us seem to care — but nonetheless, scientists are increasingly convinced that the world is barreling towards what has been called a “sixth mass extinction” event. Simply put, species are going extinct at a rate that far exceeds what you would expect to see naturally, as a result of a major perturbation to the system.
In this case, the perturbation is us — rather than, say, an asteroid. As such, you might expect to see some patterns to extinctions that reflect our particular way of causing ecological destruction. And indeed, a new study published Wednesday in Science magazine confirms this. For the world’s oceans, it finds, threats of extinction aren’t apportioned equally among all species — rather, the larger ones, in terms of body size and mass, are uniquely imperiled right now.
From sharks to whales, giant clams, sea turtles, and tuna, the disproportionate threat to larger marine organisms reflects the “unique human propensity to cull the largest members of a population,” the authors write.
“What to us was surprising was that we did not see a similar kind of pattern in any of the previous mass extinction events that we studied,” said geoscientist Jonathan Payne of Stanford University, the study’s lead author. “So that indicated that there really is no good ecological analogue…this pattern has not happened before in the half billion years of the animal fossil record.”
The researchers conducted the work through a statistical analysis of 2,497 different marine animal groups at one taxonomic level higher than the level of species — called “genera.” And they found that increases in an organism’s body size were strongly linked to an increased risk of extinction in the present period — but that this was not the case in the Earth’s distant past.
Indeed, during the past 66 million years, there was actually a small link between smaller body sizes and going extinct, marking the present as a strong reversal. “The extreme bias against large-bodied animals distinguishes the modern diversity crisis from all potential deep-time analogs,” the researchers write.
The study also notes that on land, we’ve already seen the same pattern — and in fact, we saw it first. “Human hunting has been extensive for many thousands of years on land, whereas it’s been extensive for a couple of hundred years in the oceans,” says Payne.
Thus, humans already drove to extinction many land-based large animal species in what has been dubbed the Late Quaternary extinction event as the most recent ice age came to a close.
“These losses in the ocean are paralleling what humans did to land animals some 50,000 to 10,000 years ago, when we wiped out around half of the big-bodied mammal species on Earth, like mammoths, mastodons, saber-tooth cats and the like,” said Anthony Barnosky, executive director of Stanford Jasper Ridge Biological Preserve, who was not involved in the study but reviewed it for the Post. “As a result, terrestrial ecosystems were locked into a new trajectory that included local biodiversity loss over and above the loss of the large animals themselves, and changes in which kinds of plants dominated.”
Barnosky was the co-author of a study published last year that found an “exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way.”
A particular problem, says Payne, is that if you take out all the top predators, then the species they used to prey upon can run amok and explode in population, having large reverberating effects on the entire ecosystem.
“The preferential removal of the largest animals from the modern oceans, unprecedented in the history of animal life, may disrupt ecosystems for millions of years even at levels of taxonomic loss far below those of previous mass extinctions,” the authors write.
Interestingly, if climate change was the key driver of species losses, you’d expect to see a more evenly distributed set of risks to organisms.
“I’ve worked on the Permian mass extinction quite a bit, it shows environmental evidence of ocean warming, ocean acidification, and deoxygenation, the loss of oxygen from seawater,” says Payne. These are the very same threats to the oceans that we’re worried about now due to ongoing climate change. But the Permian extinction, some 250 million years ago, did not feature a selective disappearance of large-bodied organisms, Payne says.
Thus, as previous work has also suggested, the current study underscores that ecosystem risks are not being principally driven by a changing climate — yet. Rather, they’re being driven more directly by which species humans hunt and fish, and where they destroy ecosystems to build homes, farms, cities, and much more. But as climate change worsens, it will compound what’s already happening.
“The losses the authors describe in the oceans do not include the extinctions expected from business-as-usual climate change,” said Barnosky. “Adding those human-triggered losses onto those we’re already causing from over-fishing, pollution, and so on is very likely to put the human race in the same class as an asteroid strike–like the one that killed the dinosaurs–as an extinction driver.”