September 15, 2016 — 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.