It’s tricky to turn a metric into a mascot, but the climate movement has done it. Over the past few decades, “2 degrees Celsius”—the threshold of global warming we don’t want to surpass—has gone from a back-of-the-envelope calculation to a rallying cry, serving as the cornerstone for a number of international agreements and playing a starring role on protest signs and in explainer videos.
One number can’t capture all the intricacies of the climate change crisis. (Experts don’t even agree that two degrees is the right one—more and more, the consensus is pushing towards 1.5.) But it can give organizers, legislators, and communicators something to hold onto as they raise awareness and fight for change. In a recent article in Science, a group of conservationists argues that we need a similar number for another disaster: the biodiversity crisis. And they, too, propose a threshold: twenty or fewer extinctions per year.
Like climate change, the biodiversity crisis is complex. Across the world, animal, plants, and fungi are disappearing for a long list of overlapping reasons, including habitat loss, resource extraction, wildlife trafficking, competition from introduced species, and climate-change exacerbated pathogens and diseases.
Addressing the overall issue involves working on each of these problems, and current initiatives often focus on habitat conservation as a kind of umbrella approach. “Some have called for bold targets to protect half the earth, others to protect intact wilderness,” writes Richard Gregory, one of the authors of the new paper, in a blog post. “But most of these describe actions and means to protect biodiversity, and not biodiversity itself.”
In other words, they’re a step removed from what the authors describe as “the most fundamental aspect of biodiversity loss:” extinction. If a population diminishes or disappears, a species can theoretically recover. But if every single member of a species dies, it means the end of many things: a group of creatures unique to this planet; the rich evolutionary and genetic history that brought them here; all the contributions they made to their surrounding ecology and culture; a whole set of futures. It is, the authors write, “an irreversible loss.”
A hard line of twenty extinctions is meant to cut to the chase, and provide “a single outcome-oriented target,” Dr. Gregory writes. In the paper, the authors call it “simple,” “measurable” and “actionable.” The goal is for the number to be “readily communicated to galvanize both political will and public support,” as two degrees Celsius is for the climate crisis. Nations can develop their own targets based on the overall figure, and more complex management plans can be built up around this straightforward aim. Once we’ve achieved it—in a century, the authors hope—we can aim for a lower extinction count, closer to the background rate of between zero and two species per year.
On its face, it feels a bit strange to have a “goal” of twenty extinctions per year rather than, say, zero. One of the Aichi Biodiversity Targets—proposed in 2010 by the Convention on Biodiversity, and meant to be achieved by 2020—is that “the extinction of known threatened species has been prevented.”
But we’ve failed at this almost completely. In 2019 alone, nearly two dozen species officially died out, including the Catarina pupfish (killed during groundwater extraction), the po’ouli (routed by invasive feral pigs) and the Bramble Cay melomys (drowned by rising seas). Many, many extinctions likely occurred without us knowing. To bring the death count down, we might have to admit that twenty per year is ambitious—and start writing it into legislation and shouting it in the streets.
Source: Rounsevell, Mark, et. al. “A biodiversity target based on species extinctions.” Science, 2020.
Image: Paula Olson/NOAA
