Across an estimated 58 percent of the planet's land surface, biodiversity—the rich variety of species that inhabit a given environment—has dropped below what scientists consider "safe" levels.
That doesn't just spell trouble for the multitude of plant and animal species that are vanishing. It will be disastrous for us humans, too.
But there are a lot of big questions around whether there's even such thing as a "safe" level for species loss to begin with, and where it should be set. Isn't any species loss too much?
"Biodiversity is essential for human well-being," landscape ecologist Tom Oliver of the University of Reading, who wrote a commentary that accompanies the new study (both are published in Science), told me. Take, for example, the species we rely on to pollinate our food crops, he pointed out: Colony collapse disorder has been a major concern for years now, as bees die off.
In this study, an international group of researchers looked what was previously defined as a "safe limit" for biodiversity loss, set at 10 percent. In other words, if species abundance dips below 90 percent of where it stood before humans came in and started using up the land, the species upon which we rely—and, by extension, us too—may become critically threatened. The Science study drew on over 2 million records for nearly 40,000 terrestrial species.
The authors found that, because of land use and other pressures, biodiversity has already dipped below the proposed threshold across an estimated 58 percent of the world's land surface—where 71 percent of humans live.
The boundary itself—the idea that there's a "safe" limit to which biodiversity could drop—is actually controversial
"It's an impressive data set," said Oliver, who works with long-term species monitoring data, mainly butterflies. Some parts of the world, he noted, are more data-deficient than others, although the authors accounted for this in their work.
The boundary itself—the idea that there's a "safe" limit to which biodiversity could drop—is actually fairly controversial. "These boundaries are subject to huge uncertainty," Oliver told me. If the "safe" limit is really somewhere around 70 or 80 percent, it would change the picture quite a bit. "If the safe limit is down to 80 percent, we could lose a bit more [biodiversity] and still be safe," he said. "It does matter, where we set those boundaries."
Some scientists don't see a use in establishing a "safe limit" at all. "We don't know what the safe limit is. We shouldn't even try to find that out," Erle Ellis of the University of Maryland told Discover. "Our goal is not to change the planet up to its safe limits."
Oliver told me that the idea of a "tipping point" is hotly contested by scientists. And we don't even know whether thresholds for tipping points are planetary, or regional. "At the planetary level, we could hit a catastrophic collapse of biodiversity, which sounds terrifying," he said.
But if tipping points are more likely to be regional or local, maybe certain parts of the world—refuges from climate change and other pressures—will stay relatively safe.
While the "58 percent" number is definitely concerning, Oliver agreed with me that there are still plenty of questions left to be answered. "There are certainly improvements that can be made, as more data come in," he said. "But the key thing to note is that we can't wait until all the data come in, before we can start producing the evidence. Because we could be waiting tens of years."
In that time, more species will be lost.