A global assessment of ecosystems across the planet shows that "exploitation of terrestrial systems"—in other words, human land use from road-building to industrial agriculture—has pushed biodiversity below "safe" levels.
The study, published Thursday in the journal Science, finds that for 58 percent of the world's land surface, which is home to roughly 71 percent of the global population, the level of biodiversity loss is "substantial enough to question the ability of ecosystems to support human societies."
"Until and unless we can bring biodiversity back up, we're playing ecological roulette."
—Andy Purvis, Imperial College London
The researchers cite "land use and related pressures" as the cause of this decline, with grasslands, savannas, and shrublands most affected by biodiversity loss, followed closely by many of the world's forests and woodlands.
"Biodiversity supports a number of functions within ecosystems, things like pollination, nutrient cycling, soil erosion control, maintenance of water quality," lead author Tim Newbold of the United Nations Environment Programme and University College London told the Washington Post. "And there's evidence that if you lose biodiversity, that these functions don't happen as well as they would have done in the past."
Newbold added in a press statement: "The greatest changes have happened in those places where most people live, which might affect physical and psychological wellbeing."
As the Christian Science Monitor explains, the assessment "was based on a Biodiversity Intactness Index (BII), which measures changes in species abundances across the globe and places a safe limit of species decline at 90 percent of the total population that would exist in a pristine version of that habitat—one free of human activity. Once populations drop below that threshold, scientists fear there will be a tipping point at which one species goes extinct, causing a domino effect that leads to the collapse of the ecosystem."
Indeed, co-author Andy Purvis, a professor at Imperial College London and research leader at the Natural History Museum, told the BBC: "Once we're the wrong side of the boundary it doesn't mean everything goes wrong immediately, but there is a markedly higher risk that things will go badly wrong."
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"Decision-makers worry a lot about economic recessions," Purvis said, "but an ecological recession could have even worse consequences—and the biodiversity damage we've had means we're at risk of that happening. Until and unless we can bring biodiversity back up, we're playing ecological roulette."
Bradley Cardinale, the director of the Cooperative Institute of Limnology and Ecosystem Research at the University of Michigan, who did not take part in the study, told The Verge that the level of decline revealed in the study was "little bit higher than we've estimated in the past."
"But it's also the most rigorous dataset," Cardinale added, "so there's no doubt that this is the correct estimate of where we're at."
Meanwhile, professor Bill Sherwin of the Ecology and Evolution Research Centre at Australia's University of New South Wales, provided the following critique:
Future developments of Newbold's model will hopefully add effects of climate change. To take an Australian example, it is forecast that soon many areas of Australia will have a combination of physical conditions that is unlike anything that currently exists anywhere in Australia.
This means that currently existing biodiversity will need to show rapid adaptive changes, or move elsewhere. In other words, models such as Newbold's cannot simply consider the biodiversity that is currently functioning in each area, but will need to think about the different array of species that will be able to function there in the future. This is likely to be true for all continents.
In any case, Newbold said: "We're crossing into a zone of uncertainty."