Our planet is losing so many species that we might be putting ourselves at risk. New research shows that across half of the Earth’s land surface, biodiversity — all the different animals, plants, and other species that make up an ecosystem — has dropped below a threshold that some scientists consider "safe."

Understanding how much biodiversity we’re losing is key to our own survival. We depend on biodiversity to live. Natural processes like crop pollination, waste decomposition, and regulation of the global carbon cycle all depend on biodiversity. For example, more than 240 crop species around the world, including many fruits, need pollinators like bees and butterflies to survive. "The possible consequences of things going wrong are very large," says study author Tim Newbold, who researches biodiversity at University College London.

"The possible consequences of things going wrong are very large."

Some studies have already shown that extinction rates are increasing and that biodiversity is being affected in areas like the Amazon rainforest. But today’s study, published in Science, is a very comprehensive quantification of how much biodiversity has been lost all over the world, as more and more land is changed by people — turned into pastures, fields, or cities. And quantifying that loss is key to better understand the damage, and then hopefully create strategies to reverse it.

"Unless we really convey ... the implications of that biodiversity loss for human well-being, for livelihood, then we can’t really expect to be able to explain to decision makers why they should be paying money to conserve biodiversity," says Tom Oliver, an associate professor in landscape ecology at Reading University in the UK, who did not take part in the study. "It’s a difficult step to take, but it’s very necessary."

To quantify the global biodiversity loss, the researchers analyzed more than 2.3 million records of nearly 40,000 species living in 18,600 sites around the world at different times. Their model revealed that, globally, biodiversity has fallen on average to 84.6 percent of what it was before people changed the landscape. If new species are accounted for, the average loss is at 88 percent. Both figures of biodiversity loss are below the threshold considered safe by some scientists. And local biodiversity has dropped below safety levels across 58 percent of the world's land surface.

We depend on biodiversity to live

"It’s a little bit higher than we’ve estimated in the past, so that’s a little bit surprising," says 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. "But it’s also the most rigorous dataset, so there’s no doubt that this is the correct estimate of where we’re at."

Today’s calculations are based on the theory that a given habitat should retain 90 percent of its original species, before humans changed the land, to remain intact. According to this theory, the "safe" threshold for biodiversity loss is at 10 percent in the so-called Biodiversity Intactness Index, or BII. (Some researchers, however, believe a reduction as high as 70 percent is still safe, the study authors write.) Once that threshold is crossed, then we can’t be sure that the ecosystem will function properly. "If you lose more biodiversity than that, humans are going to start suffering," says Cardinale. "We’re not going to be able to produce the air, water, and food that we need to survive."

The researchers found that tundra and boreal forests are the least affected areas, while grasslands — where most of the world’s agriculture happens — are the most affected. That creates a vicious cycle. "We obviously need the agriculture areas to support the human population, but as we get more agriculture areas, then we lose biodiversity and then the ability of biodiversity to support that agriculture starts to come into question," says Newbold. "The bottom line is that we need to preserve what natural habitats we have left."

"We need to preserve what natural habitats we have left"

The calculations in today’s paper have limitations. They’re based on statistical analyses and assumptions about how the land has changed and how many species have disappeared. There’s even uncertainty about what the "safe" threshold for biodiversity loss should be. The 90 percent limit in BII is considered on the precautionary side, so our planet’s biodiversity might not be in such dire condition as the paper suggests. In other words, we might have not crossed the "safe" line yet.

Other researchers believe that, even though we’re losing biodiversity, we might be able to preserve those key ecological functions by conserving the species that we need the most. For example, we could save crop pollination by identifying which pollinators do most of the work. The theory is that "there are certain species that might be more important than diversity per se," Cardinale says.

"We better start worrying. We better start restoring biodiversity."

Even with all its assumptions, which are normal in any kind of statistical analysis, today’s study is a necessary one, according to Oliver. Other fields, like climate change, have had to deal with uncertainty as well. With biodiversity, it’s harder to make predictions because ecosystems are very complex, and it’s hard to know how a changing habitat or a disappearing species might affect an ecosystem and its functions. At the end of the day, however, scientists’ predictions will have to inform lawmakers in order to create the targeted policies we need to protect the environment — and our own survival.

If we passed the safe threshold, "We better start worrying. We better start restoring biodiversity," Cardinale says. "Otherwise, within this generation, we’re going to experience some real problems in providing for human needs."