In the most comprehensive study of its kind, scientists have merged big data with our knowledge of ecosystems to reach a troubling conclusion — the abundance of the overall number of animal and plant species across the majority of Earth’s land surface has fallen bellow what biologists consider the “safe” limit.

Image credits geograph.org.uk

The cause at least is clear: from grasslands to tropical forests, humans are taking over ever more land to use in agriculture, for roads, infrastructure, and housing. This requires clearing the land so we can use it — an act which doesn’t cause ecological collapse in itself, but it does reduce the abundance of species that make up an intact ecosystem. If biodiversity falls too low, ecosystems lose their resilience to external factors (such as shifting climate) or, in extreme scenarios, cease to function altogether.

“Exploitation of terrestrial systems has been vital for human development throughout history, but the cost to biosphere integrity has been high,” notes the study led by Tim Newbold of the United Nations Environment Programme and University College London with colleagues representing several British, Australian, Danish and Swiss universities and institutions.

The team analyzed over 1.8 million measurements of the abundance of species (39,123 different species to be exact) from almost 19 thousand locations across the world. Because of the sheer quantity of data used, an accompanying essay in the journal Science by University or Reading Tom Oliver calls it the “most comprehensive quantification of global biodiversity change to date.”

From the results in these points, the researchers extrapolated for the whole area of the planet, then compared the figures to a “Biodiversity Intactness Index” to find out where species decline could have destabilizing effects on ecosystems. This approach is based on a “planetary boundaries” concept, which according to lead author Newbold:

“.. attempts to set some sort of safe limit to the amount of biodiversity we can lose, while biodiversity still supports important ecosystem functions.”

It’s important to note that “safe” here means the ecosystem can still support the processes humans require for a normal continuation of our lives — not that the ecosystem is unaffected. The concern is that as these systems lose their biodiversity, the processes that allowed them to supply stored carbon, clean water, fertile soil, and every other bio-related resource will falter — this is bad news for both humans and the animals that rely on these ecosystem “services.”

“Biodiversity supports a number of functions within ecosystems, things like pollination, nutrient cycling, soil erosion control, maintenance of water quality,” Newbold said. “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.”

The researchers assumed that a decline of more than 10% in the abundance of species — after human interference — in a given area corresponds to a dangerous drop in biodiversity. The study worryingly found that on average, global decline is already inching close to 15 percent, meaning original species are roughly 85 percent as abundant (85.6 percent to be exact) as they were before we started changing the land. Overall, 58 percent of the Earth’s surface has fallen below 90% intact biodiversity, and into the danger zone.

Places with little human habitation naturally fare better than the rest — the team found that northern tundra and boreal forest ecosystems are relatively intact, as was much of the Amazon rain forest. On the other end of the spectrum, large swaths of central North America show areas with less than 60% of their original biodiversity, stretching all the way from Canada to Texas. The correlation is staggering — that 58 percent I mentioned in the previous paragraph houses 71 percent of the human population, the team reports.

Total abundance of species occurring in primary vegetation (areas above safe limit in blue)
Image credits Newbold et al/Science

Critics point out that there are some major uncertainties and matters left open to interpretation in the study. For example, is the 10 percent biodiversity loss threshold that the researchers picked accurate? Does it vary from case to case? And moreover, humans don’t only remove species from ecosystems — they also introduce non-original or “invasive” species — is this a benefit or a further strain on these systems?

The authors openly admit these criticisms and have actually considered some of the points being raised. They found that if new species are considered to benefit ecosystems, or if they set the danger threshold lower, so ecosystems can ‘safely” reach 80 or 70 percent of their original species abundance, then considerably less of the world is in trouble. But as a personal note, everything is over the bar if you set the bar low enough. In the end, it all comes down to how much of a risk we want to take with mother nature’s resilience.

So in the end, we won’t start seeing ecologic collapse all around us as ecosystems reach exactly 90 percent intact biodiversity. But it does mean that they are considerably weakened, and less likely to successfully withstand future hurdles like global warming, according to Newbold.

“We’re entering a space where things become more uncertain, and we expect that things will be less resilient in the face of other changes,” he said.

Mark Urban, who directs the newly founded Institute of Biological Risk at the University of Connecticut which focuses on biodiversity losses, believes that the study will set a stepping stone in our fight to preserve the environment around us and ourselves along with it, but has some criticisms of his own to add:

“Newbold and colleagues find sobering evidence that we have already crossed that line in terrestrial ecosystems,” said  “Human land use has reduced local populations to 85 percent of original abundances on average. What this means is we have not only crossed a planetary boundary, but have kept going. At least now we’re looking back.”

However, he also noted that “this result ignores the accelerating threat from a warming climate,’ focusing on land use. “Climate change is about to make things more complicated as we try to pull back from the edge of the Earth’s resilience.”

In the end, even if the details are not yet clear the general picture is what matters — and this one paints a warning, at least in my eyes.

The full study titled “Has land use pushed terrestrial biodiversity beyond the planetary boundary? A global assessment” has been published online in the journal  .