Zack Sheppard from San Francisco, USA, CC BY-SA 2.0 <>, via Wikimedia Commons

Part 2: The Science of Half-Earth

It was the work of biologist Edward Wilson that inspired Tony Hiss, and it is important to understand the science behind Wilson’s seemingly audacious goal of protecting half of earth’s land and sea for nature by the year 2050.

Biologists have found that as the area of a habitat is reduced, the percentage of species that can survive in what remains is also reduced—but surviving species will decrease more and more rapidly as the area shrinks.

If the area of a habitat is reduced by half, the fraction of species that can survive in what remains is about 85% of the original number of species that were found there. Note that this fraction is for the number of species, not the number of individual plants and animals within a species. In other words, a much greater percentage of individuals will be lost, likely even the majority of them, but about 85% of species will have enough individuals left to be able to avoid extinction. So holding habitat reduction to 50% is the goal of the Half-Earth Project.

When 90% of a habitat is lost, the species survival plunges into a downward slope so steep that all remaining species may completely die out. Many of the world’s regions with the greatest biodiversity are at exactly this point.

The Half-Earth projection is not exact; it is an estimate. The actual area needing protection depends on the species and the details of their range. For example, some species may only need 30% of their original range to survive, while others may need 70%. But even saving a very large area may not be enough if, for example, it’s divided by a highway that keeps species from reaching their breeding grounds or from making their seasonal migrations. Protected spaces must be carefully designed according to the overall ecology of the area.

It would be a disaster to lose 15% of the Earth’s species and many more of its individuals, but we are on our way to something much worse. The Half-Earth Project acknowledges that a high level of extinction is already inevitable. But by identifying and protecting critical habitats, we can prevent an even more calamitous mass extinction. Right now plants and animals are going extinct at a rate hundreds, or even thousands, of times faster than they would if not for the impact of humans on their environments. And the full shock of climate change is yet to be felt.

The impending extinction has many drivers – overhunting and overfishing, pollution, and more – but the primary cause is habitat destruction. It is surprising that when 90% of a habitat is lost, more than half of the species can still survive. But note from the chart that at that point the species survival plunges into the sharp downward slope of the curve. The cliff quickly becomes so steep that all remaining species may completely die out. Many of the world’s regions with the greatest biodiversity are at exactly this point.

Madagascar, one of the world’s richest and most irreplaceable natural areas, has lost 90% of its forests to “slash and burn” agriculture. It is one of a handful of areas of that have been identified as a “hotspot” — an environment with unusually great biodiversity that has lost most of its habitat and is under threat to lose even more. Such a biologically rich area is well suited to farming, hunting and fishing, and forestry. It is possible to for these activities to be done sustainably, but they cannot be permitted in any hotspot. These areas have already lost too much. They are the places that most need protection.

We have an even more urgent reason for preservation in the face of global warming. These natural areas are especially good at absorbing the excess carbon that‘s heating our planet, locking it away in great quantities in soil and deep waters. It is estimated that the earth’s soil contains far more carbon than all the planet’s atmosphere and plant life combined. Since the 1850’s, when the Industrial Revolution began the current period of global warming, the soil, forests and oceans have captured more than half of the extra carbon that humans released into the air.

In this way the planet has so far shielded us from the full effects of global warming. At some point — and perhaps very soon — these carbon-storing natural areas will be saturated, and the process may even be reversed, the stored carbon being released back into the air in enormous quantities. To protect ourselves from the worst effects of global warming, we must ensure that the large areas of land and sea that have so far been spared exploitation, like the boreal forests of Canada, are protected and preserved.

Why half?

Watch: The Half-Earth Project Map

Walter Jetz, Esri User Conference 2019
This presentation by the Scientific Chair of the E.O. Wilson Biodiversity Foundation explains what the Half-Earth Project is, why it is so important to preserving earth’s species, how the Half-Earth Map helps us to understand how the project is working for all of us—and how we can each help in this important endeavor.

For more information, visit the Half-Earth Project

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