Earth is a big place, but size isn't everything. The planet's richest ecosystems are in rapid decline, forcing us to acknowledge the elephant in the room: Elephants, along with countless other creatures worldwide, are running out of room.

Habitat loss is now the No. 1 threat facing wildlife on Earth, and the main reason why 85 percent of all species on the IUCN Red List are endangered. It comes in many forms, from outright deforestation and fragmentation to less obvious effects of pollution and climate change. Every species needs a certain amount (and type) of habitat to find food, shelter and mates, but for a growing number of animals, the space where their ancestors found those things is now overrun by humans.

As habitats shrink and splinter, animals also grow more vulnerable to secondary dangers like inbreeding, disease or conflict with people. And so, despite plenty of physical space on Earth, wildlife around the world finds itself painted into a corner. Scientists now widely agree we're seeing the early stages of a mass extinction, with species vanishing at hundreds of times the historical "background" rate, largely due to a shortage of ecological real estate. Earth has suffered five mass extinctions before, but this is the first in human history — and the first with human help.

Like climate change, mass extinction is a global problem. It threatens wildlife all over the world, from iconic elephants, lions and pandas to obscure amphibians, shellfish and songbirds. And while it will take lots of local efforts to save those animals, it will also take a bigger, more ambitious approach than we've used in the past.

According to many scientists and conservationists, our best strategy is surprisingly simple — at least in theory. To avoid a catastrophic loss of biodiversity, we need to set aside half of Earth's surface area for wildlife. That might sound like a big sacrifice at first, but upon closer inspection, it's still an incredibly sweet deal for us: One species gets half the planet, and all other species must share the other half.

Amazon rain forestThe Amazon rain forest spans 40 percent of South America and hosts 1 in 10 of Earth's known species. (Photo: Shutterstock)

Half an Earth is better than none

This idea has been around for years, manifested in programs like the WILD Foundation's "Nature Needs Half" campaign, but it has gained more traction recently. And it may now have one of its most eloquent arguments yet, thanks to a new book by renowned biologist E.O. Wilson titled "Half-Earth: Our Planet's Fight for Life."

"The current conservation movement has not been able to go the distance because it is a process," Wilson writes in the book's prologue. "It targets the most endangered habitats and species and works forward from there. Knowing that the conservation window is closing fast, it strives to add increasing amounts of protected space, faster and faster, saving as much as time and opportunity will allow.

"Half-Earth is different," he adds. "It is a goal. People understand and prefer goals. They need a victory, not just news that progress is being made. It is human nature to yearn for finality, something achieved by which their anxieties and fears are put to rest. We stay afraid if the enemy is still at the gates, if bankruptcy is still possible, if more cancer tests may yet prove positive. It is further our nature to choose large goals that while difficult are potentially game-changing and universal in benefit. To strive against odds on behalf of all life would be humanity at its most noble."

Today, protected areas cover about 15 percent of Earth's land area and 3 percent of its oceans, according to the U.N. Environment Program. Raising that to 50 percent would be no small feat, but it's not like we'd need to start from scratch. We would, however, need to move a little more quickly than we have so far.

urban wetlandA gray heron wades through reeds at a 100-acre urban wetland in London. (Photo: Dan Kitwood/Getty Images)

Missing forests for trees

Of course, no one is suggesting humans move to one hemisphere and all other animals relocate to the other. The two halves would be interspersed, and would inevitably overlap. The Half-Earth concept relies heavily on wildlife corridors, and not just the tunnels and bridges that help animals cross highways (although those are important). In conservation ecology, "wildlife corridor" also refers to larger-scale tracts of habitat that connect two populations of a species, thus enabling a broader habitat network with more shelter, food and genetic diversity.

Those kinds of networks used to be the norm, before Earth's biggest biomes were bisected by things like roads, farms and cities. Animals are now increasingly separated from others of their kind, leaving them little choice but to inbreed or risk their lives by dashing across roads or traipsing through civilization.

About 60 percent of the U.S. Southeast was once longleaf pine forest, for example, which spanned 90 million acres from modern-day Virginia to Texas. After 300 years of land change for timber, agriculture and urban development, less than 3 percent of the region's signature ecosystem is left. A lot of biodiversity still persists in its remaining pockets — including up to 140 plant species per square kilometer — but large animals like Florida panthers and black bears are frequently killed by road traffic as they try to improvise their own makeshift wildlife corridors.

Florida panther crossing signA sign in Everglades National Park warns motorists to watch for Florida panthers. (Photo: Everglades NPS)

Because ecosystems are so interwoven, the loss of one species can start a horrible chain reaction. When the American chestnut tree was driven to near extinction 100 years ago by an invasive Asian fungus, Wilson notes, "seven moth species whose caterpillars depended on its vegetation vanished, and the last of the passenger pigeons plunged to extinction." Similarly, the modern decline of monarch butterflies is largely linked to the decline of milkweed, on which their larvae rely for food.

On Half-Earth, human society wouldn't be cleaved from non-human society — we'd still be living among milkweed and monarchs, and even sometimes among bears, panthers, lions and elephants. The difference, however, is wildlife would also have a safe, stable home of its own, occasionally wandering into our midst rather than being forced there by a lack of options. And that overlap is important, since humans are animals, too, and we rely on ecosystems just like everyone else.

"Biodiversity as a whole forms a shield protecting each of the species that together compose it, ourselves included," Wilson writes. "As more and more species vanish or drop to near extinction, the rate of extinction of the survivors accelerates."

birds flying over Los AngelesEven for urban-adapted birds, air pollution and climate change can pose grave threats. (Photo: Frederic Brown/Getty Images)

How the other half lives

Although we need to think bigger about habitat conservation, preserving tracts of wilderness is still a local struggle. If we set aside enough half-yards, half-towns, half-nations and half-regions for nature, Half-Earth should start to take care of itself.

"Many assessments over the last 20 years have determined that nature needs at least half of a given eco-region to be protected, and needs to be interconnected with other such areas," explains the WILD Foundation, "in order to maintain its full range of life-supporting, ecological and evolutionary processes, the long-term survival of the species that live there, and to ensure the system's resilience."

Half-Earth, therefore, isn't so different from today's Earth. We're already doing many of the right things, as Wilson recently told the University of California-Berkeley's "Breakthroughs" magazine. We still have a few big biodiversity zones left, and others that could still recover. We just need to protect as many wilderness areas as we can, fill in gaps wherever possible and do no further harm.

"I'm confident we can go from 10 percent to 50 percent coverage, land and sea," Wilson says. "It could be immense reserves that still exist, like in the Altai Mountains of Mongolia, in the taiga, the major wilderness areas of Congo, in Papua New Guinea, the Amazon — these can be made inviolate reserves; they can be pieced together.

"Likewise for smaller reserves," he continues, "all the way down to 10 hectares granted to the Nature Conservancy somewhere."

That kind of patchwork strategy is already working in many places. Wildlife corridor projects have become a mainstream conservation tactic lately, as seen in places like India's and Nepal's Terai Arc Landscape, Central and South America's Jaguar Corridor Initiative, and North America's Yellowstone-to-Yukon artery. Conservationists are also working to relink longleaf pine forest, including efforts by the Nature Conservancy, Nokuse Plantation, the Florida Wildlife Corridor Expedition and others.

In fact, as Wilson notes in "Half-Earth," our conservation efforts thus far may have already reduced extinction rates by as much as 20 percent. We have proven conservation can work; we've just done it at too small of a scale. And since old-growth forests are being felled to bring us beef, palm oil and other products, the key to expanding conservation is to crowd-source it: As each person shrinks his or her ecological footprint, our species' demand for space dwindles, too.

What might compel us to cut back? Why go out of our way to protect half the planet for other species, rather than letting them fend for themselves as we've had to do? There are plenty of economic reasons, from the ecosystem services offered by forests and coral reefs to the ecotourism revenue that can make elephants worth 76 times more alive than dead. But as Wilson argues, it really boils down to our nature as social — and moral — animals, now at a pivotal stage in our ethical evolution.

"Only a major shift in moral reasoning, with greater commitment given to the rest of life, can meet this greatest challenge of the century," Wilson writes. "Like it or not, and prepared or not, we are the minds and the stewards of the living world. Our own ultimate future depends upon that understanding."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.