snow leopard

Habitat loss is the main threat to many endangered land animals like snow leopards, the WWF warns. (Photo: Tambako the Jaguar/Flickr)

Earth is most likely experiencing its sixth mass extinction. It has been through five such catastrophes before, but this is the first one in human history — and the first one with human fingerprints.

A new report by the World Wildlife Fund (WWF) offers some sobering details about this decline, which has already halved the planet's known population of vertebrate wildlife in just 40 years. The 2014 Living Planet Report reveals the troubling extent of this and other environmental crises around the world, but it also sheds light on the ways we can still protect and rehabilitate what's left.

"We're gradually destroying our planet's ability to support our way of life," WWF CEO Carter Roberts says in a statement about the report. "But we already have the knowledge and tools to avoid the worst predictions. We all live on a finite planet and it's time we started acting within those limits."

The full report spans 180 dense pages in a 35-megabyte PDF, and WWF chief scientist Jon Hoekstra acknowledges "it can seem very overwhelming and complex." Here are a few key takeaways:

fox

Red foxes are one example of urban animals that have successfully adapted to live near humans. (Photo: Shutterstock)

1. Earth's wild vertebrate population — which includes all mammals, birds, reptiles, amphibians and fish — declined by 52 percent between 1970 and 2010. In other words, the total number of vertebrates in the wild has been cut in half within a single human lifetime.

2. Researchers calculated this by analyzing 10,380 animal populations from 3,038 species. They used these data to create a representative "Living Planet Index" for 45,000 known vertebrates.

3. The No. 1 cause of wildlife declines is habitat loss and change, which is reducing 45 percent of the animals studied. Hunting and fishing, both intentionally and via "bycatch," is next at 37 percent.

4. The most severe loss of wildlife has taken place in South America. Vertebrate species fell 83 percent from 1970 to 2010 in the neotropics, a bioregion that includes most of South America, Central America and the Caribbean as well as parts of lowland Mexico and South Florida.

araripe manakin

Brazil's araripe manakin is critically endangered, due mainly to habitat loss. (Photo: Wikimedia Commons)

5. Low-income countries have seen a 58 percent decrease of vertebrate wildlife since 1970, compared with an 18 percent drop in middle-income countries. The wealthiest nations have seen a 10 percent increase, although their rising resource use correlates with the global trend of biodiversity loss. "In effect, wealthy nations are outsourcing resource depletion," says WWF's Keya Chatterjee.

6. It would require 1.5 Earths to regenerate all the natural resources currently consumed by humanity every year. "This means we are eating into our natural capital," the WWF warns, "making it more difficult to sustain the needs of future generations."

7. The fastest wildlife decline is in freshwater habitats, which have lost 76 percent of their vertebrates.

gray whale

Western Pacific gray whales are considered critically endangered, with fewer than 100 left. (Photo: Omar Torres/Getty Images)

8. Marine animals — ranging from tiny sardines to the enormous baleen whales that eat them — have suffered a 39 percent population loss over the past four decades.

9. Populations of land animals have also fallen 39 percent overall. But they only declined 18 percent within land-based protected areas, less than half the rate for land animals as a whole. That suggests nature preserves can be an effective way to slow down wildlife declines.

10. Tigers are rapidly vanishing from much of their range, for example, but their numbers have risen 63 percent since 2009 within several protected areas in Nepal. Such success hinges on enforcement, though, as seen in the rampant poaching of rhinos and elephants in some African nature preserves. The number of rhinos poached in South Africa alone rose from 13 in 2007 to more than 1,000 in 2013.

11. There are examples of rare wildlife thriving near dense human populations, however. Fewer than 1,000 mountain gorillas are left on Earth, squeezed into islands of forest that are "surrounded by a rising tide of humanity," as the WWF puts it. Yet thanks to conservation efforts that engage local residents and promote eco-tourism, mountain gorilla populations have increased by nearly 30 percent in recent years. Gorilla tourism fuels a $200 million industry in Rwanda, for example, where communities near national parks share 5 percent of the money generated by park permits.

mountain gorillas

Mountain gorillas are stable, thanks to eco-tourism and the work of forest rangers in central Africa. (Photo: Brent Stirton/Getty Images)

"The scale of biodiversity loss and damage to the very ecosystems that are essential to our existence is alarming," says Ken Norris, director of science at the Zoological Society of London, which contributed to the 2014 Living Planet Report. "This damage is not inevitable, but a consequence of the way we choose to live. Although the report shows the situation is critical, there is still hope. Protecting nature needs focused conservation action, political will and support from businesses."

Related wildlife stories on MNN:

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