Earth is most likely experiencing its sixth mass extinction. The planet has been through at least 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 2016 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.

"For the first time since the demise of the dinosaurs 65 million years ago, we face a global mass extinction of wildlife," says Mike Barrett, director of science and policy at WWF-UK, in a statement. "We ignore the decline of other species at our peril — for they are the barometer that reveals our impact on the world that sustains us."

This is the first edition since 2014 of the Living Planet Report, which the WWF releases every two years. The full report spans 140 dense pages in a 20-megabyte PDF, and as WWF chief scientist Jon Hoekstra acknowledged in 2014, these reports "can seem very overwhelming and complex." Here are a few key takeaways:

Hainan gibbon The critically endangered Hainan gibbon lost about 80 percent of its total population over the past 50 years. It's now considered the rarest ape on Earth, with just 28 living in a single nature preserve. (Photo: Jessica Bryant/ZSL)

1. Earth's population of wild vertebrates — all mammals, birds, reptiles, amphibians and fish — declined 58 percent from 1970 to 2012. (That's up from 52 percent in the 2014 report, which spanned 1970 to 2010.) In other words, the total number of wild animals with backbones has fallen by more than half within one human lifetime.

2. The data show an average annual decline of 2 percent, and "there is no sign yet that this rate will decrease," according to the WWF report. If it doesn't decrease soon, global wildlife populations could drop by 67 percent as early as 2020.

3. Researchers calculated these statistics by analyzing a total of 14,152 animal populations from 3,706 species.

4. The No. 1 cause of the decline is habitat loss and degradation, which is reducing nearly half of the species studied. "The principal causes of habitat loss appear to be unsustainable agriculture and logging," the report states, "and changes to freshwater systems." Other top threats include overexploitation (hunting and fishing as well as accidental "bycatch"), climate change, invasive species and pollution.

5. The fastest wildlife decline is in freshwater habitats, which lost 81 percent of their vertebrate populations between 1970 and 2012. The total number of freshwater vertebrates drops by about 4 percent each year.

Shenandoah salamander The Shenandoah salamander is an endangered species that exists only in Virginia's Shenandoah National Park. About half of all salamander species on Earth are now considered threatened with extinction. (Photo: Brian Gratwicke/Flickr)

6. Habitat loss is the main problem for 48 percent of threatened freshwater species, including direct threats like wetland development and indirect threats like interrupted river flows. Invasive species and disease also pose widespread danger, affecting 25 percent of threatened amphibian species. That's largely due to chytridiomycosis, an invasive fungal disease that's obliterating amphibians worldwide.

7. Marine animals — ranging from tiny sardines to the enormous baleen whales that eat them — have suffered a 36 percent population loss over the past four decades. Nearly one-third of all commercial fish stocks are currently at biologically unsustainable levels, according to the WWF, and are therefore overfished.

8. Populations of land animals have fallen 38 percent overall, but their decline was about half that in land-based protected areas. This suggests nature preserves can be an effective way to slow down wildlife declines.

green sea turtle at Papahānaumokuākea A green sea turtle swims under Midway Island Pier at Hawaii's Papahānaumokuākea Marine National Monument, which was expanded in 2016 to become one of the largest ocean sanctuaries on Earth. (Photo: Greg McFall/NOAA)

9. Protected areas now cover 15 percent of the Earth's land surface, including inland waterways. Only 4 percent of the oceans are under protection — but an array of new marine refuges have been created in recent years, raising hopes the tides are turning.

10. Tigers offer one example of what habitat protection can do. The big cats have vanished from much of their historical range, but their numbers have risen 63 percent since 2009 within several protected areas in Nepal — and a 2016 study suggested they still have enough intact habitat to bounce back in other places, too. 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 1,100 in 2015.

11. Still, there are examples of rare wildlife thriving near dense human populations. Fewer than 1,000 mountain gorillas are left on Earth, for example, 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 gorillas have increased by nearly 30 percent in recent years. Gorilla tourism fuels a $200 million industry in Rwanda, where communities near national parks share 5 percent of the money generated by park permits.

mountain gorillas Mountain gorilla populations have become relatively stable, thanks largely to eco-tourism and the work of heroic forest rangers in central Africa. (Photo: Brent Stirton/Getty Images)

"A strong natural environment is the key to defeating poverty, improving health and developing a just and prosperous future," says Marco Lambertini, WWF director general, in a statement. "We have proven that we know what it takes to build a resilient planet for future generations; we just need to act on that knowledge."

For more information — including ideas about what can be done to save the wildlife we have left — take a look through the full 2016 Living Planet Report. And for a quicker overview, check out this new WWF video about the report:

Editor's Note: This article has been updated since it was first published in October 2014.

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

11 startling stats about Earth's disappearing wildlife
Our planet has lost 58 percent of its vertebrate animals since 1970, but there is still time to save the rest.