"In less than two human generations, population sizes of vertebrate species have dropped by half." That's the startling conclusion offered by the World Wildlife Foundation, as they release their biennial "Living Planet Report." But what does that mean?

Every couple years, the WWF assesses the status of 10,380 representative populations of 3,038 mammal, birds, reptiles, amphibians, and fish. What they report this year is that compared with 1970, those populations have declined by half. That might seem like an unbelievable figure, but here's how they arrived at that statistic:

"There is a lot of data in this report and it can seem very overwhelming and complex," said Jon Hoekstra, chief scientist at WWF. "What's not complicated are the clear trends we're seeing — 39 percent of terrestrial wildlife gone, 39 percent of marine wildlife gone, 76 percent of freshwater wildlife gone – all in the past 40 years."

If you take the average of 39 percent, 39 percent, and 76 percent, you get 51.3%. Simple enough. Whether that method of calculating wildlife abundance is convincing for you isn't important. It is indeed an average of averages of averages, which provides little more than a rough trend. However, the pattern is clear: wildlife is disappearing, and fast.

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The 180 page report is a daunting read. We've culled out some of the most interesting facts and figures for you. The graph below is where that 50% figure comes from. If the LPI in 1970 was set at 100%, then today it's around half that.

To calculate the LPI, the researchers divided the planet into "realms" and taxonomic groups. The main drivers of loss come from freshwater ecosystems - that's fish and amphibians - and from the tropics of Central and South America.

It appears as if North America and Europe are a bit over-represented in the dataset. Blue dots are marine, green dots are terrestrial, and orange dots are freshwater populations.

The biggest threats to wildlife? Unsustainable exploitation and habitat degradation.

The LPI also measures the extent to which humans are using up our planet's resources faster than the planet can regenerate those resources. One and a half Earths would be required to sustainably meet the demands that we are currently placing on the only Earth we have.

What do those demands look like? Our utilization of fishing grounds, agricultural lands, and so on haven't increased all that much in 50 years. It's carbon that makes the largest, fastest growing chunk of our ecological footprint. And it's primarily through the burning of fossil fuels: oil, coal, and natural gas. In 1961, carbon accounted for 36% of our species' footprint. By 2010, it comprised 53%.

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"The sum of all human demands no longer fits within what nature can renew," says the report. "The consequences are diminished resource stocks and waste accumulating faster than it can be absorbed or recycled, such as with the growing carbon concentration in the atmosphere."

The US and China make the biggest demands on our planet, with India not far behind.

However, Kuwait, Qatar, and the UAE blow the rest of the world out of the water when it comes to Ecological Footprint per capita. And that's driven by carbon, ostensibly thanks to oil extraction, and perhaps due to the development of cities. Not that the US is doing much better: "If all people on the planet had the Footprint of the average resident of Qatar, we would need 4.8 planets. If we lived the lifestyle of a typical resident of the USA, we would need 3.9 planets," says the report. (click to enlarge)

Biodiversity loss is driven more by low and middle income countries, while high income countries actually show a net gain in wildlife of around 10%. However, that may be deceiving. For one thing, the LPI only began in 1970, and it's likely that if data began being collected earlier, even the wealthiest countries would show a decline. In addition, richer countries may be better able to allocate resources for domestic biological conservation or restoration.

In the chart below, blue is high income, purple is middle, and green is low.

More insidiously, according to the report, "they may also reflect the way these countries import resources – effectively outsourcing biodiversity loss and its impacts to lower-income countries."

Because those in high income countries can afford to outsource biodiversity loss, their per capita ecological footprint is actually higher. Compare the chart below to the chart above (the green line represents the world's total biocapacity). Note that the per capita consumption of wealthy nations more or less tracks with the global economy. Dips in consumptions are seen following the oil crises of the 1970s, and recessions in the 1980s and 2000s. Though they're followed by increases as economies have recovered.

"India, China and the USA – the three countries with the highest water footprint of production – also contain 8 of the top 10 most populous basins experiencing almost year-round water scarcity," says the report.

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That's not just a problem for the populations there, because the majority of our water is used for agriculture. The US, for example, is the world's largest exporter of cereal crops. Recent droughts have meant lower crop yields, which impacts not just food prices, but also food availability. As climate change exacerbates these problems, it will be a problem for importing countries that rely on these water-intensive crops to meet their basic needs.

Taken together, the trends described by the report are absolutely sobering. We've overutilizing the resources provided by our planet in a way that's not only unsustainable for our own needs but is rapidly diminishing the habitat available for wildlife.

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But there is a silver lining, which is that in many cases these losses are not irreversable. While many of the populations assessed by the WWF have decreased, most have not decreased to the point where extinction is an inevitability. And indeed, there are some places where local communities have rallied around conservation- and biodiversity-related causes. For example, while big cat numbers are down worldwide, they've actually increased roughly fivefold over the last decade in Nepal. That's thanks to grassroots activism and increased education, but especially due to ecotourism. Not only does wildlife tourism allow animals to be worth more alive than dead, but it also encourages the preservation of large swaths of habitat, and results in the creation of jobs. A similar story is told for the mountain gorillas of Uganda and Rwanda. The numbers are dire, but there is reason to be optimistic.

All graphics via WWF's Living Planet Report 2014; Header photo of an African Lioness taking a swipe at remote camera, Masai Mara National Reserve, Kenya, copyright naturepl.com / Anup Shah / WWF-Canon.