Shutterstock The red panda is ranked "vulnerable" by the International Union for Conservation of Nature.

From red pandas to golden-striped salamanders, Earth's wildlife is in trouble.

Many scientists believe our planet is in the early stages of a mass extinction, an event defined by a loss of 75% of species on Earth. It will be the sixth one to occur in the planet's 4.5 billion year history — and the first to be caused by humans.

But just how fast are species disappearing from Earth, and how much should we be worried?

Information recently compiled by the journal Nature, the World Wildlife Fund, and the International Union for Conservation of Nature (IUCN) sheds some light on these questions. It's not a pretty picture.

Drawing from the IUCN's "Red List," a catalogue of species considered in danger of extinction, Nature recently published a detailed analysis of threatened animals on Earth. The report concluded that 26% of all known mammals, 13% of birds, and 41% of amphibians are in jeopardy. Scientists don't have enough data for fish and reptiles to make an assessment for them, and insects got off comparatively easy — an estimated 0.5% of known species are thought to be facing extinction.

But these are just the species that we know of. There are about 1.7 million species of animals, plants, and fungi that humans are aware of, but scientists estimate there are millions more yet to be discovered, and we have no idea what kind of shape their populations will be in if we ever do discover them before they die off.

Wikimedia Commons The Andean flamingo is threatened by habitat destruction.

And there's more bad news where that came from.

Scientists aren't completely sure how fast all these species are disappearing from the planet, but the fastest estimates — which suggest 690 extinctions take place every week — indicate that the mass extinction could be complete in the next 200 years. (Slower estimates give us several more hundred years before 75% of life on Earth is gone, and the most conservative guesses allow us thousands.)

In fact, research from the World Wildlife Fund suggests that the number of vertebrates on Earth (excluding humans) is only half what it was 40 years ago.

The Living Planet Index, an assessment of vertebrate populations, shows that between 1970 and 2010, terrestrial and marine vertebrate populations both declined by 39%, and freshwater vertebrates declined by a whopping 76%. Altogether, the total rate of decline for vertebrates was 52%, meaning their populations have been cut in half since 1970.

AP Photo/Andre Penner Deforestation in Brazil. Habitat destruction is a major threat to animals on Earth.