The World Wildlife Fund, an international conservation group, says more than two-thirds of the world's wildlife could be gone by the year 2020.
The organization says there's already been a 58 percent decrease in the number of fish, mammals, birds and reptiles since 1970. The number of elephants as we've reported has dropped drastically, in part because of poaching. And the WWF says overfishing could make a third of the planet's sharks extinct.
Looking at wildlife as a whole, one major reason for all this is habitat loss. A WWF conservation scientist says it's because people are using so much of the planet and destroying so much of the animals' habitat. His organization suggests more government laws protecting the environment and that people look for more sustainable products and renewable energy.
But not all scientists totally agree with the WWF report. A conservation ecology professor at Duke University says it's true that wildlife is declining, but he called some of the WWF's numbers sketchy and said its research is flawed, in part because most of it focuses on Western Europe.