According to the Proceedings of the National Academies of Science, more than 40 percent of mammal species have experienced severe population declines over the last century, meaning that their range has shrunk more than 80 percent.Almost 200 species of vertebrates have gone extinct over the last 100 years, a rate of about two extinctions per year. That’s 100 times the historic rate. Previous mass die-offs have been caused by asteroids, volcanos and other natural catastrophes. But this one has been triggered by human population growth, development, and climate change, scientists have concluded.