The population of planet Earth reached 8 billion people late last year. By the year 2100, we're headed for 2 billion more.

What does that mean for us and our planet?

Elizabeth Hadly is a professor of biology at Stanford University, and director of the Jasper Ridge Biological Preserve in California's Santa Cruz Mountains.

For four decades, she's been an eyewitness to dramatic changes in the plant and animal kingdoms caused by human beings.