Monarch butterflies are dying at an alarming rate around the world -- a looming extinction that could also put human life at risk. But we have just the thing to help save these insects, says author Mary Ellen Hannibal: citizen scientists. Learn how these grassroots volunteers are playing a crucial role in measuring and rescuing the monarch's dwindling population -- and how you could join their ranks to help protect nature. (You'll be in good company: Charles Darwin was a citizen scientist!)