Steve Brill who many New Yorkers know as "the Wildman" has been doing these tours of Central Park vegetation and at one point was even an outlaw. In 1986, he was arrested for eating a dandelion. Think about that, 1986, all sorts of crimes were going down, and this guy got arrested for eating a plant!
He began his walks on nature in Central Park in 1981. The City's Parks Department ultimately hired him to do the walks he was arrested for. He still provides nature walks to this day, for which you can get more information about on his site.