Zebra mussels, poisonous giant toads and melaleuca trees are all invasive species which cause millions of dollars of damage and the elimination of native creatures all over the globe. Some say invaders are the biggest threat to our environment this century. Host Steve Curwood talks to Alan Burdick, senior editor at Discover Magazine, who has written a book on the subject. Burdick’s central theme “what is nature?” asks us to question our assumptions of whether natural landscapes even exist anywhere but in our own desires.