Scientists estimate there are 10 to 30 million plant and animal species on the planet, most of them unidentified. Each year as many as 50,000 species disappear. Most die off, Tilman says, because of human activity....... Scientists say wildlife extinction rates are soaring. The die-off, they claim, threatens the planet's web of life or biodiversity which sustains farming, forestry and oceans. At a Paris meeting last week scientists called on world leaders to catalog and save species. One of the speakers was University of Minnesota ecology professor David Tilman. He's known around the world for his research showing the effects...