Yet Sawhill says even that estimate may be much too low, as scientists are just beginning to catalog the nation's vast stores of insects and microscopic life forms. Every year, some 30 new species of plants are being discovered in North America, Sawhill says. That's the real point. The report is intended to help guide land managers and legislators in making informed conservation decisions. But the exceptional richness of the U.S. is in danger, with one-third of U.S. species at risk.
Aquatic life is particularly vulnerable.
Almost 70 percent of the nation's freshwater mussel species and more than half its crayfishes are in trouble, for example. The single biggest threat to species survival is loss of habitat. Alien species invasion, such as zebra mussels and Asian longhorned beetles, are second only to habitat loss as a cause of species declines. Yet there is still time to protect the nation's natural heritage, the report's authors note hopefully. About one-third of U.S. species appear to be at risk due to their rarity or threats from human activities.