The world is facing the largest wave of plant and animal extinctions since the demise of thedinosaurs 65 million years ago. Surveys show that scientists regard extinction as one of the biggest environmental threats to the planet. They predict that 50 percent of all species may be lost during the 21st century, complicating efforts to feed Earth's growing population as well as develop new medicines. In the United States, environmentalists urge the government to take immediate steps to preserve biodiversity. But Congress remains divided over the Endangered Species Act, and business leaders say that the nation should focus on economic growth instead of setting aside habitat for species that are so similar to one another that the loss of some will have no discernible impact.