For perhaps the first time, an environmental group, the Nature Conservancy, has landed among the top 10 of U.S charities ranked by income. The conservancy sat at No. 9 on a listing by the NonProfit Times, with a total income in 1999 of $704 million. The group claims it devoted more than 93 percent of the money to its mission — acquiring and managing land important to biodiversity — even though the Internal Revenue Service does not interpret things that way. The National Audubon Society, meanwhile, fell from the list of the top 100 charities.