After U.S. politicians on both sides of the aisle and scientists across the world raised the roof about a plan to close a wildlife conservation center in Virginia, the secretary of the Smithsonian Institute, Lawrence Small, said yesterday that he was withdrawing the proposal. Small had proposed to save the Smithsonian $2.8 million a year and shift some of the Conservation and Research Center’s work on endangered species and habitat to the National Zoo in Washington, D.C. But opponents to the plan said science would suffer if the center closed, and they succeeded in making the brouhaha a national story.