Federal wildlife agents and rangers — the folks who protect U.S. public lands from poaching, endangered species smuggling, illegal off-road activity, and other no-nos — were in short supply long before Sept. 11. But now their ranks are even thinner, after scores of agents were re-assigned to serve as sky marshals or guard federal buildings in the nation’s capital. The shift has been largely clandestine, with the federal government declining to make a public announcement, give specific figures on how many officers have been pulled from land and wildlife posts, or address the resulting enforcement problem on public lands.