BOULDER, Colo. - A 20-year plan to thin the burgeoning elk herd in Rocky Mountain National Park could cost $18 million to kill some animals and disperse others, park officials said. An estimated 2,200 to 3,000 elk live in the park, overgrazing vegetation that is also important to other wildlife including songbirds, beavers and butterflies, biologists say. Elk numbers have escalated because the animals have few predators and no hunting is allowed in the park. The park's goal is a herd of 1,200 to 1,700 elk. Park officials outlined the proposed program and its estimated costs during a public meeting Monday. The park's favored plan would involve killing up to 700 elk annually for four years. After that, an additional 25 to 150 elk would be culled annually for 16 years. The costs would come from hiring extra staff or a contractor to shoot elk, building fences to protect vegetation, transporting carcasses, testing them for disease and processing the meat. "Doing something like this is not going to be cheap, for sure," said park Superintendent Vaughn Baker. "But we're talking 20 years." The park's preferred plan calls for killing elk at night with silencer-equipped guns in part to minimize disturbances to park visitors. Park officials said they recognize that some people are upset by the prospect of killing elk in the park. While most recognize that something needs to be done to manage the population, there are contentious disagreements over the best method, said park biologist Therese Johnson. "For and against wolves. For and against hunting. And we have heard from people who prefer fertility control to killing the elk," she said. Congress would have to approve any plan to allow hunting in a national park. A draft elk-management plan released last month did not suggest releasing wolves in the park, but park officials have said wolves would best meet environmental objectives and do the least damage. Any proposal to release wolves in Colorado would have to be considered by federal and state agencies and likely would meet strong opposition from ranchers and others. Some people at Monday's meeting expressed dismay at the thought of killing elk. Others questioned why the park waited so long to do something about the growing elk population. "Fewer elk are going to help all of us," said Wally Wedel, who owns a cabin near the park. Wedel said elk are crowing out deer and damaging private property. The park is accepting public comments until July 4. It has scheduled other public meetings this week in Loveland, Grand Lake and Estes Park.