The killing is scheduled to start on Oct. 1. It would be particularly destructive, argues the lawsuit filed by ecologists last week, because the Conata Basin in the Buffalo Gap National Grassland is also home to the endangered black-footed ferret, which eats prairie dogs.

The U.S. Forest Service in Denver had no comment on the lawsuit. The U.S. Fish and Wildlife Service and the Animal and Plant Health Inspection Service were also named as defendants.

Ranchers, plagued by a fifth year of drought, say the prairie dog population is growing at such a rapid pace that the animals are spilling onto already parched private grassland, leaving little for cattle that graze the area.

"We have really worked hard to have a plan which has allowed us to be good stewards of the land. We have really worked hard to provide a balance. It's a bad situation," said Mark Johnston, spokesman for South Dakota Gov. Mike Rounds.

Last month, the U.S. Fish and Wildlife Service removed the black-tailed prairie dog as a candidate for listing under the Endangered Species Act.

Plans were then put in place to carry out the shooting and poisoning in the Conata Basin, part of the desolate looking 600,000 acre grassland area in southwestern South Dakota that surrounds the Badlands National Park.

Hunting prairie dogs in the basin has been forbidden since 1998, but is allowed in the spring on other federal lands.