RUSSELL SPRINGS -- Prairie dog populations at the main black-footed ferret reintroduction site are so low that poisoning might be limited to the 30-foot grass barriers that ring much of the 10,000-acre ranch.

Typically, an additional 600 to 700 acres of grass are poisoned annually to help reduce the density of prairie dog populations and, as a result, the pressure to flee onto adjoining lands.

Drought and natural forces, however, have combined to reduce the number of prairie dogs on the Haverfield-Barnhardt complex that Larry Haverfield oversees.

Overall, Haverfield is estimating the 10,000-acre complex only has about a fourth of the prairie dogs it had when the U.S. Fish and Wildlife Service agreed to start releasing the highly endangered black-footed ferrets on the ranch.

He suspects the existing prairie dog population had only a fourth of its normal reproduction, the result of dry conditions that were so harsh the animals were forced into an almost unheard of hibernation to survive the weather.

Wildlife biologists had expected reproduction rates to fall, but they didn't know by how much.

FWS also has released ferrets on the Smoky Valley Ranch owned by the Nature Conservancy, and the numbers there -- for both prairie dogs and ferrets -- are markedly lower.

"They'll use the least poison they ever have this fall," Haverfield predicted.

This year, he's suggested the prairie dogs within the 30-yard-wide barriers be poisoned as soon as possible.

"And then wait until December and come back and do 30 yards inside the barrier," Haverfield said.

That would mean poisoning back prairie dogs along a 60-yard-wide border.

The vegetative barriers are an area that has been cordoned off from grazing cattle, allowing the grass to grow and serve as a visual barrier to migrating prairie dogs.

Haverfield has 25 miles of vegetative barriers on the ranch, adjacent to adjoining landowners.

There's another 10 miles of specially designed fences that were installed by the Audubon of Kansas.

"The fence, it does help," he said. "Especially if you get a little bit of trash and stuff that blows up in there. If everything's clean, they go under the wire."

Even blowing thistles -- small enough not to take out the fence -- are enough to serve as a barrier, Haverfield said.

He also contends the vegetative barriers are working.

"It looks to me like 30 yards is adequate," he said, especially if the animals brave enough to venture into the taller grass are poisoned.