Bison herd at Rocky Mountain Arsenal refuge gets more room to roam

COMMERCE CITY — Federal wildlife managers whose herd of 89 bison is exceeding capacity rounded them up Tuesday for health tests and returned 74 to habitat that is 28 percent larger.

And U.S. Fish and Wildlife Service officials said by the end of 2016, they will double the size of the Rocky Mountain Arsenal National Wildlife Refuge habitat to 6,373 fenced acres, letting bison roam grassland likely visible from the road to Denver International Airport.

A four-year plan aims at sustaining a herd of 180 to 210 bison on 12,000 acres — if enough native grasses can be coaxed to replace thriving weeds.

“The bison like to have space. They’re curious. And, generally, when we allow them new space, they go out and get used to it and build it into their routines,” refuge manager Dave Lucas said on a catwalk above bison banging in a pen. “They want to know where their boundaries are.”

Government biologists recently released 30 captive-bred endangered black-footed ferrets at the refuge, where badgers and foxes have taken up residence, along with raptors and prairie dogs, able to crawl under 18-inch gaps in bison fencing.

And refuge planners are continuing a focus on declining species, preparing to introduce plains sharp-tailed grouse and lesser prairie chickens. Another Fish and Wildlife project in the works will add pronghorn antelope to the mix.

But bison, drawing 330,000 visitors so far this year, still anchor the 27-square-mile arsenal refuge — 8 miles from downtown Denver — which once ranked among the nation’s worst environmental disasters, a site where the Army made chemical weapons and Shell produced pesticides.

For decades, weapons and pesticides production leaked toxic chemicals that contaminated soil and water, forcing a $2.1 billion cleanup. Fish and Wildlife since 2007 has been working to establish a wildlife refuge. A fenced 1,084-acre area, where waste is buried beneath a clay cap, remains under Army control.

Restoring prairie grasses is proving difficult, with land managers using controlled fire and herbicides to promote growth.

Each foraging bison eats about 40 pounds a day of dry grass. Rainfall this year helped. Refuge crews worried during recent droughts but now face weeds.

They’ve been expanding bison habitat by installing 8-foot-high steel fences. With the addition of 700 acres Tuesday, the current fenced habitat covers 3,200 acres and links west and east sides of the refugee and is seen as sufficient for 64 bison. By the end of next year with 6,373 fenced acres, the herd is expected to grow to between 88 and 120 bison.

The ultimate goal is up to 210 bison roaming 12,000 acres.

The wildlife crews Tuesday plucked 15 calves for removal to a bison preserve in Nebraska, anticipating 14 more calves soon will be born north of Denver. They’re testing bison genetics and blood as part of health monitoring.

“Realistically, our goal is to create a short and mixed-grass prairie here,” Lucas said. “We’re going to try to get as close as possible to a native prairie again. That’s a very high bar.”

Bruce Finley: 303-954-1700, bfinley@denverpost.com or @finleybruce