Do coyotes need "control' on the refuge?

  • Antelope fawn remains found on the Hart Mountain Refuge

    USF&WS photo
 

Note: this article is a sidebar to this issue's feature story.

Mike Nunn and Dan Alonso stop their rig on a punishing track in the southeastern corner of the Hart Mountain National Antelope Refuge.

They have sighted two female pronghorn, just dark dots on the landscape to untrained observers. The does head toward a distant ridge, accompanied by a single fawn. The men watch with satisfaction. It is June, several weeks after pronghorn calving season, and they have just spotted their first fawn of the season.

The coyotes that decimated this year's crop of newborn fawns, and last year's as well, are nowhere to be seen. But Nunn and Alonso can feel their presence; the men are in the second year of a two-year study to nail down the reasons for the high death toll among newborn pronghorn.

An explosion in coyote numbers is at the top of the list.

Fawn survival rates for Hart Mountain's pronghorn herd have been low since at least the beginning of the decade, and the size of the herd has dropped from 1,900 to 1,300 since 1990. Nearly all the killed fawns fell victim to coyotes.

The crash in antelope fawn survival rates isn't unique to Hart Mountain. It's occurring in the larger herd that ranges between Hart Mountain and the Sheldon National Wildlife Refuge in northern Nevada, and east to the Malheur Wildlife Refuge. The size of that herd has fallen from 7,200 to 5,300 since 1990, according to state and federal wildlife tallies. The decline is occurring wherever cattle graze in the Great Basin, Nunn says. Until recently, that was just about everywhere.

Most pronghorn does give birth to twins, nature's way of compensating for the natural high mortality of their fawns. Biologists believe a survival rate of 25 to 30 fawns per 100 does is needed to assure the survival of the herd. At Hart Mountain, fawn survival rates fluctuated from 14 to 44 fawns per 100 does between 1991 and 1994, then dropped to an all-time low of 0.8 per 100 does in 1995.

In 1994, shortly after he came to Hart Mountain, Nunn noticed that coyotes were visible everywhere. At the time there were plenty of pronghorn, as well.

But the following year, when the refuge recorded its record-low survival, only eight of an estimated 1,700 fawns survived. Monitoring of scent posts, aerial surveys of dens, and howling counts indicated that coyotes were unusually plentiful, occupying the refuge at a density of about one per square mile.

The Oregon Department of Fish and Wildlife had done a predator-control experiment on BLM land north of the refuge. The study, never published, involved killing half the coyotes in the area before pronghorn calving, then fitting fawns with radio transmitters at birth and following them in their first days. With predator control, survival rates increased from 14 to 80 fawns per 100 does.

Nunn saw no reason why that shouldn't work on the refuge. But when he announced plans in December 1995 to shoot coyotes from airplanes shortly before pronghorns gave birth, the response was instantaneous.

Environmentalists denounced the plan, and animal-rights activists threatened to sue.

More damaging was the response from veteran biologists familiar with wildlife population dynamics at Hart Mountain. Many believe recent low survival rates are part of a natural boom-and-bust cycle that brings balance to populations of pronghorn, coyotes and small mammals over time.

Former refuge manager Barry Reiswig says the arid refuge is at the very edge of the pronghorn's range and provides only marginal habitat even in the best years. Jim Yoakum, a pronghorn specialist now retired from the Bureau of Land Management, told Nunn he "flat didn't have the data" to launch a predator control program.

Some game biologists supported predator control as a way to restore balance to an unbalanced system. Nevertheless, within weeks, the agency announced that it would suspend its plan for two years while it gathered more data.

In spring 1996, biologists attached radio transmitters to 52 pronghorn fawns; all but eight died. Most were killed by coyotes. Refuge-wide, the survival rate after two months was only 17 fawns per 100 does. This past May, 52 newborn fawns again were fitted with transmitters. Eight survived until July. Biologists are still analyzing the causes of death.

Studying antelope to death

In their zeal to study the factors affecting the fawns, wildlife biologists using helicopters and nets inadvertently caused the deaths of several does and fawns this year.

In March, they captured 20 of the high-strung pregnant does with nets and flew them by helicopter to refuge headquarters, where state wildlife biologists were to examine them and draw blood for lab tests.

Fourteen of the pregnant does died within two to 10 days after their capture, victims of the syndrome known as "capture myopathy." Their unborn fawns died with them. The study was abandoned. "In retrospect, we just tried to do too much," Nunn admits.

Nunn and his staff also miscalculated pronghorn longevity and coyote distribution, as he now concedes. They estimated the life span of pronghorn at eight years; in fact, at Hart Mountain, many live 13 to 15 years.

That means the herd can sustain high mortality for a few years without its survival being threatened. They also overestimated the number of coyotes they would need to shoot.

But killing fewer coyotes won't placate opponents of predator control. And biologist David Dobkin, who helped develop the Hart Mountain plan, points out, "The question of fawn survival is not the issue. The issue is, what is the carrying capacity of the land? Even if you get high fawn survival, the habitat has to be there to support them."

That conclusion finds support in the 1994 management plan for the refuge. Based on research by Yoakum and Bart O'Gara, an expert on pronghorn-predator relationships, the plan says:

"A predator control program on the refuge during the 1950s and 1960s resulted in increased fawn survival. However, only slight increases in pronghorn populations were noted between 1955 and 1969. Apparently fawns that survived because of predator control died of other causes ... The cumulative effects of predation ultimately appear (to be) regulated by habitat quality."

This year, the survival rate rose to 31 fawns per 100 does. Biologists say that may allow the herd to maintain its size through next spring - if the winter is not too severe.

Nunn won't make a decision on reviving the predator control proposal until later this fall, but he believes he now has the data he needs.

"Scientists studied more than 30 factors that might account for low survival rates," he said. "The only thing that correlates with low pronghorn survival is high coyote numbers."

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