Biologists: Wildfires give habitat, northern Colorado bighorn sheep a boost

(Photo by Bob Drieslein)

BOULDER, Colo. — After struggling for decades, the bighorn sheep population in the Big Thompson and St. Vrain canyons finally appears to be stabilizing. While it is difficult to pinpoint the exact cause, wildlife biologists at Colorado Parks and Wildlife have ironically linked at least part of the herd’s success to wildfire.

“Sheep typically don’t like closed in areas with a forested canopy so when you have major landscape changes such as a fire, it’s usually pretty beneficial for the sheep,” said Ben Kraft, a wildlife biologist for Colorado Parks and Wildlife. “The Big Elk Fire and the fire on Kenny Mountain (in 2002) certainly benefited these sheep.”

Once so abundant in the late 1800s, bighorn sheep were declared Colorado’s state animal, but by the early 1900s the population was nearly wiped out due to unregulated hunting and the spread of diseases from an increasing number of livestock located close to their habitat along the Front Range, the Daily Camera reported.

By 1940, the population of historical herds in the Big Thompson and St. Vrain canyons had dropped to such a point that Colorado Parks and Wildlife, along with the Rocky Mountain Bighorn Society, began transplanting sheep from the more robust herds deeper in the mountains to repopulate those along the Front Range.

Since then, more than 100 transplants have taken place throughout the state, including 26 sheep that were transferred to the Big Thompson and St. Vrain canyons from the Mummy Range in 1987 and 22 from Georgetown in 2000.

While the transplants appeared to work extremely well at first, bringing the herd back to around 100 animals, a large die-off, due to pneumonia likely contracted from domestic sheep, decimated the population in the late 1990s, prompting a second transfer. Since additional habitat was created after the wildfires in 2002, however, the Big Thompson and St. Vrain herds are able to spend more time in the high mountain regions and away from diseased livestock, allowing them to recover quite well.

“When disease comes into a bighorn population you have the potential for all age die-offs for up to five years and after that, you can see lamb recruitment really fall off,” Kraft said. “Sometimes that reduced lamb recruitment can last for several decades.”

Despite their recent recovery, Parks and Wildlife continue to survey the herds each winter to monitor their population and look for signs of disease.

For the most part, the population surveys are a lot of sitting and waiting, trying to stay warm in early morning sunlight, scanning steep hillsides with a pair of binoculars, looking for any signs of movement or the sheep’s distinctive white butts. Then, all of the sudden, the majestic creatures appear as if emerging from the rocks themselves and roam the sheer cliff faces with their lambs in search of food or a late-season mate.

Along with determining the age of each animal based on the color of their coat and the size of the horns, the key to the surveys is determining the ratio between lambs and ewes, so biologists can accurately predict future populations.

If the ratio is high, Parks and Wildlife will issue additional hunting permits for both rams and ewes in an attempt to slow the growth of the herd and limit the animals’ need to sprawl outward and risk interaction with humans or livestock. If it’s too low, Parks and Wildlife will reduce the number of permits and, if observers suspect disease is to blame for the drop-off, they might try to remove certain sheep from the herd.

For the last five years the population of the Big Thompson and St. Vrain herds has remained fairly stable at 60 animals. This year the lamb-to-ewe ratio was calculated to be 70-to-100, which is high, suggesting the health of the herd has continued to improve.

“The take-home message is we’ve got a stable herd out there thanks to very carefully managed hunting, but that there is probably room to grow,” said Janet George, the Senior Wildlife Biologist for Colorado Parks and Wildlife’s Northeast Region. “The area could support over 100 animals in total and we’re hoping that we get there at some point, but a lot of times the population trend is more important than the total numbers. There’s also evidence of lion predation that may be contributing to the reduced size of the herds.”

The best place for those looking to view the bighorn sheep is Big Thompson Canyon, as the narrow canyon limits the area in which they can hide, but sheep can often be seen in the St. Vrain Canyon near Kenny Mountain.

“The best way to view sheep is from a high vantage point across the valley,” George said. “In the fall you can even catch the males butting heads if you’re lucky. They are truly incredible animals.”

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