Bighorn Sheep Winter Range Project
Weeds like leafy spurge and spotted knapweed are known as "habitat transformers" because they change the physical structure and forage availability in habitats, eventually pushing resident wildlife out.
In 2010, we identified the winter range of the Spanish Peaks bighorn sheep herd as critical wildlife habitat compromised by invasive species. Montana Fish, Wildlife and Parks lists bighorn sheep as a "species of conservation concern," meaning those species for which population viability is threatened as evidenced by a significant downward trend in population or a significant downward trend in habitat capacity.
The combination of compromised habitat and a species at risk prompted the Alliance to take action to improve the winter range of our local and beloved herd of bighorn sheep.
Big Sky's Bighorn
Bighorn sheep are high-country ungulates that thrive on steep mountainsides and require a combination of four habitat elements:
Ample wild grasses and forbs
Reliable water sources
Wide visibility so they can see predators
Steep, bare slopes nearby for escaping danger
According to wildlife biologist Julie Cunningham, the Spanish Peaks herd is comprised of roughly 140-150 individuals, and is capable of producing trophy quality rams.
Bighorn sheep face a multitude of threats to their winter range:
Habitat loss due to human development and sprawl
Spread of invasive species (noxious weeds)
Competition with other grazing animals (both domestic and wild)
In 2010, the Alliance set out to identify wildlife habitat areas compromised by noxious weeds. The area northwest of Lone Mountain Trail and Highway 191 is well-known bighorn sheep winter range, and is considered critical habitat by Montana Fish, Wildlife and Parks.
The Alliance found high densities of noxious weeds (houndstounge, musk thistle, hoary alyssum, spotted knapweed, and others) that initially established on private property and quickly spread to adjacent public lands.
As abundant native forage is the critical factor in bighorn sheep survival during winter months, and the presence of noxious weeds results in fewer resources and space for critical native grass and forb production.
Since 2010, the Alliance has lead an annual effort to manage invasive species on the steep, rocky bighorn sheep winter range with the following partners:
Gallatin County Weed District
Gallatin National Forest
Montana Conservation Corps
Montana Department of Transportation
Montana Fish, Wildlife, and Parks
Montana State University
Natural Resources Conservation Service
Wild Sheep Foundation
Successful weed management includes assessing progress on an ongoing basis. While the partners were seeing a reduction in noxious weeds (in density, abundance, and status), it was important to see if the native vegetation was rebounding in their absence.
In 2014, with the help of rangeland experts, partners visited the site and concluded that current range conditions indicate a downward trend, unfavorable to the needs of bighorn sheep during winter months.
Utilization of existing desirable grass species is high, as almost no litter from previous year’s growth is present
Production was estimated at only 1/6th of potential
Only 25-30% of the kinds and amounts of potential native plants on the site
Seed bank of desirable grasses is likely non-existent due to repeated grazing (no or few seeds produced) and it's reasonable to expect the seed bank of desirable species is not adequate to maintain a sustainable native vegetation population
Downy brome was present in high densities in areas that should be occupied by desirable grass species, further indicating a downward trend.
Tree encroachment caused bighorn to concentrate in opening, exerting more pressure on critical plant communities and exacerbating the potential for invasive plant establishment.
This use was determined to be unsustainable. With that assessment came action.
In 2015, the Alliance established vegetation test plots to determine what native plants (grass and forbs) could compete on the shallow soils and steep hillside, and provide winter forage for bighorn sheep.
In the fall of 2015, the site was prepared and seeded with native grasses and forbs. Starting in 2016, test plots were evaluated annually for stand density (plants/ft²) to determine what species established best (# plants/ft2), and if grazing exclosures were effective.
In 2018, results were analyzed. As expected, native species had low establishment.
All species increased over time except for slender white prairie clover.
Prairie coneflower, Lewis flax, and thickspike wheatgrass were the seeded species with the highest densities. They established well on the weedy, southern aspect.
Protecting seeded species from grazing improved the density of all species.
Cheatgrass, common mullein, musk thistle, and other weeds species re-established on the sites at approximately 70% canopy cover.
Prairie coneflower was the only seeded species being grazed by wildlife.
Additional weed management for cheatgrass prior to seeding, or using a herbicide with residual to control cheatgrass and other weeds for a longer period of time, may improve seeded species establishment.
The Next Step
With data in hand, the Alliance is working with our partners to plan the next step.
With multiple agencies, public and private land, and complex ecosystem dynamics, there is no easy or inexpensive answer to improve the bighorn's winter range.
The Alliance is planning on continued noxious weed management on public lands, experiments looking at reducing cheatgrass, and working to connect with and educate private landowners located adjacent to the range.