Bighorn Sheep Habitat
Weeds like leafy spurge and spotted knapweed, called "habitat transformers," alter habitat structure and forage, displacing wildlife.
In 2010, we identified the Spanish Peaks bighorn sheep winter range as critical habitat threatened by invasive species. Listed as a "species of conservation concern" due to declining populations, these sheep inspired us to act to restore their habitat.
Partners
Since 2010, the Grow Wild team 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
Custer Gallatin National Forest
Montana Conservation Corps
Montana Department of Transportation
Montana Fish, Wildlife, and Parks
Montana State University
Natural Resources Conservation Service
Private Landowners
14
YEARS
8
PARTNERS
$15,879
OUR INVESTMENT
$160,679
MATCHING FUNDS
Big Sky's Bighorn
Bighorn sheep are high-country ungulates that thrive on steep mountainsides and require a combination of four habitat elements:
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Ample wild grasses and forbs
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Reliable water sources
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Wide visibility so they can see predators
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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:
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Habitat loss due to human development and sprawl
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Spread of invasive species (noxious weeds)
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Conifer encroachment
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Overgrazing
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Climate change
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Competition with other grazing animals (both domestic and wild)
Winter Range
In 2010, we 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 team found high densities of noxious weeds (houndstongue, musk thistle, hoary alyssum, spotted knapweed, and others) that initially established on private property and quickly spread to adjacent public lands.
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.
Cheatgrass
A Serious Threat to Wildlife
Cheatgrass is the most widespead invasive plant in the western U.S., outcompeting native plants that sustain wildlife.
Cheatgrass is highly flammable and can double the risk of wildfires. It dries out early in the summer and can ignite easily. Cheatgrass also grows densely, which helps fires spread faster.
Habitat Improvement Efforts
Effective weed management requires ongoing progress assessment. By 2014, partners noted reduced noxious weeds but, with rangeland experts, found range conditions declining, posing challenges for bighorn sheep in winter.
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Utilization of existing desirable grass species is high, as almost no litter from previous year’s growth is present
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Production was estimated at only 1/6th of potential
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Only 25-30% of the kinds and amounts of potential native plants on the site
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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
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Downy brome was present in high densities in areas that should be occupied by desirable grass species, further indicating a downward trend.
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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.
Field Trial #1
In 2015, we created test plots to identify native grasses and forbs suited for steep, shallow soils to provide winter forage for bighorn sheep. The site was prepared and seeded that fall, and starting in 2016, annual evaluations measured plant density and the effectiveness of grazing exclosures.
In 2018, results were analyzed, and native species had low establishment.
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All species increased over time except for slender white prairie clover.
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Prairie coneflower, Lewis flax, and thickspike wheatgrass were the seeded species with the highest densities. They established well on the weedy, southern aspect.
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Protecting seeded species from grazing improved the density of all species.
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Cheatgrass, common mullein, musk thistle, and other weeds species re-established on the sites at approximately 70% canopy cover.
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Prairie coneflower was the only seeded species being grazed by wildlife.
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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.
Field Trial #2
The 2018 evaluation showed the 2015 treatment sites were 70% dominated by invasive species like cheatgrass, mullein, and musk thistle.
To address this, the study tests Indaziflam (Esplanade 200 SC), a pre-emergent herbicide that targets invasive seed banks by inhibiting cellulose biosynthesis in seedlings. This approach aims to suppress invasives, enabling native plants to regenerate and outcompete them. Residual control is expected to last 8 months, with site conditions and plant occupancy monitored throughout.
The goal is to restore native plant frequency, improve wildlife habitat, and reduce erosion, water contamination, and weed invasion.
Study Methods
Treatment sites, like the 2015 plots, are on the southwest slope north of the Big Sky Spur Road. Cheatgrass plots (1–5%, 6–25%, 26–50%, 51–100% canopy cover) were treated with Indaziflam to determine the maximum cheatgrass cover that allows native plant regeneration for land-use goals.
In fall 2022, plots were cleared of residual biomass, and Indaziflam was applied to mineral soil at three rates, replicated across cover classes. Untreated control plots were paired with treated ones for comparison.
Treatment and control plots will be surveyed pre-treatment to establish baseline species data. Over 2–3 years (2022–2024), species composition will be monitored using permanent line-point intercepts, comparing treated plots with controls to assess native species response to cheatgrass control with Indaziflam.
Unanswered Questions
Will the desired species fill in after the cheatgrass is controlled, or are there no desired species remaining to fill the gaps that cheatgrass occupied?
In the latter situation, revegetation by artificially seeding adapted native species may be required.
