NEPA--Environmental Assessment

the other two species due to the concern regarding possible disease transmission between domestic
sheep and bighorn sheep.
Existing habitat for sensitive species was determined by the use of computer modeling using
vegetative information described in Forest-wide MIS Assessments on National Forest System lands.
Habitat modeling was conducted using habitat structural stage matrices described by Towry (1984).
In addition, information on species’ distribution, professional judgment of FS wildlife biologists,
coordination with Colorado Division of Wildlife (CDOW) biologists, coordination with the U.S.
Fish and Wildlife Service, and field reconnaissance of the project area was also used.
Bighorn Distribution
Rocky Mountain bighorn sheep were historically distributed across the mountainous portions of
Colorado and much of the San Juan Public Lands (Beecham et al. 2007). Desert bighorn sheep are
not known or thought likely to occur in the Silverton Landscape. The Rocky Mountain bighorn
sheep is designated a sensitive species in the U.S. Forest Service Rocky Mountain Region. This
designation is based primarily on potential threats to the long-term viability of bighorn sheep
populations in the Rocky Mountain Region, including diseases transmitted from domestic sheep,
lack of connectivity and/or loss of genetic variability (fitness) due to habitat fragmentation, habitat
loss, increased human disturbance on summer and winter grounds, competition for forage with
domestic livestock, and predation on small isolated herds.
Scientific observation and field studies demonstrate that contact between domestic sheep and
bighorn sheep is possible under range conditions. This contact increases risk of subsequent bighorn
sheep mortality and reduced recruitment, primarily due to respiratory diseases (WAFWA 2007). The
complete range of mechanisms and/or causal agents that lead to disease events in bighorn sheep
cannot be conclusively proven at this time and not all bighorn sheep disease events can be attributed
to contact with domestic sheep (Onderka and Wishart 1984, Aune et al. 1998). However, when
contact between wild sheep and domestic sheep and goats has been documented, the severity of the
wild sheep die-off is typically more pronounced (Aune et al. 1998, Martin et al. 1996). For these
reasons, it is prudent to implement management actions designed to reduce or eliminate the potential
for contact between wild sheep and domestic sheep.
The Silverton Landscape intersects the mapped range of two bighorn sheep herds, S71 the Animas
Canyon Herd, and S33 the Pole Mountain/Upper Lake Fork Herd. There is about 41,430 acres of
mapped overlap between these two herds and domestic livestock allotments in the landscape.
However, it is equally apparent that most domestic sheep grazing and trailing activities in the
Silverton Landscape occurs outside of currently mapped bighorn sheep summer range. Concern for
contact between domestic sheep and bighorn sheep outside of mapped summer range is low because
domestic sheep are present in the landscape only for about a two month period during summer, and
bighorn occurrence outside of mapped summer range is irregular and unpredictable.
The 2007 estimated population size of the Pole Mountain/Upper Lake Fork Herd was about 90
animals (George et al. 2008). Although not well documented, evidence from the Colorado Division
of Wildlife (Diamond 2005) suggests that the S33 Pole Mountain/Upper Lake Fork Herd
experienced a significant and widespread die-off in the late 1980’s. No specific disease monitoring
has occurred in S33, nor have these sheep ever been treated with fenbendazole, an inoculant against
respiratory disease. Based on the high level of recruitment observed in S33, it appears that this
bighorn sheep herd is not experiencing any additive mortality from predation. Habitat quality in the
unit is excellent (Beecham et al. 2007).
Monitoring by the Colorado Division of Wildlife, along with recent (2009) sighting reports indicate
this herd is in an expansion phase (Wait pers. comm.). CDOW believes it is likely that individual
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