NEPA--Environmental Assessment

equires that a cause and effect relationship be established that is not merely speculative or based on
remote possibilities. Principals of population ecology using the concept of species limiting factors
as they relate to reproduction, growth, mortality rates, and distribution of MIS are applied whenever
possible.
Most MIS species analyzed in detail have either been observed or reported on NFS lands in the
Silverton Landscape. Additionally, most MIS species have habitat that is well distributed across the
San Juan National Forest, with the exception of Colorado River cutthroat trout and Uncompahgre
fritillary butterfly. It should also be noted that within and adjacent to NFS lands in the Silverton
Landscape there are large amounts of habitat in similar condition, and this habitat is well distributed
across the landscape and connected to the larger National Forest administrative unit. The Silverton
Landscape does not provide unique or isolated habitats within which discrete populations are
restricted. Aside from Canada lynx and southwestern willow flycatcher, most MIS are not species at
risk nor are they species that are trending towards protected status and are well distributed across the
San Juan National Forest.
For some species, such as mule deer and elk, there appears to be no relationship between habitat
trends and population trends, with population trends regulated by State hunting season structures.
For other species, such as Canada lynx and river otter, population trends are dependant primarily on
the results of Colorado Division of Wildlife reintroduction efforts, rather than the amount or
distribution of habitat on National Forest System lands. For other species, such as deer mouse,
populations are highly variable and regulated primarily by local annual weather patterns, rather than
the amount or distribution of habitat. Again, MIS regulations do not apply to non-National Forest
System lands and thus this analysis will be limited solely to those species and acres of habitat that
occur on National Forest System lands.
AFFECTED ENVIRONMENT
There are 23 species identified as MIS in the SJNF Forest Plan. Some species are not present in the
Silverton Landscape due to the absence of suitable habitat, or, suitable habitat is present in the
landscape but the proposed action (domestic sheep grazing) will not affect the species or its key
habitat components. Table C-3 (Appendix C) provides rationale for why some MIS species were
brought forward for detailed project analysis and other species were not. A total of five species have
habitat present in the Silverton Landscape and may be affected by domestic sheep grazing (Table 3-
1, below). The five species brought forward for detailed analysis for this range analysis project
include: Canada lynx, elk, mule deer, green-tailed towhee, and mountain bluebird. Affects of
domestic sheep grazing to federally listed species (Canada lynx) was also discussed in the project’s
Biological Assessment. Affects to Forest Service designated Sensitive Species were also discussed
in the project’s Biological Evaluation.
Existing habitat for each MIS was determined by the use of a computer mapping model 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 across the Forest, professional judgment of FS
wildlife biologists, coordination with CDOW biologists, coordination with the U.S. Fish and
Wildlife Service, and field reconnaissance of the project area was also used.
A brief description of suitable habitat for each MIS species brought forward for detailed project
analysis, their likely period of use, and Forest-wide habitat and population trend estimates for each
of these MIS species are provided in the Wildlife Review (Schultz 2009c) in the project record.
More detailed information about species natural history requirements, distribution, and habitat and
population trends is on file at the Columbine Ranger District Office located in Bayfield, Colorado
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