NEPA--Environmental Assessment

Silverton Grazing Risk Assessment
The current San Juan National Forest Land and Resource Management Plan of 1983
(Forest Plan), amended in 1992, and the BLM’s San Juan/San Miguel Resource
Management Plan of 1984 (RMP), along with Allotment Management Plans (AMP) and
Grazing Permits, regulate the current numbers and type of livestock, dates of use,
salting, vegetation manipulation and other activities undertaken for the purpose of
grazing domestic livestock on public lands.
Domestic Sheep are the only livestock permitted to graze in this landscape, and their
principle forage areas are in the alpine zone. Alpine rangelands have been used for
grazing domestic sheep in this Landscape since the late 1800’s. Prior to government
control, sheep were herded in tightly grouped bands and continuously bedded in the
same location for several nights in a row, which resulted in large forage losses and soil
damage from trampling and excessive trailing. Some sites in the Silverton Landscape still
display these historic effects. There are no additional bands of domestic sheep being
grazed on adjacent or intermingled non-federal lands, in addition to the bands permitted
to graze on the Federal allotments under analysis in this document.
Rocky Mountain bighorn sheep are native to the Silverton Landscape. Desert bighorn
sheep are not known or thought likely to occur in the Landscape. For this reason, this
analysis refers only to Rocky Mountain bighorn sheep.
One of the primary management concerns associated with domestic sheep grazing in the
Silverton Landscape is the potential for physical contact between domestic sheep and
bighorn sheep. This contact increases risk of subsequent bighorn sheep mortality events
and reduced recruitment, primarily due to respiratory diseases (Western Association of
Fish and Wildlife Agencies 2007). The complete range of mechanisms and/or causal
agents that lead to disease events in bighorn sheep is still under debate, and not all
bighorn sheep disease events can be attributed to contact with domestic sheep (Colorado
Division of Wildlife 2009, Aune et al. 1998, Onderka and Wishart 1984). 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
(Colorado Division of Wildlife 2009, Western Association of Fish and Wildlife Agencies
2007).
This “Risk Assessment” analysis is focused on the “risk of contact” between bighorn sheep
and domestic sheep. No presumption is made that contact will lead to disease
transmission or a subsequent bighorn sheep mortality event. However, the assumption is
made that physical contact between bighorn sheep and domestic sheep results in an
increased risk to bighorn sheep of disease transmission and therefore it is prudent to
reduce the risk of contact and/or increase the distance and/or degree of effectiveness of
separation between the two species (Colorado Division of Wildlife 2009, Western
Association of Fish and Wildlife Agencies 2007, USDI Bureau of Land Management 1998).
The goal of this “Risk Assessment” is to provide the decision maker with an objective
evaluation of the risk of contact between bighorn sheep and domestic sheep in each active
and each vacant domestic sheep grazing allotment in the Silverton Landscape. The
decision maker will then use the results of this “Risk Assessment” as an important factor
of consideration in their decision regarding domestic sheep grazing in the Silverton
Landscape.
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