NEPA--Environmental Assessment

CUMULATIVE IMPACTS
Global warming is a contentious issue with a great deal of uncertainty about what likely outcomes
might be. However, there is little doubt that plants and animals found almost exclusively in the
alpine zone may be the first to decline or go extinct as a result of changes in global climate. Most
predictions about global climate change predict a gradual loss of alpine habitats as treeline moves
upward in response to a generally warming climate. For white-tailed ptarmigan, the effects of global
warming have the potential to have far greater consequences than the combined effects of grazing,
recreation, mining, and other human impacts. As such, global climate change may be the most
serious threat to long-term persistence of ptarmigan populations in the Silverton landscape.
More immediate and localized threats to alpine species, including ptarmigan, wolverine, and bighorn
sheep, include mining, water development, and motorized and non-motorized recreation. While
alpine ecosystems are hardy and resilient to natural environmental factors, they are particularly
vulnerable to human related disturbances and may require decades to recover. Although substantial
progress has been made in developing techniques to restore damaged alpine landscapes, this
technology is still not capable of restoring alpine plant communities to their pre-disturbance
condition (Hoffman 2006).
One of the largest impacts on the environment in this landscape has been past mining activities and
other activities associated with mining, such as road building (see watershed section above). One
hundred years of surface and subsurface mining operations have likely contributed to substantial
cumulative effects to habitat for sensitive. Some of the impacts of past mining activities have been
reduced or mitigated through natural re-vegetation of formerly impacted areas, and substantial
improvements in downstream water quality have been achieved as abandoned mines have been
reclaimed. Bighorn sheep populations around active mining areas were likely heavily impacted by
miners hunting for food before hunting restrictions were adopted in the early 1900’s.
The Colorado Division of Wildlife (CDOW) continues to manage the S33 Pole Mountain/Upper
Lake Fork bighorn sheep herd for possibility of providing hunting opportunities. Between 1982 and
1990 CDOW issued between 2 and 5 ram tags annually. S33 was closed to hunting between 1991
and 2005, then re-opened in 2006 with 3 ram tags issued annually since then. Hunting success rate
averages 30% for this unit (Diamond 2005). No ewe licenses have ever been issued for this herd.
The CDOW opened S71 for hunting for the first time in 2007 with 1 ram tag issued annually.
As the number of off-highway vehicles (OHV’s) continues to increase on most roads in the Silverton
Landscape, the potential for disturbance to bighorn sheep using areas that are adjacent to popular
OHV routes also continues to increase each year. The continual annual increase in OHV use
observed over the past 5-10 years is likely to continue for the foreseeable future. Increased
motorized disturbance to bighorn sheep in places such as Minnie Gulch may force animals away
from preferred foraging areas and into areas with lower quality forage or areas where animals are
more vulnerable to predation, leading to increased predation or mortality.
White-tailed ptarmigan populations in the southwest Colorado ore belt, roughly between Telluride,
Silverton and Lake City, including most of the Silverton Landscape, are thought to not be self
sustaining. The cause of low ptarmigan populations appears to be cadmium toxicity induced renal
failure and brittle bones that significantly reduces adult female over-winter survivorship (Larison et
al. 2000). Cadmium naturally occurs in high concentrations in the Colorado ore belt and is readily
mobilized by mining. Cadmium is taken up by willows and biomagnified in the buds which are the
primary fall and winter food source for ptarmigan (Braun et al. 1976). Larison et al. (2000) found
adult female survivorship in the southwest Colorado ore belt was reduced by more than half causing
highly skewed sex ratios and low productivity. They also documented high rates of immigration
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