watervulnerability

Sawtooth National Forest Watershed Vulnerability Assessment, Intermountain Region (R4)
management activities that cumulatively impact habitat annually. This assumption is supported by studies
that found that populations in complex habitats are more stable than populations in simple ones because
they have greater capacity to buffer the effects of environmental change (Schlosser 1982; Saunders et al.
1990; Sedell et al. 1990; Schlosser 1991; Pearson et al. 1992). Neville et al. (2006) also showed that
small, isolated populations were at increased risk of extinction because of demographic and genetic
factors associated with their reduced population sizes and loss of interpopulation connectivity.
There are, however, limitations with this approach, as follows
1. Bull trout may persist in streams that commonly exceed their perceived thermal limits (Zoellick
1999) because of increased availability of food, lack of competition with other species, or
adaptations that better exploit thermal refugia or shift timing of life history transitions (Crozier
and others 2008; Jonsson and Jonsson 2009).
2. Baselines and management threats were assumed to remain at present levels. In reality, some
threats will diminish due to restoration or changed management approaches, some will persist due
to a lack of political/social will to change, and new unexpected threats will emerge. As a result,
baseline conditions will also not stay constant.
3. It was assumed that species and populations will continue to use and respond to the environment
as they have in the recent past. In some instances, biological adaptation to changing environments
could mitigate some of the challenges organisms face.
4. Finally, there are many complex interactions between physical changes brought on by climate
change and species’ responses to these changes. While the model is a good start, it oversimplifies
these interactions and may inaccurately project future persistence.
Figure 15. Bayesian belief network for determining bull trout population persistence
Currently there are 14 patches in the Upper Salmon on the Sawtooth RNA that have reproducing bull
trout populations. Bull trout in three of these patches are “functioning at unacceptable risk”, six patches
are “functioning at risk,” and six are “functioning appropriately.” Populations in unacceptable or at-risk
conditions are due to low population sizes, competition/hybridization risks with brook trout, poor habitat
conditions, and/or moderate/high management risks. Bull trout populations in a better condition are
characterized by relatively good habitat, larger populations, low to moderate management risks, and/or no
brook trout present.
174 Assessing the Vulnerability of Watersheds to Climate Change