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A Gap Analysis of the Lewis & Clark 
Trail: Fort Peck Dam to Great Falls, 
Montana

PATRICK CRIST and MICHAEL JENNINGS
National Gap Analysis Program, Moscow, Idaho

In the summer of 1999, the National GAP office was contacted by
the Department of the Interior to see what type of analyses we could
produce for a portion of the Lewis & Clark Trail. We previously
had produced a stewardship map of the western half of the trail, and
now we were charged with conducting a gap analysis and prelimi-
nary biodiversity core reserve identification project in three weeks
time. The data source was Montana GAP and included land cover
derived from TM satellite imagery, predicted animal species distri-
butions, and land stewardship. The data layers were then clipped to
the study area boundary, which was essentially a 20-mile buffer on
each side of the Missouri River from Fort Peck Dam to Great Falls,
Montana (Figure 1).

• The species is a terrestrial vertebrate mapped by Montana GAP.

• Less than 10% of the species’ distribution in Montana falls on
current reserve lands (GAP status 1 or 2).

• Species are state-threatened status S–3 or lower.

Thirty-six terrestrial vertebrates were identified that met the crite-
ria above. The species distributions were clipped to the study area
boundary and then combined for the reserve selection process.

The reserve selection demonstration used an iterative
“complementarity analysis” with the objective of finding the com-
bination of places that represent at least one occurrence of every
species included in each taxonomic group. Jason Karl of the Uni-
versity of Idaho’s Landscape Dynamics Lab developed the ARC/
INFO programming to conduct the analysis. This began with se-
lection of the largest land cover patch containing the most at-risk
species. Then the next largest patch containing the most at-risk
species was selected, and so on until a collection of potential re-
serve locations was identified to represent all at-risk terrestrial ver-
tebrates in each major taxonomic group. Time constraints did not
allow further analysis that would incorporate other considerations,
such as species viability, reserve connectivity and configuration for
metapopulations, or socioeconomic factors. Those additional analy-
ses would be included in a longer-term reserve selection and design
process.

The three maps (Figure 2) depict the preliminary proposed reserve
areas for the three taxonomic groups in relation to the four
biodiversity management categories used by GAP. Status 1 & 2
areas, shown in dark grey below, are existing reserves such as na-
tional parks and wilderness areas; status 3 areas, shown in light
gray, are public multiple-use lands; and status 4 areas, shown in
white, have no known management for biodiversity. The potential
reserves identified on these maps fall within status 3 & 4 lands.
They show opportunities for avoiding possible future conservation
crises by changing the management of these areas.

GapBulletin856-00.jpg 727x423

Figure 1. Study area boundary.

Because of the very limited time available, the next step was to
narrow the number of species considered from the large number
originally mapped for the area. The criteria for selection were:

GapBulletin857-00.jpg 541x229 GapBulletin857-01.jpg 367x151 GapBulletin857-02.jpg 539x239

Figure 2. Potential future reserves for a) birds, b) mammals, and c)
amphibians and reptiles.

Within the potential reserves identified, the U.S. Fish and Wildlife
Service is the largest owner of public lands. In this case the USFWS
would bear the largest agency responsibility for biodiversity stew-
ardship. Most of the potential reserves that were identified, how-
ever, fall within status 3 & 4 lands, suggesting the need for a change
in land ownership or management for these areas.

Conclusions
This project was a rapid gap assessment and reserve selection con-
ducted over three weeks. There are many limitations to the results,
but the project demonstrates the utility of Gap Analysis data and
methods to evaluate current biodiversity management status, iden-
tify biotic elements that require increased management and protec-
tion, and identify potential new reserve locations. A more robust
analysis would include the full complement of plant communities
and species mapped by GAP as well as “enduring features” and

evaluation of species viability, connectivity, and quality. Reserve
selection would incorporate evaluation of parcel-level land owner-
ship, specific management practices, and socioeconomic consider-
ations.

Acknowledgements
Project conducted by the National Gap Analysis office in coopera-
tion with Conservation Imaging, Inc. and the Landscape Dynamics
Laboratory of the University of Idaho Cooperative Fish and Wild-
life Research Unit. GAP data produced by the Wildlife Spatial
Analysis Lab, MTCFWRU, University of Montana (Redmond et
al. 1999).

Authors: Michael Jennings, GAP Director; Patrick Crist, National
GAP Coordinator; Philip Tanimoto, Conservation Imaging; Jason
Karl, Landscape Dynamics Lab; Jeremy Knudsen, GAP.

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