Gap analysis:  A geographic approach


Scott, J. M., F. Davis, B. Csuti, R. Noss, B. Butterfield, C.
Groves, H. Anderson, S. Caicco, F. D'Erchia, T. C. Edwards, Jr.,
J. Ulliman, and R. G. Wright.  1993.  Gap analysis:  A geographic
approach to protection of biological diversity.  Wildlife
Monograph No. 123.  Reprinted by U.S. Fish and Wildlife Service,
Environmental Management Technical Center, Onalaska, Wisconsin,
May 1993.  EMTC 93-R010.  41 pp.   (NTIS #PB94-123148) 


ABSTRACT

The conventional approach to maintaining biological diversity
generally has been to proceed species by species and threat by
threat.  We suggest that piecemeal approaches are not adequate by
themselves to address the accelerating extinction crisis and,
furthermore, they contribute to an unpredictable ecological and
economic environment.  Here, we describe a methodology called Gap
Analysis, which identifies the gaps in representation of
biological diversity (biodiversity) in areas managed exclusively
or primarily for the long-term maintenance of populations of
native species and natural ecosystems (hereinafter referred to as
biodiversity management areas). Once identified, gaps are filled
through new reserve acquisitions or designations, or through
changes in management practices.  The goal is to ensure that all
ecosystems and areas rich in species diversity are represented
adequately in biodiversity management areas.  We believe this
proactive strategy will eliminate the need to list many species as
threatened or endangered in the future.  Gap Analysis uses
vegetation types and vertebrate and butterfly species (and/or
other taxa, such as vascular plants, if adequate distributional
data are available) as indicators of biodiversity.  Maps of
existing vegetation are prepared from satellite imagery (LANDSAT)
and other sources and are entered into a geographic information
system (GIS).  Because entire states or regions are mapped, the
smallest area identified on vegetation maps is 100 ha.  Vegetation
maps are verified through field checks and examination of aerial
photographs.  Predicted species distributions are based on
existing range maps and other distributional data, combined with
information on the habitat affinities of each species. 
Distribution maps for individual species are overlaid in the GIS
to produce maps of species richness, which can be created for any
group of species of biological or political interest.  An
additional GIS layer of land ownership and management status
allows identification of gaps in the representation of vegetation
types and centers of species richness in biodiversity management
areas through a comparison of the vegetation and species richness
maps with ownership and management status maps. Underrepresented
plant communities (e.g., present on only 1 or 2 biodiversity
management areas or with a small total acreage primarily managed
for biodiversity) also can be identified in this manner.  
Realization of the full potential of Gap Analysis requires
regionalization of state data bases and use of the data in
resource management and planning.  Gap Analysis is a powerful and
efficient first step toward setting land management priorities. 
It provides focus, direction, and accountability for conservation
efforts.  Areas identified as important through Gap Analysis can
then be examined more closely for their biological qualities and
management needs.  As a coarse-filter approach to conservation
evaluation, Gap Analysis is not a panacea.  Limitations related to
minimum mapping unit size (where small habitat patches are
missed), failure to distinguish among most seral stages, failure
to indicate gradual ecotones, and other factors must be recognized
so that Gap Analysis can be supplemented by more intensive
inventories.  


KEYWORDS

Gap Analysis, GIS, biodiversity, species richness, species
diversity, land cover, Landsat