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Using Ecological Systems as Land Cover Map Units for GAP


1Association for Biodiversity Information, Minneapolis, Minnesota, 2Kansas Biological Survey, University of Kansas, Lawrence

The U.S. National Vegetation Classification (USNVC), which is maintained by the Association
for Biodiversity Information (ABI), is used by the Gap Analysis Program (GAP) as the
classification standard for mapping existing vegetation. In the past, GAP projects have used
many methods to derive map units, from mapping associations (finer than alliances) to
combining alliances into coarser units that encompass different upper physiognomic units.
Different states may choose contrasting methods of combining alliances, which can lead to
problems when cross-walking map units across state borders.

One approach to potentially solve these problems is the use of Ecological Systems. While
mapping alliances continues to be the goal for vegetation classification for GAP, in some
instances this will not be possible. The use of ecological systems is being developed to maintain
consistency with the USNVC and to facilitate regionalizing GAP data across states. Use of
ecological systems is not a substitute for the alliance-level goal, and the finer-level alliance data
will always be retained as part of the GIS data sets.

Ecological Systems have recently evolved from several years of work by ecologists from ABI,
The Nature Conservancy, and the Natural Heritage Network. Systems are currently defined as
conceptual vegetation units, unified by similar ecological conditions and processes (e.g.,
disturbance), environmental features (e.g., soils, geology), environmental gradients (e.g.,
elevation, latitude), and broadly similar plant species composition. Ecological Systems can form
spatially contiguous units and provide a standardized classification structure for combining
associations or alliances that share ecological processes. Ecological Systems alleviate
inconsistent aggregation from state to state. They are similar in concept to “complexes” and
“compositional groups” but provide richer context and data because they are more directly linked
to alliances and associations. They are more intuitive and so will be readily understood by
outside partners and clients. The use of Ecological Systems in an individual state usually results
in fewer units compared to associations or alliances. For example, there are approximately 250
Ecological Systems for the Great Plains, Midwest, and Southeast regions (25 states), compared
to about 800 alliances and 2000 associations. The ecological construction of Systems renders
them as ideal map units for GAP projects that use remotely sensed satellite data or aerial
photography. Systems integrate ecological elements such that many animal and plant species
will be limited to one or a few Ecological Systems. Other vertebrates that use a wider variety of
Ecological Systems can be easily modeled using Systems. Thus, Systems provide a way to
“regionalize” these maps while still maintaining conservation units that are ecologically

Significant progress has been made towards the conceptual development and implementation of
Systems in the Midwest and Great Plains regions. However, the utility of Systems in developing
spatial units for projects such as GAP needs to be addressed in more detail. To that end we hope
to evaluate the use of Ecological Systems with the current Kansas GAP land cover map.
Ecological Systems could be a valuable tool in unifying state GAP maps across a region by
developing more ecologically meaningful map units.

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