INHS Reports September-October 1998
Mapping the Diversity of Amphibians and Rep tiles in Illinois
Because Illinois is at the crossroads of several different biogeographic
realms, it possesses a rich diversity of amphibians and reptiles. The
Illinois
Natural History Survey (INHS) and the Museum of Natural History (MNH) at the
University of Illinois have an extensive repres entation of this diversity in
their scientific collections. There are approximately 11,000 preserved
specimens, collected from 1842 to the present, in these two collections.
These
collections are invaluable because they allow us to track individual species
and overall species diversity through time and across a landscape that has
been
dramatically changed by agriculture. To mainta in and potentially enhance
this
diversity, it is important to know where each species exists and its habitat
requirements.
One of the goals of the National Gap Analysis ( GAP) program is to identify
gaps
in the representation of the nation's biodiversity. Using geographic
information system (GIS) technology, we can use digital maps to identify
individual species, species-rich areas, and vegetation types that are
unrepresented or underrepresented in existing protected areas. Specifically,
the GAP program in Illinois involves vegetation mapping, identifying land
stewardship management, and mapping distributions of mammals, birds,
amphibians, and reptiles. Currently, we are concentrating our efforts on
mapping the locations of Illinois specimens in the INHS and MNH amphibian and
reptile collections.
Our approach involves identifying a location for each specimen based on
information collected with the specimen. Each specimen may have a "common"
location description (e.g., 2 miles east of Champaign) or a legal description
(e.g., section 6 of township 34 north, range 5 east), or a town name (e.g.,
Bement). Using our GIS and computerized U.S. Geological Survey quad maps
called digital raster graphics (DRG), we are able to locate roads, streams,
and
other landmarks used in the location description to determine the collection
site. We also have a computerized map of the township, range, and section
information for the entire state.
Western chorus frog,
Pseud-
acris triseriata.
Using GIS, we can overlay this map layer
onto
the DRGs to determine the collection site from the legal description. When
only a town name is available as the collection site, we use our GIS to match
the location of the town hall in our computerized database of town names with
the town name in the collection database. Using these various methods and
employing GIS technology enables us to build a map of collection sites for
all
the amphibian and reptile specimens. Additionally, we assign an accuracy
code
that reflects how sure we are of our location assignment for each
specimen.
Each specimen collection site falls within a 63 5-km2 hexagon,
which
is the mapping unit used by the GAP program. Hexagons were chosen over
counties because they are equally sized and easily connect to adjacent
states.
It is within these hexagons that we determined species richness for
amphibians
and reptiles.
To build a species richness map, which is the first step in identifying areas
of high species diversity, we use GIS and overlay the collection sites onto
the
GAP hexagons. To determine species richness, we sum up the number of
different
species that occur in each hexagon. From the species richness map we can
identify species as well as groups of species that are not represented in the
current network of management areas.
Species richness maps based on preserved specimens in natural history
collections can misrepresent species diversity because not all areas of
the
state were visited with equal frequency by collectors. We know, for example,
that collection locations are often clustered around large cities,
universities, or outstanding natural features. Some hexagons in the species
richness map may have never been visited by collectors. In addition, there
are
problems with the scale at which the data are displayed. The hexagons cover
a
large area, which may not adequately represent the diversity of a small pond
in
someone's backyard.
It is important to be aware of these limitations; however, the ability to map
the species richness of amphibians and reptiles for the entire state is a big
accomplishment. Our next step will be to use our vegetation mapping and
management practices on public lands to identify areas throughout the state
that need better management to preserve the biodiversity of amphibians and
reptiles in Illinois.
Jocelyn Aycrigg, Center for Wildlife Ecology; Chris Phillips, Center for
Biodiversity; Tim Maples, Center for Wildlife Ecology; and Tom Kompare,
Center
for Biodiversity
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