Eastern Indigo Snake

Using habitat modeling to guide
Eastern Indigo Snake conservation in Georgia

When
most people think of a “wide-ranging” species the first thing that comes to
mind is probably a grizzly bear or a jaguar, not a snake. However, the Eastern
Indigo Snake is capable of surprisingly extensive movements. Recent radio
telemetry research has shown that Eastern Indigo Snakes in southern Georgia may
travel almost five miles between overwintering and summer habitat and may cover
an area over five and a half square miles during a single year.

On a
pound-per-pound basis, that is equivalent to a human walking almost 200 miles
or covering 228 square miles.

When
it comes to conservation, wide-ranging species present a unique set of challenges.
These species often require separate habitats for different activities, such as
foraging, raising young, or overwintering. In order to protect the habitat that
these species depend on, one must not only protect many different habitats, but
large amounts of them. As today’s natural landscapes become smaller and more
fragmented, determining where to allocate resources for land protection is a
major challenge for conservation organizations and government agencies.

Eastern
Indigo Snakes in southern Georgia present such a challenge. This species is
dependent upon gopher tortoise burrows for overwintering sites and these
burrows are typically clustered on dry, open sandhills. During the summer, Eastern
Indigo Snakes also forage in flatwoods, bottomland forests, or around wetlands,
thereby requiring them to move off of the sandhills. This requirement for multiple,
often spatially separate habitats, combined with large home range sizes, make Eastern
Indigo Snakes very susceptible to habitat loss, fragmentation, and degradation.
Because of these, and other factors, Eastern Indigo Snakes were listed as
Threatened under the U.S. Endangered Species Act in 1978. Although factors,
such as over-collecting, were addressed by their listing, habitat related
threats continue to threaten Eastern Indigo Snakes throughout their range.

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Habitat fragmentation is a serious
threat to the Eastern Indigo

The
Orianne Society’s (TOS) Land Protection Program is dedicated to providing
protection and connectivity for Eastern Indigo Snake habitat in the Altamaha
River Drainage of southern Georgia. The Altamaha River Drainage is a stronghold
for Eastern Indigo Snakes in Georgia, providing quality habitat in the form of
dry, open sandhills and networks of flatwoods, bottomland forests, and
wetlands. However, determining where these quality habitats are remains an
obstacle to their protection. Fortunately, several tools have emerged that
allow researchers to model the distribution of suitable habitat using their
knowledge of a species’ ecology.

One
approach is the Habitat Suitability Index or HSI. Developed by the U.S. Fish
and Wildlife Service in the 1980’s, the HSI creates an index of habitat quality
(ranging from 0-1) using data from the scientific literature and expert opinion
from biologists. This index is typically used to create a grid-based map of
habitat quality. However, one important shortfall of this approach is that it
does not model habitat accessibility. For example, if a patch of good-quality
winter habitat is separated from a patch of summer habitat by a mile of
flatwood forest, an Eastern Indigo Snake could easily access that habitat. But
what if there is a four-lane highway or a cotton field within that one mile?
That summer habitat would no longer be accessible to an Eastern Indigo Snake.
Without access to summer habitat, the winter habitat no longer becomes suitable
as well. TOS therefore needed a tool that could model habitat suitability on
the basis of habitat quality at a given location and the accessibility of
additional habitat in the surrounding landscape.

To
help us with this task, TOS turned to the Landscape Ecology Laboratory at the
University of Massachusetts (UMass) under the leadership of Dr. Kevin
McGarigal. Dr. McGarigal’s lab has been involved in a range of projects focused
on habitat modeling, landscape change, and assessing conservation priorities
and actions. UMass Research Associate Brad Compton and Dr. McGarigal have
developed a model called HABIT@ (pronounced habitat) to provide a way of
incorporating habitat quality and accessibility, using both expert opinion and
empirical data collecting from the field. To develop a habitat model for Eastern
Indigo Snakes in the Altamaha River Drainage using HABIT@, TOS worked with Dr.
McGarigal, Mr. Compton, and graduate student Willem Sytsma, as well with biologists
from Georgia Department of Natural Resources.

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Homerange modeling

HABIT@
begins with grid-based Geographic Information System (GIS) data, such as
landcover type, canopy cover, roads, or soil type. These data sets are used to
first model the habitat quality of each grid cell based on the characteristic
of that cell. For example, a cell containing sandhill soil, low canopy cover,
and no roads would be considered high quality winter habitat for an Eastern
Indigo Snake. Because Eastern Indigo Snakes use different habitats in the
winter and the summer, this process was done separately for both seasons to
produce separate habitat grids. In this modeling process, grid cells with no
sandhill soils were automatically assigned a value of zero to reflect the
necessity of sandhills and tortoise burrows for Eastern Indigo Snakes in
Georgia.

At
this point, the model is similar to a standard HSI. The next step is what distinguishes
HABIT@. The winter and summer habitat grids are summarized into what is called
a Homerange Capability (HRC) grid, which is defined as the capability of an Eastern
Indigo Snake home range centered on a given grid cell to provide all the
resources an Eastern Indigo Snake in that home range would need. The HRC uses a
concept called a resistance kernel to simulate an Eastern Indigo Snake home
range over every grid cell and then assigns each cell a value based on the
suitability of all the cells in that home range. But what if parts of this simulated
home range are not accessible? What if a housing development blocks off part of
the home range? This is where the resistance part of the resistance kernel
comes in. Using a GIS layer of landcover type, resistance values were assigned
to each cover type, with more disturbed cover types, such as roads and urban
areas, receiving higher resistance values. This resistance surface allows the
simulated home range to only include the areas that would be accessible to an Eastern
Indigo Snake. These HRC grids were calculated for both winter and summer and
then combined into a single measure of year-round habitat quality.

The
resulting model now allows TOS to guide their Land Protection Program more
efficiently by targeting areas with the highest predicted habitat quality. By
using HABIT@, TOS will know that predicted habitat quality reflects the quality
and accessibility of both winter and summer habitat. The model also revealed
some interesting patterns about the state of Eastern Indigo Snake habitat in
the Altamaha River Drainage. Although this area is considered a stronghold for Eastern
Indigo Snakes, the model showed that over 50% of the Drainage was unsuitable
for Eastern Indigo Snakes and less than 1% of the study area could be
considered moderate to good quality. This highlights the importance of both
preserving remaining tracts of suitable habitat and restoring habitats that
were formerly suitable, which is a goal of TOS’ Land Management Program. This
project provides an example of how TOS uses state-of-the-art science to direct
on-the-ground conservation efforts of the Eastern Indigo Snake.

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