The purpose of this layer is to depict information relevant to the
conservation of the desert tortoise (Gopherus agassizii) under the Desert
Renewable Energy Conservation Plan (DRECP). Its extent includes all desert
tortoise conservation areas (TCAs) identified in the US Fish and Wildlife
Service's (USFWS) recovery plan for the desert tortoise and a series of least
cost pathway linkages that connect these TCAs. The least cost pathway linkages
were mapped by the USFWS Desert Tortoise Recovery Office (DTRO) and the
University of Redlands. For the purpose of DRECP planning, this layer provides a
more detailed assessment of this broad conservation network and identifies how
different geographic areas within this network will be treated relative to
conservation reserve establishment and application of biological goals and
objectives.
The "Desert Tortoise TCA Habitat Linkages" layer for the DRECP was created in the following manner:
The "20130410 Desert Tortoise Reserve" dataset was
created by merging a layer that depicts the Tortoise Conservation Areas (TCAs)
identified in the USFWS's Recovery Plan for the Mojave Population of the Desert
Tortoise (USFWS 2010) with a layer depicting least cost pathway linkages between
TCAs. The least cost pathway linkages were created by the USFWS Desert Tortoise
Recovery Office and the University of Redlands. The least cost pathway model
identifies areas between the TCAs with the highest relative potential to support
desert tortoises based on the U.S. Geological Survey's Desert Tortoise Habitat
Model (Nussear et al. 2009). In developing the least cost pathway linkages the
DTRO and University of Redlands zeroed out the habitat potential values for any
desert tortoise habitat identified in off-highway vehicle areas and military
bases to ensure that linkages were not modeled across these land use types. In
addition to the least cost pathway linkages and TCAs, additional localized areas
were mapped and merged with the TCA/least-cost pathway layer based on USFWS
staff input and expertise. These include 1) specific linkages in the Pinto Wash
area (southeast of Joshua Tree National Park), 2) upper alluvial fan areas in
the vicinity of the Whipple Mountains, and 3) some areas of higher desert
tortoise habitat potential south of the Chemehuevi Desert Wildlife Management
Area.
Because DRECP will have no management
authority on military basis, all overlap of Department of Defense Lands with any
portion of the final merged layer were removed. In addition, any area identified
as "highly converted" in the Nature Conservancy's ecoregional assessments for
the Mojave and Colorado Deserts were removed from the layer because they no
longer had the potential to support desert tortoises. The merged layer was
categorized in the "Reserve_Ca" field of the attribute table to identify which
portions of the reserve are derived from TCAs, least cost pathway linages, or
other high priority desert tortoise habitat. Each TCA and linkage was also given
a specific name in the "Name" field for reference during the development of
biological goals and objectives.
After the merged layer was created, further
analysis was performed on two least-cost pathway linkages that contained
substantial amounts of human disturbance and habitat degradation. In these
linkages, USFWS staff determined that more detailed mapping of disturbed,
intact, and lost habitat was needed to precisely identify primary and secondary
reserve areas and more accurately define the geographic application of specific
biological goals and objectives. These included - 1) Ord-Rodman to
Fremont-Kramer linkage, 2) Ord-Rodman to Joshua Tree National Park linkage. To
begin, extensive analysis of aerial photography was performed across the entire
extent of these two linkages using imagery available as an online basemap layer
for Arc Map (viewed at 1:10,000 scale). A point layer was developed through this
analysis that includes point features for all portions of the linkages that were
composed of houses, bare areas, agricultural areas, or major roadways (i.e.,
interstate highways). For areas of extensive habitat loss, numerous points were
added to cover as much of the extent of the bare area as possible. For areas of
scattered housing, one point feature was created per house. Using this point
layer, a point density raster layer was then developed to identify areas with
higher levels of human disturbance and habitat loss. A preliminary threshold was
then selected as a cut off within this layer to identify where disturbance and
habitat fragmentation was too great to reliably contribute to desert tortoise
conservation in the long-term.
Field reconnaissance of both linkages was
then performed. During this reconnaissance, additional areas were identified
within the linkage that were heavily disturbed by off-highway vehicles or that
did not contain suitable habitat (i.e., dry lake beds, dune systems, Mojave
River). The reconnaissance visit was also used as a means to ground-truth our
threshold cut off for areas that were too fragmented to contribute to long-term
conservation.
Based on information from the field
reconnaissance, the fragmentation threshold cut-off was adjusted and used in
some areas to form the boundaries between "fragmented" and "intact" habitat. In
other areas, it was not used at all, but served only as a guide for the
refinement of this boundary, which was done based on field notes, additional
aerial photography analysis, and staff input. In addition, portions of the two
linkages that were heavily impacted by off-highway vehicles or that did not
contain desert tortoise habitat were mapped based on field notes and additional
analysis of aerial photography. Following the mapping of these areas, a layer
was created that buffered the disturbance point layer created using aerial
photography.by 100 meters. This layer of buffered points, was mapped as areas of
lost or severely disturbed habitat.
Based on the geographic location of the
remaining portions of the layer, it was determined that detailed analysis of
human disturbance and further field reconnaissance was not warranted because
these areas were either already under conservation protection and management or
they comprised linkages that were in areas that contained little if any human
development. However, some work was done to remove dry lake beds, lava fields,
open pit mines, interstate highways, and other isolated areas of disturbance.
This was done using aerial photography.
The above analysis resulted in the
categorization of the "20130410 Desert Tortoise Reserve" into 7 categories
identified in the "Habitat_Ca" field of the attribute table. These include - 1)
fragmented desert tortoise habitat, 2) intact desert tortoise habitat, 3) lost
or severely disturbed habitat, 4) not habitat - playa, 5) not habitat - dune -
sand, 6) not habitat - Mojave River corridor, 7) significant OHV impacts to
Desert Tortoise Habitat. For some linkages and TCAs where detailed mapping of
the boundaries of intact habitat was not performed, the record is left blank for
the "Habitat_Ca" field.
The layer was also categorized based on the
role a given area would play in the DRECP reserve design. This categorization is
provided in the "Essential_" field of the attributes table. These categories
include "Primary" reserve, "Secondary" reserve, and "Not in Reserve". All
portions of the layer outside of the Ord-Rodman to Fremont_Kramer and Ord-Rodman
to Joshua Tree Linkages were identified as "Primary" reserve except for those
areas mapped as "lost or severely disturbed habitat" in the "Habitat_Ca" field.
Within the Ord-Rodman to Fremont_Kramer and Ord-Rodman to Joshua Tree Linkages
all areas of "Intact Desert Tortoise Habitat", as identified in the "Habitat_Ca"
field, were determined to be either "primary" or "secondary" reserve. All other
areas were identified as "not in reserve".
References:
Nussear, K.E., Esque, T.C., Inman, R.D., Gass, Leila, Thomas, K.A., Wallace, C.S.A., Blainey, J.B., Miller, D.M., and Webb, R.H., 2009, Modeling habitat of the desert tortoise (Gopherus agassizii) in the Mojave and parts of the Sonoran Deserts of California, Nevada, Utah, and Arizona: U.S. Geological Survey Open-File Report 2009-1102, 18 p.