Desert Tortoise TCA Habitat Linkages, DRECP

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.