We calculated a total footprint of 31,994 ha for proposed solar energy generation facilities under verified Right of Way applications on BLM lands and on private lands of the western, central and south-central subregions of the ecoregion. Meeting compensatory mitigation needs for these proposed projects would contribute more to regional conservation goals if mitigation is not restricted to private lands. For example, if we use the “future” mitigation ratio and restrict mitigation investment to private lands, there will not be enough higher conservation value private land in the central Mojave subregion to offset impacts for five conservation targets, including the desert tortoise, which falls short of the mitigation need by 38% (23,104 ha) (Figure 7). In contrast, if public lands are also eligible for investment, mitigation requirements under the future ratio could be met for all but two targets (playa is short by 601 ha and desert pavement is short by 30 ha) (Figure 7). Moreover, in the private land only scenario, lands selected for mitigation at both ratio levels are more fragmented than the mixed ownership scenario (as reflected in higher edge length of the full selected network, 15% higher for current ratios and 52% for future ratios). The areas selected in the private land only, current scenario are slightly more degraded (11%, as indicated by the average Marxan “cost” per selected assessment unit) than the mixed ownership solution (Table 3). This difference in degradation jumps to 60% using the future ratios, which is largely due to Marxan seeking to meet the mitigation goals for tortoise, by having to include areas that may be relatively more impacted.
The ideal arrangement of places for mitigation differs depending on what lands are available. The percentage overlap of the mitigation solutions for the mixed ownership and the private land only scenarios is low: the Jaccard similarity index  was 0.29 for the current mitigation ratio and 0.42 for the future ratio (Figure 7). A similar comparison of total area
FIGURE 7: Scenarios of suitable mitigation areas using the future ratios. This map shows the private land-only (medium gray) and the mixed ownership (dark gray) scenarios, with planning units that are shared in both scenarios (with outline). The private land-only solution is more dispersed and was not able to offset impacts for five targets in a subregion (grey outlines, labeled in Figure 2), most notably a deficit of over 23,000 hectares of suitable desert tortoise habitat in the Central Mojave subregion, north and east of Barstow, CA. Urbanized areas are shown in lighter grey. The extent of Ecologically Core (darker green) and Ecologically Intact (lightest gray) is shown for reference (adapted from Randall et al. 2010).
needed for both ownership scenarios could not be performed for the future ratio solutions because mitigation goals could not be met in the private land only, future scenario (Table 3).
TABLE 3: Performance of compensatory mitigation scenarios.
Assessment unit costs are the sum of the “cost" values, a unitless index used in Marxan as a proxy for anthropogenic disturbance. The number of assessment units is the number selected in the most efficient scenario of 100 model runs. Boundary length is the total edge length of the selected assessment units and is a proxy for the dispersion of the selected network of areas. Goal attainment refers to whether the mitigation goals for the targets are met in the given scenario. See Supporting Information S2 for full description of Marxan settings.