A range‐wide model of contemporary, omnidirectional connectivity for the threatened Mojave desert tortoise

Abstract

As habitat destruction leads to species extinctions globally, conservation planning that accounts for population‐level connectivity and gene flow is an urgent priority. Models that only approximate habitat potential are incomplete because areas of high habitat potential may be isolated, whereas intermixed areas of lower habitat potential may still be critical for maintaining connectivity between and among populations. We developed a range‐wide, omnidirectional (coreless) connectivity model and map for the threatened Mojave desert tortoise at a high spatial resolution (30 m), based on empirical movement data and a circuit‐theoretic approach to estimating connectivity. Specifically, we first estimated habitat potential (i.e., quality) for tortoise movement (as distinct from habitat potential more generally) across its range using hypotheses based on the published literature, linear mixed models, multiple environmental factors derived from remotely sensed data, and recent solar and wind development footprints. The resultant raster output was used to represent landscape conductance in a circuit‐theoretic model of connectivity, which relates the flow of electrical current through a circuit to the movement of tortoises through the landscape. We then modeled potential connectivity across the range of the tortoise using Circuitscape software and the Julia numerical programming language. Intermediate distances from minor roads, intermediate values of annual average maximum temperature, and increasing density of desert washes were among the strongest predictors of movement habitat quality. There was also strong evidence for increased habitat quality for movement with increasing amounts of vegetation cover. The resulting connectivity model and map was determined to accurately reflect important areas for tortoise movement, but we encourage others to do their own evaluation of the model within local areas of interest and as more data become available. Accordingly, the map can provide an important component to improve management decisions that have the potential to influence the conservation of connected desert tortoise populations throughout the range.

Characterizing multispecies connectivity across a transfrontier conservation landscape

“…While this approach is frequently used to evaluate connectivity between a single source‐destination pair, the application of circuit theory has expanded to multiple source‐destination pairs and to wall‐to‐wall omnidirectional approaches used to model current flow potential across continuous landscape gradients (e.g. Dickson et al, 2019; Gray, Dickson, Nussear, Esque, & Chang, 2019; Koen, Bowman, Sadowski, & Walpole, 2014; McRae et al, 2016; Pitman et al, 2017). The wall‐to‐wall omnidirectional approach provides a framework for evaluating current flow potential (i.e.…”

“…connectivity) within and between any source and destination pair across a landscape (e.g. Gray et al, 2019; Pitman et al, 2017), which is particularly appealing in large landscapes such as KAZA where not all potential sources and destinations of animal movement are understood (e.g. where potential range shifts or recolonization may occur).…”

Using movement to inform conservation corridor design for Mojave desert tortoise

“…We derived topographical covariates from the 30-m USGS Digital Elevation Model [50]. Slope is an important factor in tortoise habitat suitability as well as movement ability [51,52]. Desert washes and ephemeral streambeds are prominent landscape features in the Mojave Desert and are important in desert tortoise ecology, both as foraging areas and potentially as movement corridors [53,54].…”

“…Desert washes and ephemeral streambeds are prominent landscape features in the Mojave Desert and are important in desert tortoise ecology, both as foraging areas and potentially as movement corridors [53,54]. We used a wash layer derived using USDA National Agricultural Imagery Program (NAIP) [55] and classified pixels by the "intensity of wash characteristics" using a random forest model at a 1-m resolution (further details in [48,52]). Washes typically have different substrates and vegetative communities than the surrounding uplands; pixels scored as high wash intensity in the model had lower vegetation cover.…”

“…Perennial shrubs are important for desert tortoises as thermal refugia and provide integrity for burrow construction [51,56,57]. To approximate shrub availability, we used a vegetation layer derived from early summer 2016-2017 NAIP NDVI (normalized difference vegetation index) imagery, to capture the variability in perennial shrub cover within and between our sites while eliminating annual variation in spring and late-summer annual plant growth (further described in [52]).…”