The Tehachapi Connection: A Case Study of Linkage Design, Conservation, and Restoration

The Tehachapi Mountains of California (Figure 1) form the spine of a landscape-scale linkage that has been the subject of much conservation planning and successful implementation. The Tehachapis and their contiguous foothill and valley habitats form the sole wildland connection between 2 major mountain systems, the Sierra Nevada to the east and the southern Coast and Transverse ranges to the west. This region of California is notable for the convergence of several major geomorphic and biogeographic provinces (Schoenherr 1992, White et al. 2003, Baldwin et al. 2012). The Tehachapi Connection is a landscape linkage generally bounded by the extensive agriculture of the southern Great Central (San Joaquin) Valley to the north and urban and exurban development in the western Mojave Desert (Antelope Valley) to the south. Land uses within the linkage are primarily livestock grazing, scattered agriculture, rural residential development, and increasingly, development for wind and other forms of renewable energy.

The Tehachapi Connection is the only relatively intact landscape (i.e. not experiencing significant land cover changes) between the Coast/Transverse ranges and the Sierra Nevada, and it is the only mountainous connection between these ranges for over 640 km. The terrain within the linkage has a broad elevational range, where elevations average approximately 150 m on the northern San Joaquin Valley side, climb to peaks between 1,980—2,440 m, and fall to 900 m on the southern Mojave Desert side. This landscape supports a broad range of life zones, extending north to south from southern San Joaquin Valley grasslands, through foothill oak woodlands and montane conifer and mixed conifer forests, to desert shrublands and grasslands on the bajadas and terraces of the Antelope Valley. Populations of several special-status and regionally endemic species have been documented in the linkage, making the Tehachapi Connection a critical conservation landscape.

Figure 1.

Geographic setting of the Tehachapi Connection, which provides a linkage between conserved lands at the west (e.g., Los Padres National Forest and privately owned Wind Wolves Preserve) and east (e.g., Sequoia National Forest) ends of the Tehachapi Mountains. The Tehachapi Connection is bounded by extensive agricultural and urban land uses to the north and south.

Planning for the Tehachapi Connection was conducted as part of the South Coast Missing Linkages Project, a collaborative multi-agency effort that developed designs for several landscape linkages within or associated with the South Coast ecological region (Baldwin et al. 2012). This network of linkage designs was based on the habitat requirements and movement needs, as described in the scientific literature, of 109 focal species across diverse taxonomic groups. These focal species were considered to serve as an umbrella for all native species and ecological processes of interest in the region. Penrod and colleagues (2003) designed the Tehachapi Connection based on the needs of 34 of these focal species (Table 1). Penrod and colleagues (2003) conducted least-cost corridor analyses for 9 of the 34 focal species in the Tehachapi Connection. Least-cost corridor analysis is a GIS technique that models the relative resistance (or cost) of species movement between 2 target areas as a function of various landscape characteristics (e.g., vegetation type, topographic position, elevation, road density). In the analysis, the landscape is portrayed as a raster grid of individual pixels. Resistance refers to the difficulty of moving through a pixel and cost is the cumulative resistance incurred in moving across the landscape between targeted endpoints. For each species, we used cores and patches of potential breeding habitat within each target area as the termini for the analyses. We developed a resistance grid for each species that represents the per-pixel cost of movement across the landscape for species movement or gene flow based on literature-based species-specific information. The least cost corridor minimizes the cumulative resistance to each species movement across the landscape. The 9 least-cost corridors were then combined into a Least Cost Union. We then analyzed the size and configuration of suitable habitat patches within the Least Cost Union for all 34...