Designing Wildlife Crossings to Meet Focal Species Needs: An Example from Interstate 15 Connectivity Planning efforts in Southern California

The Nature Conservancy (TNC) and the California Department of Transportation (Caltrans), along with landowners including San Diego State University, California Department of Fish and Wildlife, Western Riverside Regional Conservation Authority and Riverside County Flood Control District are working on the planning of three proposed wildlife crossing infrastructure projects along a 3-mile stretch of Interstate 15 (I-15) in the Santa Ana-Palomar Mountains Linkage (Linkage) in southern California. These crossings will provide a critical missing link that will help reconnect wildlife in the coastal Santa Ana Mountains west of I-15 with those in interior Palomar and Eastern Peninsular ranges to the east of I 15. The Linkage supports intact and diverse habitats including coastal sage scrub, grasslands, chaparral, and oak and riparian woodlands, and has been a focus of regional conservation efforts for the last 30 years.
The three wildlife crossing infrastructure projects include enhancement of the existing Temecula Creek I-15 Bridge, construction of a new vegetated wildlife overcrossing, and construction of a new stand-alone wildlife culvert.
Given the challenges and level of financial investment required to secure wildlife crossings for I-15 in the Linkage, The Nature Conservancy and Caltrans proposed that planning efforts would benefit from engaging with focal species experts in a workshop process to evaluate and provide recommendations for wildlife crossing concepts to ensure that ultimate crossing designs meet the needs of the broadest range of wildlife. While wildlife crossings are becoming more common, the design of crossings to meet the needs of a variety of wildlife species is largely unknown and can be site specific. To address this challenge, this workshop brought together over 50 wildlife experts to brainstorm and identify specific design considerations for various focal wildlife species groups (medium/large mammals, small animals, birds, bats, plants and invertebrates) that might use the identified I-15 wildlife crossings.
Lead experts for each focal species group were enlisted and worked together to identify specific wildlife crossing features or attributes for evaluation by species group experts for each of the three proposed wildlife crossings.
Specific attributes evaluated by experts for each crossing type and species group included, at a minimum:
• Crossing Structure Attributes;
o Size
o Habitat (cover, habitat structure, substrate, moisture, light and noise mitigation)
• Crossing Approach Area Features;
o Habitat features (cover type/density, substrate, water, light and noise mitigation)
• Barrier design to reduce roadkill and/or to funnel wildlife to the crossing;
• Additional research to resolve critical uncertainties related to crossing design.
Based on the input and design recommendations provided by species experts for each potential crossing type being considered, the experts then weighed in on the suitability of existing location and probability of use by their focal species or group of species.