California State Route 74 (Ortega Highway) is one of two transportation corridors providing interregional access between coastal Orange County and the Inland Empire. Completed in 1932, Ortega Highway serves as a vital connection between large metropolitan centers on both sides of the rugged Santa Ana Mountains. As the human populations of these metropolitan centers have increased over the years, so has the use of this two-lane highway. Increased traffic and collision rates have necessitated various roadway improvement projects over the last several decades. Because much of Ortega Highway is surrounded by open space including sensitive habitats that support special-status wildlife species, the California Department of Transportation (Caltrans) has had to carefully plan every project implemented along this roadway. This case study focuses on one effective long-term mitigation strategy developed by Caltrans that can be applied in places where transportation infrastructure interfaces with sensitive aquatic habitats.
A large segment of Ortega Highway closely parallels San Juan Creek, which provides some of the most suitable (and least disturbed) habitat for amphibian breeding and larval development in Orange County. Various native amphibian species are known to occur within San Juan Creek, including the federally-listed as endangered arroyo toad (Anaxyrus californicus; ARTO) that occupies designated critical habitat within the study area. As is the case throughout aquatic ecosystems within the western United States, each of these species are predated upon by the introduced American bullfrog (Lithobates catesbeianus) and other nonnative predators such as red swamp crayfish (Procambarus clarkia), which pose a threat to the continued existence of some of the less-abundant native amphibians (including ARTO).
Most Caltrans projects that have occurred on Ortega Highway since 1994 (the year that ARTO was listed) have had to implement protective and compensatory measures due to the potential for direct and indirect impacts to ARTO and its habitat during construction activities. Beginning in 2007, Caltrans has implemented a focused effort to eradicate invasive predatory species from San Juan Creek as part of mitigation for small impacts to suitable ARTO habitat that have occurred as a result of several highway safety improvement projects. This multi-year Caltrans-funded effort has removed thousands of bullfrogs and crayfish, and likely eradicated African clawed frog (Xenopus laevis) from San Juan Creek given its lack of observation since 2009. Over a decade's worth of data collected during these efforts show a substantial increase in the numbers of ARTO detected within San Juan Creek, and have contributed significantly to our understanding of the species within the watershed. Our presentation will provide these data in context, showing the tremendous benefits of long-term invasive predator removal as mitigation for transportation projects in sensitive areas.