Developing Partnerships to Support Arroyo Toad Recovery in San Diego County
The Arroyo Toad (Anaxyrus californicus) was once a widespread inhabitant of arid riparian systems in California ranging from Monterey County to Northern Baja Mexico including river systems that drain into the Mojave Desert on the eastern side of the mountain ranges in Southern California. As its common name implies, the Arroyo Toad inhabits low gradient, intermittent streams and rivers characterized by open braided channels with large areas of sandy sediment deposits that are subject to periodic and often intense flooding. These river systems are common to the arid areas of California and provided significant areas of habitat for the toad. However, the toad has been extirpated from large areas of its original range as a result of urbanization and agricultural land uses, vegetation encroachment and habitat changes and flood control projects that have channelized stream systems and impounded water sources needed by the toad for survival. As a result, the Arroyo Toad is now a federally endangered species in both the United States and Mexico.
The Arroyo Toad is a reclusive nocturnal species that borrows into stream sides and terraces during the day and becomes active at night to forage and breed. The toad is dependent on shallow pools and slow moving water for breeding, egg laying and larval development. Low gradient arroyos provide this availability in flood disturbed areas from the first significant rain events and continue to provide suitable habitat until the water disappears in the spring and summer. During the drier periods of the year, the toad takes advantage of the extensive terrace systems and sandy substrate to dig burrows into the cooler and moister soils and enter a period of dormancy, called estivation, where they remain inactive and lower their metabolic rate until the rainy season returns. While in this period of dormancy, toads are susceptible to a wide array of additional threats including off-highway vehicle use, trampling and predation. As a result, the once widespread range of Arroyo Toads has been reduced to 25-35% of its historic range.
Working with local Arroyo Toad researchers with the US Geological Survey and other partnering agencies such as the Wildlife Conservation Board and the City of San Diego, we have identified locations and methods of habitat restoration for the toad in the San Dieguito River and San Pasqual Valley of San Diego County. Building on the expertise and research conducted on this species in the local area, the partnership has been evaluating different locations along the river that could provide the greatest benefit to toad recovery and working to identify the best combination of plant species for restoration areas.
Understanding the variety of habitat types used by the toad during its different life stages and seasons has shown us that successful toad habitat restoration projects must encompass a mosaic of different vegetation types and densities that take advantage of site-specific considerations, as well as the differences between historic and current hydrologic conditions. When combined with the control and removal of non-native species of plants, the goal of our toad habitat restoration project is to provide a suite of habitat types that maximizes usable habitat for all of the toad’s various habitat needs. Work is ongoing at this stage but it is hoped that this project can serve as a model for possible future habitat restoration projects for Arroyo Toads in Southern California.
(Above) Native river bed terrace habitat used by Arroyo Toad for forage and breeding consisting of a mosaic of open areas, patches of mulefat and low growing forbes.
(Above) Terrace habitat that has been degraded by encroachment of non-native grasses in the foreground and invasive tamarisk in the background.
The above article originally appeared in the Summer 2016 issue of the River Partners Journal.