This post was originally published on an older version of sdrp.org in Fall 2004 by Shawna Anderson, SDRP Principal Planner.
There are several threatened and endangered species that reside within the San Dieguito River Valley. One of them is the Arroyo Toad (Bufo californicus) listed as Endangered by the U.S. Fish and Wildlife Service. Historically extending from the upper Salinas River in San Luis Obispo County to well into Baja California, its distribution has declined by more than 75%. The River Park’s population of Arroyo toads resides along the Santa Ysabel, Guejito, and Santa Maria creeks and along the San Dieguito River (which originates where the Santa Ysabel and Santa Maria creeks join) in San Pasqual Valley, and extends further east into the Boden Canyon area. Actually, the toad doesn’t just use the creek as its home but spends most of its time on the sandy terraces that surround the river. They may even travel into surrounding upland scrub or woodland habitat or agricultural lands. In fact, the Arroyo toad’s habitat requirement is perhaps the most specialized of any amphibian found in California! This specialized habitat preference, coupled with many years of habitat destruction and disturbance has left the Arroyo toad among the most vulnerable too. And when environmental factors occur such as the several years of drought that we’ve been experiencing in San Diego County the Arroyo toad doesn’t do well. Arroyo toad breeding activity has virtually halted since 1998 when the last measurable above ground flow existed within San Pasqual Valley.
To breed, survive and multiply, this nocturnal toad requires an environment that includes exposed shallow, sandy or gravelly pools (with little marginal vegetation) in which to breed and lay eggs and for toad larvae to survive. These pools must be in the vicinity of juvenile and adult habitat, which consists of sandy terraces with an overstory of scattered shrubs and trees such as mule fat, cottonwoods and willows, usually with an almost barren understory. Adult Arroyo toads also travel into the surrounding upland habitat to burrow and forage. Much of the San Pasqual Valley meets these specific requirements. Sufficient rainfall is a necessity for sustaining suitable breeding habitat, which is why breeding activity has not occurred in the last five or six years. It is thought that these small toads (2-3 inches in length) may only live 7 or 8 years at the most.
Species that require specialized habitat conditions are often particularly vulnerable to human interference. The toad population in San Pasqual Valley has certainly been affected by many land use activities that occur in the valley such as habitat destruction, off-road vehicle activity, manipulation of the hydrologic regime, exotic plant infestations (arundo, aka giant reed, and pepperweed) that choke out native vegetation, cattle grazing, and introduction of exotic predators. These activities can have a devastating effect on a species’ survival particularly when the species is already stressed. Because toads burrow in sand bars and travel along farm roads and open areas, trampling by vehicles or humans and livestock often occurs. The City of San Diego has been conducting Arroyo Toad breeding surveys in San Pasqual through the MSCP. Because the valley is within an agricultural preserve, which has also preserved the riparian corridor along the creeks and river, agriculture and Arroyo toads appear to be able to co-exist. Hopefully a better understanding of the toad population in the valley and its threats to survival will help to determine if other measures need to be taken to ensure the Arroyo toad’s ultimate survival.
Thanks to Bill Haas of Pacific Coast Conservation Alliance and City of San Diego MSCP staff for providing information for this piece.
Photo credit: Bradford Hollingsworth, San Diego Natural History Museum