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The resulting projects provide comprehensive inventory work and
detailed ecological
studies of endangered
or unique species.
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Even one large exotic bullfrog in a pool can
eat all of the male Arroyo Toads in a few nights.
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CCBER Curator of Herpetology Professor Sam Sweet has conducted ecological studies of the southern California endemic Arroyo Toad (Bufo californicus) since 1988, with a focus on reproductive biology and habitat use by juvenile and adult toads. This work established the localized nature of surviving populations and identified a suite of anthropogenic effects contributing to this species' rapid decline. Based on this information, Arroyo Toads were placed on the federal endangered species list in late 1994. The US Forest Service was able to promptly address a number of land-management problems identified by Sweet's research, and toad populations have since stabilized or increased.
The most critical factors were breeding failures and losses of adult male toads to exotic bullfrogs. Arroyo toad eggs are laid in strings in shallow flowing water on the sandy edges of pools, and are quite vulnerable to being stranded or washed downstream by water level changes. The tadpoles are durable once they hatch and disperse, but newly-metamorphosed toads are very sensitive, since they occupy lightly-vegetated sand and gravel bars bordering the larval pools for several weeks in early summer.
Intensive recreational use (including OHV travel) in breeding pools and abrupt water level changes resulting from upstream dam operations, were preventing recruitment of young toads. Male Arroyo Toads call each night for several weeks from habitual sites on pool margins during the breeding season. Even one large exotic bullfrog in a pool can eat all of the male Arroyo Toads in a few nights. Since bullfrogs do not move among pools until after Arroyo Toads have finished breeding, targeted removal of single bullfrogs is an effective short-term solution.
Increased protection of the riparian zone through relocation of OHV trails and streamside campgrounds, rescheduling of water releases to less sensitive times of the year, and bullfrog removal programs have dramatically improved survival and recruitment of Arroyo Toads. The long-term solution (for nearly all exotic riparian plants and animals) is the restoration of natural flow regimes, including scouring floods in winter and allowing streams to go dry in late summer and fall. Endangered listing for Arroyo Toads drives this restoration process on public lands, to the benefit of all other native riparian species.
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Arroyo toads breed in exposed sites in very shallow water, such as along
the near edge of this pool on Piru Creek in Ventura County. |
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