Foothill Yellow-Legged Frog Conservation Assessment in California
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United States Department of Agriculture Foothill Yellow-Legged Frog Conservation Assessment in California Forest Pacific Southwest General Technical Report August Service Research Station PSW-GTR-248 2016 In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, the USDA, its Agencies, offices, and employees, and institu- tions participating in or administering USDA programs are prohibited from discriminating based on race, color, national origin, religion, sex, gender identity (including gender expres- sion), sexual orientation, disability, age, marital status, family/parental status, income derived from a public assistance program, political beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or funded by USDA (not all bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident. 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Submit your completed form or letter to USDA by: (1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: program.intake@ usda.gov. USDA is an equal opportunity provider, employer, and lender. Technical Coordinators Marc P. Hayes is a senior research scientist, Washington Department of Fish and Wildlife, 600 Capitol Way North, Olympia, WA 98501; Clara A. Wheeler is an ecologist, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, 1700 Bayview Dr., Arcata, CA 95521; Amy J. Lind is a wild life biologist, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, 1731 Research Park Drive, Davis, CA 95618, currently located at U.S. Department of Agriculture, Forest Service, Pacific Southwest Region, 631 Coyote St., Nevada City, CA 95959; Gregory A. Green is a wildlife ecologist, Tetra Tech EC, 19803 North Creek Parkway, Bothell, WA 98011, currently located at Owl Ridge Natural Resource Consultants, Inc., 22116 45th Ave. SE, Bothell, WA 98021; and Diane C. Macfarlane is a wildlife biologist, U.S. Department of Agriculture, Forest Service, Pacific Southwest Region, 1323 Club Drive, Vallejo, CA 94592. Cover photo: Foothill yellow-legged frog (Rana boylii), by Ryan Peek. Abstract Hayes, Marc P.; Wheeler, Clara A.; Lind, Amy J.; Green, Gregory A.; Macfarlane, Diane C., tech. coords. 2016. Foothill yellow-legged frog conservation assessment in California. Gen. Tech. Rep. PSW-GTR-248. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 193 p. The foothill yellow-legged frog (Rana boylii) is a stream-breeding amphib ian that has experienced significant population declines over a large portion of its historical range. This frog is nearing extirpation in much of the Sierra Nevada region where existing populations are sparse. Water development and diversions are likely to be the primary cause of population declines and are currently a prominent risk factor because they result in hydrological changes that chronically affect several aspects of the species’ life history. Other primary risk factors include climate change, mining and suction-dredging, introduced species, and habitat loss. Conservation approaches could include restoration of hydrologic attributes such as flow and thermal regimes on regulated rivers, restoration of associated uplands and connecting riparian corridors, and management of flow regimes to retain or restore favorable habitat conditions. Keywords: Rana boylii, Sierra Nevada, risk factors, water development and diversions. Executive Summary The foothill yellow-legged frog (Rana boylii) is associated with lower elevation streams draining the Pacific slope from west-central Oregon to northwestern Baja California. This assessment focuses on what is known about the natural history of this frog rangewide, with special focus on risks and management issues that pertain to the Sierra Nevada region at the heart of the species’ range. Historically common throughout its geographic distribution, the foothill yellow-legged frog now appears to be near extirpation over at least two-thirds of its range. The area over which this frog nears extirpation includes the Sierra Nevada, an area that comprises the east ern portion, or roughly one-quarter of the historical distribution. In areas where the species persists in the Sierra Nevada, populations are sparse. This pattern suggests that the species is at risk of extirpation regionally and the regional risk contributes to a broader risk of extinction. Therefore, the U.S. Forest Service initiated a multia gency effort to develop a conservation assessment focused on attenuating causative factors. This assessment has three parts: (1) a synopsis of foothill yellow-legged frog ecology designed to identify areas of vulnerability, (2) a review of foothill yellow-legged frog status across the national forests and national parks in California and specifically in the Sierra Nevada, and (3) an evaluation of current and future risk factors likely to impinge on the foothill yellow-legged frog and its habitat. This assessment is intended to form the basis for a conservation strategy for the species. Foothill yellow-legged frogs are currently recognized as one taxonomic unit, but genetic data reveal substantial genetic variability across their geographic range that may conceal unrecognized taxa. Foothill yellow-legged frogs occupy a diverse range of ephemeral and permanent streams, rivers, and adjacent moist terrestrial habitats over the course of their complex life history. Small streams often have dense canopies that limit the light needed by algae, the food resource of tadpoles. Adults can migrate down the drainage network to channels that are broad and more sunlit. Occupied streams are often partly shaded, low gradient, and dominated by coarse, unconsolidated rocky substrates. Seasonal variation in streamflow has a strong influence on life history and movement. To avoid disturbance and optimize feeding by tadpoles, adults breed and tadpoles develop in slow water velocity habitats. Reproduction occurs in synchrony with the transition from winter and spring snowmelt freshets to summer drought. The period of fastest tadpole growth and development coincides with blooms of algae and diatoms and warm water tem peratures; tadpoles do not overwinter. Mortality is high through the juvenile stage because of abiotic factors such as stranding and scouring of egg masses as well as losses to predators. Early life stages, as prey items, are likely important to trophic transfer within stream food webs. Fall rains trigger movement of recently meta morphosed juveniles upstream and away from the active channel. Postmetamorphic stages occupy terrestrial stream-margin habitats as well as springs and seeps at varying distances from breeding and rearing sites. Radiotelemetry demonstrates that foothill yellow-legged frogs travel distances far enough to allow movement between breeding sites along a watercourse, but gene flow between breeding popu lations is impaired in river systems in which stream reaches have been fragmented by reservoirs. Historical records document species presence for every major Pacific-slope Sierra watershed between the upper Sacramento River and the Tehachapi Moun tains, at elevations ranging from the Central Valley floor to around 2000 m; how ever, Central Valley records are sparse and may not reveal the presence of effective breeding populations. In California, a little over 30 percent of the historical range of foothill yellow-legged frogs is on national forest lands. The remainder is on private, state, and other federal lands. Occupancy surveys that covered historical sites and potentially suitable habitat suggest that the species is much less widespread than it was historically in the Sierra Nevada. Fewer than 10 recent records from the Cosumnes River southward suggest near-extirpation in the southern portion of their Sierran range. Records from the 1960s and 1970s imply that populations were robust until that time, but lack of monitoring prevents precisely identifying when declines began. A similar geographic pattern of extirpation exists within California in general, with few populations remaining south of San Francisco Bay. The most robust data implicate water development and diversions as the primary cause of declines in foothill yellow-legged frogs. Water development and diversions are a prominent risk because they result in hydrological changes that chronically affect several aspects of the frog’s life history. Recent studies from both regulated and unregulated rivers have demonstrated that small-scale changes in local habitat conditions,