| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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horizons Silent Nights in Yosemite The four million people expected to visit Yosemite National Park this year will gaze at the splendor of Sierran granite, giant sequoias, and thundering waterfalls. Many will lug their backpacks to the alpine scenery of Tuolumne Meadows. The really lucky ones will hear or see some frogs. Stories of disappearing amphibians from scattered parts of the globe have appeared frequently in scientific and popular media for the past several years. But a study published in the April issue of Conservation Biology documents for the first time a dramatic decline in an entire native frog and toad fauna. The study by National Biological Service (NBS) researchers Charles Drost and Gary Fellers gives cause for concern because the same region of Yosemite and both sides of the Sierra Nevada had been surveyed intensively for vertebrates in 1915 and 1919. A team led by biologists Joseph Grinnell and Tracy Storer visited 40 sites ranging from oak savanna and sagebrush steppe to montane forest, alpine meadow, and tundra. At that time, each of the seven species in question was more abundant within its range. That was then, and this is now. "Doing the work at Yosemite made it very clear to me that these declines are taking place," says Fellers, a biologist based at Point Reyes National Seashore. "The declines are occurring in some of our largest parks and wilderness areas--the very places where we would presume they are best protected." A California native, Fellers spent part of each summer of his youth hiking in Tuolumne Meadows and elsewhere in the Sierra and became intimately familiar with the region's frogs. After retracing some of his early treks in the course of this study, he now feels a deep sense of loss. Drost and Fellers consulted the original expedition's field notes and photographs at the California Academy of Sciences and U. C. Berkeley's Museum of Vertebrate Zoology for all references to amphibians in order to relocate the same specific sites. Then, in 1992 and 1993, they followed a nearly identical 90-mile transect from the eastern edge of the Central Valley, through Yosemite along the Merced River, over the Sierra crest, and down to Mono Lake. They conducted field surveys focused only on amphibians, specifically the California red-legged frog (Rana aurora draytoni), the foothill and mountain yellow-legged frogs (R. boylii and R. muscosa), western and Yosemite toads (Bufo boreas and B. canorus), the Great Basin spadefoot toad (Scaphiopus intermontanus), the Pacific treefrog (Hyla regilla), and the non-native bullfrog (Rana catesbeiana). Though they located 38 of 40 sites visited by Grinnell and Storer, Drost and Fellers didn't find many frogs. They searched streams, ponds, lakes, and moist meadows for adults, tadpoles, or eggs. They waited into the night, listening for a chorus of calls. Time and again, their observations contrasted starkly with the survey done decades earlier and indicated a "strong downward trend" for nearly all species. Take the Yosemite toad, which Drost and Fellers documented at just over half of the 1915 survey sites, but consistently in low numbers during their season of peak activity. Or take the western toad, called "exceedingly abundant" in 1915, where it thrived at six sites in the Sierran foothills; the new survey found them only in Yosemite Valley: two females (one of which was dead) and a few dozen tadpoles. The mountain yellow-legged frog may have vanished from as many as half of the sites in the Sierra where it once lived. When the Grinnell and Storer team explored Yosemite, this was the most common amphibian around at high elevations. The 1992 survey located only a pair of individuals--a tadpole at Mono Meadow and an adult female at Evelyn Lake--at sites visited in 1915, but found a small population along the Glacier Point Road and 113 tadpoles in a glacial tarn on Mount Hoffmann. The foothill yellow-legged frog fared even worse. According to Drost and Fellers's report, Rana boylii has now disappeared from the transect area entirely, and R. muscosa has gone from being the most abundant species to one of the rarest." Other biologists had already detected a decline in both yellow-legged species in the central and southern Sierra, and during a subsequent survey Fellers found only a single subadult of the foothill species anywhere south of Calaveras County. Also largely unaccounted for were the red-legged frog and Great Basin spadefoot. In 1915, the red-legged occurred at only three foothill stream sites surveyed--two more places than where Drost and Fellers saw it. A broader search of the transect area located some tadpoles in a Tuolumne River tributary, but the largest native frog in the West was otherwise absent. The spadefoot had been seen at three desert sites near Mono Lake in 1915, but Drost and Fellers failed to find it. (The survey included only a small area at the edge of the spadefoot's range, and the secretive species is known to still survive nearby.) Even the Pacific treefrog, which has previously seemed less vulnerable to whatever has been plaguing amphibians, showed signs of decline. In 1915 the treefrog was ubiquitous throughout Yosemite, and it remains California's most widespread and abundant frog. Drost and Fellers noted no up or down trend at low-elevation sites, but at all except one high-elevation site the treefrog's numbers had dropped dramatically. Fellers calls that finding--something of a surprise to the biologists--"particularly worrisome" because recent research suggests that Pacific treefrog eggs can tolerate higher doses of UV radiation better than eggs of other native frogs (see "Horizons," Pacific Discovery Summer 1994). Since 1992, Fellers has continued making frog surveys across California. He has now visited more than three thousand sites and has seen nothing to refute the results from Yosemite. In May, a month after the Conservation Biology study appeared, the California red-legged frog became the first species added to the Endangered Species List since President Clinton lifted a year-old moratorium on new listings. The U.S. Fish and Wildlife Service had been petitioned by scientists four years ago to consider listing this once-abundant frog, which was already identified by California wildlife authorities as a "species of special concern." The Service proposed it for endangered status in February 1994. Subsequent fieldwork revealed that the frog was more common within its current range than had been thought, so, though it has disappeared from 70 percent of its original range, the Service opted to designate the frog as threatened, a not-so-urgent category and one that is less restrictive to property owners and land managers. The California red-legged frog currently occurs in 240 streams and drainages, mainly in coastal central California from Point Reyes, where the subspecies remains relatively common, south to Ventura. Nevertheless, according to the Service, only three places in that range may harbor more than 350 adult frogs. Red-legged frogs were once a favored food, with an estimated 80,000 frogs annually collected to meet demand for frog legs in the last decade of the nineteenth century. Its more recent decline has been blamed on large-scale water-control projects and predatory, habitat-usurping bullfrogs--introduced to California a century ago, ironically, as a culinary substitute for the overharvested red-legs. The threatened listing should free up funds to help this species recover, but Fellers points out that a lot of critical research still needs to be done first. "I think we need to find out why they are declining," he says. "We need to look at cause and effect." Yet, after three years of steady funding from the NBS, Fellers received substantially less money this year to pursue just such studies. As for possible causes of the Yosemite decline, bullfrogs may be impacting populations at elevations up to four thousand feet but not at the high-elevation haunts of the Yosemite toad, Pacific treefrog, or mountain yellow-legged frog. Bullfrogs had not yet entered Yosemite at the time of the Grinnell and Storer survey. Drost and Fellers found bullfrogs at four survey sites in the western foothills and Yosemite Valley. Years of drought prior to 1992 probably perpetuated the decline but are unlikely to have precipitated it, since the earlier Yosemite survey took place during a similar dry spell. One potential cause could be pesticide residues wafting upslope from the San Joaquin Valley. Pesticides do concentrate in the mountains after winter rains, but no study has yet examined the effect on wildlife. More certain culprits are introduced fish, specifically trout, which eat eggs, tadpoles, and frogs. Glaciation kept the Sierra's high-altitude lakes and streams out of the reach of fishes, but intensive stocking of trout in the park began in the 1920s with numbers exceeding a million fish per year. Grinnell and Storer conducted their survey before the peak decades of trout stocking, but they noted that water bodies containing fish basically lacked frogs. At Merced Lake, where stocking pre-dated the 1915 survey, only the Pacific treefrog survived. This species and the Yosemite toad lay eggs in ephemeral ponds and transform from tadpoles to adults in late summer of the same year. However, the mountain yellow-legged appears to be particularly vulnerable to fish predation because its tadpoles overwinter for one or two years before becoming adults, so it must lay eggs in deep water bodies unlikely to freeze--the same places frequented by trout. Perhaps, Drost and Fellers suggest, the mountain yellow-legs became split up into isolated populations occupying fish-free patches, and these small populations, blocked from dispersing or returning to former breeding areas, may now be suffering ecological attrition due to other factors. But voracious fish still can't explain the absence of frogs from places too small to support fish and others that have never been stocked. Whatever the cause, the Yosemite decline mirrors results from other parts of California and western states and should serve as a wake-up call if a nocturnal chorus of ribbeting is to continue to be part of a wilderness experience.
Blake Edgar is Assistant Editor of California Wild. |
Fall 1996
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