A Leap of Faith

Embracing Our Native Frogs

by on April 01, 2002

 

A Pacific treefrog peeks out from behind ripening blackberries in Ledson Marsh, Annadel State Park, Santa Rosa.

Photo by Glenn McCrea.

 

 

Walking in the East Bay hills one hot April afternoon, I happened on a small, nearly evaporated puddle containing newly hatched tadpoles. Since I had a just-emptied Peet’s Coffee cup in hand, a puddle-to-cup tadpole transfer occurred before mature reflection could intervene.

I might have released my captives if I’d seen a bigger puddle on the way back to my car, but I didn’t, so I arrived home with a dozen or so wriggling brown amphibians the size of pinheads in an inch of diluted adobe mud and cappuccino foam.

Amphibians always have fascinated me. There’s something magical about the way their strange shapes and colors materialize out of water or earth. But childhood attempts of keep them as pets were smelly failures, so I had misgivings about my acquisitions. I dumped the tadpoles in a small aquarium anyway, and sprinkled some TetraMin Baby Fish Food for Egglayers on the water. They went after the stuff as though they’d evolved to eat “fishmeal, torula dried yeast, ground brown rice, potato products, wheat gluten, dehulled soybean meal, and natural and artificial colors,” swimming busily upside down to suck it into swelling paunches. Such ready enthusiasm for my largess seemed admirable, and their vitality and adaptability were encouraging at a time when the news about frogs is generally of their imminent demise from pollution and habitat loss.

Pacific treefrog
Pacific treefrog, Ledson Marsh in Annadel State Park. Photo by Glenn McCrea.

When the water started to smell, I used a turkey baster to change it, but that was all I did. The tadpoles responded with a complexity that seemed disproportionate to my kitchen technology. They developed daily, and with bewildering diversity, deploying a range of genetic variation that probably contributes to multiple adaptive strategies in the wild. Some stayed dark brown and pinhead-sized. Some stayed dark brown but grew within weeks to sunflower-seed-size. Others turned greenish or beige and continued growing.

Hind legs soon appeared on the larger dark brown tadpoles, then on the green and beige ones. By early June, individuals that had grown front legs and resorbed their tails lurked near the aquarium top, ready to bounce halfway across my study when I opened the cover at feeding time; even the pinheads had begun to metamorphose into miniatures of their larger siblings. I had liberated the whole brood in my backyard by the Fourth of July. In August, I started finding half-dollar-sized frogs in flowerpots, some dark brown or greenish, some pink or mottled green and beige. During the first fall rains, as the local termite nests produced their sparkle-winged wedding flights of queens and drones, happy croaks issued from the shrubbery. The frogs were such an interesting presence that my wife and I built a four-foot-long “vernal pool” beside our toolshed, hoping they’d breed therein. We dug a hole, installed a vinyl pool liner, and floored the bottom with rocks to give the frogs and their offspring hiding places from Berkeley’s ubiquitous raccoons.

The creatures’ joie de vivre made me wonder how the native frogs and toads (technically called “anurans,” that is, no-tails) are doing here. With a long dry season and only about 20 inches of rainfall a year, the Bay Area is no amphibian paradise compared to a tropical rainforest. Yet all five frog and toad species known to have inhabited the region when Europeans arrived still survive here, although precariously in some cases.

My backyard frogs belonged to the West Coast’s most common and adaptable species, the Pacific treefrog (Hyla regilla). More resistant to pollution and habitat modification than others, it will breed in almost any kind of fresh water, and lived everywhere in the Bay Area before urbanization destroyed much of its habitat. It is even counterattacking against suburban growth in some cases, invading ornamental lakes in gated subdivisions, disturbing new-economy slumbers with clamorous water sports. Philip Northen, professor of biology at Sonoma State, told me he gets phone calls from developers and property owners asking how to exterminate frogs. “I just tell them they should learn to love nature,” he said.

Pacific treefrog calls are amazingly loud, as anyone who has been near a breeding pond knows. (In a sense, the calls are heard around the world, since Hyla regilla supplies Hollywood’s typical “ribit” frog sound effects.) What is less well known is that the calls are not simply chaotic screeching, but a “language” that organizes breeding. Northen says that their vocabulary’s function is not completely understood, but that male Pacific tree-frogs make five calls, including the familiar “ribit,” each adapted to particular functions in attracting females and repelling rival males. The pond calls are distinct from the ones they make on land, whose function is still unknown. Pacific treefrogs spend most of their lives on land, passing the dry season in rodent burrows and other refuges. Their terrestrial adaptability can be remarkable. When I lived in Round Valley in northeast Mendocino County, tree-frogs inhabited my tin-roofed storage shed during summers when the ambient temperature climbed into the 90s.

A melodious chirping sometimes accompanies treefrogs in breeding pools. This is the call of the western toad, Bufo boreas, the Bay Area’s second most adaptable species. It has a similar way of life, staying in burrows and other refuges during dry times and moving to quiet waters to breed in late winter or early spring. Western toads are greenish or brownish gray with a light dorsal stripe and warts that are often rusty-colored. The warts, and “parotoid” glands behind the eyes, exude a milky toxin that can poison attackers, although some determined predators, such as ravens and raccoons, have learned to avoid the skin and eat the internal organs. Growing to three times treefrog size, western toads are much more conspicuous as they emerge at twilight to look for prey. They can cover a lot of ground, walking as well as hopping, and will attack formidable victims, including wasps and scorpions.

Pacific treefrogs and western toads seem to be the most numerous anurans in the Bay Area now, but in the past the region’s largest native frog, the California red-legged frog, Rana aurora draytonii, may have been almost as common. Red-legged frogs have low-pitched calls and spend most of their lives in and around water, so people encounter them less often than treefrogs or toads. The species likes to breed in big wetlands, a type of habitat more available in the past, so there must have been quite a few red-legged frogs once upon a time. Mark Jennings, a herpetology research associate at the California Academy of Sciences, has researched commercial exploitation of the species in the late 19th century. He said California diners consumed the legs of an estimated 60 to 80 thousand red-legged frogs a year until the wild “froggery” collapsed after a record capture of 118,000 in 1895. The industry then turned to “ranching” imported bullfrogs. Escaping from cultivation, the larger bullfrogs hastened the native frogs’ decline, preying on them and pre-empting breeding habitat. The California red-legged declined so markedly that in 1996 it became the first Bay Area anuran to be federally listed as a “threatened” species.

Another native anuran species, the foothill yellow-legged frog, Rana boylii, spends its entire life around year-round streams, not an abundant habitat in the Bay Area. A medium-sized, mottled, tan or grayish frog, it is usually seen only briefly as it jumps into the water and hides under a rock. The species is heard even less than it is seen, and was once thought to vocalize very little, which would make sense, given its habitat of running water. However, Northen and a student recently discovered that male yellow-legged frogs call to females underwater, where sound carries much better, and that they do so rather eloquently. Northen said male yellow-leggeds have seven calls that they use to attract females and repel rivals.

The western spadefoot toad, Spea hammondii, can be quite noisy; its call has been compared to someone sawing wood. People seldom hear it, however, because the species has very restricted habitat locally, living only in isolated areas along the Central Valley’s western margin. It is a stout, cat-eyed little creature, beloved of nature documentaries because of its explosive breeding after thunderstorms in desert regions of the Southwest. In that habitat, spadefoots can develop from eggs to baby toads in three weeks after a storm, the larger tadpoles helping themselves along by devouring smaller siblings. Summer cloudbursts are rare here, however, and knowledge about local spadefoot breeding is limited. Robert Stebbins, emeritus professor of biology at U.C. Berkeley, has been observing Bay Area amphibians since the 1940s and says that the spadefoots are most active from April to June, breeding opportunistically in streams or ponds as well as in ephemeral pools. Mark Jennings says they might breed after the first fall rains as well.

Larry Serpa, a Nature Conservancy biologist, has found spadefoot tadpoles, but not adults, in stock ponds east of Henry Coe State Park. Serpa raised the tadpoles on fish food, then returned the baby spadefoots to their natal ponds, where they immediately burrowed into the mud, disappearing from sight within a half hour. The behavior points up a major difference between spadefoots and the other Bay Area species, which may remain active through the year when the weather isn’t too hot or cold. Fully adapted to arid climates, spadefoots spend eight to nine months of the year dormant in the soil, emerging only to breed and feed when a profuse spell of warm rain occurs. Not surprisingly, they are fast eaters. A spadefoot can gulp prey equivalent to 11 percent of its body mass in one meal.

A flexible lot, Bay Area frogs and toads weathered civilization’s onslaught better than mighty condors, elk, and grizzlies. Urbanization, wetland destruction, and stream diversion reduced some populations, but irrigation and water impoundments must have provided new habitat for others. Anurans (native frogs and toads are technically called “anurans,” that is, no-tails) remained relatively common into the mid-twentieth century; their main decline here seems to have come after 1970. David Wake, a leading herpetologist and professor of biology at U.C. Berkeley, sadly watched western toads and Pacific treefrogs dwindle in the Oakland and Berkeley hills. “I haven’t seen a toad there in years,” he said. Treefrogs can still be heard in Tilden Park, but Steve Bobzien, ecological services coordinator for East Bay Regional Park District (EBRPD), does not have records of western toads in the Berkeley or Oakland hills for the past several years. “There’s a strong indication that toads are losing parts of their range,” he said.

Western spadefoot toad
Western spadefoot toad. Photo by Chris Brown, USGS.

Ironically, the Bay Area’s only federally listed anuran, the California red-legged frog, is not the rarest species here, remaining widespread—if inconspicuous—in creeks, stock ponds, and other quiet waters of parks and rural areas. Foothill yellow-legged frogs and western spadefoots aren’t listed because populations not known to be at risk exist outside the Bay Area, but the two species certainly seem at risk here. I’ve heard of only two or three streams in the East and South Bay where sizeable populations of foothill yellow-legged frogs survive, although they may be doing better in the North Bay. The spadefoot toad’s situation may be even more precarious, since its main local population at Corral Hollow is under pressure from increased traffic, off-road vehicles, and pet collecting. (“Spadefoot toads are cute,” Mark Jennings told me.) Larry Serpa said the spadefoot population east of Henry Coe is “pretty minimal.”

Urban sprawl and its attendant habitat destruction, water pollution, and traffic are obvious factors in anuran decline. But the creatures have dwindled even in natural areas. Thin-skinned amphibians may be especially vulnerable to what Stebbins called the “witch’s brew” of airborne farm and urban chemical pollutants, as suggested by their decline in places where prevailing winds concentrate it, such as the Sierra Nevada. Jennings said med-fly spraying in the 1980s may have impacted Bay Area anurans seriously. Not only bullfrogs but introduced fish like bass and perch (even the little mosquitofish distributed by insect-abatement agencies) can reduce breeding natives. Increased ultraviolet radiation induced by atmospheric ozone depletion may sterilize eggs, as studies in Oregon seem to show, although Jennings said other studies have failed to duplicate those results.

Whether native anurans will continue to hang on in the face of these threats is an open question. Much of the effort to protect anurans recently has gone toward the red-legged frog because it is federally listed. A recent court decision resulting from a lawsuit brought by the Center for Biological Diversity declared much of the East Bay “critical habitat” for the species, forcing federal agencies to take that into account in all permitting decisions. This is having an impact in places such as Corral Hollow, where the state’s plan to extend an off-road vehicle area was sent back for further review because it includes potential frog habitat.

The best hope for anurans lies in parks and other protected lands, and management agencies are looking at ways of being frog- and toad-friendly. But, again, doubts about the causes of decline muddy the waters. Stebbins told me that East Bay Municipal Utility District restored a red-legged frog population when it fenced cattle out of a creek. Cattle trampling and overgrazing certainly can degrade riparian and wetland habitat. On the other hand, Bobzien told me that cattle don’t seem to be a major factor limiting the species on EBRPD lands. He said that 75 of the 265 ponds on EBRPD properties support red-legged frogs (65 have breeding populations), and the most valuable ones aren’t fenced for cattle. Gary Fellers, a U.S. Geological Survey research biologist at Point Reyes National Seashore, said that red-legged frogs inhabit most of the stock ponds of the Seashore’s dairy farms, and that there are more red-legged frogs on grazed lands than ungrazed. Jennings says that frogs and cows can coexist, but proper management to ensure the health of riparian vegetation and a minimum of siltation is essential.

In an attempt to better understand the relationship between cattle and red-legged frogs, Fellers has been conducting radio tracking studies of the frogs’ movements—they can travel as much as a mile away from breeding waters during winter floods. I asked Fellers how he attaches transmitters to frogs. “We have little belts that we put on them,” he told me. “You just straighten the legs out and slip them on.” He also plans to fit frogs with infrared lights so he can study their nocturnal behavior.

Everybody concurs, at least, on the threat bullfrogs and exotic fish pose to red-legged frogs. In some cases, Bobzien told me, EBRPD has restored red-legged populations by removing bullfrogs and exotic fish from ponds, and “redesigning” the ponds to favor native species. Bobzien’s counterpart at the Mid-Peninsula Regional Open Space District, Stan Hooper, has begun to implement similar plans.

Management agencies seem less clear about conserving the four nonlisted species. Bobzien said EBRPD is trying to link protection of its one substantial foothill yellow-legged frog population with a steelhead trout restoration project, since a stream that has the water quality and quantity to support steelhead breeding will probably support yellow-legged frog breeding. So far, the main strategy for protecting the spadefoot toad, as with western toads and Pacific treefrogs, seems to be simply preserving as much habitat as possible.

Bay Area frogs and toads are definitely down in the 21st century, but that doesn’t mean they’re out. After all, they are old hands at survival. Fossils indicate they appeared in the Jurassic period, during the time of the dinosaurs, and remained abundant through the next 200 million years while dozens of majestic organisms perished in mass extinctions around them. As Robert Stebbins cautioned, predicting anuran demographics is tricky. Populations can burgeon or shrink mysteriously, and may even reappear where they’ve seemed long gone. The larger species begin to breed when they’re two or three years old and have relatively long life spans: California red-legged frogs can live for up to eight years in the wild, and toads have been known to inhabit the same garden for a decade or more. Treefrogs have shorter lives but start breeding even sooner.

The possibility that native anurans may survive in the remoter parts of the Bay Area doesn’t really compensate, however, for the absence of treefrogs calling outside the window on a rainy night, or of toads snapping up garden bugs on a summer evening. (A book published in 1961 estimated the value of a single toad’s insect-control function at 20 to 50 dollars a year, a value that doubtless has multiplied along with local real estate prices.) It is possible to re-establish anuran populations in suburbs, at least temporarily. I’ve seen native frogs and toads living in an Ohio backyard, whose owners told me that American toads, Bufo americanus, had been returning to breed there for several years. But it’s not easy, as I found with my Pacific treefrogs.

I had hopes for a while after I released the brood. However, the neighborhood cats soon became quite vigilant in the garden; unlike western toads, Pacific treefrogs are non-toxic. At night I heard raccoons yammering and splashing, and one morning I came out to find all the stones I’d used to floor our vernal pool pulled up and piled in the middle, with a proprietary scat on top. A bigger, deeper pool would have been better, since raccoons have trouble foraging in water over their heads, but we don’t have room in our yard. When I asked Mark Jennings for advice on treefrog gardening, he suggested chicken wire.

Still, one of the treefrogs—a big dark brown one—lived in our toolshed for over a year. We’d see him sometimes on the top shelf and hear his croaks even in dry weather. He finally seems to have disappeared, but I’m keeping my eye on the wired-over vernal pool.

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