Rigor Vitae: Life Unyielding

Thursday, February 28, 2008


Posts have been sparse around here as of late, and today's a special day, so I'm indulging in a particularly long and in-depth one. It's Leap Day: a day that has been deemed, appropriately enough, The International Day of the Frog. This dubious honor was bestowed on the order Anura to bring attention to the dramatic decline that frogs and toads have suffered over the past quarter-century. In the spring of 1972, I noticed that virtually all of the Northern Leopard Frogs (Lithobates pipiens) in my area failed to rouse from hibernation. At the time I assumed it was a local crisis, and more than a decade would pass before I learned that biologists had been chronicling similar phenomena across the Americas and in Australia.

In searching out the roots of this dramatic decrease, ultraviolet radiation was one of the first suspects. A study of Western Toads (Bufo boreas) in the Cascade Mountains found adults to be mating normally and laying viable eggs. At the age of only a few days though, the tadpoles were turning white and dying. The direct killer was a fungus, Saprolegnia ferax, which seemed to attack tadpoles that had been weakened by UV exposure in the egg. In the laboratory, eggs of Western Toads and two other anuran species from the Cascades were subjected to high levels of UV radiation. The other two species were the Cascades Frog (Rana cascadae), also declining in numbers, and the Pacific Treefrog (Pseudacris regilla), whose population was holding steady. The irradiated eggs of the Pacific Treefrogs seemed unaffected, while the eggs of the other two species showed severely reduced viability. Subsequent tests with two dwindling salamanders of the area, the Pacific Salamander (Ambystoma gracile) and the Long-toed Salamander (A. macrodactylum) also showed their eggs to be vulnerable to UV radiation.

This was surely not the whole story, though. Many frogs lay their eggs in protected sites, well shielded from the sun’s damaging rays. Two declining Costa Rican frogs are good examples: the Spiny-headed Treefrog (Anotheca spinosa) lays its eggs in tree cavities, and the Tilarán Rain Frog (Eleutherodactylus angelicus) lays them in subterranean burrows.

It was in Costa Rica that the story unfolded further. In the late 1980s a herpetologist named J. Alan Pounds and his colleagues were surveying the herpetofauna of the Monteverde Cloud Forest Preserve, where weather patterns and topography conspire to create a practically permanent cloud that rests high on the mountainside. In their thirty square kilometer study area, 20 of the 50 resident frog species disappeared within a few years. Included in this dubious list of twenty was the famous Golden Toad (Bufo periglenes), known only from Monteverde. This toad once congregated in large breeding masses. In 1987 1,500 individuals were counted. The following two years only a single male was seen. He was likely the last member of his species.

Pounds noted that this frog crash coincided with abnormally dry years at the site. He posited that a warming trend was causing Monteverde’s cloud to sit higher on the mountain, thus drying up the lower regions. Many bird species moved uphill, but the frogs, which are not equipped to make such a transit, simply died. He also suggested that the drier habitat could concentrate dissolved toxins from air pollution in the sparser mist.

In 1998, another neotropical biologist, Karen Lips, was surprised to find quantities of dead frogs in her study area in Panama. Since frog corpses don’t last long in the jungle, she presumed that what she saw was just a microcosm of what was actually happening. Inspection of the dead amphibians revealed that they had all been attacked by a chytrid fungus, which was christened Batrachochytrium dendrobatidis. Today this fungal infection, known as Chytridiomycosis, or more simply, Bd, is generally considered the biggest threat to amphibian populations, though it is still poorly understood. It has been strongly implicated in the catastrophic decline of Stub-footed Toads (Atelopus spp.), which consist of around a hundred species, all of which have declined by 80% or so in just a few generations. Perhaps half of Atelopus species are now extinct in the wild. Ten years ago, hordes of Varied Harlequin Toads (A. varius) – pictured above – congregated streamside during the Costa Rican dry season in many parts of the country. Today it is little more than a memory. For more about Bd, check here, here and here.

A dramatic chapter in this saga began in 1995, when eight middle school students from Henderson, Minnesota began catching Northern Leopard Frogs at the edge of a pond during a field excursion. It soon became apparent that something was very wrong with these frogs. Many of them had deformed, missing, or extra hind legs. In fact, of 22 frogs the kids caught, half of them showed such deformities (of course the affected frogs were far more likely to be caught). The media latched onto this story with the enthusiasm of a Rottweiler. Word of the freakish Minnesota frogs spread quickly, and soon ponds in Vermont, Ontario, Wisconsin and beyond were yielding anurans with similar afflictions. By the year 2000, such frogs had turned up in forty-three of the fifty American states. The villain here turned out to be a fluke – a flatworm of the class Trematoda, the group that contains the agent of the dreaded tropical disease schistosomiasis.

This particular fluke was called Ribeiroia ondatrae. Like many parasites, its life cycle seems impossibly complex. The eggs hatch in open water, and emerge in a free-swimming stage called a miracidium, which must find its first host, an aquatic snail, within hours or die. While inside the snail it undergoes further physiological change, including an amplification that can result in the emergence of hundreds of the next free-swimming stage, the cercarium. It is this stage that attacks tadpoles, burrowing into their flesh, tending to attack the region of the hind limb buds. Once ensconced within its amphibian host, the cercarium forms a cyst, and enters a state of dormancy. It is the trauma caused by these cysts that induces leg deformities, and this works out nicely for the flukes, since it makes the adult frog more vulnerable to capture by R. ondatrae’s next host: a heron or other predaceous water bird. The cercarium finally metamorphoses into an adult within the bird, falls in love, and eggs are laid, to be ejected with the bird’s feces, hopefully into a body of water where the cycle can start again.

Records of frogs with legs to spare go back to the Civil War, but they are becoming increasingly common. One theory posits that eutrophication, or over-fertilization, caused by phosphate- and nitrate-rich runoff from chemical fertilizers has encouraged algal blooms, and in turn, a population boom of algae-eating snails and R. ondatrae.

Experimental evidence suggests that the presence of certain chemicals in the water increases the likelihood that parasitized tadpoles will manifest deformities. These chemicals include the popular insecticide Malathion, the synthetic pyrethroid insecticide Esfenvalerate, which has gained favor recently for being less toxic to mammals and birds than it is to insects, and the weed killer Atrazine, over sixty million pounds of which is applied to the United States every year. The levels of Esfenvalerate and Atrazine necessary to induce fluked-up limbs fall well below the EPA standards for human drinking water. Incidentally, low levels of Atrazine also appear to cause severe testosterone reduction in male frogs, causing them to become reproductively functionless.

The logic of how these chemicals increase a tadpole’s chances of developing crazy legs is admittedly elusive. It’s doubtful that it has anything to do with weakened antibodies, since tadpoles have no known antibody system to speak of, and the fluke cysts cloak themselves in an antigen-resistant pellicle, anyway. For more on Atrazine, see here, and click here to watch Tyrone Hayes' excellent lecture about the herbicide.

In short, there is no simple culprit of the current frog decline, but a partially understood collection of interrelated factors. Acid rain has been implicated in crashes of Natterjack Toads (Bufo calamita) in Southern England and Red-legged Frogs (Rana aurora) in central California. PCBs and organochlorides are blamed for the decline of Mountain Yellow-legged Frogs (Rana muscosa) in the Sierra Nevada Mountains. Aerosolized clouds of these chemicals blow up the mountains from cities and agricultural areas to the west, to fall with the rain and ultimately settle into pond bottoms, where they are absorbed by hibernating tadpoles ensconced in the mud.

Invasive plant and animal species have hurt many amphibian populations, including fellow anurans like the American Bullfrog (Lithobates catesbianus) and the Cane Toad (Bufo marinus). In western North America, the introduction of trout, bass, and other game fish has led to the disappearance of native amphibians from many waters. Pressure from commercial hunting for frog legs has devastated populations of the Pig Frog (Lithobates grylio) in the United States, the Edible Frog (Rana esculenta) in Europe and R. tigrina and R. hexadactyla in India and Bangladesh.
Intensive logging also has adverse effects. Not only does it destroy microhabitats directly, but it does other damage as well, such as overall desiccation of the area and soil compaction. It’s estimated that logging in the southern Appalachians has caused a 9% overall decrease of salamander populations in the region.
Nothing has had a bigger impact on the ecology of the arid western United States than water redistribution, and that has been disastrous for amphibians. Las Vegas, with its fabulous fountains and golf courses, depleted the waters of that region like a gaudy Tiddalik, causing the total extinction of the Vegas Valley Leopard Frog (Lithobates fischeri) in the 1940s.
Today, a similar growth boom is threatening the unique ecology of the Virgin River drainage in southwestern Utah and adjacent Arizona and Nevada. The once charming town of St. George, Utah currently boasts thirteen thirsty golf courses in a region nearly as hot and dry as Death Valley. We usually see beauty in chlorophyll’s verdancy, but I can think of few sights more vulgar than an emerald golf course imposed on a redrock desert. I will happily go to my grave having never hoisted a golf club, nor rubbed elbows with the greedy golf entrepreneurs who suck the lifeblood from our deserts. I cannot imagine contributing a nickel to their cause, unless it was delivered via slingshot through a clubhouse plate glass window. The Relict Leopard Frog (Lithobates onca), endemic to the Virgin River area, is surely doomed to become another of their victims.
Livestock grazing, bête noir of the conservation movement, has a mixed record with respect to anurans. Moderate levels of manure in breeding pools encourage growth of algae and invertebrates, important food sources for tadpoles. Larvae of a few species, such as the Syrian Spadefoot Toad (Pelobates syriacus), will thrive on a cow pie diet. In Scandinavia the Green Toad (Bufo viridis) is dependent on cattle to enrich breeding pools to a point where its tadpoles can flourish, and to maintain the open meadow habitat favored by the adults. In the same area, Firebellied Toads (Bombina bombina) fail to persist where cattle have dwelt. The kind of insanely intensive grazing that we happily see much less of in North America these days, tends to wipe amphibians out altogether.

The true (and still poorly understood) picture of amphibian decline is a mosaic of interlocking factors. Warming and drying trends in the climate, habitat alteration, runoff of pesticides, fertilizers, and other chemicals, invasive species and pathogens and plain old stress combine in various ways to strike at their victims in ways that can differ substantially in different situations. The plights of many species look hopeless, and a concerted, global effort is being put into captive propagation of the most critical cases. To find out more about this effort, visit the Amphibian Ark site, and what better way to celebrate International Frog Day than to make a donation?
upper: BLUE-LEGGED TREEFROG (1998) acrylic 7" x 7"
second: VARIED HARLEQUIN TOAD (2008) digitally colorized acrylic underpainting 20" x 15"
third: PAINTED REED FROG (1999) acrylic 7" x 7"
lower: GREEN PADDY FROG (1999) acrylic 7" x 7"