The Fall of the Frog

Carel Brest van Kempen - AFC
March 31, 2011 share
Artists for Conservation

 

I turned 14 in May of 1972, and like any self-respecting country boy of that age, I looked forward to the impending emergence of hibernating frogs. Three ponds near my home harbored good populations of Northern Leopard Frogs,  and each summer I brought home several eggs to watch the twitching embryos grow within their gelatinous orbs before bursting from them and metamorphosing into small frogs over the summer.

That year, though, no more than half a dozen adults roused from their winter's sleep, and no eggs were in evidence. Those few Northern Leopard Frogs were the last I ever saw in the region. More than a decade would pass before I learned that this was but one of many local frog extinctions occurring across the Americas and in Australia during that period.

In most cases, the causes of these extinctions are still unclear. Ultraviolet radiation was one of the first suspects. In the Cascade Mountains, the eggs of Western Toads and Cascades Frogs have shown reduced viability from increased UV exposure, and after hatching, the young tadpoles of the former species showed increased vulnerability to attack by the fungus Saprolegnia ferax.

In Costa Rica's Monteverde Cloud Forest, climate change has been blamed for the mysterious 1989 extinction of the Golden Toad and the declines of several other frog species, as the persistent cloud that creates that environment slips up the mountainside.

In the mid '90s, Northern Leopard Frogs, with deformed and extra limbs, made the headlines in the upper Midwest. These anomalies were caused by the flatworm Ribeiroia ondatrae, which attacked the limb buds of tadpoles. The worms require snails as hosts in an earlier stage of their complex life cycle, and eutrophication, or over-fertilization, caused by phosphate- and nitrate-rich runoff from chemical fertilizers seems to have encouraged algal blooms, and in turn, a population boom of algae-eating snails and R. ondatrae.

Affected frogs are more likely to be preyed on by herons and other birds, the final hosts needed to complete the worm's life cycle. The presence of other pollutants, such as the popular insecticides Malathion and Esfenvalerate, and the weed killer Atrazine, [http://en.wikipedia.org/wiki/Atrazine] over 60 million pounds of which is applied to the U.S. every year, appears to increase the likelihood that parasitized tadpoles will manifest deformities.

The powerful endocrine disrupting effects of very low levels of Atrazine has also been shown to destroy the reproductive function of male frogs. Chemicals diffuse very easily through amphibian skin, making them especially vulnerable to environmental toxins.

In 1999, a new amphibian threat was identified, in the form of a previously unknown fungus which was christened Batrachochytrium dendrobatidis. Researchers have tried in vain to discourage the use of the term "Chytrid" (which describes a large family of fungi) for this infection, preferring instead the abbreviation for the specific Latin binomial: "Bd."

Bd appears to have been literally carried around the world on the backs of African Clawed Frogs, [http://en.wikipedia.org/wiki/African_clawed_frog ] which were used in pregnancy tests during much of the 20th century. The disease is believed to have caused the recent extinctions of well over 100 frog species, and has devastated more, including the Central American Varied Harlequin Toad and the Australian Corroboree Toadlet, both of which are essentially extinct in the wild.

The pandemic of Bd is considered an ecological emergency, and an international project, the Amphibian Ark,  is dedicated to the captive propagation of the most critically affected frog species.

Although Bd has understandably received the focus of recent conservation efforts, 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, diversion of waterways, 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. It's only through continued research and observation that the total picture will become more clear - then we can begin to ponder the question of how frog decline affects ecosystems in toto.

For more information on Bd, see herehere and here.

For more information on Atrazine see here.

To watch Tyrone Hayes' excellent lecture on Atrazine see here.

 

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