Mark Twain's frog is understood to have been the California red-legged frog, the largest native frog west of the Continental Divide. This frog was common at that time to the wetland areas near Angels Camp, a fading gold mining camp in the Sierra foothills where the story takes place. Its habitat extended throughout California's Coast Range and Sierra foothills and around the northern end of the Central Valley, and on down into Baja California. Today it is largely limited to parts of the Coast Range. Although "Twain's frog" is a folk hero regularly evoked by boosters in Calaveras County, and although frog jumping contests are still a popular event at the annual county fair, the red-legged frog is extinct in the Central Valley and has vanished from 99% of its Sierra range.

The decline of the red-legged frog matches that of amphibians generally. Of the world's 6260 documented amphibian species, 2030 – nearly one-third – are threatened with extinction or have recently become extinct, and a significant proportion of the remaining species are likely to be so.1 Entire communities of amphibians are facing ecological collapse.

Amphibians are especially vulnerable to environmental change. Since they live both on land and in water, they are exposed to habitat changes and contaminants in both places. Their skin is highly permeable and readily absorbs pollutants in the water. They are unprotected by hair or feathers. Their eggs, too, are vulnerable; they are not encased in hard shells, but in many cases are laid in soft masses that float on the surface of the water, attached to vegetation to keep them in place, until they hatch. A rampant fungal infection, chytridiomycosis, believed to have been spread by a South African frog widely used for pregnancy tests, is also devastating amphibian populations.2

In the case of the red-legged frog, a number of possible causes for its decline have been investigated, including habitat loss, predation by non-native species (especially the bullfrog), exposure to agricultural chemicals, and UV-B radiation. There is evidence that each of these has played a role. One study demonstrated a link between extinction sites and upwind farmland where pesticides were heavily used, especially in the Central Valley and the Sierra foothills; it also found a link with higher elevations where the increase in ultraviolet radiation is especially noticeable.3 Other studies have found similar results. But scientists broadly agree that the major culprit is habitat loss and fragmentation. Great swaths of wetland are being filled to enable the construction of homes, shopping malls, reservoirs, and roads, and the siting and approval of these developments have not always complied with the requirements of the Endangered Species Act (ESA).

The red-legged frog was formally listed as an endangered species under the ESA in 1996. Listing requires that critical habitat be defined and protected, and that a recovery plan be drawn up. It has been shown that species that have enjoyed critical habitat protection for two or more years are more than twice as likely to show increasing population numbers.4 Yet typically there is at best a considerable lag between the listing and the defining, protecting, and drawing up.

The process began well enough for the red-legged frog. In 2004 the U.S. Fish and Wildlife Service proposed to protect over 4 million acres of critical habitat for the animal, in keeping with scientific recommendations. But by 2006, due to a lawsuit by developers and pressure from Administration officials, this number had been revised to about 450,000 acres – one-tenth of what had been called for. And development projects went ahead accordingly.

In response to allegations of political corruption and another lawsuit, a new analysis resulted in the 2010 decision to protect about 1.6 million acres, 2764 of which are in Calaveras County, where previously there had been none.5

A separate lawsuit, which addressed approval by the Environmental Protection Agency of toxic pesticides in and near red-legged frog habitat, led to a 2013 settlement that will help protect the frogs from seven of the chemicals most toxic to amphibians.6 Given that the original list included 64 pesticides of concern, it remains to be seen if this will prove adequate.

In the meantime, a few years earlier, two children discovered a small population of red-legged frogs near a pond on their family's Calaveras County ranch, the first sighting in the county since 1969.7 The family has been working with biologists to find the best way to combine cattle ranching with protecting the frogs and their habitat. With this combination of enthusiasm and expertise, there may be cautious hope for this celebrated critter.

1 The IUCN Redlist of Threatened Species, 2016-1.

2 Whittaker, K and Vredenburg, V. “An Overview of Chytridiomycosis.” Amphibiaweb, 17 May 2011.

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3 Davidson, C; Shaffer, HB; & Jennings, MR. "Declines of the California red-legged frog: climate, UV-B, habitat, and pesticides hypothesis." Ecological Applications 11(2): 464-479. April 2001.

4 Taylor, MFJ; Suckling, KF; & Rachlinski, JJ. "The effectiveness of the Endangered Species Act: a quantitative analysis." BioScience 55(4): 360-367. April 2005.

5 Dept of the Interior, Fish and Wildlife Service. “Endangered and Threatened Wildlife and Plants; Revised Designation of Critical Habitat for the California Red-Legged Frog.” Filed 16 Mar 2010.

6 Center for Biological Diversity v. United States Fish and Wildlife Service et al. Case3:11-cv-05108-JSW, Document764, Nov 2013.

7 Ezell, W. “Red-legged frogs found on west Calaveras County ranch.” Calaveras Enterprise, 2 Dec 2003.