The California Red-legged Frog
The California red-legged frog has been protected as a threatened species by the Endangered Species Act since June 1996. It was the first species to be added to the list of threatened and endangered species following a year-long Congressional moratorium on listings. The moratorium began in April of 1995 and was lifted on April 26, 1996. The moratorium imposed by Congress prevented final listings of endangered and threatened species and final designations of critical habitat.
Threatened species are animals and plants that are likely to become endangered in the foreseeable future. Identifying, protecting, and restoring endangered and threatened species is the primary objective of the U.S. Fish and Wildlife Service’s endangered species program.
The California red-legged frog is the largest native frog in the western United States. It is one of two subspecies of the red-legged frog found on the Pacific coast; the other is the northern red-legged frog Rana aurora aurora. The California red-legged frog once ranged across much of California, including portions of the Sierra Nevada Mountain Range, where it is believed to be the title character of Mark Twain’s famed short story, “The Celebrated Jumping Frog of Calaveras County.” In 1865, when the story was written, red-legged frogs were the largest frogs in the state; bullfrogs were not introduced to California until 1896.
The historic range of the California red-legged frog extended coastally from the vicinity of Point Reyes National Seashore, Marin County, and inland from the vicinity of Redding, Shasta County, in California southward to northwestern Baja California, Mexico.
In the mid-1800s, whole mountains were washed away by placer mining, and almost every stream east of the Central Valley was choked with mud, silt, and rock, destroying thousands of acres of frog habitat. It was an unprecedented ecological catastrophe, and led to some of the first environmental laws.
During the late 1800s and early 1900s, the California red-legged frog was harvested for food in the San Francisco Bay area and the Central Valley, with approximately 80,000 frogs harvested annually. As the frog become more rare, the market for them declined. Bullfrogs (Rana catesbiana) were introduced in California around 1896 to help satisfy the demand for frog legs as the red-legged frog population dwindled. Ironically, the native red-legged frog soon become prey for the much larger bullfrog, a threat to the red-legged frog’s existence that continues today.
At about the same time the frog population began its decline, Central Valley wetlands and riparian habitats were being converted to agricultural land. Streams were denuded of riparian vegetation and channelized. Livestock grazing removed vegetation cover and undercut the banks along streambanks, causing increased water temperatures and lack of cover. These changes resulted in a loss of more than 90 percent of historic wetlands, with the majority of that loss occurring before 1939. California red-legged frogs were eliminated from the Valley floor by 1960. Those populations that remained in the Sierra Nevada foothills were separated from other populations and nearly eliminated from this area as a result of reservoir construction, introduction of exotic species, and drought.
In Southern California, urbanization with its resulting infrastructure, including road construction, channeling of streams, and reservoir construction, had a devastating impact on red-legged frogs. The frog was once common in parts of Los Angeles, San Bernadino, Orange, Riverside, and San Diego counties. In general, this species has been extirpated from these counties although a few populations still exist. A recent population was discovered in East Las Virgenes Creek in near the Ventura and Los Angeles county border and adjacent to the Santa Monica Mountains National Recreation Area. South of the Tehachapi Mountains, two populations can be confirmed. One is at the Santa Rosa Reserve in western Riverside County, which is managed by The Nature Conservancy and another is in Los Angeles County at Sierra Pelona near Palmdale.
Range and Habitat
The present distribution ranges from Sonoma and Butte Counties in the north to Riverside County in the south, primarily in the western counties.
California red-legged frogs have been eliminated from more than 70 percent of their historic habitat. Surveys indicate the frogs are present in about 10 percent of their historic locations. California red-legged frogs are found primarily in wetlands and streams in coastal drainages of central California.
Today they are known to occur in about 238 streams or drainages in 23 counties. Monterey, San Luis Obispo, and Santa Barbara counties support the greatest amount of currently occupied habitat. Only four areas within the entire historic range of the subspecies may currently support more than 350 adults.
Red-legged frogs require aquatic habitat for breeding but also use a variety of other habitat types including riparian and upland areas. Adults often utilize dense, shrubby or emergent vegetation closely associated with deep-water pools with fringes of cattails and dense stands of overhanging vegetation such as willows.
Adult frogs that have access to permanent water will generally remain active throughout the summer. In cooler areas, they may hibernate in burrows or other refugia in the winter. Red-legged frog adults may move both upstream and downstream of their breeding habitat to forage and find refuge.
California red-legged frogs range from 1.5 to 5 inches in length. The belly and hind legs of adult frogs are often red or salmon pink; the back is characterized by small black flecks and larger dark blotches on a background of brown, gray, olive or reddish-brown. They can also be recognized by their low, staccato grunts, heard during the few weeks between late winter and early spring when they breed. This is one of the features that separates this subspecies from its silent cousin, the northern red-legged frog, which has no vocal sacs.
California red-legged frogs are relatively prolific breeders, usually laying egg masses during or shortly following large rainfall events from late December to early April. Females can lay between 2,000 and 5,000 eggs in a single mass. The eggs are attached to vertical emergent vegetation such as bulrushes or cattails.
The eggs hatch in 6 to 14 days and, approximately 3.5 to 7 months later, the tadpoles develop into frogs. The highest rates of mortality for this species occur during the tadpole stage; less than 1 percent of eggs hatched reach adulthood.
Tadpoles and young frogs thrive on invertebrates, which they catch with their mouths. They hunt day and night. This constant activity makes them visible, and, therefore, more vulnerable to predators. About half of the prey mass in the diet of adult frogs consists of Pacific tree frogs and California mice; insects are the most common prey items. Adults feed and are active largely at night.
Over the last two decades, scientists have noted a widespread decline of frogs and other amphibians, the causes of which are not fully understood. The decline of the California red-legged frog is attributed to the spread of exotic predators such as bullfrogs, and the widespread changes that have fragmented habitat, isolated populations, and degraded streams. Bullfrogs, introduced to California from Maryland and Florida in 1896, are more suited to survival in human-disturbed areas than the native ranids, partly because they can tolerate warmer water temperatures. The decline signals a loss of diversity and environmental quality in wetlands and streams that are essential to clean water and to the survival of most fish and wildlife species.
What is going on to help the population recover?
- Recovery Plan - The U.S. Fish and Wildlife Service’s draft recovery plan for the California red-legged frog was released for public review on May 12, 2000. Public comments are being reviewed. The strategy for recovery will involve: protecting existing populations by reducing threats; restoring and creating habitat that will be protected and managed in perpetuity; surveying and monitoring populations and conducting research on the biology and threats of the species; and re-establishing populations of the species within the historic range.
- Critical Habitat - The Fish and Wildlife Service published the critical habitat for the California reg-legged frog in the Federal Register on September 11, 2000. Public comments are being reviewed.
- Captive Breeding - The Service is interested in working with the Mexican government, University of California at Davis, Los Angeles Zoo, and the Nature Conservancy to augment the dwindling population in Southern California. Tentative plans include obtaining genetic samples by toe clippings from a population of red-legged frogs in the San Rafael drainage near Colonet, Baja California (Mexico) to determine if the population is compatible with the population in Riverside. If it is, then the Service would like to obtain some of the Mexican frogs and transport them to the Zoo for captive breeding. The captive-reared frogs could then be released at The Nature Conservancy's Reserve.