USGS Frequently Asked Questions

Answer:

Declines in several amphibian species have been studied for the past 20 years. More recent concern about deformed frogs arose with the discovery of large numbers of deformed leopard frogs in Minnesota in 1995. But scientists who have studied amphibian declines and deformities agree that deformities are unlikely to have caused the extensive, well-documented declines of many amphibian species worldwide. USGS scientists have described severe declines of several amphibian species in Colorado and California. Although they have observed that many species are absent from historically documented localities, often in national parks and wilderness areas, they have rarely observed deformities. For example, USGS scientist Dr. Gary Fellers has studied declining amphibian populations in California for the last 9 years. He and his field crews have visited over 4,500 sites and examined over 8,000 frogs and toads. Fewer than 10 of these individuals had any type of deformity.

The recent research on deformities provides additional evidence that deformed frogs may occur naturally as a result of parasite infections. But it does not prove that all deformities are caused by parasites nor that global species declines are associated with deformities. It is probable that deformities in different localities have different causes. In the Upper Midwest, deformities similar to those in wild frogs have been produced in laboratory-reared frogs exposed to water and soil extracts from sites where deformed frogs have been studied. Extracts did not contain the parasite (metacercariae) but did contain chemicals present when samples were taken. Thus, chemical causes of deformities should continue to be explored. In studies of deformed leopard frogs in Midwestern and Northeastern states, scientists at the USGS National Wildlife Health Center did not find parasites (metacercariae) in the limb buds of the developing tadpoles and they have not yet found evidence that a parasite influenced limb development in these tadpoles.

A variety of mechanisms, including habitat alteration and destruction, introduction of exotic predators, disease, and possibly airborne contaminants, have brought about amphibian declines. For several declining species, such as the boreal toad in the Rocky Mountains or several species studied by Dr. Fellers in the Sierra Nevada, the agents have not been discovered. This is cause for concern because these studies have been conducted in undisturbed or protected locations.

The USGS is continuing research on the causes of amphibian declines. We believe that this research will lead to a better understanding of why we are losing frogs and toads in the very parks and wilderness areas where we would expect them to be best protected.

If you have an interest in these topics, we suggest that you visit http://biology.usgs.gov/frog.html, http://www.nwf.org/frogwatchusa/, and http://www.frogweb.gov/index.html. All of these sites contain many links to related amphibian information.