- In recent years, the frequency of malformed frogs, toads,
salamanders and other amphibians found with missing limbs, extra limbs,
and skin webbings has increased. The shrinking populations of many North
American amphibian populations underscore the need to understand the causes
and implications of this phenomenon. Now a new study suggests that a parasite
may be to blame for many of the abnormalities found in amphibians of the
western United States.
-
- In the research article "Parasite (Ribeiroia Ondatrae)
infection linked to amphibian malformations in the western United States,"
appearing in the May issue of the Ecological Society of America's journal
Ecological Monographs, Pieter Johnson of the University of Wisconsin-Madison
and colleagues describe the results of their broad-scale field survey.
Covering parts of California, Oregon, Washington, Idaho, and Montana, the
team of researchers looked for malformations in over 12,000 amphibians
representing 11 species of amphibians. The group looked at the relationships
between the frequency and severity of abnormalities and a variety of factors
in a particular aquatic site, including the abundance of a parasite (Ribeiroia)
and pesticide contamination.
-
- The collaborative and interdisciplinary effort, which
included academic researchers, as well as federal scientists from the U.S.
Geological Survey and the U.S. Fish and Wildlife Survey, found malformed
amphibians at a wide variety of aquatic sites, ranging from montane lakes
and ephemeral pools to irrigation canals and impoundments. While the researchers
did not find a relationship between pesticides and the frequency of malformed
amphibians, they did find a striking connection between malformed amphibians
and the presence of Ribeiroia.
-
- "The presence of this parasite was a powerful predictor
of the presence and frequency of malformed amphibians in an aquatic system.
The greater an amphibian population's infection with Ribeiroia, the more
frequent and severe the population's limb malformations," said Johnson.
-
- Amphibians at sites supporting the parasite exhibited
6 times as many abnormalities as the average number of malformations recorded
at sites without the parasite. The researchers found the parasite embedded
around the base of amphibians' limbs and tails, where they form cysts beneath
the skin. The frequency of abnormalities varied substantially among sites
and species, ranging from 0 to nearly 90 percent. Pacific treefrogs (Hyla
regilla) exhibited the greatest number of abnormalities, with more than
1000 abnormal tadpoles and young frogs found at 55 sites. When they co-occurred
at an aquatic site, Pacific treefrogs exhibited more abnormalities than
did western toads, marginally more than bullfrogs, and less than California
newts.
-
- In order to understand if and why the parasite may have
become more common in recent years, the research team also gathered information
on the ecology and life history of Ribeiroia, which has a multi-host life
cycle.
-
- The paper notes that the final stage of the parasite's
life cycle is dependent upon predation; in order to complete its final
developmental stage, the parasite depends upon a bird or mammal to eat
an infected amphibian or fish. The parasite then sexually matures and releases
eggs via bird or mammal feces. When the eggs hatch they invade the tissue
of Planorbella, an aquatic snail.
-
- The study revealed that this aquatic snail is a significant
indicator of both the presence and abundance of the parasitic infection.
The presence and abundance of this snail were the only two factors related
to the presence or abundance of Ribeiroia.
-
- The researchers note that both Planorbella and increased
parasitic diseases are associated with artificially created wetlands, which
have been on the increase, often replacing natural wetlands.
-
- "People assume that parasites are "natural"
and therefore of no conservation concern," says Johnson. "However,
we suspect that nutrient pollution from fertilizers and cattle may be increasing
the numbers of snails, parasites, and therefore malformed amphibians."
-
- Johnson notes that the parasite may be of particular
concern to declining amphibian populations, such as Western Toads and Columbia
Spotted Frogs, which were often found infected and malformed.
-
- Founded in 1915, the Ecological Society of America (ESA)
is a scientific, non-profit, organization with over 7500 members. Through
ESA reports, journals, membership research, and expert testimony to Congress,
ESA seeks to promote the responsible application of ecological data and
principles to the solution of environmental problems. For more information
about the Society and its activities, access ESA's web site at: http://www.esa.org.
___
-
- Editor's Note: The original news release can be found
at http://www.esa.org/pao/pr041802.htm
-
- Note: This story has been adapted from a news release
issued by Ecological Society Of America for journalists and other members
of the public. If you wish to quote from any part of this story, please
credit Ecological Society Of America as the original source. You may also
wish to include the following link in any citation: http://www.sciencedaily.com/releases/2002/04/020419064712.htm
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