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Out-of-balance ecosystems play role in demise of amphibian populations
Jim
Barlow, Life Sciences Editor
217-333-5802; jebarlow@illinois.edu
2/10/04
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CHAMPAIGN,
Ill. — During the last decade, Val Beasley of the University of
Illinois College of Veterinary Medicine has led a team wanting to know why the world’s amphibian populations
have been dwindling or riddled with limb deformities.
Evidence from his and other teams points to increasing numbers of common
parasites as an important cause. However, the problems facing amphibian
habitats really pose a poignant example of ecosystems out of balance
because of human activity, according to Beasley, a professor of veterinary
biosciences and executive director of the Envirovet
Program in Wildlife and Ecosystem Health.
Beasley’s main collaborators, postdoctoral researcher Anna M.
Schotthoefer and Rebecca A. Cole, an adjunct professor of veterinary
pathobiology at Illinois and scientist with the National Wildlife Health
Center in Madison, Wis., say they agree.
A series of their research projects – two published in 2003 and
another that will appear as a book chapter this year – dramatically
refine the data on parasitic activity and argue that physically and
chemically induced changes of aquatic habitats are taking a toll.
“We
have to be asking what human activities are contributing to imbalances
in these ecosystems to set the stage for more severe infections,”
Beasley said. “It’s becoming a serious question of how we
can better manage landscapes, streams, wetlands, ponds and lakes. Frogs
are among the first animals that young children see in a healthy wild
place, but they are not finding them in the same numbers as in past
years.”
Tadpoles of many species feed on algae and periphyton – plant
slime that grows on other surfaces – converting the material into
the protein, fat and other nutrients that are needed by other creatures
higher in the food chain, Beasley said. Amphibians are clearly important
players in ecosystem functioning, he added.
“The frogs that develop from tadpoles subsequently devour thousands
of insects,” he said. “The frogs themselves are also important
prey for reptiles, birds and mammals.”
In 1999, two teams of researchers documented in Science that the larval
form of Ribeiroia trematodes (parasitic flatworms) cause limb deformities
in frogs. The same researchers recently implicated excess nutrients
in the water as a contributing cause. While these parasites are to blame
for many of the malformed frogs observed, Schotthoefer said, the studies
at Illinois indicate that the timing of infection is critically important.
Schotthoefer, in her two studies published last summer, provided the
first stage-specific data involving two types of trematode parasites
common to frogs, Beasley said.
In the Canadian Journal of Zoology, a team that included Schotthoefer
and Cole, working with funding from the U.S. Geological Survey’s
Amphibian Monitoring and Research Initiative, showed that about 16 percent
of northern leopard frog tadpoles infected with Ribeiroia during the
limb-bud stage had major deformities. However, when infected earlier,
in the pre-limb stage (two weeks of age), tadpoles suffered massive
tissue destruction that resulted in almost 100 percent mortality.
Also last summer, Schotthoefer, Beasley and Cole, in work funded by
the U.S. Environmental Protection Agency, found that another parasite,
in a group known as echinostomes, is more common than Ribeiroia among
the same species of frogs in Minnesota. These parasites attack the kidneys
rather than the limb-bud areas of tadpoles.
In the Journal of Parasitology, they reported that when echinostomes
attack the kidneys in early development, about one week after hatching,
infections lead to the blockage of kidney function and result in a death
rate of virtually 100 percent. Tadpoles infected later, after advanced
kidneys had developed, survived.
“This all leads to more questions,” Schotthoefer said. “Since
there appears to be specific vulnerable periods for infections of Ribeiroia
or echinostomes, we need to know whether changing environmental conditions
are widening the window of susceptibility.”
It may be that tadpoles are developing more slowly so that they are
at the most susceptible stages for longer periods of time than in the
past, she said.
“Or are conditions bringing susceptible early life stage tadpoles
into contact more frequently with the parasites at critical times so
that we are seeing more deformities and, perhaps of greater importance,
more rapid die-offs?” she said.
Beasley, who previously documented the presence and impact of echinostomes
on cricket frogs in Illinois, said the emerging findings also raise
a question about which family of parasites may be of greater importance
in terms of amphibian survival. The parasite that attacks the kidneys
is far more widespread than the one causing deformities.
A central question is why are more of these parasites being found in
frogs. They mature in vertebrate hosts such as birds or mammals or frogs.
Their eggs are shed in the feces of the hosts. The eggs hatch and enter
snails, where their numbers increase through asexual reproduction. They
then emerge into the water to seek out tadpoles.
Research by Beasley, Schotthoefer, Cole and colleagues that focuses
on Illinois cricket frogs will appear this year as a chapter of “The
Status and Conservation of United States Amphibians,” a book edited
by Michael Lannoo of the Indiana University College of Medicine in Muncie
and published by the University of California Press.
In the study funded by the John G. Shedd Aquarium and Chicago Zoological
Society, Beasley’s team noted that Midwest landowners excavate
their ponds so that deep water prevents the establishment of aquatic
plants.
Such a practice allows for easier boating, swimming and fishing, but
by eliminating aquatic plants, the habitat becomes structurally barren.
Herbicide use also aggravates the problem and produces chemical changes.
Plant-derived oxygen is reduced, and herbicides also may have direct
toxic effects on tadpoles and other animals, Beasley said.
“These changes may set the stage for more severe trematode infection,”
he said. “In this study, herbicide impacts on aquatic plants were
associated with an increased frequency of severe trematode infection
in the kidneys of tadpoles.”
The lack of vegetation also may expose tadpoles to more trematodes coming
from the infected snails, because the tadpoles can’t hide or use
the vegetation to brush off the parasites, Schotthoefer said.
So while the parasites seem to contribute to declines and deformities
in frog populations, the challenge is to find management approaches
that put balance back into amphibian ecosystems, Beasley said.
“What are the natural controls of these parasites? Can we re-establish
them?” Beasley said. “Are there things missing or reduced
from these ecosystems that used to eat both the snails and the larval
form of the trematodes? Are there plants missing that are needed to
provide both oxygen and cover?
“In all likelihood, we are seeing concurrent changes in all of
these factors, and they may have additive or synergistic effects, resulting
in increased parasite infections and associated impacts on amphibians,”
he added. “Re-establishing the structural and biological diversity
of amphibian habitats is clearly worth a closer look.”
