Frog Deformities Blamed on Farm and Ranch Runoff
Date: 26-Sep-07
Country: US
Author: Will Dunham
These nutrients from fertilizers and animal waste create
dramatic changes in aquatic ecosystems that help a certain type
of parasitic flatworm that inflicts these deformities on North
American frogs, researchers said.
"You can get five or six extra limbs. You can get no hind
limbs. You can get all kinds of really bizarre, sick and
twisted stuff," Pieter Johnson, an ecologist and evolutionary
biologist at the University of Colorado at Boulder who led the
study, said in a telephone interview.
Many ecologists have expressed alarm over the plight of the
world's amphibians and the role of human activities in their
declining populations.
"We continue to see malformed amphibians all over the place
and yet very little is being done to address those questions or
even understand them," Johnson said.
While scientists had blamed parasitic infections for
deformities seen in recent years in some types of amphibians,
this study documented how runoff from farms and livestock
ranches drives the process.
The runoff sets in motion a series of events in lakes and
ponds where frogs live, the researchers said.
The nutrients stimulate the growth of algae, which in turn
increases the population of snails. Microscopic parasitic worms
called trematodes infect the snails -- and more snails means
more worms.
ATTACK OF THE 'ZOMBIES'
The worms reproduce asexually inside the snails, which
Johnson said are turned into "zombies" castrated by the
parasites, allowing the worms to expel thousands of offspring.
The worms then swarm over tadpoles -- the water-dwelling
larvae of frogs -- and burrow at the spots where limbs are
developing, forming cysts and causing developmental
deformities.
But how would a worm benefit from an amphibian having such
deformities? Predators such as birds eat the infected frogs and
spread the worm back into the ecosystem through defecation.
Deformed frogs are more easily caught and eaten, benefiting the
worm's life cycle, Johnson said.
To examine the role of nitrogen and phosphorus runoff on
the process, the researchers created 36 ponds in Wisconsin and
stocked them with snails and frog tadpoles. They added nitrogen
and phosphorus and observed the consequences.
The ponds with added nitrogen and phosphorus had their
snail population, parasitic worm egg production and infection
rate of frogs increase greatly, according to the study in the
Proceedings of the National Academy of Sciences.
Johnson said an important area of research is tracking
connections between nutrient runoff from all kinds of sources
into aquatic environments, and the emergence of disease in
people or wildlife.