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Horrific deformities in frogs are the result of a series of events that starts when nitrogen and phosphorus from farming and ranching bleed into lakes and ponds, say researchers.
These nutrients from fertilisers 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.
Many ecologists have expressed alarm over the plight of the world's amphibians and the role of humans 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.
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 lifecycle, 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 published 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.
- Reuters