Washington: The richer the assortment of amphibian species in a pond, the more protection that community of frogs, toads and salamanders has against a parasitic infection that can cause severe deformities, including the growth of extra legs, a study has found.
The findings support the idea that greater biodiversity in large-scale ecosystems, such as forests or grasslands, may also provide greater protection against diseases, including those that affect humans.
A larger number of mammal species in an area may curb cases of Lyme disease, while a larger number of bird species may slow the spread of West Nile virus.
"How biodiversity affects the risk of infectious diseases, including those of humans and wildlife, has become an increasingly important question," Pieter Johnson, an ecologist and evolutionary biologist at the University of Colorado Boulder, and the lead author of the paper said.
Researchers have struggled to design comprehensive studies that could illuminate the possible connection between disease transmission and the number of species living in complex ecosystems.
Part of the problem is the enormous number of organisms that may need to be sampled, and the vast areas over which those organisms may roam.
This study overcame that problem by studying smaller, easier-to-sample ecosystems, the scientists said.
"The research reaches the surprising conclusion that the entire set of species in a community affects susceptibility to disease," Doug Levey, program director in the National Science Foundation (NSF)'s Division of Environmental Biology, which funded the research said.
Johnson and colleagues visited hundreds of ponds in California, recording the types of amphibians living there as well as the number of snails infected by the pathogen Ribeiroia ondatrae.
Snails are an intermediate host used by the parasite during part of its life cycle.
"One of the great challenges in studying the diversity-disease link has been collecting data from enough replicate systems to differentiate the influence of diversity from background 'noise,'" Johnson said.
The researchers buttressed field observations with laboratory tests designed to measure how prone to infection each amphibian species is, and by creating pond replicas using large plastic tubs stocked with tadpoles that were exposed to a known number of parasites.
All the experiments told the same story.
Greater biodiversity reduced the number of amphibian infections and the number of deformed frogs.
The results showed that ponds with half a dozen amphibian species had a 78 percent reduction in parasite transmission compared to ponds with just one amphibian species.
The reason for the decline in parasitic infections as biodiversity increases is likely related to the fact that ponds add amphibian species in a predictable pattern, with the first species to appear being the most prone to infection and the later species to appear being the least prone.
Therefore, in a pond with greater biodiversity , parasites have a higher chance of encountering an amphibian that is resistant to infection, lowering the overall success rate of transmission between infected snails and amphibians.
The findings are published in the journal Nature.
First Published: Monday, February 18, 2013, 09:01