February 14, 2013
Amphibian Biodiversity Makes Entire Ecosystem More Robust
April Flowers for redOrbit.com - Your Universe Online
A new study from the University of Colorado, Boulder shows that the richer the biodiversity of amphibian species living in a pond, the more protection that ecosystem has against parasitic infections.
The finding of the study support the broader theory that greater biodiversity in large-scale ecosystems such as forests or grasslands may also provide greater protection against diseases, including those that attack humans. A larger number of mammals in an area, for example, may curb cases of Lyme disease, while a great variety of bird species can 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,” said Pieter Johnson, an assistant professor in the Department of Ecology and Evolutionary Biology at UC Boulder and head of the Johnson Lab. “But as it turns out, solidly testing these linkages with realistic experiments has proven very challenging in most systems.”
Designing comprehensive studies that might illuminate the possible connection between disease transmission and the number of species living in a complex ecosystem has been a challenge for researchers in the past. The vast numbers of organisms that need to be sampled as well as the large areas over which they might roam pose formidable challenges to this type of large-scale study.
Johnson´s study overcomes that problem by using smaller, easier-to-sample ecosystems. The team visited hundreds of small California ponds to record the types of amphibians living there as well as the number of snails that were infected with the pathogen Ribeiroia ondatrae, a species of parasitic flatworms that can cause missing and deformed limbs in a number of amphibians. The parasite uses snails as an intermediate host during 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´,” explained Johnson. “By collecting data from hundreds of ponds and thousands of amphibian hosts, our group was able to provide a rigorous test of this hypothesis, which has relevance to a wide range of disease systems.”
The team used two additional research methods to buttress field observations: laboratory tests designed to measure how prone to infection each species was, and the creation of pond replicas using large plastic tubs stocked with tadpoles. The tadpoles had all been exposed to a known number of parasites. According to Johnson, the experiments all told the same story — great biodiversity reduces the number of amphibian infections and the number of deformed frogs.
The experiments took three years, with the research team sampling 345 wetlands and recording all amphibian malformations — including missing, misshapen or extra sets of hind legs — that were caused by parasitic infections in 24,215 amphibians. The team also cataloged 17,516 snails. They found that ponds with a half dozen amphibian species had a 78 percent reduction in parasite transmission when compared to ponds with just one species. The team also set up additional experiments both in the laboratory and outdoors using 40 artificial ponds stocked with 60 amphibians and 5,000 parasites each.
DISEASE VULNERABILITY AND REPRODUCTIVE SPEED: A STRATEGIC TRADEOFF
They believe the reason that parasitic infections decline with greater biodiversity has to do with the fact that ponds add amphibian species in a predictable pattern. The first species to appear is the most prone to infection, and the later species are much less prone. In a pond with just one type of amphibian, for example, that amphibian was almost always the Pacific chorus frog. This frog is able to rapidly reproduce and colonize wetland habitats but is especially vulnerable to infection and parasite-induced deformities.
The California tiger salamander, however, is typically one of the last species to appear in a pond community and is the most resistant to parasitic infection. This shows that in a pond with greater biodiversity, parasites have a higher chance of encountering an animal that is resistant to infections. This lowers the overall success rate of transmission between infected snails and amphibians.
Johnson believes that this same pattern — less diverse communities being more susceptible to disease infection — could play out in more complex ecosystems as well. Species that disperse quickly across ecosystems appear to make a tradeoff between the ability to reproduce quickly and a greater vulnerability to disease.
“This research reaches the surprising conclusion that the entire set of species in a community affects the susceptibility to disease,” said Doug Levey, program director for the National Science Foundation´s (NSF) Division of Environmental Biology, which helped fund the research.
The very large scope of this study reinforces the connection between deformed frogs and parasitic infection, a source of concern for ecologists who noticed that the number of frogs with extra, missing or misshapen legs skyrocketed in the mid-1990s. This phenomenon even attracted widespread attention in the media and provided an incentive for experts trying to figure out the cause. Johnson was among those who helped discover the link between frog deformities and Ribeiroia infections. The rise in reports of deformations as well as their causes, however, remain a point of controversy.
The team warns that though the study has wider implications beyond parasitic infections in amphibians, the findings do not always mean that an increase in biodiversity always results in a decrease in disease. Johnson cautions that there are other factors that affect disease transmission rates as well.
“Our results indicate that higher diversity reduces the success of pathogens in moving between hosts,” he explained. “Nonetheless, if infection pressure is high, for instance in a year with high abundance of vectors, there will still be a significant risk of disease; biodiversity will simply function to dampen transmission success.”
A report of the team´s study was published recently in the journal Nature.