Young CU ecology prof garners two high honors

Increases in the diversity of parasites that attack amphibians cause a decrease in the infection success rate of virulent parasites, including one that causes malformed limbs and premature death, according to an award-winning study by a CU-Boulder researcher and his collaborator. Frog image copyright David Herasimtschuk, Freshwaters Illustrated. Parasite images courtesy Pieter Johnson.

Pieter Johnson, assistant professor of ecology and evolutionary biology at CU-Boulder, is having a pretty good year. He and a co-researcher have won an award recognizing outstanding contributions to ecology, and he has been named an Early Career Fellow by the Ecological Society of America.

During the Ecological Society of America’s 98th Annual Meeting next month in Minneapolis, Johnson and co-winner Jason Hoverman will accept the George Mercer Award, which recognizes an outstanding and recently published ecological research paper by a young scientist.

Pieter Johnson

Johnson and Hoverman, with Purdue University, are cited for using a novel approach in their 2012 Proceedings of the National Academy paper “Parasite diversity and coinfection determine pathogen infection success and host fitness.”

Increases in the diversity of parasites that attack amphibians cause a decrease in the infection success rate of virulent parasites, including one that causes malformed limbs and premature death, Johnson and Hoverman found.

Commenting on that study, Johnson said scientists are concerned about how changes in biodiversity affect the risk of infectious diseases in humans and wildlife. Charting the relationships between parasites and amphibians is important, since few studies have examined the influence of parasite diversity on disease, and the fact that amphibians are declining faster than any group of animals on the planet due to human activities like habitat loss, pollution and emerging diseases, Johnson said.

In the PNAS study, the team sampled 134 California ponds for the parasites, known as trematodes, comparing their abundance and distribution with the health of more than 2,000 Pacific chorus frogs.

The CU-Boulder team combined the field studies with extensive lab experiments that charted the health of the frogs in the presence of different combinations of the six most common amphibian parasites, including the Ribeiroia group whose larvae burrow into tadpole limb regions and form cysts that disrupt normal frog and toad leg development, causing extra or missing limbs.

The study showed when the chorus frogs were exposed to all six trematode types simultaneously, the infection success rate was 42 percent lower than for frogs exposed to only a single species of parasite.

“Our results show increases in parasite diversity consistently cause a decrease in infection success by the most virulent parasite,” said Johnson.

While the six parasites used in the study are responsible for about 95 percent of trematode infections in the wild, most of the world’s parasites cause limited damage to host individuals, said Johnson. In the PNAS study, only two parasites, Ribeiroia and a parasite group called Echinostoma — which can trigger amphibian mortality — were known to be particularly dangerous to their host species.

The primary study results support the idea that higher biodiversity can help protect against certain diseases, but few previous studies had considered the diversity of the parasites themselves.

Because many parasites compete with each other, ecological systems richer in parasites can act as a buffer against virulent pathogens. Johnson said the combination of extensive field and lab work helped strengthen the study results.

One surprising study finding was that under certain conditions, increases in parasite diversity could increase or decrease host disease. In that aspect of the study, the infection rates were dependent on the order in which the six parasite species were added to the habitats of the frogs, and whether newly added parasite species replaced other parasites or were added alongside them, he said.

If a dangerous parasite is first on the scene, it tends to be replaced when less dangerous species are added, decreasing the odds of host disease.

But if a dangerous parasite species is added to an environment already harboring parasites, the study showed either a neutral effect or an increase in disease, Johnson said.

“Collectively, our findings illustrate the importance of considering the hidden role of parasite diversity in affecting disease risk,” said Johnson.  “While our study was on amphibian diseases, there is ample evidence to suggest similar processes can be occurring in humans and other groups of animals.”

The Ecological Society of America said this of the work: “Their study demonstrates how an ecological approach can contribute deeper understanding of biomedical questions.”

The Mercer Award is given for an outstanding ecological research paper published by a younger researcher (the lead author must be 40 years of age or younger at the time of publication).

Early Career Fellows are typically within eight years of receiving their Ph.D. and have begun making and show promise of continuing to make outstanding contributions to a wide range of fields served by ESA. They are elected for five years.

“I am both flattered and honored to receive these distinctions,” said Johnson. “In the case of the Mercer Award, it is particularly rewarding and more than a bit daunting to be recognized alongside the ranks of previous recipients, who are some of the most accomplished ecologists in the U.S. I am fortunate to have such a dedicated lab, generous colleagues and a supportive university, without which this would not have been possible.”

Jim Scott, senior science editor of CU’s Office of News Services, contributed to this report.

July 2013

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