Parasites seem to be each other's worst enemy, and a frog's best friend, new research indicates. Increases in the diversity of parasites that attack amphibians cause a decrease in the infection success rate of those parasites.

"Collectively, our findings illustrate the importance of considering the hidden role of parasite diversity in affecting disease risk," study researcher Pieter Johnson, of the University of Colorado, said in a statement. "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."

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 new study, the team sampled 134 California ponds for parasites, known as trematodes, comparing their abundance and distribution with the health of more than 2,000 Pacific chorus frogs in those ponds. The researchers compared their data from the field studies with extensive lab experiments that charted the health of the frogs in the presence of different combinations of six common amphibian parasites.

The new study showed when the chorus frogs were exposed to all six parasites 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," Johnson said.

The study was published today, May 21, in the journal Proceedings of the National Academy of Sciences.

The primary study results support the idea that higher biodiversity of a host animal 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.

The new study has implications for the declining biodiversity being seen across the planet as a result of human activities, including among amphibians. Roughly 40 percent of amphibian species around the world are in decline, and more than 200 have gone extinct since the 1970s.