“When I open up a frog, it’s literally just bursting with parasites — thousands of them, and dozens of different species,” the evolutionary biologist Pieter Johnson says.

David Herasimtschuk/Freshwaters Illustrated (frog); Pieter Johnson/ University of Colorado (parasites)

Hardly an appealing image. But the observation led Dr. Johnson, an assistant professor at University of Colorado, Boulder, to pose an intriguing question: When might more parasites be a good thing?

The results of his research appeared this week in the Proceedings of the National Academy of Sciences.

Amphibians are in a conservation crisis, having experienced striking declines in biodiversity over the last 30 years, and many species are on the brink of extinction. The leading cause of species loss is still believed to be habitat destruction combined with the devastating effects of the fungal skin disease chytrid.



Parasites can also play into this, especially those which cause severe limb deformation. While all wild animals are loaded with parasites, amphibians get a particularly diverse dose because they are exposed to a whole suite of them from both land and water.

The research team combined field observations with controlled laboratory experiments to tackle the question of whether parasites can actually be beneficial. In the lab, they focused on six of the most common parasites, including the ones that cause extra or missing limbs, to examine how diversity affects the abundance of the most dangerous parasites and the overall fitness and survival of the host species — in this case, the Pacific chorus frog.

The bottom line? Increases in parasite diversity almost always decreased the level of parasite infection, including that of the most dangerous species. For example, when a chorus frog was introduced to all six types of parasites simultaneously, the infection rate was 42 percent lower than for frogs infected with just one type of parasite.

The effects on the overall fitness of the host frogs were more variable, depending largely on the order in which the parasites were introduced. If one of the more dangerous parasites was introduced first, as is usually the case in natural systems, it was often supplanted by the more benign species added later. But if the harmless parasites were first on the scene, disease rates increased in some cases.

The implications for amphibian conservation are manyfold. Parasites have typically been thought of as harmful by definition. Consequently, disease control efforts have often been very nonspecific — wiping out all parasites, for example, which may have the unintended effect of making a bad infection worse by eliminating inter-parasite competition.

“It’s a case of ‘The enemy of my enemy is my friend,’ ” Dr. Johnson said. “Not all parasites cause harm, and it’s possible that some are actually helping keep their more dangerous friends in check.”

On another level, there is growing interest on the relationship between biodiversity and disease management. It’s still a controversial theory, but there is evidence for what scientists are calling the dilution effect, with greater biodiversity leading to lower rates of infection. The best-studied example involves ticks and humans.

Researchers have found that in more highly diverse communities with more species of birds and mammals, the abundance of the ticks that are infected with the bacterium that causes Lyme disease is greatly reduced, which provides a buffer against infection for humans.

Conversely, in highly disturbed areas with lower diversity, there is a greater abundance of hosts like the white-footed mouse, which is particularly adept at transmitting Lyme disease back to ticks, leading to a greater risk of infection for humans.

“We know so little about parasite diversity because it is so cryptic,” Dr. Johnson said. “There’s the potential for it to have all these interesting effects that people historically just haven’t paid attention to, because, well, it’s hard to tease apart. ”