Big apex predators may get all the attention and all the TV documentary time, but it's the small predators that can determine the health of any ecosystem, researchers say.

That's the key finding of one study of aquatic ecosystems that showed that their biodiversity of small predators including dragonflies or other aquatic insects that will attack and eat parasites, could benefit the well being of amphibian populations living in the particular ecosystem, they say.

The work suggests a possible link between the decline of global biodiversity and an unprecedented worldwide increase in infections diseases, says Jason Rohr, a University of Southern Florida integrative biology professor.

"The controversial 'dilution effect hypothesis' suggests that the two phenomena might be linked, or that biodiversity often decreases disease risk," he says.

In their particular study, the researchers reported, a number of field surveys, laboratory experiments and computer modeling showed that the incidence of species of dragonfly larvae present reduced infections seen in frogs that parasitic flatworms known as trematodes can cause.

The parasites can penetrate tadpoles, often killing them or at least weakening them through tissue damage or causing severe deformities in limbs when they become frogs.

Other vertebrates can be infected by trematodes in ponds or streams, including wildlife, herd animals or humans -- mostly children -- who are often affected by the parasitic disease schistosomiasis found in tropical regions of the globe.

While most similar investigations of biodiversity look at the diversity of parasite hosts, the new study highlights the significance of focusing on the diversity of species that can attack and kill parasites, Rohr says.

"In our wetland survey, our microcosms and disease models, we discovered that there were fewer flatworms in frogs where there were more species of flatworm predators," he says. "Additionally, the field study indicated that the diversity of these predators was a better predictor of flatworm infections than nutrients, frog immunity or the diversity and abundance of hosts."

Parallels between their study findings and studies on the roles small predator populations can play in helping keep down crop pests suggest efforts to control crop pests and initiatives to prevent many parasitic diseases can be directed by the requirement to conserve small predator biodiversity, the scientists say.

The researchers presented their study findings in the Proceeding of the National Academy of Sciences.

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