Having an abundance and diversity of predators - such as dragonflies, damselflies and aquatic bugs - to eat parasites is good for the health of amphibians, a group of animals experiencing worldwide population declines.
University of South Florida biologists and colleagues from four other universities has shown this using a combination of experiments, field studies and mathematical models.
Moreover, their focal disease system has a comparable human system in the tropics and their results having striking similarities to research on the role of predators in controlling crop pests. Thus, the researchers are hopeful that their research can guide the management of predator species to reduce human and wildlife diseases and pest populations.
"In the last century, there has been an unprecedented, global increase in infectious diseases and a concomitant decline and homogenization of biodiversity," says study lead author Dr. Jason Rohr, an associate professor in the Department of Integrative Biology at USF. "The controversial 'dilution effect hypothesis' suggests that the two phenomena might be linked, or that biodiversity often decreases disease risk."
According to Rohr, most dilution effect research has focused on host diversity rather than predator diversity. In fact, until the work of Rohr and colleagues there were no studies that evaluated the relative importance of predators of parasites to disease risk.
"In our wetland survey, we discovered that there were fewer flatworm infections in frogs where there were more species of flatworm predators. Additionally, the diversity of these predators was a better predictor of flatworm infections than nutrients, frog immunity, or the diversity and abundance of hosts," says Rohr.
By conducting a follow-up experiment, Rohr and colleagues provided evidence that these field patterns were likely causal. They manipulated the density and diversity of predators of the free-swimming flatworms and showed that, on average, an increase in predator density or diversity reduced tadpole infections. However, not all predators diluted disease risk equally.
'Predators that only consumed parasites were better diluters than predators that ate tadpole hosts and parasites, and our mathematical model confirmed the generality of this finding,' says co-author David Civitello of the Department of Integrative Biology at USF.
"When we returned to the field patterns, virtually all of the dilution effect was driven by predators that predominantly eat the parasite rather than the amphibian host," concludes Rohr.
Thus, the researchers believe they identified a general trait of predators - how much they consume parasites or pests relative to hosts - that can be used to assess their ability to control parasite and pest populations.
"We revealed an important negative relationship between biodiversity and disease risk that is consistent with several host-biodiversity-dilution-effect studies. However, we also have identified when exceptions to the dilution effect occur, specifically when the diversity of predators that eat both hosts and parasites is high," adds Civitello.
"We hope to have improved the management of pests and wildlife and human parasites by providing decision makers, wildlife managers, and farmers with guidance on how to manipulate predators to reduce pests and disease," ends Rohr.