By Roberta Attanasio
Biodiversity is a term coined to describe the diversity of all living things, from human beings to microorganisms. A New York Times editorial published almost two decades ago aptly describes the importance of the biodiversity concept: “Biodiversity is a hugely important concept that stresses the coherence and interdependence of all forms of life on earth and a new willingness to appraise the meaning of that interdependence, not just for humans but for every one of life’s component parts.”
The editorial goes on to illustrate the alarming effects of biodiversity loss: “Biodiversity is a way of talking about what scientists have long understood and a way of reminding the rest of us of a cardinal fact: that we are standing in the midst of the earth’s sixth great extinction of diverse species, that this extinction is driven by us and that we are not now and will never be immune to its effects.”
One of these effects is the worldwide spike in infectious diseases, as suggested by a study recently published in the journal Proceedings of the National Academy of Sciences. The study (Predator diversity, intraguild predation, and indirect effects drive parasite transmission) explores how the diversity of small predators shapes the transmission of parasites in wetlands.
Lead author Jason Rohr said in a press release by Penn State: “In the last century, there has been an unprecedented global increase in infectious diseases and a concomitant decline in and homogenization of biodiversity. The controversial ‘dilution effect hypothesis’ suggests that the two phenomena might be linked, or that biodiversity often decreases disease risk.”
The study, which included a series of laboratory experiments, field surveys and mathematical modeling, shows that — in presence of various species of dragonfly larvae — there is a reduction of frog infections caused by trematodes, which are parasitic flatworms also known as flukes. The dragonfly larvae are small predators that eat trematodes.
Val Beasley, senior author of the study, said in the press release that various species of trematodes penetrate tadpoles. The trematodes sometimes kill the tadpoles. In other instances, the trematodes weaken them by causing tissue damage, kidney failure, or severe limb deformities while the tadpoles develop into frogs. He added that other vertebrate species commonly catch trematode infections from bodies of water. These vertebrate species include wildlife, domestic animals and humans — mostly children — who are commonly affected by schistosomiasis in tropical parts of the world.
Schistosomiasis is a parasitic disease carried by freshwater snails infected with one of the five varieties of the parasite Schistosoma, a type of trematode. Although the worms that cause schistosomiasis are not found in the United States, more than 200 million people are infected worldwide.
While most biodiversity research focuses on variations of the parasite hosts, this study highlights the role that the diversity of the predator species (in this case the diversity of dragonflies, which attack the flatworms) play on infectious diseases.
Rohr also said: “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. 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.”
Further work on the diversity of predator species able to decrease parasite populations may be crucial for the management of infectious diseases.