The blood-sucking consequences of biodiversity decline

People often describe an ecosystem thriving with a diverse set of species as “healthy,” but they may not realize just how apt that label is. A new study shows that biodiversity serves as a bulwark against the transmission of diseases and parasites, and suggests this phenomenon may even play a role in human health.

The idea that diverse communities inhibit the spread of parasites and diseases is known as the dilution effect hypothesis. Scientists have posited this relationship to help explain why infectious disease outbreaks are increasing in ecosystems around the world at the same time that biodiversity is declining. But they have debated how general the effect is, especially whether it is at play in the transmission of parasites from animals to humans. The new study, published June 11 in the Proceedings of the National Academy of Sciences, is the first to rigorously test whether the dilution effect applies broadly across many types of species and ecosystems.

In short, it does. To reach this conclusion, researchers from the University of South Florida in Tampa conducted a type of study known as a meta-analysis. They assembled data from 202 previously published assessments involving 61 parasite species, including 47 that infect wildlife species only and 14 that also infect humans.

More diverse systems show less circulation of parasites, the researchers found. This pattern recurs regardless of factors like study design (the meta-analysis included experiments in which researchers manipulated species composition as well as observational studies of real ecosystems), density of host species, or type of parasite involved.

The mechanisms of this relationship are not yet clear, the researchers note, and there may be exceptions to the rule. But overall, the results indicate that the dilution effect is not an isolated, idiosyncratic phenomenon but something more like a general property of ecosystems. And that leads to the sobering implication that biodiversity loss may have a kind of snowball effect, contributing to disease outbreaks in the species that remain.

The researchers conducted a similar meta-analysis of plants and the herbivores that graze on them, encompassing 136 assessments of the effects of 39 herbivore groups. Once again, the same pattern emerged: biodiversity keeps herbivory in check, a relationship that in the case of plants is known as associational resistance. Because of this effect, human-caused biodiversity declines could result in decreased crop and timber production.

The dilution effect holds for parasites that are transmitted from animals to humans, not just those that circulate among wildlife species, the researchers found. This suggests that by being careless with the earth’s biodiversity, we put our own health at risk.

On the flip side, though, this also means that biodiversity conservation may reduce the risk of human disease outbreaks. That, at least, should be an easy pill to swallow. – Sarah DeWeerdt | 23 June 2015

Source: Civitello D.J. et al. 2015 Biodiversity inhibits parasites: Broad evidence for the dilution effect. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1506279112

Header image:  Anopheles mosquito, a species that transmits a parasite — which causes malaria — to humans. Credit: Centers for Disease Control and Prevention Public Health Information Library via Wikimedia commons.

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