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Biodiversity inhibits parasites: Broad evidence for the dilution effect
Edited by Simon A. Levin, Princeton University, Princeton, NJ, and approved May 15, 2015 (received for review March 30, 2015)
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Significance
The dilution effect hypothesis suggests that diverse ecological communities limit disease spread via several mechanisms. Therefore, biodiversity losses could worsen epidemics that harm humans and wildlife. However, there is contentious debate over whether the hypothesis applies broadly, especially for parasites that infect humans. We address this fundamental question with a formal meta-analysis of >200 assessments relating biodiversity to disease in >60 host–parasite systems. We find overwhelming evidence of dilution, which is independent of host density, study design, and type and specialization of parasites. A second analysis identified similar effects of diversity in plant–herbivore systems. Thus, biodiversity generally decreases parasitism and herbivory. Consequently, human-induced declines in biodiversity could increase human and wildlife diseases and decrease crop and forest production.
Abstract
Infectious diseases of humans, wildlife, and domesticated species are increasing worldwide, driving the need to understand the mechanisms that shape outbreaks. Simultaneously, human activities are drastically reducing biodiversity. These concurrent patterns have prompted repeated suggestions that biodiversity and disease are linked. For example, the dilution effect hypothesis posits that these patterns are causally related; diverse host communities inhibit the spread of parasites via several mechanisms, such as by regulating populations of susceptible hosts or interfering with parasite transmission. However, the generality of the dilution effect hypothesis remains controversial, especially for zoonotic diseases of humans. Here we provide broad evidence that host diversity inhibits parasite abundance using a meta-analysis of 202 effect sizes on 61 parasite species. The magnitude of these effects was independent of host density, study design, and type and specialization of parasites, indicating that dilution was robust across all ecological contexts examined. However, the magnitude of dilution was more closely related to the frequency, rather than density, of focal host species. Importantly, observational studies overwhelmingly documented dilution effects, and there was also significant evidence for dilution effects of zoonotic parasites of humans. Thus, dilution effects occur commonly in nature, and they may modulate human disease risk. A second analysis identified similar effects of diversity in plant–herbivore systems. Thus, although there can be exceptions, our results indicate that biodiversity generally decreases parasitism and herbivory. Consequently, anthropogenic declines in biodiversity could increase human and wildlife diseases and decrease crop and forest production.
Footnotes
- ↵1To whom correspondence should be addressed. Email: civitello{at}usf.edu.
↵2J.C., H.F., N.T.H., J.L., T.A.M., C.N.O., E.L.S., T.S., and S.Y. contributed equally to this work.
↵3Present address: Department of Biology, University of Tampa, Tampa, FL 33606.
Author contributions: D.J.C. and J.R.R. designed research; D.J.C., J.C., H.F., N.T.H., J.L., T.A.M., C.N.O., E.L.S., T.S., S.Y., and J.R.R. performed research; D.J.C. and J.R.R. analyzed data; and D.J.C. and J.R.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
See Commentary on page 8523.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1506279112/-/DCSupplemental.
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