Biodiversity inhibits parasites: Broad evidence for the dilution effect

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.

Keywords: associational resistance; biodiversity; dilution effect; meta-analysis; parasitism.

Fig. 1.

Results of the meta-analysis of the generality of the dilution effect hypothesis. (A) Overall, there was a strong negative relationship between host diversity and parasite abundance. (B–F) The strength of dilution effects did not differ between (B) parasites that infect only wildlife and those that infect humans, (C) parasites with complex and simple lifecycles, (D) microparasites and macroparasites, (E) specialist and generalist parasites, or (F) manipulative and observational studies. Despite this lack of differences between groups, all groups exhibited significant evidence for the dilution effect [asterisks indicate significant (P < 0.05) differences from zero]. Error bars represent ± SE.

Fig. 2.

Results of the metaregressions relating the strength of diversity to the (A) frequency and (B) density of focal host individuals in the experimental communities. (A) Dilution effects were significantly stronger (i.e., more negative Hedges’ g) when the frequency of focal hosts was lower in the diverse treatments. (B) In contrast, the strength of dilution was unrelated to the density of focal hosts. Solid lines indicate the fit of a multiple linear regression model while holding the other factor constant at its mean value. Dashed lines indicate the 95% confidence interval for these model fits.

Fig. 3.

Results of the meta-analysis of dilution effects in host–parasite and plant–herbivore systems. Both natural enemy types were significantly inhibited by host diversity [asterisks indicate significant (P < 0.05) differences from zero]. Further, there was no significant difference in the strength of dilution effects between parasites and herbivores. Error bars represent ± SE.