New invited PNAS commentary! Lose biodiversity, gain disease
Prof. McCallum authors an excellent commentary about the recently published meta-analysis of the dilution effect by Civitello, et al (2015). The abstracts and links are below.
There has been a vigorous and sometimes acerbic debate about the generality of the “dilution effect”: the notion that biodiversity inhibits infectious disease, and conversely that loss of biodiversity increases disease risks to humans and livestock. In PNAS, Civitello et al. (1) report a meta-analysis of more than 200 individual effect sizes for 61 parasite species, and find strong support for the dilution
effect. One of the key utilitarian arguments for preserving biodiversity is that it provides “ecosystem
services,” services that are essential for human survival and well-being (2, 3). If the dilution effect is indeed a general phenomenon, it would be an important ecosystem service, with wide public policy implications. The hypothesis that biological diversity limits infectious disease goes back to at least
the middle of last century. In his classic paper on the epidemiology of malaria, MacDonald (4) suggested controlling malaria through “zooprophylaxis,” the provision of alternative hosts (such as cattle) that mosquito vectors prefer to bite over humans, thereby limiting transmission to humans. In a similar but more general vein, Elton (5) suggested that simple communities could be invaded more
easily than complex communities. In an influential body of work, summarized in ref. 6, Ostfeld and Keesing have trenchantly argued that biodiversity is an effective buffer against infectious disease threats.
Civitello, D. J., Cohen, J., Fatima, H., Halstead, N. T., Liriano, J., McMahon, T. A., Ortega, C. N., Sauer, E. L., Sehgal, T., Young, S., and Rohr, J. R. (2015) Biodiversity inhibits parasites: Broad evidence for the dilution effect. Proceedings of the National Academy of Sciences, United States of America. 10.1073/pnas.1510607112 (link).
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 metaanalysis 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.