Dilution Effect Debate Continues!

Admit it, the title of this post made you cringe! Or it set off a little fuse in your brain and you’re just seconds away from your head exploding. Take a zen moment, and then continue reading for some interesting science.

If you don’t know what the dilution effect hypothesis is or why disease ecologists are debating the hypothesis, you might want to check out some of my previous posts before reading this one. But briefly: scientists have found a negative relationship between host biodiversity and the risk of infection to particular host species in some disease systems in some areas. That negative relationship is called the “dilution effect,” because host biodiversity is “diluting” parasite transmission. The debates have arisen because disease ecologists can’t agree on is how often the dilution effect occurs in natural systems: always, sometimes, or never? I’ve described the core arguments on both sides of the debate in this post.

Meta-analyses are one way to figure out how commonly the dilution effect occurs in natural systems. By collecting all of the available empirical and/or observation  studies that consider biodiversity-disease relationships and lumping them together  into one analysis, we can figure out whether the dilution effect always, sometimes, or never occurs in natural systems. Before my last post on the dilution effect debates was published, two such meta-analyses argued that the dilution effect only sometimes occurs (it’s “idiosyncratic”) in systems where the focal hosts are humans and non-human primates. Additionally, in the meta-analysis regarding how biodiversity influences human risk of infection with zoonotic pathogens, Salkeld et al. (2013) found evidence of a publication bias for studies that find a dilution effect, suggesting that studies finding neutral or amplification effects are less likely to be published.

Fast forward to Civetello et al. (2015), who did a larger meta-analysis that included more studies in more host-parasite systems. As I posted about a few weeks ago, Civetello et al. (2015) found broad support for the dilution effect, which I suppose we can say means that the dilution effect “often” or “usually” occurs, which puts us somewhere between “always” and “sometimes.”

Now back to the “debate” part of this post: Salkeld et al. (in press) responded to the Civetello et al. (2015) paper with some concerns regarding the larger meta-analysis. In particular, they worried that including laboratory studies might muddle the analysis, because the way that we manipulate systems in the lab doesn’t necessarily correspond to what really happens in nature. Also, they pointed out that if there is a publication bias – as they previously found – then it might not be particularly meaningful that the dilution effect is commonly reported in the ecological literature. However, Civetello et al. (2015) didn’t evaluate whether there was a publication bias in their analysis. (Note that McCallum et al. 2015 also pointed out some of these possible concerns.)

Civetello et al. (in press) responded to these worries by doing another meta-analysis. They used only a subset of the studies from their previous paper, so that they were including only field studies of human pathogens, like in Salkeld et al. (2013). Civetello et al. (in press) still found an overall dilution effect, and they suggested that adding in the studies published since Salkeld et al. (2013) provided more statistical power to see the dilution effect than Salkeld et al. (2013) had. Civetello et al. (in press) also looked for publication bias in their subsetted dataset and didn’t find any evidence for bias, but they point out that the analysis to look for a publication bias had to violate some assumptions of independence, so it might not be particularly meaningful.

It’s unlikely that things are totally resolved here, but I think everyone is on the same page regarding the future directions for diversity-disease relationships: we’ve spent time looking for general trends and determining how common the dilution effect is in natural systems, and now it’s time to switch our focus to the mechanisms underlying the dilution effect.

References:

Civitello DJ, et al. (2015) Biodiversity inhibits parasites: Broad evidence for the dilution effect. Proc Natl Acad Sci USA 112:8667–8671.

McCallum H. (2015) Lose biodiversity, gain disease. Proc Natl Acad Sci USA 112: 8523–8524.

Salkeld DJ, Padgett KA, Jones JH (2013) A meta-analysis suggesting that the relationship between biodiversity and risk of zoonotic pathogen transmission is idiosyncratic. Ecol Lett 16(5):679–686.

Salkeld DJ, Padgett KA, Jones JH, Antolin MF (2015) Public health perspective on patterns of biodiversity and zoonotic disease. Proc Natl Acad Sci USA, 10.1073/pnas.1517640112.

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