To say that the dilution effect is a hot topic in disease ecology would be an understatement. For instance, the posts that I wrote in 2013 about the dilution effect debates are some of the most frequently accessed posts on this blog. Since those debates, the dilution effect literature has been pouring in, and I thought I would compile some of the most notable papers (in my opinion) that I haven’t covered yet in one post.
But first: is there still a debate? Or have we all come to an agreement?
I’ve been putting off posts about the dilution effect for some time now, because I find the tone of some of the literature to be tedious. I tried to humorously represent how cutthroat the literature has been with my mortal kombat cartoons in my previous posts, but that might have unintentionally come off as me being antagonistic instead of just being silly. I think the tone has calmed down a little now, but we still have some distance to cover before people agree on what kinds of systems we expect to find the dilution effect in and at what spatial scales we might observe the dilution effect in those systems. But that’s completely understandable, because this is ecology, where the answer to all questions is, “Well, it depends….” 😛 In the meantime, it has been really neat to see (1) the recent compilations of data and (2) the frameworks that have been proposed to explain where and when we will see a given diversity-disease relationship and/or the relative strengths of mechanisms underpinning the diversity-disease relationships in some systems. For instance:
Civetello, D.J., J. Cohen, H. Fatima, et al. 2014. Biodiversity inhibits parasites: Broad evidence for the dilution effect. PNAS 112(28): 8667–8671.
One of the most talked about recent diversity-disease papers is this meta-analysis by Civetello et al. (2014). Looking across 202 effect sizes and 61 parasite species, they found strong support for a broad-scale, negative relationship between diversity and focal host disease risk. Very surprisingly, the strength of this relationship didn’t vary according to whether the pathogen infected only wildlife or wildlife and humans, whether the pathogen was a micro or macro parasite, whether the pathogen had a simple or complex life cycle, whether the pathogen was a specialist or generalist… or basically any other predictor variable. This is surprising because theory predicts that the diversity-disease relationships that we observe should depend on characteristics of the host-pathogen system, like whether transmission is density or frequency dependent and whether host species are added additively or substitutively to the system. But as Civetello et al. (2014) explain, this broad negative relationship between diversity and disease doesn’t tell us about the mechanisms underpinning that relationship, so we really need to start digging into the mechanisms.
Strauss, A.T., D.J. Civetello, C.E. Caceres, and S.R. Hall. 2015. Success, failure and ambiguity of the dilution effect among competitors. Ecology Letters 18(9): 919-926.
Speaking of mechanisms, Strauss et al. (2015) demonstrate how a really neat framework can give rise to amplification, neutral, and dilution effects. They suggest that by taking into account the focal host’s ability to spread the disease (R0), the competitive ability of the focal host (R*), and the ability of dilutor hosts to vacuum up parasites (encounter dilution), we can determine which diversity-disease outcome should occur. For instance, when focal hosts are strong competitors and the R0 is high, dilutors won’t be able to suppress focal host populations or vacuum up enough parasites to have an effect, so we won’t see a dilution effect. This is cool stuff!
Wood, C.L., K.D. Lafferty, G. Deleo, H.S. Young, P.J. Hudson, and A.M. Kuris. 2014. Does biodiversity protect humans against infectious disease? Ecology 95:817–832.
How often do we expect a negative relationship between host diversity and infection risk for human pathogens, in particular? Wood et al. (2014) considered the ecology/epidemiology of 69 human pathogens and determined whether human infection should increase or decrease with animal diversity based on existing diversity-disease theory. For instance, for pathogens that are directly transmitted among humans and don’t use any wildlife/environment reservoirs, wildlife diversity shouldn’t play a role in pathogen transmission. Wood et al. (2014) concluded that the dilution effect is not expected to occur for the majority of the most important human pathogens, and “there will be winners and losers in environments subject to anthropogenic change.”
Johnson, P.T.J., R.S. Ostfeld, and F. Keesing. In press. Frontiers in research on biodiversity and disease. Ecology Letters.
This paper starts out with an overview of the history of the dilution effect literature, including the dilution effect debates, and some background on how the dilution effect hypothesis fits into existing community ecology theory. Then the authors suggest some future directions for field studies, experiments, and models aimed at understanding the diversity-disease literature.
Finally, I just want to point out that my experience at ESA 2015 suggests that there is a lot more cool diversity-disease work in the pipeline. If you missed those talks, this is a good place to start perusing.