Biodiversity decreases disease through predictable changes in host community competence

Johnson, P. et al., 2013. Nature 494: 230-233. DOI:10.1038/nature11883. Biodiversity decreases disease through predictable changes in host community competence

Below, we give our first impressions of this article. Please comment below, or tweet Will or Lynsey (maybe use #pegejc). Think of this as a journal club discussion group!

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Will Pearse

Neither of us know anything about disease transmission, but we’d probably both claim to be ‘biodiversity scientists’ (whatever that means), so I picked this paper to shake things up a little bit. The paper is a very large-scale empirical demonstration that more diverse communities have fewer pathogens, implying that biodiversity in of itself helps stop disease transmission. It focuses on a single system, and they have measured one heck of a lot of frogs!

I have quite strong views about Ricklefs – I love his work, but I find his emphasis on pathogens strange. In Ricklefs’ view of the world, pathogens are the hidden puppeteers that control species diversity and many evolutionary dynamics. I argue that there is some truth to this, but it’s just too easy to say that observed patterns are because of something we can’t easily test. However, this paper demonstrates this effect rather clearly – not only is the rate of abnormalities reduced in more diverse systems when density is controlled for, but also the species found in the depauperate assemblages are the ones most tolerant of infection. I’m not convinced that pathogens are the only things driving this pattern; we might expect generally more ‘hardy’ individuals to survive in difficult environments, and if conditions are particularly harsh we might expect individuals to either die, or survive and thrive because they’re tolerant of the conditions but have reduced competition. However, this does paper does provide hope that we might actually be able to start measuring pathogen infection and explicitly linking infection with observed ecological patterns.

I’m not wildly enamoured with figure 3b – I think there’s a lot of scatter in the lower-left corner of the plot, and that the figure is influenced by a few outliers in the top-right. This means I’m not convinced there’s as strong a connection between snail density and amphibian density as the authors, but I’m willing to admit I’m being snarky. I’m much more excited about the experimental results that suggest this increase in transmission rate is not just a consequence of density changes – that there’s something about being in a diverse assemblage that gives a system more resilience to pathogens. This leads to a second question, I think – what’s the effect of increasing diversity in the pathogens? Is a more diverse pathogen community able to overpower a power diverse host community? While we’re on this subject (and perhaps this is something disease ecologists know a little more about), what is a pathogen community?

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Lynsey McInnes

Wow, thanks Will! When I choose papers they fall right into his area of expertise – we he chooses papers, they fall right between either of our areas of expertise. Moving on…

I enjoyed reading this paper. It struck me that the authors took a hypothesis that has been being bandied about and went through a painstaking process to unpick it using a neat set-up incorporating field observations and lab and mesocosm experiments (I love mesocosms). In the process they got to play with a lot of diseased frogs.

My understanding is that support is accumulating for the idea that diversity in a community (here in terms of species numbers, presumably acting as a proxy for functional diversity) decreases disease transmission among individuals. Yet another plus point for maintaining diverse communities.

The authors stress that this effect is not just due to changing density of the species most able to transmit disease although the numbers point to this being part of the effect. Which is fair enough I guess. Rather, there is some kind of ‘dilution effect.’ How does the effect actually work? I don’t know enough about disease transmission to understand or really even speculate.

The authors state this ‘One possible explanation for the negative relationship between a host’s competence and its assembly order is that defences are costly and may incur trade-offs with resource investment in reproduction or dispersal. Indeed, studies in both eco-immunology and conservation support linkages between a species life history traits (for example, ‘pace of life’) and its vulnerability to infection or extinction, respectively.’ But I am not sure how this really relates to their findings and (I may have missed this), but I did think the analyses were concerned with host diversity versus assembly order (which is surely another dimension of the diversity issue?). My naïve thought process would lead me to think of the opposite: in diverse assemblages, hosts devote less to defense at the expense of reproduction, dispersal (indeed to competition with other species), that they are more vulnerable? I am without a doubt that someone could explain why my reasonings is wrong, but in the space of this paper, the authors did not manage to.

I was pleased to see the authors advocate considering more aspects of community competence than simply host diversity, (‘climate’, ‘resources’, ‘habitat’ – yay, ecology!), but did wonder why they did not consider any further biotic aspects (apart from snails P/A and density) of their communities that might affect their conclusions. What of interspecific competition? Are there any other unaccounted for disease hosts? I feel strongly like I’m missing something, but why do less resistant hosts dominate in low diversity communities? What are their traits?

And, the usual, from a macro-scale biologist. How do these results scale to different systems, different sites?

An interesting paper, timely, nuanced and thought-provoking.