Does Biodiversity Prevent Pandemics?

I recently read the surprisingly well-written paper Impacts of biodiversity on the emergence and transmission of infectious diseases by Keesing et al. Here are some quick notes and highlights.

First, the bottom line, which roughly “Yes, but it’s complicated” (when is it ever not 🙂

“In principle, loss of biodiversity could either increase or decrease disease transmission. However, mounting [empirical] evidence indicates that biodiversity loss frequently increases disease transmission. In contrast, areas of naturally high biodiversity may serve as a source pool for new pathogens.”

“The researchers don’t know why the effect occurs. But they speculate that species that are better at buffering disease transmission — for example because they have low rates of reproduction or invest heavily in immunity — …
tend to die out first when diversity declines, whereas species that have high rates of reproduction or invest less in immunity — and thus are more likely to be disease hosts — survive for longer.

The review looks at the question of how pathogen transmission is affected by biodiversity loss. Intuitively, “reducing biodiversity can increase disease transmission when the lost species are either not hosts for the pathogen or are suboptimal ones. For pathogens for which transmission is a function of host density, loss of diversity is most likely to increase transmission if the loss causes an increase in the density of competent hosts.”

So the question is whether there any correlation between suitability as pathogen hosts and which species tend to die out in an area first. Unfortunately, it seems that “resilience in the face of disturbances that cause biodiversity loss, such as habitat destruction and fragmentation, is facilitated by life-history features such as high reproductive output and intrinsic rates of increase. Vertebrates with these features tend to invest minimally in some aspects of adaptive immunity; we hypothesize that this may make them more competent hosts for pathogens and vectors.”

The same process seems to recur at the micro scale: – “Changes in the composition of microbiomes are frequently associated with infection and disease. For example, corals suffering from white plague disease have microbial communities distinctly different from those in healthy corals. In some of these examples, a rich microbial community appears to regulate the abundance of endemic microbial species that can become pathogenic when overly abundant. In other cases, high microbial species diversity can prevent colonization by invasive pathogenic species. For example, the more diverse the microbiome surrounding the roots of wheat plants, the more protected the plants were against invasion by the pathogenic bacterium Pseudomonas aeruginosa.” I wonder why this happens? Since here we’re not talking about pathogen hosts presumably the mechanisms are different. There could be crowding out effects if there are limited resources to be had, but then it’s just density that matters, not diversity.

On the other hand, biodiversity hotspots and humans really don’t mix well. “Indeed, almost half of the zoonotic diseases that have emerged in humans since 1940 resulted from changes in land use.”

I didn’t realise there’s another way overusing antibiotics is bad, but apart from straight up natural selection effects, “human use of antibiotics is thought to select for resistant microbes by eliminating the great diversity of non-resistant microbial strains and species that suppress resistant strains in the absence of antibiotics.”

Actually this quote doesn’t quite make sense to me – do they mean kill off other strains of the pathogen, or kill off other harmless bacteria and reduce diversity (ala the story that sanitizing toilet seats kills off the harmless skin bacteria letting them be colonized by far more dangerous fecal bacteria)?

The paper ends with three interesting recommendations:

First, potential emergence ‘hotspots’ could be predictable on the basis of land-use change and underlying biodiversity patterns; these areas should be targeted for surveillance of endemic wildlife pathogens that have the potential to jump host species40,51.

Second, preserving and protecting intact habitats in these hotspots provides a simple, direct way of reducing human–animal contact and reduces the likelihood of emergence of new pathogens, although methods for achieving reduced contact are not always straightforward51.

And third, to reduce the probability that pathogens become established and transmissible within a new host population once spillover occurs, the husbandry of high-density monocultures of domestic animals, particularly in areas at high risk of spillover, should be subject both to more intensive surveillance and to measures that reduce contact between wildlife and livestock.

The last rec seems particularly cost-effective, since the intersection of biodiversity hotspot and “factory farming site” is probably a lot smaller than either, and high-density highly-sterile monocultures are particularly good incubation sites for pathogens.

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