The core concepts of conservation ecology are well-established: we know that habitat loss, fragmentation, invasive species, over-exploitation and of course, climate change, are bad for biodiversity. This well-quantified scientific baseline has led the discipline recently to embark on questions pertaining more to the (a) implications of biodiversity loss for humanity and (b) what we can do to offset these. A recent paper by Morand and colleagues addresses perhaps one of the most compelling reasons that human society should appreciate biodiversity beyond its intrinsic value; as biodiversity degrades, so too does human health.
Some argue that the only way to convince society in general that biodiversity is worth protecting is that we link its loss directly to degrading human health, wealth and well-being. Confirmation of such relationships at a variety of spatial and temporal scales is therefore essential. Morand and colleagues used data from a variety of sources to test two predictions: (1) that the number of infectious disease should increase as overall biodiversity increases and (2) that biodiversity loss, inferred from species threat and deforestation data, should increase the number of infectious disease outbreaks in humans. Using data from 28 countries in the Asia-Pacific region, they confirmed both predictions.
While the authors rightly controlled for various confounding effects such as population density, country area and health investment, their statistical approaches were basic and potentially prone to misinterpretation. While I’m reasonably confident that their overall conclusions would hold, it would be prudent to re-analyse the dataset using more sophisticated methods (e.g., accounting for spatial autocorrelation and model uncertainty). It would also be prudent to expand the rather small sample size to more than 28 countries.
These analytical issues aside, the potential mechanisms underlying the correlative phenomena are interesting. While it’s not unexpected that pathogen diversity should increase as overall biodiversity increases (more diversity, full stop), it is ecologically fascinating how biodiversity degradation could lead to a higher incidence of infectious disease in humans. Two mechanisms are proposed. One is the direct ‘dilution’ effect where a high number of potential hosts reduces the chances that any one (including humans) becomes infected with the pathogen. Another is the indirect dilution effect whereby higher host diversity reduces susceptible host density via intensified interspecific competition. Clearly the mechanisms underlying the correlations require much more work; regardless, the paper’s message represents an essential piece in the ecosystem services puzzle.