The Dilution Effect (in Primate Disease)

Some Gorillas munchin’ on grass. Photo Credit WWF.

(For my more recent posts about the dilution effect, see here and here.)

The “dilution effect” hypothesis is big in disease ecology right now.  The hypothesis posits that host biodiversity is negatively correlated with disease risk.  This is an appealing idea because if the dilution effect is real and common, then we can kill two birds with one stone when we conserve biodiversity.  And of course, conservation efforts always go over better if there is an anthropogenic benefit to the action, such as reducing human disease risk.

Here’s the caveat.  The dilution effect should only happen if the species that are lost first from disturbed habitats are the “low competency” hosts.  In some systems, this does happen. For instance, with Lyme disease, the most resilient host is usually the white-footed mouse, which is also the most competent host (here’s a related summary).  So, we know that community disassembly rules really matter, and an important next step in disease ecology is actually figuring out the disassembly rules for different disease systems.

There is a prediction that the most competent hosts might also be the most resilient in many systems.  The hypothesis is that relatively ‘r-selected’ type species (e.g., mice) probably do best in disturbed environments because of their fast reproduction rates and short life spans, and they also might invest the least in immune response to parasites/pathogens.  It’s still up in the air as to whether that actually happens or not.

The white-footed mouse:  a species that is resilient to disturbance and is a highly competent Lyme disease vector. Photo from Wikipedia.

Which brings me to a 2013 Ecology Letters (EDIT: Fixed link) paper that I just read.  Young et al. (2013) found that in primates, there was no relationship between host resilience and parasite infection.   There was also no relationship between host resilience and immune response (white blood cell count) in zoos.  Then, in their various analyses, they found that sometimes the dilution effect hypothesis was supported, but other times the converse (the amplification effect) was supported.  So complicated!

At the end of the paper, Young et al. (2013) suggested that we need to do more meta-analyses that synthesize system-specific studies so that we can figure out whether the dilution effect is common.  You can check out my post about the dilution effect debates to learn more!

Reference:

Young, H., R.H. Griffin, C.L. Wood, and C.L. Nunn. 2013. Does habitat disturbance increase infectious disease risk for primates?  Ecology Letters.