|Frogs in California killed by the chytrid fungus |
(source: National Geographic News)
Suggested mechanisms behind the dilution effect include 1) that diluter species (i.e. not the focal host) reduce parasite encounters for focal hosts, with little or no risk to themselves (resistant); and 2) diluters may compete for resources or space against the focal host and so reduce the host population, which should in turn reduce density dependent disease risk. But, if these are the mechanisms, there are a number of corollaries that should not be ignored. For example, what if the diluter species is the poorer competitor and so competition reduces diluter populations? What if diluter species aren't completely resistant to disease and at large populations are susceptible? The cost/benefit analysis of having additional species present may differ depending on any number of factors in a system.
- Dilution failure: the result of a host genotype that is a strong competitor, and a large epidemic (low R*, high R0).
- Dilution success: the result of a host that is a weak competitor and a moderate epidemic (host has high R*, moderate R0).
- Dilution irrelevance: the outcome of a host that is a weak competitor, and a small epidemic (high R*, low R0).
|From Strauss et al. 2015. The y-axis shows percent host population infected, solid lines show the disease prevalence without the diluter; dashed show host infection when diluter is present.|
Of course, all models are simplifications of the real world, and it is possible that in more diverse systems the dilution effect might be more difficult to predict. However, as competition is a component of most natural systems, its inclusion may better inform models of disease risk. Other models for other systems might suggest different outcomes, but this one provides a robust jumping off point for future research into the dilution effect.
Alexander T. Strauss, David J. Civitello, Carla E. Caceres and Spencer R. Hall. 2015. Success, failure and ambiguity of the dilution effect among competitors. Ecology Letters. 18: 916–926.