"Effects of species diversity on disease risk".
Reviewed 10/28/11
How does the presence of disease within an organism affect their ability to tolerate and survive? More broadly how does the presence of disease in a population or community of hosts affect the dynamics of that system, and how does the host community affect the pathogen? The study of the movement of disease in a community has long been an area of interest in ecology. The authors of this paper looked specifically at many recent experiments designed to tease out the role of host species diversity on disease prevalence and risk for a community.
Simple mathematical models were used at each stage in their discussion to illustrate how proposed mechanisms could be projectable onto a real ecosystem. For example an increase in the number of non-host species in a community could affect the encounter rate of a host and its pathogen. There are a number of ways it is possible to imagine how this would occur, if a predator induced its prey to move less and hide more, the prey would have fewer interaction events with its disease enemy and so disease risk would be driven down.
This is just one example of 'Encounter Reduction' that the authors propose as a mechanism for how diversity increases could affect the role of disease in a community. However, a rising level of diversity does also have the potential to increase disease risk. This is possible in systems where there is only one host of study, but it is maybe easier to imagine in a system with multiple hosts. If a community already has an abundance of low quality host species, and rising levels of diversity introduce a higher quality host, then the net effect would be one of an amplification of disease within the system.
The concept of a 'dilution host' was introduced by the authors in the later stages of the paper. As I see it this type of host introduction could be thought of as something of a keystone species. Just as we have included the concept of a keystone predator into our ecological vocabulary it is intriguing to think abou the possibility of a keystone disease regulator. A host that, when present in the system, regulates the prevalence of disease across the entire community. This could occur if the species is a poor reservoir for a pathogen, but a high-quality host for a predatory vector. It is unclear how common this type of species is in natural communities, but it is nevertheless an interesting venue to consider for conservation techniques.
The models described relied heavily on the ability to connect a mechanism for diversity effects with the net change in the density of infected hosts. There are other means of tracking disease risk in a system, and these include prevalence of the pathogen and density of infected vectors, among others. One area of future research would be to look at how using any of the various metrics as the measure of disease risk alters the outcome of the net effect of diversity on disease. Though the authors claimed to use the rate of change of infected hosts they often referred to disease risk in the paper simply as disease prevalence. I am wondering if their is a scenario that could arise within a natural system where the prevalence of disease remains high while the future risk of disease begins to decline. This scenario seems possible especially when the dynamics of a disease are primarily caused by non-population level drivers, such as seasonal or environmental. If this is indeed the case, than it seems as though using disease prevalence and disease risk interchangeably may not be the most appropriate. Future empirical or theoretical studies could illuminate this problem.
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