Seabloom, E.W., Borer, E.T., Mitchell, C.E., & Power, A.G. Ecology. (2010).

"Viral diversity and prevalence gradients in North American Pacific Coast grasslands".

Reviewed 10/27/11

Host-pathogen interactions do not exist inside a bubble, they must be considered within a larger community and geographical context. It is possible to find multiple pathogens coexisting inside a single host species and/or to observe pathogens across a wide range of host species. The authors of this paper collected observational data at 26 separate field sites, analyzing the role of four Barley and Cereal Yellow Dwarf Viruses (B/CYDVs) on three grass host species.  These sites spanned a breadth of close to fifteen degrees latitude and represent a wide range of environmental variables and community composition.
The key questions the researchers hoped to answer revolved around how host identity, environmental attributes, and host community of the site affected pathogen prevalence and diversity. Lastly researchers were interested in the coinfection of pathogens and whether or not that was regulated by the total available pathogen pool at a site. This coinfection can be thought of as synonymous with alpha diversity, a traditional measure of local scale diversity in ecology. Beta diversity in this study was the total possible pool of pathogens at a site, and as these viruses are aphid vectored, the turnover among co-inhabiting host species is of particular importance.
For this study system, prevalence was not found to correlate with coinfection. Statistical analyses of the model variance showed that the two metric of pathogen dynamics could have some minor drivers in common, but that overall the majority of the variation seen likely arises from different primary drivers, ie what primarily causes patterns of coinfection does not cause patterns of prevalence.Prevalence was found to be largely a result of site precipitation and soil nitrate patterns, and coinfection was most correlated with latitude.
Coinfection levels increased significantly the farther north the site occurred, and as global gamma diversity remained largely the same across all sites, this resulted in a net decrease in beta-diversity, lower pathogen turnover. The authors posit that this is the result of a higher density of generalist vectors for the pathogens found in the north. A higher rate of vector herbivory on the grass hosts in general would lead to an increase in the overall transmission of the separate pathogen species and therefore a higher level of coexisting pathogens within a single host. More experimental work would be needed to prove this mechanism however.
It is possible to think of latitude as being a collation of many possible environmental variables (such as sunlight, temperature, etc), however the fact that coinfection was strongly correlated with this metric and not with host species type lends evidence to the idea that maybe the viral coexistence within a single host species is a function of the underlying site-level environmental conditions. Viral coexistence and within host niche allocations is not a traditional example used to think about niche-theory, but if pathogen coexistence is somehow determined by resource availability, mediated by their mutual host of course, this raises many interesting questions for future work, including: what categories of pathogen might be able to coexist with one another (rust, viral, bacterial, etc) based on their differential niches? This sort of question can still apply to viral species such as B/CYDVs.