Fields full of herbaceous plants such as these can be incredibly diverse and complicated ecosystems, and the multitudes of species that inhabit them can influence the magnitude of disease that the organisms that inhabit it may encounter (Image Credit: LudwigSebastianMicheler, CC BY-SA 4.0, Image Cropped)

Past is prologue: host community assembly and the risk of infectious disease over time (2018) Halliday, F.W. et al., Ecology Letters, 22, https://dx.doi/10.1111/ele.13176

The Crux

Everything in ecology is based around the environment that a focal organism inhabits, including the interactions it has with other organisms and the non-living aspects of the habitat itself (temperature, water pH, etc.). That being said, it’s no surprise that disease dynamics are likely to depend on the environment that a host inhabits, and that the environment itself is a product of what came before. That is to say, the group of organisms that originally populate a given ecosystem can have an effect on how that ecosystem will look in the future (lakes with freshwater mussels will have clearer water than those without).

The scientific literature is full of experiments, observations, and hypotheses about which environmental conditions lead to fluctuations in disease dynamics. As such, it is difficult to come to a consensus with a “one-size-fits-all” rule for disease dynamics and community structure. The authors of today’s study used a long-term experiment to determine what exactly moderates disease over time. 

What They Did

The main questions were how the initial number of host species and resource supply affected, indirectly, disease dynamics of the host plants. To accomplish this, the authors used two resource levels (with fertilizer and without), two host species richness levels, and six different community compositions (each community had a different group of host species). In total, the study utilized 120 plots planted in an old-field research station in North Carolina, and lasted for three years.

Disease is quantified as the mean leaf area damaged by the parasites (insects and microbes) on each of the plant species. Because all of the plant species used in the study senesce (lose their leaves) during the winter, this design allowed for accurate assessment of damage on a yearly basis.

Did You Know: Disease Ecology

In this article I’ve used the terms parasite and disease interchangeably, and while these organisms are not technically the same, the effects they have on hosts and the functions they serve in an ecosystem are the same. Diseases tend to be caused by bacteria and viruses, and are transmitted from one host to another through a variety of different methods. Parasites, on the other hand, tend to have a free-living stage that seeks out a host to feed off of, and some parasite species only feed off of other organisms for part of their lives, becoming full-blown predators as adults.

What They Found

Initial host species diversity increased disease magnitude in the first year of the study, while decreasing it in both the second and third years. The supply of resources did not have an effect on final community parasite load (the number of parasites found on all of the host plants) until the third year, where it indirectly increased parasite load via an increase in exotic (invasive) host abundance.

Problems

This study addressed some large-concept, general topics from disease and community ecology, but it was a study that utilized one study system over a single three-year period. It’s difficult to say whether or not the results here are applicable to other study systems, or if these results could even be replicated again using the exact same experiment!

So What?

Although they can be financially and logistically demanding, long-term studies like this one that use a well-designed experimental protocol are necessary for making any kind of progress into understanding the natural world. It may only apply to this system, but this study has shown that the effects of host species diversity and resource availability on disease are not only context dependent, but they can also vary over time.

If these results hold in other systems, it may give us as humans a leg up on combating future disease outbreaks due to our understanding of what conditions may promote disease.