Reference: Liu, X., L. Chen, M. Liu, G. García–Guzmán, G. Gilbert, and S. Zhou. 2020. Dilution effect of plant diversity on infectious diseases: Latitudinal trend and biological context dependence. Oikos 129: 457-465. https://doi.org/10.1111/oik.07027
Over the last several weeks epidemiology has become a popular topic in the public eye, as talk about what increases disease risk and spread permeates a variety of news sources. A few of these stories have brought the connection between poor environmental health and disease prevalence to light as a sobering reminder that the degradation of the natural world around us will inevitably lead to our own self-destruction. As an ecologist who thinks a LOT about how and why we should preserve biodiversity, particularly in the world of plants, I started to wonder about all of the ways that diversity can affect the health of a wide range of organisms. This linkage to environmental health may be instrumental for the motivation and implementation of better policies for the protection of biodiversity. So let’s take a journey through the background of biodiversity-disease research all the way to hot-off-the-press discoveries in the realm of plant diversity and disease suppression.
The “Dilution Effect”
Researchers in both ecology and human health have increasingly become interested in the connection between biodiversity and disease. So much so, that they coined a term to describe how biodiversity might help slow disease occurrence- the “dilution effect.” Essentially, a greater diversity of animals, insects, or any potential biological host in a given area decreases the likelihood of an infectious agent succeeding because the infectious agent enters a less suitable host more often. Therefore, disease spread is stopped or slowed by the “decoy” host. This concept has also been recently applied to research in regards to zoonotic diseases (diseases that pass from animal/insect to humans), a group which includes the now notorious coronavirus. Here, the impact of biodiversity on human health could be much larger than previously realized, if the dilution effect plays out to be true. In fact, some scientists believe that the global rise in disease outbreaks may be a direct result of environmental degradation and destruction of biodiversity. While there is still debate over the biodiversity-human disease link, a few studies have already identified a decrease in transmission rates of zoonotic diseases such as Hantavirus and Lyme disease where biodiversity of potential hosts is high.
More reasons to take biodiversity loss seriously
Biodiversity-disease research has also ventured into other areas of interest, including conservation, ecosystem health, and agriculture. In many cases, it has been found that biodiversity is important to the reduction of disease. In particular, it is important to determine how plant biodiversity relates to the occurrence and impact of plant diseases because plants form the base of food chains and provide shelter for wildlife. Habitat degradation has also been determined as a cause of increases in zoonotic disease spread because shrinking habitats force viable hosts into tighter quarters that are often near human settlements.
A comprehensive study of plant diversity and disease
So, does plant diversity decrease the impact of disease after all? Does the dilution effect hold true in all cases? Liu and colleagues (2020) analyzed 21 different studies to answer these questions, comparing the effect of diversity on disease occurrence and severity (“impact”) in different ecosystem types (e.g. forest, grassland) at different latitudes with varied regional biodiversity levels and climates. The researchers also examined whether diversity decreased disease occurrence for different pathogens (e.g. viruses, fungi) with different specializations and methods of infection. For instance, if a pathogen only has one or few hosts (specialist) then it might be more likely for a high diversity plant community to “dilute” disease because the likelihood of a pathogen reaching a suitable host will be especially low. In contrast, dilution may not occur if the pathogen has a lot of possible hosts (generalist). Relative size of a plant host (e.g. grass vs. tree) and climate of the region could also determine how strong the dilution effect is.
Overall, the researchers found that plant diversity significantly suppressed disease impact across studies but also that the reduction strength depended on the specifics of the study location and pathogen type examined. One major discovery was that the ability for plant diversity to reduce disease was much stronger at lower latitudes regardless of ecosystem type, climate, or pathogen type. It is thought that this might have occurred because pathogens at lower latitudes tend to specialize on particular hosts and because plant diversity is typically higher at lower latitudes, which could make dilution easier. The reduction of disease impact was also consistently greater in grasslands than in forests. These results enforce the importance of understanding contexts when making assertions about how diversity impacts disease and have the potential to guide conservation plans to reduce disease occurrence in plant communities.
Setting Our Sights on Conservation
There is obviously a lot more for scientists to explore in regards to the biodiversity-disease relationship, though the findings presented here give us even more reasons to put the protection of biodiversity at the top of our social change bucket lists. Today, as I finish this article on the 50th anniversary of Earth Day, I hope that these scientific discoveries remind us how interconnected all living things are and inspire us to take action to preserve biodiversity!