If you’ve ever had Lyme disease, you might be glad to know that the bacteria responsible didn’t enjoy the infection any more than you did. That’s because humans are what’s known as a “dead-end host.” In other words, when you get Lyme disease, you’re highly unlikely to give it to anyone else. Many other animals are dead-end hosts for Lyme disease too, and studying the disease transmission patterns amongst them has given rise to a controversial theory called the “Dilution Effect.”
The dilution effect theory argues that greater host diversity reduces disease spread. As an example, let’s consider in more detail what the dilution effect looks like for Lyme disease.
Lyme disease is caused by a bacterium called Borrelia burgdorferi that lives, grows, and reproduces in white-footed mice. This makes mice what ecologists would call the primary “reservoir” of Lyme disease — the chronically infected species that seeds disease spread in other organisms. Ticks then act as “vectors” of the disease, picking up the bacteria by feeding on infected mice, and then spreading them by biting other animals. When ticks have less contact with white-footed mice, and instead feed on other species that are less likely to transmit Lyme (shrews, squirrels, raccoons, and other small mammals), the presence of these alternative food sources for the ticks can dilute the spread of Lyme disease (LoGiudice et al 2003) — a perfect example of the dilution effect in action.
More broadly, the dilution effect theory suggests that increased biodiversity increases dead-end hosts and reduces contact between reservoirs and vectors, thereby lowering disease transmission. However, in practice, this chain of events is not guaranteed.
For one thing, while the dilution effect often holds true in diseases that spread through frequency-dependent transmission, it is much less common in diseases with density dependent transmission (Espira et al 2022). What does this mean? Frequency-dependent transmission refers to disease spread that doesn't depend on the overall density of the population, but on the rate of contact between individuals (i.e. the frequency of interactions). Sexually transmitted diseases, for example, spread more rapidly when sexual activity increases, but not necessarily when population density increases. The common cold, by contrast, spreads in a density-dependent fashion, where spread increases with the number of individuals present (a sneeze infects more people in a packed room than an uncrowded one). The dilution effect is most often observed in diseases whose spread is frequency-dependent, since the total number of interactions is constrained.
Another complication is that the opposite of the dilution effect — the so-called “amplification effect” — has occasionally been observed, as in the case of hantavirus pulmonary syndrome (Luis et al. 2018). This can happen when increased biodiversity also increases contact events between vectors and hosts, especially when the introduction of new species modifies predator-prey dynamics in the food web of an ecosystem, resulting in species coming in contact that might otherwise not have, and ultimately increasing disease transmission (Gómez-Hernándes et al. 2023).
As an analogy for how all this works, imagine you love eating chips. When you (disease vector) buy pringles (reservoir), you share them with all your friends (transmission), but with other brands, you keep them to yourself (dead-end host). If the store increases the number of chip brands it sells, they compete for your attention, so you’ll be less likely to buy pringles (and, thus, less likely to transmit to your friends). This is the dilution effect in action! However, for this to actually hold true, you need to keep buying the same, finite number of chip bags (frequency-dependent spread). If, instead, when the number of chip brands increases, you proportionately increase your chip-buying budget (density-dependent transmission), then, since you’re buying more chips, you might still share just as many pringles with your friends. Furthermore, if the new variety of brands brings more shoppers to the store to buy chips, or if the new competing brands become involved in a corporate merger with the pringles manufacturer (predator-prey dynamics), the store might actually end up selling more pringles as a result of the increased diversity of chip brands— an amplification effect.
Recent studies are developing new models that reconcile all these complexities, exploring how dilution and amplification effects interact with each other and which dominate in different situations (Espira et al 2022). In the meantime, the uncertainties around the dilution effect may force us to adopt a more nuanced view of biodiversity.
In recent years, “biodiversity” has become something of a buzzword in environmental circles, hailed as an unmitigated good. The complicated nature of the dilution effect— and the existence of concurrent amplification effects in some cases— reminds us to examine the situation a bit more carefully, remembering that even though biodiversity loss is generally harmful, increasing biodiversity may not always be the cure-all it seems to be. (And the next time you take a walk in the forest, don’t forget to check for ticks!)
Works cited
Espira, L. M., Brouwer, A. F., Han, B. A., Foufopoulos, J., & Eisenberg, J. N. S. (2022). Dilution of Epidemic Potential of Environmentally Transmitted Infectious Diseases for Species with Partially Overlapping Habitats. The American naturalist, 199(2), E43–E56. https://doi.org/10.1086/717413
Gómez-Hernández, E. A., Moreno-Gómez, F. N., Bravo-Gaete, M., & Córdova-Lepe, F. (2023). Concurrent dilution and amplification effects in an intraguild predation eco-epidemiological model. Scientific reports, 13(1), 6425. https://doi.org/ 10.1038/s41598-023-33345-2
LoGiudice, K., Ostfeld, R. S., Schmidt, K. A., & Keesing, F. (2003). The ecology of infectious disease: effects of host diversity and community composition on Lyme disease risk. Proceedings of the National Academy of Sciences of the United States of America, 100(2), 567–571. https://doi.org/10.1073/pnas.0233733100
Luis, A. D., Kuenzi, A. J., & Mills, J. N. (2018). Species diversity concurrently dilutes and amplifies transmission in a zoonotic host-pathogen system through competing mechanisms. Proceedings of the National Academy of Sciences of the United States of America, 115(31), 7979–7984. https://doi.org/10.1073/pnas.1807106115