Diverse Ecosystems Control Infectious Diseases

These white tailed deer aren't just gorgeous, they may protect against Lyme disease. Orhan Cam.

Even if you don't care that the world is losing some of the great creatures of the Earth, you may want to value biodiversity for your health. A much mocked theory suggesting protection of animal species can limit the spread of infectious diseases to humans has gained support from a study published in the prestigious Proceedings of the National Academy of Sciences.

The Dilution Effect hypothesis states that parasites are more likely to spread diseases when they are restricted to a small number of species to prey on. It is an idea that many people find deeply counter-intuitive. When diseases cross over from animal hosts to humans, there are almost always calls for mass culling of the animals. So the idea that we should actually be encouraging fellow parasite targets is strange indeed.

Nevertheless, evidence for the theory has been found when it comes to the black-legged tick and the Lyme disease-inducing bacteria (Borrelia burgdorferi) it carries. Host species can interact with parasites in different ways, and while an increase of potential hosts often leads to more parasites that can jump to humans, in some cases it can also decrease the chance that the parasites will be carrying diseases. For example, ticks can pick up Lyme disease from mice but not deer, so an increase of deer populations may boost the tick numbers, but sharply decrease the proportion that carry the disease

Critics have argued that, while in very limited circumstances the Dilution Effect may apply, in the real world, the effect will be very rare

However, University of South Florida biologists found when they investigated 202 examples of 61 host-parasite systems, and the ecological conditions in which they occurred, they found something very different.

“Our study found broad evidence that species-rich communities suffer less infectious disease, and the magnitude of this effect was independent of host density, study design, type and specialization of parasites, and whether the parasite infected humans or wildlife, indicating that dilution was robust across all ecological contexts examined,” said lead author Dr David Civitello. The authors backed this up with an analysis of interactions between plants and herbivores, showing that where there is a greater variety of plant diversity, pest infestations are less intense.

If the paper is right, the implications are huge. “Maintaining biodiversity in nature could reduce the abundance of many parasites of humans and wildlife,” Civitello pointed out. “Conversely, human-induced declines in biodiversity could contribute to increases in both human and wildlife diseases.” Equally, the findings support advocates of farming regimes, such as permaculture, that promote devoting land to non-food plants in close conjunction with food crops.

In other words, killing Bambi's mother wasn't just cruel, it could have made the hunter's family and friends very sick.

Critics of the Dilution Effect have called it “Panglossian” for the implication that species diversity is always best. Even its creators admit this is unlikely to be true, but the latest work suggests we may benefit from being surrounded by other species far more often than we suffer.

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