Dwindling biodiversity might make you sick.

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When the COVID-19 pandemic struck the world in 2020, it drew attention to the ways environmental damage can set the stage for disease outbreaks. Scientists pointed to the potential roles of urbanization, habitat loss, and trade in live animals for helping to fuel a disease that many scientists think leapt from wild animals to people.

While all those factors might have influenced this particular pandemic, they aren't the main ways that environmental destruction threatens to amplify infectious disease. It turns out loss of biodiversity, global warming and the spread of non-native species are the biggest factors, according to a sweeping new analysis of research from around the world.

"We did not know which global change drivers most increased or decreased infections," said Jason Rohr, an infectious disease ecologist at Notre Dame University who lead the new study. "Disease control efforts were partially flying blind."

Studies of individual outbreaks have delved into links to a fast-changing world, such as connections between deforestation, hunting and the deadly spread of Ebola in  parts of Africa. Scientists have also drawn connections between problems such as climate change and a whole host of diseases.

But this is the first time that scientists have tried to compare and rank the disease-altering effects of major forces changing the environment today: dwindling biodiversity, climate change, chemical pollution, habitat loss and the spread of invasive species.

To tackle such a massive subject, the researchers relied on the work of hundreds of other scientists. Rohr and his collaborators amassed 972 studies that assessed the effects of at least one of these environmental factors and their links to diseases affecting both humans and other organisms. They then analyzed whether an effect was found in each of them, and how strong the effect was.

Biodiversity loss, invasive species and climate change emerged as the three environmental problems with the strongest link to increased disease. Loss of biodiversity stood out. It was associated with a 65% greater increase in disease than invasive species and a 111% greater increase than climate change, the scientists reported last week in Nature.

So how would biodiversity connect to infectious diseases? A key factor is the way loss of biodiversity can shift the abundance of disease-carrying species, explained Rohr. There's an evolutionary advantage for parasites to target more abundant species, while less common species often have greater resistance to parasites. When rarer species disappear, more common species are left behind and may expand in numbers. A classic example is abundant white-footed mice that are carriers of Lyme disease, according to Rohr.

The phenomenon "is a well-known mechanism by which biodiversity loss increases disease," he wrote in an email response to questions.

Climate change, meanwhile, had widespread effects in a variety of settings, the researchers found, meaning it wasn't confined just to specific situations. "The lack of context dependencies for climate change suggests that disease increases in response to climate change will be consistent and widespread," Rohr said.

Not all environmental damage translated to more disease, however. The rise of cities, in fact, was tied to a decrease in infectious disease. That's might happen because urbanization often means better basic infrastructure such as water and sewage management, the authors suggest.

This also points to another factor that outranks all these environmental problems when it comes to the spread of infectious diseases. The intersection of rural life and poverty "is the strongest predictor of environmentally transmitted infectious diseases on the planet," Rohr and his collaborators write.

The results of this paper could help policymakers and scientists decide where to focus their efforts, said Erin Mordecai, a Stanford University scientist who studies ecology and infectious disease. She wasn't an author on the new paper. "I hope this evidence can be used in international policy, such as in the United Nations Intergovernmental Panel on Climate Change and Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services reports, to spur action on climate change and biodiversity loss," she said.

Mahon, et. al. "A meta-analysis on global change drivers and the risk of infectious disease." Nature. May 8, 2024.

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