Human-driven changes to the planet are bringing widespread and sometimes surprising effects—including shifting the Earth’s rotation, hiding meteorites in Antarctic ice and, potentially, supporting locust swarms.
Now, a large-scale analysis of nearly 1,000 scientific studies has shown just how closely human activity is tied to public health. Published in the journal Nature, the findings suggest anthropogenic environmental changes are making the risk of infectious disease outbreaks all the more likely.
The biodiversity crisis—which has left some one million plant and animal species at risk of extinction—is a leading driver of disease spread, the researchers found.
“It could mean that by modifying the environment, we increase the risks of future pandemics,” Jason Rohr, a co-author of the study and a biologist at the University of Notre Dame, tells the Washington Post’s Scott Dance.
The analysis centred on earlier studies that investigated at least one of five “global change drivers” affecting wildlife and landscapes on Earth: biodiversity change, climate change, habitat change or loss, chemical pollution and the introduction of non-native species to new areas.
Based on the previous studies’ findings, they collected nearly 3,000 data points related to how each of these factors might impact the severity or prevalence of infectious disease outbreaks.
Researchers aimed to avoid a human-centric approach to their analysis, considering also how plants and animals would be at risk from pathogens. Their conclusions showed that four of the examined factors—climate change, chemical pollution, the introduction of non-native species to new areas and biodiversity loss—all increased the likelihood of spreading disease, with the latter having the most significant impact.
Disease and mortality were nearly nine times higher in areas of the world where human activity has decreased biodiversity, compared to the levels expected by Earth’s natural variation in biodiversity, per the Washington Post.
Scientists hypothesize this finding could be explained by the “dilution effect”: the idea that pathogens and parasites evolve to thrive in the most common species, so the loss of rarer creatures makes infection more likely.
“That means that the species that remain are the competent ones, the ones that are good at transmitting disease,” Rohr tells the New York Times’ Emily Anthes.
For example, white-footed mice, the main carriers of Lyme disease, have become one of the most dominant species in their habitat as other, rarer animals have disappeared—a change that might have played a role, among other factors, in driving rising rates of Lyme disease in the United States.
One global change factor, however, actually decreased the likelihood of disease outbreaks: habitat loss and change. But here, context is key. Most habitat loss is linked to creating a single type of environment—urban ecosystems—which generally have good sanitation systems and less wildlife, reducing opportunities for disease spillover.
“In urban areas with lots of concrete, there is a much smaller number of species that can thrive in that environment,” Rohr tells the Guardian’s Phoebe Weston. “From a human disease perspective, there is often greater sanitation and health infrastructure than in rural environments.”
Deforestation, another type of habitat loss, has been shown to increase the likelihood of disease. The incidence of malaria and Ebola, for example, worsens in such instances.
The new work adds to past research on how human activity can prompt the spread of disease. For instance, climate change-induced permafrost melt may release pathogens from the Arctic, a concern that’s been well-documented in recent years. Both habitat loss and climate change may force some animals to move closer together—and closer to humans—increasing the potential for transmitting disease.
Additionally, the research signals the need for public health officials to remain vigilant as the effects of human-caused climate change play out, experts say.
“It’s a big step forward in the science,” Colin Carlson, a global change biologist at Georgetown University who was not an author of the new analysis, tells the New York Times.
“This paper is one of the strongest pieces of evidence that I think has been published that shows how important it is health systems start getting ready to exist in a world with climate change, with biodiversity loss.”
