Several large-scale, human-induced changes to the planet — including climate change, biodiversity loss and the spread of invasive species — are making infectious diseases more dangerous to people, animals and plants, according to a new study.
Scientists have documented these effects in the past in more targeted studies that have focused on specific diseases and ecosystems. For example, they found that a warming climate may be helping malaria spread in Africa, and that declining wildlife diversity may be boosting Lyme disease outbreaks in North America.
But the new research, a meta-analysis of nearly 1,000 previous studies, suggests that these patterns are relatively consistent around the world and across the tree of life.
“It’s a big step forward in science,” said Colin Carlson, a biologist at Georgetown University who was not an author of the new analysis. “This paper is one of the strongest pieces of evidence that I think has been published and shows how important it is for health systems to start preparing to exist in a world of climate change, of biodiversity loss.”
In what is likely to be a more surprising finding, the researchers also found that urbanization reduced the risk of infectious diseases.
The new analysis, published in Nature on Wednesday, focused on five “global change agents” that are altering ecosystems across the planet: biodiversity change, climate change, chemical pollution, introduction of non-native species, and habitat loss or change.
The researchers gathered data from scientific papers that looked at how at least one of these factors affected various infectious disease outcomes, such as severity or prevalence. The final dataset included nearly 3,000 observations of disease risks to humans, animals and plants on every continent except Antarctica.
The researchers found that, in general, four of the five trends they studied – biodiversity change, the introduction of new species, climate change and chemical pollution – tended to increase the risk of disease.
“It means we’re probably selecting for general biological patterns,” said Jason Rohr, an infectious disease ecologist at the University of Notre Dame and senior author of the study. “It suggests that there are similar kinds of mechanisms and processes that are likely to occur in plants, animals and humans.”
Biodiversity loss played an especially large role in increasing disease risk, the researchers found. Many scientists have argued that biodiversity can protect against disease through a phenomenon known as the dilution effect.
The theory holds that parasites and pathogens, which rely on abundant hosts to survive, will evolve to favor species that are common rather than those that are rare, Dr. Rohr said. And as biodiversity declines, rare species tend to disappear first. “That means the species that remain are the competent ones, the ones that are really good at transmitting disease,” he said.
Lyme disease is a frequently cited example. White-footed mice, which are the main reservoir for the disease, have become more dominant in the landscape as other rarer mammals have disappeared, Dr Rohr said. This change may partly explain why rates of Lyme disease have increased in the United States. (The extent to which the dilution effect contributes to Lyme disease risk has been debated, and other factors, including climate change, are likely to play a role as well.)
Other environmental changes could enhance disease risks in a wide variety of ways. For example, introduced species can bring new pathogens with them, and chemical pollution can stress organisms’ immune systems. Climate change can alter animal movements and habitats, bringing new species into contact and allowing them to exchange pathogens.
In particular, the fifth global environmental change the researchers studied — habitat loss or change — appeared to reduce disease risk. At first glance, the findings may appear to contradict previous studies, which have shown that deforestation can increase the risk of diseases ranging from malaria to Ebola. But the overall trend toward reduced risk was driven by one particular type of habitat change: increasing urbanization.
The reason may be that urban areas often have better sanitation and public health infrastructure than rural areas—or simply because there are fewer plants and animals to serve as disease hosts in urban areas. The lack of plant and animal life “is not a good thing,” Dr. Carlson said. “And it also doesn’t mean that animals in cities are healthier.”
And the new study doesn’t invalidate the idea that forest loss can cause disease. Conversely, deforestation increases risk in some cases and reduces it in others, Dr. Rohr said.
Indeed, while this type of meta-analysis is valuable for revealing broad patterns, it can obscure some of the nuances and exceptions that are important for managing specific diseases and ecosystems, noted Dr. Carlson.
Furthermore, most of the studies included in the analysis examined only a single global change movement. But in the real world, organisms struggle with many of these stressors simultaneously. “The next step is to better understand the connections between them,” said Dr. Rohr.