Powdery mildew-afflicted ribwort plantain leaves. Image: Susanna Kekkonen/Science
The modern natural world is an increasingly fragmented one, with islands of ecological integrity isolated in vast sprawls of human development.
An environment arranged in such a fashion may inadvertently fuel the spread of disease, according to a new study of the interaction of plants and a fungus pathogen. Conversely, large and continuous natural areas may dampen the spread. While the observations can’t be directly extrapolated from a single system to nature at large, they hint at a potentially troubling dynamic.
“If we have these little islands of susceptible hosts in a landscape, harboring high levels of infection that transmit to everything around them, the potential is there for spillover” to agriculture and perhaps humans, said biologist Anna-Liisa Laine of the University of Helsinki.
The study, published June 12 in Science , describes 12 years of the battle between Plantago lanceolata, a herb commonly known as ribwort plantain, and leaf-blighting powdery mildew in forests on the Åland Islands in the Baltic Sea.
The researchers, led by Laine and fellow University of Helsinki biologists Jussi Jousimo and Ayco Tack, found that vulnerability to the disease changed depending on the area of ribwort populations. Plants in isolated populations were more likely to become infected, and those in larger, connected patches were more likely to resist infection.
It’s a counterintuitive dynamic: At least on the surface, conventional ecological theory predicts that disease should spread much more readily through larger patches. Instead, those regular exposures seemed to heighten plant defenses, serving almost as evolutionary inoculations.
In large patches, disease-resistant plants were more likely to reproduce and spread, and—crucially—there was a larger gene pool from which disease-resistant mutations might arise. Small-patch plants, less-exposed and with a smaller gene pool to draw from, remained susceptible to infection when it finally did occur.
'We've known for some time that habitat fragmentation exacerbates many diseases, but this study reveals a whole new mechanism.'
“I would have predicted exactly the opposite result—that hosts in fragmented populations would be protected from this fungus,” said disease ecologist Richard Ostfeld of the Cary Institute for Ecosystem Studies, who was not involved in the research.
“We’ve known for some time that habitat fragmentation exacerbates many diseases, but this study reveals a whole new mechanism,” Ostfeld added.
Among the exacerbations to which Ostfeld refers are enhanced virulence of the lethal Hendra virus in isolated populations of Australian flying foxes, and also outbreaks of yellow fever in fragmented forests in southern Brazil.
Common to these incidents is a newfound appreciation for disease dynamics, with healthy ecologies linked to evolutionary patterns that naturally reduce the dangers of epidemic outbreaks.
Disease ecologist Meghan Duffy of the University of Michigan did, however, caution against extrapolating too much from the new study. More research involving other hosts and diseases is needed to see whether similar patterns are found elsewhere, she said.
“Whether habitat fragmentation is likely to contribute to increased infectious disease in a wide range of systems is an open question,” she said, but the findings underscore how links between the environment, human impacts and disease “might be more complicated than we might initially guess.”
Laine, whose group is now studying infection dynamics of viruses in ribwort, said that one significant, broad-spectrum message is the importance of genetic diversity. “It’s enough in itself to dilute disease transmission,” she said. “We see less infection when there is a diversity in resistance traits.”
Aside from possible spillovers from habitat pockets into human areas, as with the Hendra virus, there may also be conservation implications to the new findings. Some scientists have wondered whether connecting protected areas may increase the spread of disease; instead, said Laine, connection provides protection.
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