In Cities and Farms, Disease-Carrying Animals Thrive

When humans dominate wild land, disease-carrying animals take over and biodiversity suffers

Flock of pigeons flying over pavement
As natural space is converted to cropland, pastures, cities and suburbia, certain short-lived animals like pigeons and rats, thrive. Ifrah Sanober / EyeEm

When wild land is developed, humans are unwittingly creating a paradise for pathogen-carrying animals.

As natural space is converted to cropland, pastures, cities and suburbia, certain short-lived animals like pigeons and rats, thrive. These species are more likely to carry pathogens that can spread to humans and cause widespread disease.

In a study published Wednesday in the journal Nature, a team of researchers from University College London analyzed more than 184 studies covering nearly 7,000 animal species, 376 of which are known to carry pathogens.

Researchers first sought to understand which species disappear and which thrive in both undisturbed and human-dominated areas. They used a database by the Projecting Responses of Ecological Diversity in Changing Terrestrial Systems (PREDICTS) project, containing more than 3.2 million records from 666 studies. They compared these findings with data about species known to carry pathogens to understand population changes over time.

With the exploitation of natural spaces, long-lived animals that require specialized habitats, like rhinoceroses, suffer. Meanwhile, animals like rodents, bats and songbirds more easily adapt to human-dominated environments. These animals are able to quickly reproduce and live all over the world, meaning they can overrun human-dominated environments while others risk extinction due to habitat loss.

These changes in biodiversity are comparable to the success of large chain stores at the expense of small and independent retailers, co-researcher Andy Purvis of the Natural History Museum in London tells Helen Briggs at BBC.

"It makes all towns look the same, and it's less easy to tell where you are. Likewise, people are affecting nature everywhere they go, and everywhere there are localised species which are struggling to make a living," he says.

Researchers find that these short-lived animals also tend to be more tolerant of infections than other creatures. This may be because they are more likely to invest in quick reproduction rather than immune defenses, making them susceptible to pathogens, reports Michael Le Page at New Scientist. Another possible explanation is that pathogens tend to target animals that are more abundant over time.

The study complicates the misconception that wild nature, like untamed jungles, is the greatest source of disease crossover between animals and humans. The current coronavirus outbreak is thought to have originated in bats, and other wild animals may have also played a role in transmitting the virus to humans. There are strong indications that it was linked to the wildlife trade, according to BBC.

“The COVID-19 pandemic triggered by a coronavirus of animal origin has awakened the world to the threat that zoonotic diseases pose to humans,” write ecologist Richard Ostfeld of the Cary Institute of Ecosystem Studies and biologist Felicia Keesing of Bard College in a commentary piece published in Nature. “With this recognition has come a widespread misperception that wild nature is the greatest source of zoonotic disease.”

The study finds that the animals that thrive in human-dominated areas both carry a greater variety and abundance of pathogens. When studying pathogens carried by animals in the future, the authors say scientists should pay special attention to human-dominated landscapes.

"As agricultural and urban lands are predicted to continue expanding in the coming decades, we should be strengthening disease surveillance and healthcare provision in those areas that are undergoing a lot of land disturbance, as they are increasingly likely to have animals that could be hosting harmful pathogens," Kate Jones of University of College London tells BBC.

The authors suggest that restoring degraded habitat and protecting wild land would benefit both the environment and public health. Further, reintroducing lost predators could help control populations of pathogen-spreading animals.

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