Are animals good for us? Several studies suggest that our chances of picking up an animal disease are reduced if we are surrounded by a variety of species – the thinking goes that abundant animal life acts a pool that prevents the disease from jumping to people. Others strongly disagree, arguing that more creatures means a greater variety of diseases and a higher chance that one of them will evolve to infect us.
The results of a new mathematical model presented at the Planet Under Pressure meeting in London last month suggest both sides may be right – depending on the type of biodiversity in an area. This means it may be as important for public health workers to watch for new diseases in, say, New York City where few animals live, as it is to monitor pathogens in biodiversity hotspots.
Benjamin Roche of the International Center for Mathematical and Computational Modeling of Complex Systems in Bondy Cedex, France, who generated the model, says that it shows – perhaps unsurprisingly – that a diversity of disease carriers is bad news. This is certainly true for West Nile virus, Roche says, which is found in several US states and is successful partly because it can be transmitted by many mosquito species.
The model also shows, however, that humans may benefit if surrounded by a variety of animals – provided only some of them are susceptible to a disease, therefore lowering our chance of encountering an infected animal. West Nile has been less likely to cross into human populations in areas with high biodiversity among birds, which may act as reservoirs, says Felicia Keesing of Bard College in Annandale-on-Hudson, New York. She sees this as evidence that such biodiversity keeps the virus away from people and is good for health.
Whether these effects hold true on a global scale is still hotly debated. "It's a lovely story and I wish it were true," says Peter Daszak, president of EcoHealth Alliance in New York City. In 2008, he and colleagues published a paper in Nature, DOI: 10.1038/nature06536, which found that the areas of the world where diseases jump from animals to humans tend to contain many different species.
There is a "disappointingly simple" reason why biodiversity is bad for health, Daszak says. "There is a huge diversity of viruses and microbes in the wild that we don't know about yet." Using human health as an argument for keeping developers and ecotourists out of pristine habitats, he argues, is an even better way to confine animal diseases to animals.
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.