Biodiversity—what’s it good for? Of course anyone lucky enough to catch a glimpse of an endangered Indri lemur screaming through a forest in Madagascar or humpback whale cresting in the north Atlantic knows there’s an intrinsic value to a world with species beyond Homo sapiens. But if biodiversity was just about providing a pretty backdrop to human beings, would it be worth sacrificing to protect? Is nature just a thing to be used?
But it turns out that a healthy, diverse planet may help keep all of us healthier as well. That’s the conclusion of a new article in the December 3 edition of Nature. Reviewing 24 papers that have examined the potential relationship between biodiversity and the emergence and transmissions of infectious disease, a team of researchers led by ecologist Felicia Keesing of Bard College found that the loss of species could cause more humans to contract infectious diseases like the West Nile virus or hantavirus. (Download a PDF of the paper here.) As biodiversity decreases, in many cases there’s an increase in pathogens—and more risk for human beings, as Kessing said:
We knew of specific cases in which declines in biodiversity increase the incidence of disease. But we’ve learned that the pattern is much more general: biodiversity loss tends to increase pathogen transmission across a wide range of infectious disease systems.
It sounds counterintuitive. After all, animals spread many infectious diseases to human beings, whether that’s malaria (Anopheles mosquito), Lyme disease (ticks) or even SARS (horseshoe bat), so it might be logical to think that fewer species would mean fewer threats. But Kessing and her colleagues found that the ecology of disease is a lot more complicated than that. Buffer species can actually halt the spread of an infectious pathogen through animals and eventually to human beings, and when those buffer species become endangered or even extinct, disease can spread that more easily.
Here’s an example: hantaviruses are a nasty strain of pathogens that can cause serious disease in human beings, with a fatality rate close to 40%. They’re associated with certain rodent species, which shed the virus through saliva, urine and faeces—humans contract the disease either by inhaling aerosolized rodent crap (I know, ew), or through bites. The risk of humane exposure tends to increase as the density of a host rodent population—and the percentage of rodents who are infected by hantavirus—increases. But a field study in Oregon—cited by the Nature article—found that infection prevalence in deer mice (a common hantavirus reservoir) was linked to the diversity of mammalian species in the area. (Download a PDF of the study here.) As diversity declined, the prevalence of the virus rose from 2 to 14%. The idea is that the more diverse the population in the area, the less likely intraspecies encounters became, so infected deer mice were less likely to meet up—and infect—other deer mice who were free from the virus. The conclusions of that study was backed by a field experiment in Panama, where scientists reduced mammalian diversity by trapping and found that the hantavirus became more prevalent.
The Nature article indicates that it tends to be the innocent, buffer species that often go extinct first—leaving the disease-carriers to threaten us. As Rick Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies and an author of the Nature article, told the Guardian:
The species that persist or even thrive when diversity is lost tend to be the ones that amplify the disease. These critters include various kinds of mice, urban-adapted birds, some kinds of snails, toads and grasses. When we humans do things that erode biodiversity, we’re effectively getting rid of the helpful species and favoring the nasty ones.
The same seems to go for plant diversity as well. And as human populations expand and push into wilderness, clearing forests, they can encounter new diseases—the Nature authors note that almost half of the zoonotic diseases that have emerged in humans since 1940 have resulted from changes in land use. We may end up destroying many of the species we encounter as we churn through tropical rainforests, but not before they make us sick.
In truth there’s no shortage of reasons to halt the depressing slide in biodiversity the planet has experienced over the past century. Nature—clean water, air and resources—has a value we’re just beginning to put into dollars. The plants of the forests contain the raw materials of future medicines—once lost, they’ll be lost forever. But the Nature article gives us one more reason, as the world debates climate change at the U.N. summit in Cancun. Nature can keep us healthy.