In the 1770s the British botanist and physician William Withering witnessed a woman treat a person suffering from “dropsy”—the accumulation of fluids in bodily tissue—with an herbal concoction. The patient immediately recovered. Withering was so impressed by this folk remedy that he spent the next nine years investigating the medicinal properties of the primary plant in the woman’s mixture—the Foxglove (Digitalis purpurea). Foxglove turned out to be nearly miraculous in its applications. Ever since Withering's 1785 Account of the Foxglove, modern physicians have used it to treat such life-threatening maladies as heart failure and atrial fibrillation. Contemporary Americans know the medicine by its brand name, Digitalis.
The story of Withering and the Foxglove is one example of a widespread parable that environmentalists have told to convey the importance of biodiversity to human health. Even a poisonous (if lovely) flower such as the Foxglove might contain life-saving compounds. Nature’s astonishing variety contains vast, yet-to-be-discovered medicinal riches. The Foxglove and digitalis are only one example. The periwinkle plant yields an alkaloid (vincristine) used to treat Hodgkin’s disease—and a variety of other cancers. The Pacific Yew—for decades destroyed as a worthless trash tree by the timber industry—yielded the magnificent cancer fighting compound paclitaxel (Taxol). The venom of the Malayan pit viper produces the valuable anticoagulant Ancrod (current brand name Viprinex). This list is long and ancient; healing with nature is as old as humanity. But medicine cannot derive curative compounds from extinct flora and fauna (and fungi and bacteria); their loss is the greatest opportunity cost of all. It is in the interest of people to preserve biodiversity and its untapped possibilities for lifesaving resources.
Moreover, as with Withering and the folk remedy he witnessed, environmentalists have long linked the healing potential of biodiversity with cultural pluralism—particularly the survival of indigenous cultures. When indigenous peoples are killed or absorbed into industrial cultures, their ecological knowledge base is lost. In their 1981 Jeremiad against species loss, Extinction, scientists Paul and Anne Ehrlich argued for the “preservation of human diversity,” noting that “indigenous peoples…often have a comprehensive knowledge of medical uses of other organisms.” Therefore, “preserving different human cultures will give society access to folk knowledge that would otherwise be lost.” This formula intertwines biodiversity, health, and cultural pluralism; cultures emerge from the natures in which they are enmeshed.
Losing biodiversity, then, flattens human cultures and precludes the discovery of novel medicines. But as the extinction crisis has worsened, so have the implications of biodiversity loss for human health. Recent science has shown that the rapid extermination of biodiversity not only blocks new discoveries but makes us more vulnerable to established and novel diseases. No longer are we just bearing the loss of potential medicines as severe opportunity costs; we now know that the ecological simplification of ecosystems makes humanity more vulnerable to diseases such as Covid-19. As the authors of a recent review paper argue, “biodiversity loss tends to increase pathogen transmission and disease incidence.” As with any interesting science, the evidence is complex and occasionally contradictory. But taken as a whole, it demonstrates a vital truth: reducing biodiversity makes humans more vulnerable to disease.
That vulnerability occurs in three related ways: by increasing the abundance of disease hosts and vectors, by moving vector species into new habitats, and by changing the behavior of hosts and vectors. Lyme disease, Covid-19, and hantavirus and are all examples of a new parable, one that demonstrates how species extinctions drive disease outbreaks in human populations.
The growing threat of Lyme disease illustrates the most common problem with reduced biodiversity: as ecosystems simplify, vector species are more likely to thrive. Lyme disease (caused by the bacteria Borrelia burgdorferi) is transmitted to people via the bites of blacklegged ticks (Ixodes scapularis). The ticks tend to be more or less numerous due to intermediate host species such as the white-footed mouse (Peromyscus leucopus) or the Virginia opossum (Didelphis virginiana). Mice are great hosts for the ticks; opossums are terrible hosts because as they groom they kill most of the ticks that attempt to feed on them. But the species richness of mice and opossums is not stable. It depends upon forest health. Mice thrive in fragmented and degraded ecosystems, whereas opossums are usually absent from isolated, species-poor forests. As forest biodiversity lessens, the opossums that buffer the spread of ticks are lost, while the white-footed mice that amplify tick abundance thrive. As people fragment and degrade eastern deciduous forests, then, they are creating habitat better suited to the white-footed mouse—and for the ticks and the Lyme disease that infects tens of thousands of Americans each year. The number of Americans suffering Lyme disease is expected to rise, and not only due to the degradation of deciduous forests. Ticks thrive with warmer temperatures, so the very deforestation that contributes to climate disruption will also help ticks multiply.
Like the white-footed mouse, some bat species are thriving due to changes in the global climate. Bats and the Covid-19 are an example where ecosystem change has increased the abundance of vectors in a key geographic location. Bats are particularly salient for human health. They are vast reservoirs of viral diversity; they host by far the highest percentage of zoonotic viruses among all the mammalian orders. Indeed, bats may carry up to 3000 coronavirus varieties alone.
As climate disruption transformed the southern portions of China’s Yunnan province (and adjacent regions in Myanmar and Laos) from tropical shrublands to bat-friendly tropical savannah and deciduous woodlands, the species richness of bats in Yunnan province increased substantially. According to one fascinating new study, climate disruption transformed this area into “a global hotspot of climate change-driven increase in bat richness.” As many as 40 new species of bat are found in southern Yunnan province compared to a century ago. Those newly resident bat species carried with them an estimated increase of 100 new bat-borne coronaviruses. The extinction of tropical shrublands drove faunal change that in turn drove the viral biodiversity that likely played a key role in the evolution or transmission of SARS CoVs. In the words of the paper’s lead author Dr. Robert Beyer, "As climate change altered habitats, species left some areas and moved into others—taking their viruses with them. This not only altered the regions where viruses are present, but most likely allowed for new interactions between animals and viruses, causing more harmful viruses to be transmitted or evolve.” The likely pathway of Covid-19 transmission from bats to humans involved intermediate species, but it is our species, though the inadvertent consequences of habitat change, that created the conditions for Covid-19 to thrive.
Most commonly, species richness limits the number of suitable hosts for disease microorganisms. As with white-footed mice, opossums, and Lyme disease, this is also the case for the parasitic worms that transmit schistosomiasis (which infects 200 million people each year). This phenomenon is also readily seen in the western United States with the hantavirus. Rodent hosts of the hantavirus in ecosystem patches that contain high mammalian diversity are less likely to contact other suitable hosts for the virus such as deer mice (Peromyscus maniculatus), and thus are less likely to spread the virus. When landscape patches yield little diversity, hantavirus carriers were more likely to encounter species like the Pinyon mouse (P. truei) which readily carries the virus. Species diversity reduces the probability of hantavirus transmission.
A related trend is that the species that thrive in human dominated habitats tend to be species that vector disease microorganisms. As humans destroy habitats, species go extinct, reducing biodiversity. Those species that survive and even thrive in such situations—think rats and bats (and some bird species)—are more likely to host the dangerous pathogens that might jump to humans. Known wildlife hosts of zoonotic diseases are comprising a greater proportion of species richness.
Much remains to be discovered about the relationship between biodiversity and disease. But most broadly, biodiversity seems to protect all species from the transmission of infectious disease. This means that environmentalists need to revise our parable, that is, update the way we communicate the importance of biodiversity to the public. Biodiversity is not just a repository of yet to be created knowledge. Rather, biodiversity protects us from disease. Protecting nature ensures the possibility of future discoveries; it also protects us, right here and right now, from diseases. If we want to stop the next pandemic, protecting and enhancing biodiversity should be at the top of our to-do list.