Climate change is ecosystem change. And like all change it involves myriad and complex consequences. Some consequences of climate disruption—like sea level rise—are easily measured and readily anticipated. Other consequences will stem from long and seemingly unrelated series of hidden but immensely consequential changes. Those hidden factors will likely blindside us with increasing frequency as humans further disrupt the climate. This is sometimes known as the increasing prevalence of Black Swan events. The Covid-19 pandemic is one such event. It is the culmination of many ecosystem disruptions.
By tracing those disruptions we can find the origins of the pandemic. One easily observable outcome of climate disruption—even if its effects remain unknowable—is the global redistribution of species. As humans alter the makeup of environments, their biotic communities will change. In general, flora and fauna are moving higher in both altitude and latitude, searching for climates similar to the ones for which they are adapted. Some species are expanding their ranges, bringing competition to the endemic life forms in their new territories. Others are finding the amount of livable habitat greatly reduced; climate disruption is directly linked to the extinction crisis. One recent study found that one third of the planet Earth’s flora and fauna are at near-term risk of extinction.
As life forms decline in some areas and thrive in others, they will alter not just the large, visible makeup of species diversity, but the distribution of microbiological life as well. When animals migrate into new territories, they will bring microbiological hitchhikers with them. This will affect human life in many ways, some easy to see, others of the sudden Black Swan variety. Recent research has uncovered one chain of effects that likely gave rise to the most widely felt disaster of the past year: the global Covid-19 pandemic. What is not immediately seen is that the pandemic may have come about because a heated planet is redistributing mammal populations across the globe. Welcome to a climate changed world, one in which new zoonoses—diseases shared between humans and animals—emerge with newly heated environments.
Among those migrating animals are bats. Bats are particularly salient for human health. They are the source of two previously well-publicized coronaviruses (CoV): Middle East respiratory syndrome (MERS), and severe acute respiratory syndrome (SARS). Other well-known zoonotic diseases are swine flu, transmitted to humans by pigs, and HIV, transmitted to humans via chimpanzee. But bats are a singularly important source of viruses. Flying foxes are the likely source of the Nipah and Hendra virus infections. Other bat species house Marburg virus disease, and, along with wild birds, the Influenza A virus. Bats are also famous as carriers of rabies. That viral diseases emerge from bats is not surprising. Bats 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.
Scientists are unsure why this is, but it may be related to the ability of bats to fly, a trait they share with no other mammal. Scientists hypothesize that because flight elevates body metabolism and temperature, it became a selective force for the coexistence of parasites. Because of its physiological intensity, flight may act as if it were a fever by elevating body metabolism in a manner analogous to a feverish state. Fevers inhibit pathogen reproduction and enhance immune responses. The daily flight of bats, then, might alleviate the costs of raising metabolism to activate immune responses. On an evolutionary scale the metabolic stresses associated with flight resulted in the stunning diversity of zoonotic viruses within bat species.
Yet bats themselves do not seem to suffer from hosting so many viruses. Daily flight may allow some viruses to adapt and persists within bat hosts. Bats evolved unique abilities to defend against some viruses and tolerate others, an outstanding characteristic of their 64 million years of adaptive evolution. In short, bats developed many genes responsible for protection against viruses, and that protection is quick to neutralize invading pathogens. But when bats migrate, they will bring these pathogens with them, and those pathogens will inevitably take advantage of the new environments available to them—like human bodies. In the language of science, fundamental changes in species composition and ecology will produce novel host-pathogen interactions.
Like every life form, bats will migrate into habitats that suit them. Habitat change driven by climate disruption brought many new bat species to China. The southern portions of Yunnan province and adjacent regions in Myanmar and Laos have experienced vast changes to their biodiversity. These regions have morphed from tropical shrublands, which are poor bat habitat, to bat-friendly tropical savannah and deciduous woodlands. Thus, the species richness of bats, mobile and highly adaptable animals, increased greatly in southern Yunnan province. According to a fascinating new study in Science of the Total Environment, climate change 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. (Bat species richness also increased in central Africa, and in parts of Central and South America.)
Those newly resident bat species carried with them an estimated increase of 100 new bat-borne coronaviruses. The study found that this climate-induced chain reaction of floral change driving faunal change driving viral biodiversity “may have played a key role in the evolution or transmission of the two 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.”
But the pathway from bat-housed viruses to human infection requires intermediaries. People rarely interact with bats directly. (If you do happen to come across a sick or injured bat, you should not handle it.) Thus, it is likely that another animal acted as a disease vector between bats and humans. Two likely possibilities from southern Yunnan are masked palm civets (Paguma larvata) and Sunda pangolins (Manis javanica). Captured civets and/or pangolins that were sold in the wildlife markets at Guangdong or Wuhan (the Huanan Wholesale Seafood Market) are the likely route that Covid-19 travelled from its bat hosts to human populations.
Wildlife markets are a second way that the destruction of the natural world places humans at greater risk for pathogenic infections. They are also substantial drivers of species depletion. Palm civets are hunted for their meat, which is consumed widely in southern China and Vietnam. Pangolins are especially vulnerable to the trade in wild animals. The International Union for Conservation of Nature dubs them “the most illegally traded mammal in the world” because they are prized for their meat and for the supposed medicinal properties of their scales. The wildlife trade, coupled with disease and habitat destruction, is driving many of these magnificent, fascinating animals to extinction. If the climate and market links to the coronavirus epidemic prove correct, then Covid-19 might be one in a series of synergistic Black Swan effects deriving from the multiple ways that humans are destroying the natural world. When land—habitat—is converted for human use, bats and other species encounter each other more often, providing greater opportunities for viruses to jump from species to another.
Though bats are the source for many coronaviruses, they are not responsible for the pandemic. People are. Contrary to their undeservedly scary reputation, bats provide many ecosystem services that help humans flourish. They help regulate insect populations: insectivorous bats eat their weight in bugs every day. Moreover, bats are essential pollinators. Tropical bat species pollinate valuable commercial crops such as bananas, mangos, and guavas. They are also major distributors of seeds. But human driven climate change, habitat destruction and appetite for bushmeat all converged to increase the likelihood of virus transfer from bats to humans. That is our fault, not theirs.
Advocates for the public good rightly point out the necessity of investing in public health facilities. Those investments should happen alongside massive efforts at habitat protection and rehabilitation, and a-quickly-as-possible end to fossil fuel use. As the Covid-19 epidemic demonstrates, human health will not be secure unless we secure our surrounding environment is as well.