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Diseases often spill over to humans when road building opens access to wilderness to exploit resources or when habitat becomes altered by agriculture. Such encroachment often alters species movements and habitat use in ways that increase the likelihood for direct contact with humans. Flying fox bats and fruit trees, for example, are mutually dependent on one another in their natural habitats. When these fruit trees are cultivated by agriculture, they can draw the bats in closer to humans. The bats’ presence might be encouraged because they are natural pollinators of fruit trees.
But they also carry viruses that can cause human encephalitis outbreaks. The danger to humans can become acute when fruit trees are planted near livestock farms. The close proximity between bats and livestock allows the virus to jump to livestock, in turn spilling over to humans when the livestock are slaughtered. An effective ecological fix is to deploy a form of permanent “agricultural distancing” in which fruit trees are planted far enough away from the farms to ensure the bats’ habitat use doesn’t overlap with the livestock.
Diseases spill over too when we unravel ecological dependencies. Livestock herders in West Africa often kill lions out of fear that their attacks threaten their livelihoods and personal safety. But lions naturally keep baboon populations in check. Baboons can be heavily infested with gut parasites, including worms and protozoans. Once relieved from predation threat themselves, baboons in turn move more widely, oftentimes into villages, resulting in the spillover of parasites to humans.
The toll of parasitic infections on human health and welfare in these villages can be heavy. But again, interventions that help humans coexist more safely with lions are available. Livestock can be held in corrals, built with natural materials, to protect them during times when they are at highest risk of being hunted. As well, ensuring that lions’ natural prey base is sustained through habitat conservation can help to draw predator movement and activity away from human settlements.
Microbes and parasites are a natural part of the web of life, among the millions of species with which we share the planet. Their evolved functional roles in nature are effectively no different than predator species that keep prey populations in check, or the herbivore species that graze down vegetation. And so, they too contribute to sustaining the planet. The lesson from the pandemic is not that we can or should retreat from nature. Rather we must learn to become better stewards.
Stewardship is about striving for continuous improvement of environmental performance everywhere in support of the simultaneous preservation of species and the production of natural resources and ecosystem services. It means being mindful to minimize the disruption to the ecological interdependencies in the places where we do encroach. That involves striking a balance between keeping species in place to fulfill their natural functions, while keeping them at bay to minimize the risk of disease spillover and the emergence of public health crises in places far away from nature.
This column is based on the author’s essay, “Sustaining Humans and Nature as One: Ecological Science and Environmental Stewardship,” in the book A Better Planet: Forty Big Ideas for A Sustainable Future, recently published by Yale University Press.
The views expressed are those of the author(s) and are not necessarily those of Scientific American.
ABOUT THE AUTHOR(S)
Oswald J. Schmitz is the Oastler Professor of Populatio
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