Scientists are discovering that species extinctions fuel the rise and spread of infectious diseases and hinder medical research
ON A RECENT AFTERNOON, Laura Shappell followed a slender deer trail into a thicket of invasive Japanese knotweed. The plants towered over her head, and their deer-trampled stalks crunched under her boots as she vanished into the mass of pale green leaves. “If I’m not out in 10 minutes, send help,” she called back.
A graduate student at Rutgers University, Shappell is a member of a research team exploring the link between biodiversity and human disease. Led by Rutgers wetland ecologist Joan Ehrenfeld, the group is looking at plant diversity in a variety of freshwater wetlands in New Jersey. After characterizing each plot, they will compare the diversity of its plant communities with bird diversity and the prevalence of West Nile virus within mosquito populations. The research builds on previous studies showing that areas with high bird diversity tend to have less West Nile virus, which mosquitoes can transmit from birds to people.
The link to plants? “Birds need a variety of plant species and habitats for nesting and foraging,” Shappell explains. “Reduce plant diversity and you may reduce bird diversity as well.” And across North America, nonnative invasives like Japanese knotweed pose one of the greatest threats to plant diversity. “There’s not much that can grow under this stuff,” Shappell says of the exotics overrunning the Rahway River wetland she is surveying.
The Rutgers study is just one of many new investigations into the link between biodiversity and human health. “The natural world provides so many services vital to our health,” says Eric Chivian, founder and director of Harvard Medical School’s Center for Health and the Global Environment. “But these services depend on an enormous diversity of species about whose interactions we know very little.”
Over the past decade, the Harvard center has been working with three United Nations agencies to draw together what scientists do know. In 2008, it published Sustaining Life: How Human Health Depends on Biodiversity. The 542-page tome draws on the research and expertise of more than 100 scientists. But rather than a dry academic text, it takes the form of a richly illustrated coffee table book with chapter authors writing in language accessible to a general audience as well as their fellow scientists. “Above all,” Chivian explains, “we’re trying to reach beyond specialists to help everyone grasp the urgencies involved in species loss.” It’s no coincidence that in 1985 Chivian shared a Nobel Peace Prize for cofounding the International Physicians for Prevention of Nuclear War. That same year, he recalls, scientists discovered an ozone hole widening over the Antarctic. Concerns about climate change quickly followed.
“By the 1990s, there was a growing awareness among physicians in the anti-nuclear movement that we had to enlarge our perspective to include global environmental changes,” Chivian says. “We saw these changes as Armageddon in slow motion, something that would affect the globe in ways just as catastrophic as nuclear war.” In particular, researchers began uncovering how biodiversity losses appear to fuel the rise and spread of many infectious diseases. Chivian and others are also trying to raise awareness of how widespread extinctions hamper medical research.
DIVERSITY AND DISEASE
Imagine a newly hatched tick resting on the floor of a northeastern U.S. forest, waiting for its first blood meal. If the larval tick is in a fragmented or otherwise degraded patch of woods, the first host it encounters will probably be a white-footed mouse. As a result, the tick will likely become infected with the Lyme disease microbe, Borrelia burgdorferi. In much of the United States, up to 80 percent of white-footed mice carry this disease-causing bacterium, which they pass to deer and humans via the bite of infected ticks.
Now consider the same newly hatched tick in an undisturbed forest. Here its first host is more likely to be an opossum, raccoon, ground bird, lizard or other non-mouse host. None are good carriers of the Lyme disease bacterium. As a result, the tick will likely escape infection and never transmit the disease to anyone.
In 2000, animal ecologist Richard Ostfeld described this Lyme disease-blocking dynamic and dubbed it the “dilution effect.” In essence, he explains, the greater a habitat’s biodiversity, the more likely that an animal-borne microbe such as B. burgdorferi will end up in a dead-end host—one that does not pass it along.
In theory, says Ostfeld, biodiversity loss could eliminate a species that transmits disease to people. But scientists are not seeing this, he adds. When tropical deforestation decreases mosquito diversity, for example, surviving mosquito species tend to be more-effective carriers of malaria. “It may be that there is something about the kind of weedy species that survive in disturbed areas,” Ostfeld says. One idea worth investigating, he proposes, is that fast-growing, fast-breeding organisms are by nature less resistant to disease. Making matters worse, such plants and animals often worsen diversity loss by outcompeting species more sensitive to human disturbance.
Since Ostfeld’s landmark study, other researchers have extended his findings to diseases such as West Nile virus, hantavirus pulmonary syndrome and malaria. In each case, the loss of biodiversity in an area leaves behind a few species that are more apt to transmit a disease to people. This transmission can be direct or through an intermediary such as a tick or mosquito.
In the case of West Nile, the strongest carriers tend to be the kinds of perching birds that predominate in disturbed, fragmented and less-diverse habitats. They include common backyard species such as house sparrows, grackles, robins and crows. By contrast, the types of birds that predominate in undisturbed woodlands, wetlands and prairies tend to carry little West Nile virus in their bloodstreams. “As a result, where many bird species exist, very few mosquitoes get infected, and so we humans are at lower risk,” says biologist Brian Allan of Washington University in St. Louis. Allan’s studies have shown that across the nation, bird diversity is a significant buffer against the spread of West Nile virus to humans.
Taken together, these studies suggest it’s no coincidence that the world is seeing a dramatic rise in newly emerging and reemerging infectious diseases, Ostfeld says. The first Lyme disease outbreaks, for instance, occurred in the 1970s in and around Lyme, Connecticut. The disease is now the most prevalent arthropod-borne infection in North America. (Arthropods include insects, ticks, spiders and their relatives.) West Nile first appeared in North America in 1999 in New York City. Since then it has spread across the United States, infecting tens of thousands and killing more than 1,000 people. Hantavirus appeared in the Western Hemisphere in 1993, killing several young people in the Southwest. Since then, it has infected people from Canada to the tip of Argentina, killing more than 500 in the United States alone.
THREATS TO RESEARCH
For decades, “save the rain forest” campaigns have touted the treasure chest of potential medicines lost when these biodiverse landscapes are razed. The concern is a valid one. Most drugs in common use derive directly or indirectly from natural sources. But less than 1 percent of the world’s plants have been analyzed for their medical potential. Meanwhile, plant extinction rates have accelerated to levels hundreds of times higher than those seen in preindustrial times.
But medicinal plants are far from the only—or even the most important—contribution that biodiversity makes to medical research, Chivian argues. He cites several groups of threatened plants and animals with vital importance to medical research. They include:
Amphibians: Medical researchers are racing to study a wide variety of amphibians for the vast array of chemicals they produce to protect their soft and vulnerable bodies from predators and infections. Of particular interest are natural chemicals called antimicrobial peptides, or AMPs. Already, biochemists have developed several amphibian AMPs into potent experimental antibiotics. Today the need for such drugs is particularly keen, especially in hospitals awash with antibiotic-resistant infections.
Amphibians also have been at the center of studies on early embryonic development, including research into the cause of birth defects and how they might be prevented. Some of the newest research involves frog species that can survive being frozen solid. They include wood frogs, spring peepers and chorus frogs, whose unique physiology may provide insights into the preservation of donated organs.
Yet frogs and other amphibians are in the midst of an extinction crisis worldwide. Nearly 1 in 3 amphibian species is threatened, and 122 appear to have gone extinct in the last few decades alone. Many more undiscovered amphibians are likely to have vanished as well. Threats include habitat loss, global warming, increased ultraviolet radiation due to ozone depletion and fungal infections spread by exotic and invasive species.
Bears: Among the world’s eight bear species, one (the giant panda) is listed as endangered and six are listed as vulnerable by the International Union for Conservation of Nature. Notable among them is the polar bear, whose bile contains substances that show potential for preventing pregnancy complications and for treating a lethal disease called primary biliary cirrhosis that affects some women. Medical researchers are also studying polar bear metabolism to better understand how these predators remain healthy despite not urinating, eating, drinking or bearing weight for months at a time and packing on levels of fat associated with type 2 diabetes in people.
Primates: Though many people are uneasy with medical research involving our closest animal relatives, these studies are behind the development of successful vaccines for a host of deadly diseases such as smallpox, rabies and polio. Primates are also pivotal to studies aimed at preventing or curing AIDS, hepatitis, malaria and other widespread diseases as well as countering new threats such as Ebola and Marburg disease. In addition, primates are helping scientists understand many of the brain and behavioral disorders that apes share with humans—Parkinson’s and Alzheimer’s diseases being two examples.
As no one primate species is a good model for all human disorders, future research advances may hinge on maintaining a diversity of apes, monkeys, and lemurs in the wild. Yet along with amphibians, primates rank as the most endangered of all major animal groups. Worse, most primatologists believe that without drastic action, a great wave of primate extinctions is near.
Cone snails: Long prized by shell collectors captivated with their diversity of forms, cone snails are even more prolific in their manufacture of biologically active chemicals. Of the 700 species currently known, each is thought to produce 100 to 200 different toxins. Cone snails inject these venoms into prey and would-be predators using tiny, barbed harpoons.
“This chemical-making ability dwarfs the number of alkaloids, such as morphine and caffeine, made by all known plants,” Chivian notes. Cone-snail toxins are particularly interesting to researchers because of their ability to bind to cell surfaces in ways that initiate a variety of activities within the cell. Some of the most promising may have the ability to provide powerful pain relief that does not lead to a drug becoming less effective with chronic use, as is the case with opiates.
Though scientists so far have not assessed the conservation status of cone snails, these mollusks face the threat of extinction due to worldwide collapse of coral reefs and degradation of mangrove wetlands—their two primary habitats.
Sharks: Sharks are among the most endangered of the world’s ocean fish. The main threat remains overharvesting, with some 75 million sharks caught each year. In the last 8 to 15 years, populations of virtually all the world’s 400-odd species have declined by more than 50 percent. Scientists, meanwhile, are investigating the shark steroid squalamine, which like frog toxins may have promise as a potent new antibiotic. Others are studying squalamine as a potential treatment for macular degeneration, a leading cause of blindness. Squalamine even appears to have appetite-suppressing powers—of great interest in countries such as the United States where obesity has become an epidemic health problem. And immunologists study sharks for their remarkable, disease-fighting immune systems—the most ancient among the so-called higher animals.
Beyond individual animals like sharks, biodiversity delivers many health services that humankind has long taken for granted, from cleaning our air and water to mitigating floods and moderating climate. Habitat disruption and associated extinctions can cripple these services in ways only now becoming clear. “At the same time,” says Chivian, “we know that the more diverse a habitat, the more likely that it will prove stable and resilient to change.” That resilience will only become more crucial, he adds, as global warming begins transforming our planet in ways we can only imagine.
New Jersey-based writer Jessica Snyder Sachs wrote about discoveries of new populations of imperiled plants in the April/May issue.
NWF Priority: Conserving Biodiversity
Beginning long before the word was coined, NWF has been at the forefront of protecting what is now called biodiversity. “We tend to think of the biodiversity crisis as something new,” says Bruce Stein, NWF’s associate director for wildlife conservation and global warming. “But this organization was spawned by a previous biodiversity crisis: the 1930s Dust Bowl, when prairie habitats that sustain the nation’s waterfowl were collapsing.” Since then, NWF and its affiliates have campaigned to conserve healthy populations of all native species of fish and wildlife. NWF works to protect wildlife “not only in treasured landscapes like national parks,” says Stein. “We also teach people to appreciate and nurture diversity in their own backyards.” NWF’s current focus on climate change is consistent with this legacy, he adds, because “today climate change is the single greatest threat to biodiversity.”
The United Nations has declared 2010 the International Year of Biodiversity.
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