What if the world’s parasites suddenly went extinct? Given how much work we’ve put into combating malaria-carrying mosquitoes and horrifying Guinea worms, it sounds like a reason for celebration. But think twice: Actually, losing these much-despised mooches, bloodsuckers and freeloaders could have disastrous consequences for the environment and human health.
A parasite, in essence, is any organism that makes its living off another organism (think bed bugs, leeches, vampire fish and even mistletoe). These freeloaders have been rather successful: up to half of Earth's 7.7 million known species are parasitic, and this lifestyle has evolved independently hundreds of times. But in a study published this week in the journal Science Advances, researchers warn that climate change could drive up to one-third of Earth's parasite species to extinction by the year 2070.
That kind of mass die-off could spell ecological disaster. "One thing we've learned about parasites in the past decade is that they're a huge and important part of ecosystems that we've really neglected for years," says Colin Carlson, a graduate student studying global change biology at the University of California at Berkeley and lead author on the study.
Carlson had experience researching how climate change is driving the current spate of species die-offs. But four years ago, he saw the potential to look into a lesser known group: parasites. "There has been a lot of work that's been done in the late couple of decades focused on understanding why big mammals go extinct, or how crops respond to climate change," Carlson says, "but there's a lot of types of animals and plants that we don't know a lot about."
He formed a team to find out more about how parasite species could feel the heat in the coming decades. The team based their predictions for this research on a "deceptively simple model" from a landmark 2004 study in the journal Nature, which connected species extinction rates to how much of their habitat they're expected to lose. "The problem is, we don't know very much about where parasites live," Carlson says.
The key to answering that question lay in the Smithsonian-run National Parasite Collection, a 125-year-old accumulation that contains more than 20 million parasite specimens from thousands of species dating back to the early 1800s—a massive yet still relatively small slice of global parasite diversity. Carlson knew that the collection, which has specimens primarily from North America but represents every continent, could serve as a historical database from which to figure out estimates of geographic ranges for specific parasites.
So he reached out to the curator of the collection, research zoologist Anna Phillips, at the Smithsonian National Museum of Natural History. The first step was to sort through a lot of old paper records. "Since this is such an old collection, many of these still used a precise locality written out, such as 'this stream at this crossing of this highway, 10 miles down east of this town,'" Phillips says. "While that's very helpful, usually today we prefer to have GPS coordinates."
Her team of researchers digitzed tens of thousands of specimens and their locations in an online database, creating what Carlson calls the biggest parasite record of its kind. Using this immense resource, researchers could then use computer models to predict what would happen to more than 450 different parasite species when climate change altered their habitats, based on how their ranges have changed over the past two centuries.
Their conclusion: Even under the most optimistic scenarios, roughly 10 percent of parasite species will go extinct by 2070. In the most dire version of events, fully one-third of all parasites could vanish.
This kind of die-off would have myriad unfortunate consequences. Consider that parasites play an important role in regulating the populations of their hosts and the balance of the overall ecosystem. First, they kill off some organisms and make others vulnerable to predators. For example, when infected with nematode Trichostrongylus tenuis, the red grouse bird emits more scent that helps predators find and eat it more easily, thus serving to control the bird’s population.
Parasites can also have more indirect effects. Periwinkle snails infected with the trematode species Cryptocotyle lingua, for instance, eat significantly less algae along their Atlantic coast homes, because the parasite weakens their digestive tracts. Their small appetites make more algae available for other species to consume. And there are millions of undiscovered parasite species, whose ecological niches we can only guess at.
"It's hard to predict what their impact on the ecosystem will be if we don't know about it yet," says Phillips. "That's one of the things that's scariest about these model predictions ... it creates a much more urgent feeling about the recognizing the diversity that's out there."
In the future, she and Carlson hope to do further analysis using this new database at finer scales, to predict how certain parasites will fare in different regions under climate change. They expect that, like many organisms, parasite species that are better able to migrate and adapt to new habitats will do better than those that are more tied to certain places.
But even if the parasites emerge successful, those possible geographical shifts present troubling prospects for humans. Parasites can certainly be harmful to people, as in the case of mosquitoes that transmit Zika, malaria or dengue fever. But in this case, the devil you know may be better than the one you don't.
Parasites and their hosts have often evolved together over many years to maintain a delicate balance. After all, parasites usually have little interest in killing their hosts, Phillips explains, since that would mean losing their homes and sources of nutrients. That’s why tapeworms are rarely fatal to the people who get them; the worms have evolved to travel to your gut and feed on the food you ingest, but they rarely siphon off enough calories to actually kill you.
But when a known parasite goes extinct, it creates new open niches in an ecosystem for other invasive species of parasites to exploit. That can create opportunities for new encounters between parasites and hosts that aren't familiar with each other, and haven’t yet developed that non-lethal relationship. In 2014, for instance, a tapeworm species foreign to humans was found in a man’s brain in China, leading to seizures and inflammation of the brain.
"I find that to be equally terrifying to the idea of the extinctions [alone]," Phillips says.
Kevin Lafferty, an ecologist with the U.S. Geological Survey who has extensively studied parasites and biodiversity, says the study raises important questions about our attitudes toward parasites as they face increasing risks of being wiped out. "In many cases, we have an affinity for the species or can place a human value on it," Lafferty said by email. "This motivation is less likely for parasites."
"The field of conservation biology has moved to view species neutrally when considering the need for protection," Lafferty added, "and this view requires that parasites be protected alongside their hosts."