The Perilous Future of Earth's Parasites - Pacific Standard
Climate change may drive as many as one in three parasite species extinct by 2070. That's very bad.

Whatever little attention the mass extinction crisis gets tends to focus on the large and charismatic species that are on the verge of disappearing from Earth forever. Smaller creatures—microbes, mollusks, and other invertebrates—hardly register at all on the human agenda. Their existential plight is pretty much ignored by politicians, press, and the American public alike.

Earlier this month, however, a team of scientists managed to shove this neglected topic into the spotlight when they released a paper on parasite extinction that ricocheted around the Internet and racked up quite a bit of coverage in the mainstream media. Published in Science Advances and written by a University of California–Berkeley graduate student and fellow researchers, the study offers a disturbing look at the enormous and unintended consequences of human activity on the planet's ecosystems. It finds that climate change threatens to decimate parasite species across the planet, with dangerous implications for wildlife survival and human health.

After analyzing data on more than 457 parasite species, the authors report that "conservative model projections suggest that 5 to 10% of these species are committed to extinction by 2070 from climate-driven habitat loss alone." This mass die-off could be exacerbated, moreover, if the host species that parasites rely on to survive also go extinct in the face of climate change and other anthropogenic disturbances. Indeed, under worst-case scenarios, as many as one in three parasite species could be wiped out.

When it comes to the planet's hundreds of thousands of parasite species, there are a lot of subtleties.

The study, which was the product of years of intensive research in museum collections and other data repositories, suggests these extinctions could have a complex cascade of effects. Though they have a bad reputation, parasites like ticks, mites, tape worms, fleas, and flukes play a major role in many food webs, and can regulate the immune systems of the animals that host them. They may even help prevent the emergence and spread of some pathogens.

"Parasites do a lot for ecosystems," says Colin Carlson, the paper's lead author and a Ph.D. candidate studying environmental science, policy, and management at UC–Berkeley. "They can be 80 percent of the food web links in an ecosystem. They can be the majority of species interacting with each other. And they have a huge impact on host populations."

By way of example, he points to a fascinating case on the other side of the planet. In Japan, there's a parasite that infects crickets and grasshoppers, and manipulates their behavior, encouraging them to seek water. These water-seeking crickets enter streams at much higher rates than their non-infected peers, becoming a ready and important food source for a species of endangered trout there.

"It turns out that 90 percent of food these endangered Japanese trout are eating comes from those crickets," Carlson says. "So behavioral-altered crickets are keeping endangered trout alive." The point is: When it comes to the planet's hundreds of thousands of parasite species, there are a lot of subtleties. There are a lot of things that scientists do not yet understand. And that's scary, because as parasites begin to disappear we won't really know what we are losing.

Consider, for instance, the great potential that parasites have as a source of therapy for human disease. Scientists and doctors around the world are presently studying the role that some species—and particularly helminths like the hookworm—could play in combating the rise of autoimmune diseases. When such parasites enter a human body, they seem to be able to manipulate and temper the immune system, suggesting the possibility that they could help reduce the worst effects of scourges like celiac disease, ulcerative colitis, irritable bowel syndrome, and other illnesses.

As Leah Shaffer, reporting in Undark, writes:

To researchers exploring helminths, this suggests that the human-worm relationship might sometimes be something other than one-way and purely parasitic—and even potentially mutualistic and symbiotic. We host them, and, like the best of frenemies, they help us by keeping our immune system from attacking itself.

These frenemies can't possibly help us, however, if climate change drives them into oblivion. They can't possibly play their crucial role in a vast array of ecosystems if human civilization destroys 10, 20, or 30 percent of them by 2070.

"Now that we know that there are these very real extinction rates, now that we know there could be these unpredictable health impacts," says Carlson, whose team has compiled a massive publicly available collection of data on the planet's parasite populations. "I think that makes a much stronger case for conserving parasites than we have ever had before."

The conservation community, in other words, needs to fight against a parasite-poor future, and fast. 2070 is not so far away.

Lead Photo: Two proglottids of a pork tapeworm. (Photo: Wikimedia Commons)

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