To catch a kakapo a century ago, all you had to do was go out at night in New Zealand, shake a few trees, and watch the flightless parrots come tumbling down like ripe green apples. Today, fewer than 150 kakapos remain on earth.
Decades worth of hunting, habitat destruction, and being eaten by exotic rats, cats, and stoats have limited the bumbly birds to just three predator-free islands, and since 1982, scientists have been monitoring the parrots’ every move, trying to breed them away from the brink. So far, so good for the birds, but the conservationists’ efforts haven’t been good for everybody.
Y’see, for the kakapos to have a better shot at survival, the biologists had to send another endangered species, Stringopotaenia psittacea, into oblivion.
Before I tell you what S. psittacea is exactly, I want you to think about the reasons we fight to keep kakapos, rhinos, pandas, snow leopards, Venus flytraps, and other plants and animals around. Is it because they are cute and cuddly? Interesting to observe? Play crucial roles in their ecosystem or benefit humans in some way? Or is it simply because every species has a right to exist?
S. psittacea is a tapeworm. The parasite spends most of its life attached to the kakapo’s intestines, siphoning off blood and other nutrients.
Or at least it used to. Since 1904, this long, milky-white flatworm has been seen just three times. Whatever the tapeworm’s historic density once was, its numbers likely plummeted alongside its preferred parrot. Parasites can be very picky when it comes to their hosts, and while we don’t know much about S. psittacea in particular, we do know it’s never been observed feeding off any of New Zealand’s other species. According to Hamish Spencer, a zoologist at the University of Otago in New Zealand, the last of them probably blinked out when kakapo keepers began deworming the birds as part of routine health treatments.
Now we probably shouldn’t be too hard on the kakapos’ keepers. After all, parasites can reduce fitness, impede reproduction, and generally torment their hosts, and the keepers’ job was to give the parrots every possible advantage. But some scientists, like Spencer, are starting to make the case for considering parasites more thoughtfully, lest we inadvertently lop off more branches from the tree of life.
The Plight of the Parasite
The story of the kakapo and the tapeworm is not unique. When the U.S. Fish and Wildlife Service sought to save the California condor in the 1980s, they drove the California condor louse extinct in the process—the last louse vanishing “from the Earth in a puff of carbaryl-powder fumigation.” The parasites that relied on the now-extinct Guam rail and spotted kiwi may have met similar fates.
Often, we aren’t even clear on what, if anything, has been lost. Black-footed ferrets were thought to have once harbored a unique species of weasel louse. However, after reintroduction efforts of ferrets in the 1980s, and the delousing protocols that went with them, no such black-footed ferret louse has been found. Perhaps it never existed. But if it did, it’s now extinct.
Threats to parasites are squared by the fact that they are vulnerable to all the dangers their hosts face. Poachers don’t target Lyperosia flies, for example, but they slaughter the rhinos these flies need to survive.
Indeed, any number of things people do—intentionally and not—from land-use alterations to routine veterinary practices, can eradicate parasites whole scale. Climate change can inhibit transmission by making parasites more vulnerable to predation or by wrecking the synchronized life cycle between a host and parasite that evolution has perfected over thousands of years. Pollution can lay them low while they seek new hosts or leave them homeless by killing off intermediary hosts—which are like the parasites' version of Uber.
Hell, one study investigated what would happen if we started inoculating big cats against canine distemper virus and concluded that while removing the disease would benefit both lions and cheetahs, more lions would eventually mean fewer cheetahs (during kills, the former tend to drive off the latter, making for hungry cheetahs). In fact, the authors expressed concern that removing the disease would actually cause certain isolated cheetah populations to go extinct (along with whatever species-specific parasites they carry).
The good news for all the bloodsuckers out there is that more and more scientists are starting to argue for their protection.
Save the Freeloaders!
Parasites are practically ubiquitous throughout the earth’s ecosystems. Out of the 42 broadly accepted phyla on the tree of life, 31 are composed mostly of parasitic animals. And out of all those millions of species, just one parasite has made it onto the International Union for the Conservation of Nature’s red list: the critically endangered pygmy hog sucking louse.
Parasites have a reputation as “unadulterated nasties,” says Spencer. And it’s true, mosquitoes, ticks, fleas, and other parasites kill millions of people every year by transmitting diseases, such as malaria. But Spencer says it’s also unfair to label all of them as destructive when many simply and harmlessly just gross us out.
For instance, a study of Eurasian oystercatchers (think: orange-beaked shorebirds) found that when ridded of their helminths (think: gut worms), chicks with fewer parasites actually “survived less well” (think: died). The scientists who conducted the study aren’t entirely sure why this happened, but they hypothesize that some level of parasitic infection might actually boost the birds’ immune systems.
Exposure to parasites while young might also make an animal more resistant to them later in life, and the loss of this resistance could have implications for reintroduction efforts. Say the descendants of our 150 dewormed kakapos lose whatever defenses the species has built up against S. psittacea, but the tapeworm turns out not to be extinct after all. Should the parasite find its way back into the parrot population, it may have an outsized impact on the unequipped birds.
There’s also evidence that parasites can be more boon than burden for their hosts. In a 2008 study, fruit flies infected with parasitic bacteria were less at risk of death from a range of RNA viruses. And remember the helminths? Other research has found that these intestinal worms can suck significant amounts of heavy metals out of the tissues of their host fish. Moreover, the more we begin to understand the world around us, the more we see parasites are pulling the strings.
“I would say that the number one reason to keep parasites around is that they are quietly running and structuring the ecology and evolution of the world,” says Elizabeth Nichols, a conservation biologist at Swarthmore College, “even if we are only figuring that out now.”
In fact, Nichols adds she wouldn’t be surprised to learn that parasites govern, in some manner, the actions of almost every species. Sex, predation, spatial organization, migration—pick any one of those behaviors and you can find a way that parasites assert their influence. They drive evolution and the creation of new species, so actively removing them could fundamentally alter the ecosystems they inhabit.
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It may well be too late for S. psittacea, but hundreds—probably thousands—of parasites are out there staring down extinction. The first order of business is to protect their hosts. After all, there’s little point in saving a species if it has nowhere to live, and let’s face it: Kakapos would look better on bumper stickers than tapeworms. But the mounting science shows that we should also try to preserve the world’s parasites. The question remains: Can we stomach it?
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