The Paradox of Parasitism: Balancing Co-Existence and Extinction
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Most species, it turns out, are parasitized by multiple others. Parasites proliferate because every living thing is a smorgasbord of nutrients and energy, and being a top predator isn’t the only way to get a bite of that bounty. Parasites opt out of the arms race between predator and prey entirely, choosing an easier path. It’s clever, when you think about it, and it’s exactly why parasitism is so common. Nature abhors a vacuum. If there’s an opportunity, someone’s going to evolve to fill it. Parasitism has evolved as a way of life again and again, over billions of years, from the smallest and simplest microbes to the most complex vertebrates. Yet, we have barely begun to identify all the parasites, much less learn their lifestyles or monitor their populations. One of the things Skylar Hopkins, an ecologist at North Carolina State University, and her colleagues have noticed is what they call the paradox of co-extinction. Since parasites by definition need other species, they’re particularly vulnerable to the phenomenon. Take, for example, the endangered pygmy hog-sucking louse. It lives only on another endangered species, the pygmy hog, which is disappearing from the grasslands it inhabits in the foothills of the Himalaya. Because they’re so sensitive to extinction, it seems likely that many parasite species have already disappeared.
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The poster child for parasite conservation, if there is one, is the California condor louse, an ironic victim of the conservation movement itself. In the 1970s, desperate to save the California condor, biologists began rearing the birds in captivity. Part of the protocol was to de-louse every bird with pesticides, on the assumption that parasites were bad for condors, though it’s not clear they actually were. The California condor louse hasn’t been seen since. Untold other parasitic worms, protozoans, and insects are presumed to have gone down with the ship, so to speak, as their hosts died out. While the demise of life’s hangers-on might seem like no big deal, or even something to strive for, ecologists caution that wiping them all out would probably spell planetary doom. Without parasites keeping them in check, populations of some animals would explode, just as invasive species do when they’re transplanted away from natural predators. Other species would likely crash in the ensuing melée.
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Big, charismatic predators would lose out, too. Many parasites have evolved to move into their next host by manipulating the host they’re in, which tends to drive that host into a predator’s mouth. Nematomorph worms, for instance, mature inside crickets but then need to be in water to mate. So they influence the crickets’ brains, driving the insects to jump into streams, where they become an important food source for trout. Similar phenomena feed birds, cats, and other predators the world over. Even human health wouldn’t entirely benefit from wiping out parasites.In countries such as the United States, where we have eliminated most intestinal parasites, we have autoimmune diseases that are virtually unheard of in places where everyone still has those parasites. According to one line of thinking, the human immune system evolved with a coterie of worms and protozoan parasites, and when we killed them off, our immune systems began attacking ourselves…
