Parasites Revealed to Be ‘Unseen Influencers’ of All Ecosystems

A social history of measurements, an anthology of new speculative fiction, and more book

HOMAGE TO THE BLOODSUCKERSA

crash course in the overlooked world of parasites

Parasites: The Inside Story
by Scott L. Gardner Judy Diamond and Gabor Rácz
Illustrated by Brenda Lee
Princeton University Press, 2022 ($29.95)

Growing up on a farm in Oregon’s Willamette Valley, Scott L. Gardner would comb the rolling hills for carcasses of mice, pheasants and other expired wildlife. It was not those larger animals that intrigued the young naturalist, though, but the smaller life-forms nestled inside their organs and flesh. Gardner was after their parasi

Gardner’s uncle, Robert L. Rausch, was a leading parasitologist with a career spanning more than 60 years. Gardner would regularly mail coiled worms, tear-shaped flukes and other specimens he found in his dissections to his uncle for identification help. One of them—a tapeworm Gardner fished out of a camas pocket gopher—turned out to be a species new to science. He later named it Hymenolepis tualatinensis, after the Tualatin River where he’d discovered it.

Gardner was hooked, so to speak. He went on to become a parasitologist at the University of Nebraska–Lincoln, where he also serves as curator of the Harold W. Manter Laboratory of Parasitology, one of largest parasitology collections in the world. In Parasites: The Inside Story, which Gardner co-authored with his colleagues Judy Diamond and Gabor Rácz, he shares the story of H. tualatinensis to make a larger point about parasites: these species are all around us yet woefully understudied. More and more, though, scientists are learning that parasites are facing the same survival pressures as free-living animals, including climate change, habitat loss and other anthropogenic stressors. Indeed, around his family’s farm today, the tapeworm he discovered just a few decades ago “is nowhere to be found.”

Disappearances of parasitic species such as H. tualatinensis are much more likely to go unnoticed than declines of songbirds or butterflies. Parasites are grossly overlooked by scientists and the public alike and, when acknowledged at all, are usually singled out only as agents of disease and death. Yet as the authors point out, our understanding of nature is glaringly incomplete without an understanding of parasites. These “unseen influencers” affect virtually every other species on the planet and create complex communities within every environment—communities that ultimately drive the ecology of those systems. Parasites, they assert, form “the scaffolding for all interactions among organisms.”

Parasites provides a crash course on just how ubiquitous those interactions are. The authors describe parasitoid wasps that lay their eggs inside the cocoons of other parasitoid wasps, for example, and leeches that spend their entire lives inside the anuses of hippopotamuses. Parasites also seem to have found their way to every inch of the planet, from the dry valleys of Antarctica to the highest levels of the atmosphere, where they are sometimes blown. Ecologists increasingly agree that parasites play as strong a role in shaping ecosystems as predators do.

Parasites may require up to five hosts to get from egg to larva to adult, and one of the most delightful parts of Parasites includes the 11 figures—mini comics, really—from illustrator Brenda Lee. Each one depicts a different parasite’s life cycle, from the human roundworm’s beginnings as an egg in a pile of curled feces to the prolific expulsion of tapeworm end segments from the rectum of a sperm whale that explode into millions of eggs. Outside the whale these eggs are eaten by zooplankton, where they hatch into larvae. Infected zooplankton are in turn eaten by fish that are eaten by whales, beginning the cycle all over again.

In spite of this co-dependent lifestyle, parasites can be opportunistically nimble, “switching hosts over time as possibilities for new colonization become available,” as Gardner and his colleagues write. Tapeworms, for example, survived the most recent asteroid impact that took out three quarters of all life on Earth by jumping from marine-based hosts to seabirds. Yet co-dependence can also make parasites vulnerable. If a parasitic species’ host suddenly goes extinct—or if conditions change precipitously—the parasites may disappear, too.

Parasites works best when it delves into the details of these surprising life histories and the evolution behind them. As a primer on the subject, it was a zippy read. But I would have enjoyed more firsthand narration of the authors’ experiences. Anecdotes such as Gardner’s boyhood discovery of the tapeworm H. tualatinensis are shared in the third person, which means readers are left to wonder how Gardner felt when he realized he had found a new species—and what his reaction was when it dawned on him that the species had vanished.

Gardner and his co-authors do not explain what caused the disappearance of H. tualatinensis. Most likely—as with so many other mysteries surrounding parasites—they simply do not know. They hint that it might have been the victim of “human environment mismanagement,” either switching to a new host or disappearing from the region entirely. What the authors can say for certain is that “the imminent loss of parasite diversity will forever curtail our understanding of how entire communities of organisms interact and evolve.” —Rachel Nuwer

Rachel Nuwer is a freelance journalist and author whose work has appeared in the New York Times and National Geographic..

Science and technology

Parasites Revealed to Be ‘Unseen Influencers’ of All Ecosystems