CAPTION: Chelsea Wood kneels to search for shore crabs on a beach in Tacoma, Washington. Later she will dissect the crabs for parasites.
CREDIT: Jesse Nichols/Grist
The parasites were a sign that local shorebirds were doing very well, Wood explained.
As scientists have learned more about parasites, some have argued that many ecosystems might need them to thrive. “Parasites are an indicator,” he said. “So if the parasites are there, you know the rest of the hosts are there too. And in that way they signal the health of the ecosystem.”
To understand this counterintuitive idea, it’s helpful to look at another class of animals that people used to hate: predators.
For years, many communities used to treat predators as a species of vermin. Hunters were encouraged to kill wolves, bears, coyotes, and mountain lions to protect themselves and their property. But over time, people began to notice some important consequences. And nowhere was this phenomenon more evident than in Yellowstone National Park.
In the 1920s, Gray wolves were systematically eradicated from Yellowstone. But once the wolf population was eliminated from the park, moose numbers began to grow uncontrollably. Over time, herds grazed too close to streams and rivers, driving away animals, including native beavers. Without beavers to build dams, the ponds disappeared and the water table dropped. In a short time, the entire landscape had changed.
In the 1990s, Yellowstone changed its policy and reintroduced gray wolves to the park. “When those wolves came back, it was like a green wave that rolled over Yellowstone,” Wood said. This story became one of the defining parables of ecology: predators were not just killers. In reality, they held entire ecosystems together.
“I think there are a lot of parallels between predator ecology and parasite ecology,” Wood said.
As with gray wolves in Yellowstone, scientists are just beginning to recognize the profound ways parasites shape ecosystems.
Take, for example, the relationship between nematomorphs, a type of parasitic worm, and stream water quality. Worms are born in water, but spend their lives on land inside insects, such as crickets or spiders.
Courtesy of Grist
CAPTION: A nematomorph worm swims in a beaker in Chelsea Wood’s Seattle office.
At the end of their lives, nematomorphs need to return to the water to mate. Instead of making the dangerous journey themselves, they trick their infected hosts into taking them by inducing a “water drive,” an urge on the part of the insect host to submerge itself in water. The insect will move to the edge of the water, consider it for a moment, and then jump off, to its own death, but for the benefit of this parasite.