Photo caption: Chelsea Wood kneels searching for shore crabs on a beach in Tacoma, Washington. She then plans to dissect the crab to look for parasites.
Credit: Jesse Nichols/Grist
The parasites were evidence that local shorebirds were doing well, Wood explained.
As scientists learn more about parasites, some argue that many ecosystems may actually need them to thrive. “The parasite is a bellwether,” she says. “So if the parasite is there, we know the rest of the host is there too. And in that way, they send signals about the health of the ecosystem.”
To understand this counterintuitive idea, it helps to look at another type of animal that people once hated: predators.
For many years, many communities treated predators as a type of pest. Hunters were encouraged to kill wolves, bears, coyotes, and cougars to protect themselves and their property. But eventually, people started noticing some serious consequences. And nowhere is this phenomenon more evident than in Yellowstone National Park.
In the 1920s, Gray wolves were systematically eradicated from Yellowstone. However, once the wolf population was removed from the park, the elk population began to increase unchecked. Eventually, the herds overgrazed near streams and rivers, driving out animals including native beavers. Without beavers to build the dams, the ponds disappeared and the water table dropped. Eventually, the entire landscape changed.
In the 1990s, Yellowstone changed its policy and reintroduced gray wolves to the park. “When those wolves came back, it was like a wave of green rolling over Yellowstone,” Wood said. This story has become one of the definitive fables in ecology. In other words, the predator was more than just a murderer. They were actually holding the entire ecosystem together.
“I think there are a lot of similarities between the ecology of predators and the ecology of parasites,” Wood says.
As with Yellowstone’s gray wolves, scientists are just beginning to recognize the profound ways ecosystems are shaped by parasites.
For example, consider the relationship between nematodes, a type of parasite, and water quality in a creek. Worms are born in water but spend their entire lives on land inside insects such as crickets and spiders.
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Caption: A nematode swims in a beaker in Chelsea Wood’s office in Seattle.
At the end of their lives, nematodes must return to the water to mate. Rather than making the dangerous journey themselves, they trick infected hosts into giving them a ride by inducing a “water drive,” the insect host’s urge to soak in water. The insect moves to the water’s edge, thinks for a while, and then dives. The insect itself dies, but to the benefit of this parasite.