8
How to Live in a Parasitic World
Whenever the earth changed its form of existence, the existing creations were also destroyed. The same thing occurs to the worms; when the host animal dies they are also destroyed.
On my visit to Santa Barbara, after Kevin Lafferty had showed me how parasites hold sway over a salt marsh, I spent a morning with one of Armand Kuris’s graduate students, a young man named Mark Torchin. He led me through one of the marine biology labs to a blue door in the corner. A sign marked QUARANTINE plastered the door. When Torchin opened the door and we walked into the dark, I could hear what sounded like a flowing creek. Torchin found the switch to the cold fluorescent lights, which shone down on a high table running the length of the room. On the left side were aquarium tanks full of water, with crabs skittering around inside on broken pieces of white mesh. On the right were tubs with cups stacked in them, each holding a single crab in a scoop of water. The sound of a creek came from the system of pipes that pumped sea water in from the lagoon just outside, flowing into the tanks and dribbling onto the table before heading down a drain, to flow back to the Pacific.
The crabs were Carcinus maenas, the European green crab. Some were the size of teacups, some only of shot glasses. If you walk along the coast of northern California and the Pacific Northwest, you may find green crabs, and that’s a fact that has certain people terrified. Before 1991, there were no green crabs on the California coast. Its original range was along the beaches of Europe. There it was a voracious creature; in Great Britain, biologists have watched single crabs eat forty cockles, each half an inch long, in a single day. For thousands—perhaps millions—of years, the rest of the world was spared from the green crab’s hunger, but that changed when humans invented ships. The green crab sheds thousands of nearly invisible larvae into the water, which can be easily sucked into the holds of ships when they take on ballast water. Perhaps two hundred years ago, some ship traveling to the American colonies carried green crabs to the New World. They quickly began to spread along the coast of the eastern United States, devouring shellfish in northern New England and Canada. The softshell clam, once the basis of a whole fishing industry in New England, disappeared altogether.
The crabs traveled to South Africa and Australia as well, but for centuries the west coast of the United States was spared. Despite all the ships traveling there from Europe and the eastern United States, it wasn’t until 1991 that a fisherman near San Francisco pulled up a green crab in his nets for the first time. As soon as reports spread around marine biology circles, scientists became gloomy. Almost every species of shellfish around San Francisco was suitable prey, and if the green crab should spread along the coast in the ships that traveled down to Los Angeles or up to the northwest, it could spread to new habitats, feasting on oysters, Dungeness crabs, and other valuable creatures. The burrows it dug might destabilize dikes, levees, and channels, causing even more damage. “It’s a disaster,” says Armand Kuris. “It’s all the things you want in a worst-case scenario.”
The green crabs in the quarantined lab in Santa Barbara skittered in their tanks. Some had ghostly white claws growing in the place where they had lost a previous one. And some, as I could see when Torchin pulled them out of the water and turned them upside down, their legs and claws windmilling around helplessly, carried a sac on their abdomen the color of butterscotch. They looked like normal crabs, but they had been transformed into something else. They were filled with Sacculina carcini, that degenerate parasitic barnacle of Ray Lankester’s nightmares. Torchin, Lafferty, and Kuris were trying to use Sacculina to save the Pacific coast from the green crab.
In the late 1800s, scientists sometimes referred to parasitology as medical zoology. They were referring to the way they had to understand parasites as real organisms, with natural histories of their own, before they could try to fight the diseases the parasites caused. Now, a century later, the term has taken on a new life. Now the patient isn’t a person but the natural world. Alien species are spreading uncontrollably across continents and seas; native plants and animals are falling prey to new diseases; habitats are disappearing as forests turn to stumps and coastlines to condominiums. As ecosystems have faltered, scientists have come to recognize that parasites are important to their health. A healthy ecosystem is riddled with parasites, and in some cases, an ecosystem may even depend on parasites for its health. As humans alter the world, tipping the biosphere out of kilter, it may be possible to enlist parasites to help us undo some of our mistakes and perhaps keep us from making new ones.
Scientists first conceived of using parasites against pests in the 1880s. The original idea was simple. A parasite is a cheap, never-ending pest-killer. It can seek out its host and invade it, fighting off the host’s immune system and, in many cases, leaving the host dead. Farmers who use pesticides have to spray their plants at least once a year, but parasites keep regenerating and tracking down new hosts. Simply sow the parasite, the argument went, and your troubles are over. In the early part of this century, farmers were having exactly the sort of success that had been promised. Scales and beetles and other pests were destroyed by wasps and flies and other sorts of parasites. The parasites couldn’t eradicate the pests completely, but they no longer threatened to wipe out whole fields.
In the 1930s, the agrochemical industry was born. DDT arrived on the market, a powerful pesticide that came with the luster of modern science—a synthetic creation that humans could use to master nature. As a result, biological control withered away. A few biologists in California and Australia kept studying parasites in the hope of bringing back biological control. And over the next forty years, pesticides began to falter. Insects evolved resistance to DDT. The chemical worked its way into the food chain, causing birds to lay eggs with thin eggshells. An environmental movement opposed to pesticides started up, and the aging masters of biological control saw a chance for a comeback.
“I was a graduate student at Berkeley at the time,” says Armand Kuris. “It was so interesting. These were old guys, twenty years, thirty years my senior. They were old agricultural guys with string ties and stuff like that. And there they were in the sixties with all the hippies, and they found themselves in the same bed together. In the beginning it was weird, but then they realized they were on the same side. It was one of the sidebars to the history of the sixties.”
In its second incarnation, biological control with parasites had a much more solid scientific foundation. Insects can evolve resistance to DDT, but parasites can evolve as well. They can come up with new molecular formulas for attacking their hosts, canceling out any resistance the pests may evolve. A parasite could rein in a pest, some scientists argued, by bringing back at least some balance to nature. Most pests are alien species like the green crab, brought to a new land. One reason they are so harmful is that they have escaped their parasites and can breed unchecked, while native species have to struggle against their own parasites. Introducing a parasite from the invader’s homeland, the argument for biological control goes, is really just a way to reestablish some natural restraints.