Lafferty looks at all of this, the eating and being eaten, this transmutation of sunlight into different forms of life, and doesn’t see it quite the way other ecologists might. A curlew grabs a clam from its hole: “Just got infected,” he says. He looks at the bank of snails and says, “More than 40 percent of these snails are infected. They’re really just parasites in disguise. There are boxcars of parasite biomass here.” He points to the snowy constellation of bird droppings along the bank. “Those are just packages of fluke eggs.” He hears the things he’s been saying to me and shrugs. “I have a pretty warped perspective.”
When Laffterty started graduate school at Santa Barbara in 1986, his perspective wasn’t yet warped. If someone had asked him then to figure out the ecology of this salt marsh, he would have studied the things he could see. He would have measured how much algae the snails could eat, he would have added up the number of eggs a female killifish could lay in a year, he would have recorded the number of clams a bird could eat in a day. He would, he now realizes, have completely missed the real drama of this ecosystem because he would have ignored the parasites.
There’d have been nothing unusual in that. For decades, ecologists have waded into bayous, paddled into lakes, and tramped through forests in order to look at two things: the competition for the necessities of life, such as food and water, and the struggle not to be eaten. They surveyed the density of plants and animals, their distribution from young to old, the diversity of species. They drew diagrams of food webs like tangled mobiles. But never did one of those strands lead to a parasite. Ecologists didn’t deny that parasites existed, but they thought of them as merely minor hitchhikers. Life could be understood as if it were disease-free. “A lot of ecologists don’t like to think about parasites,” says Lafferty. “Their vision of the organism stops at the exterior of it.”
Few ecologists had bothered to back up their indifference with any data. It didn’t matter to them that animals are typically overrun with several different species of parasites. On the other hand, parasitologists had been remiss as well. They had been ogling their parasites in laboratories, but they had no idea what effects they had in the real world.
It turns out that those effects can be huge. Only in the last decade, for instance, have marine biologists discovered that the oceans are swarming with viruses. They had known for a long time that viruses can infect just about any marine life form, from whales to bacteria. But they had thought that there simply weren’t many viruses, or that they were too fragile to cause much harm. In fact, viruses are rugged and abundant. Ten billion of them live in the average quart of surface sea water. Their favorite targets are bacteria and phytoplankton, since those are the most abundant hosts in the sea. They also serve as the bottom link in the ocean food chain, devoured by predatory bacteria and protozoa, which are in turn eaten by animals. Now marine biologists realize that this crucial link is very sick. As many as half the bacteria in the ocean are killed by viruses. When a bacterium dies, it bursts apart in a little organic shower. Other bacteria scoop up its remains, in many cases only to be burst open by another virus. A huge amount of the ocean’s biomass is stuck in this bacteria-virus-bacteria loop, and it can’t feed the rest of the marine food chain. If viruses were to vanish from the sea, it might become crowded with fish and whales.
On land, parasites are just as powerful ecologically. For decades, ecologists who worked on the Serengeti plains thought that the great herds of wildebeest and other grazing mammals there were controlled by two factors: the food that could support them and the predators that kept their population down. Yet, for most of this century it was actually a virus that was most powerful. Known as rinderpest, the disease came to Kenya and Tanzania when infected cattle were imported from the Horn of Africa around 1890. It jumped from the livestock to wildlife and dragged down the population of herbivores, as well as their predators, and kept them down for decades. Only when cattle began to be vaccinated in the 1960s did the mammals of the Serengeti rebound.
Parasites don’t even have to kill their hosts to have huge impacts. A parasite may cut down the competitive edge of a species so that it can’t drive out a competitor, making it possible for the two species to live side by side. Deer carry a nematode that causes them no harm, but when it gets inside moose, it crawls into their spines and makes them stumble around drunkenly before dying. Without that parasite, the deer wouldn’t be able to compete with the moose. And biologists such as Lafferty have shown that the way parasites manipulate their hosts can also have a big effect on the balance of nature.
Going into graduate school, Lafferty thought he had a pretty good idea of the ecology off the California coast, where he had scuba dived since high school (he paid his way through college by scraping mussels off oil rigs). It wasn’t until he took a course on parasitology that he had his mind changed. His teacher, Armand Kuris, stunned him by showing how parasites could be found everywhere in the sea. “Here are all these animals I knew and loved as a diver, and when you opened them up they were full of parasites. I realized marine ecology had been missing a big part of the picture.”
Lafferty began studying the parasites of the Carpinteria salt marsh. There are many to choose from at Carpinteria—a dozen flukes infect the California horn snail alone—but Lafferty chose the most common one, Euhaplorchis californiensis. Birds release Euhaplorchis eggs in their droppings, which are eaten by horn snails. The eggs hatch, and the flukes castrate the snail, producing a couple of generations before cercariae come swimming out of their host. The cercariae explore the salt marsh to find their next host, the California killifish. They latch onto its gills and work their way into its fine blood vessels; they crawl deeper into the fish, finding a nerve that they follow until they reach the brain. They don’t actually penetrate the killifish’s brain but form a thin carpet on top of it, looking like a layer of caviar. There the parasites wait for the fish to be eaten by a shorebird. When they reach its stomach, they then break out of the fish’s head and move into the bird’s gut, stealing its food from within and sowing eggs in its droppings to be spread into marshes and ponds.
Lafferty wanted to understand what effect this cycle had on the ecology of the salt marsh. Would Carpinteria look the same if there were no flukes? He began his ride around the parasite’s cycle at the snail stage. The relationship between fluke and snail is a strange one. It’s not a predator-and-prey arrangement. When a lynx kills snowshoe hares, the tender shoots that the dead hares would have eaten are eaten by the survivors, which can use the energy to raise baby hares. But the flukes of Carpinteria don’t quite kill their snails. In a genetic sense, the snails are indeed dead, because they can no longer reproduce. But they live on, grazing on algae to feed the flukes inside them. If the snails were truly dead, the algae that they ate would be left for surviving snails to graze on. Instead, the flukes-as-snails are in direct competition with the uninfected snails.