By the time scientists had recognized the fungus, it was far too late to do anything. They could only watch the parasite bound southward from mountain to mountain. “We should have known about that fungus,” says Brooks. “If we’d had an inventory of parasites of frogs, we might still have frogs on the mountaintops of Central America. We didn’t know it was there.” Humans have no special protection from parasites either, and they can come bounding out of disturbed rain forests. It won’t be doctors who figure out where the Ebola virus comes from, but zoologists who can find the animal in the African rain forest that normally harbors it.
But Brooks doesn’t look at his inventory simply as a catalog of death and destruction. It may be able to help scientists measure the ecological health of Guanacaste and other forests like it. An ecosystem is a bit like a person. In a healthy person, all the parts interact the way they should: the lungs take in oxygen and the stomach takes in food, the blood carries it all to the tissues, the kidneys flush out waste, and the brain ponders the world or what it wants for dinner. In a sick person, a few of the parts stop working, and their shutting down disrupts the person’s whole body, sometimes forcing the rest of the parts to shut down as well. An ecosystem lasts for thousands or millions of years because it has parts that work together welclass="underline" the worms aerate the soil, the fungus mingled with tree roots supplies them with nutrients and extracts carbohydrates in exchange, and so on. Water, minerals, carbon, and energy all circulate through the ecosystem like blood. And ecosystems, it turns out, can sicken. Introduce a parasite that kills koa bugs, and the damage can ripple out all the way to the trees in a forest.
Doctors don’t wait until their patients are dead to declare that something’s wrong with them. They look for early, easy-to-detect clues to trouble, even if they don’t know yet what the trouble is. If a potentially fatal colony of bacteria have established themselves somewhere in a person’s body, you don’t have to actually track the microbes down—you can just check for a fever. Ecologists want something that can tell them that an ecosystem is sick before the damage has rippled out to all the strands of its web. They have been auditioning the species that make up ecosystems in the hopes of finding one that could act as a sort of body-temperature index. Some have been looking at ants and other insects, others at the songbirds that nest on forest floors. Many candidates fall short in one way or another. It’s relatively easy to tell whether top predators such as wolves are declining, since they’re relatively few and big. But by the time the effects of some environmental stress have surged all the way up the food chain to the wolf, the ecosystem is probably already too far gone to help.
Some scientists, such as Brooks, think that parasites are a sign of ecological health, but not in the way most people would think. Until recently, most ecologists looked at parasites as nothing but a sign of environmental decline. If some pollutant wears down the immune systems of the members of an ecosystem, they become more susceptible to diseases. That does indeed seem to be true some of the time, but it’s easy—and wrong—to make it a generalization. The idea echoes all the way back to Lankester: the rise of parasites as a sign of degenerate times. The frogs Brooks and I had collected in the lower forests were healthy and so abundant that they threw themselves across our path, and they were riddled with parasites. Parasites are actually a sign of an intact, unstressed ecosystem, and the opposite, as strange as it may sound, is true: if the parasites disappear from a habitat, it’s probably in trouble.
As parasites travel through their life cycle they are often vulnerable to poisoning by pollution. A fluke, for example, hatches into a delicate form covered with hairlike cilia that swim in search of a snail; a couple of generations later, a cercaria emerges from the snail to find its mammal host. At both stages, the parasite depends on clean water to survive. That’s the theory, at any rate, and there’s some concrete evidence to show that it’s correct. The rivers of Nova Scotia have become acidified as a result of air pollution from coal plants upwind. Canadian ecologists added lime to the headwaters of one badly hit river, neutralizing the acid there, and then came back in the following years to collect the eels. They then compared them with eels from an untreated river that eventually joined the limed one. The eels from the limed river carried inside them a much richer diversity of tapeworms, flukes, and other parasites. The ecologists then expanded their survey to the rivers along much of the coast of Nova Scotia, and found that the most badly affected waters had eels that were the most free of parasites.
Parasites work well as ecological sentinels for another reason: they sit at the top of many ecological webs. If you dump nickel into a river, the little animals take up a little of it and don’t suffer too badly, but as the nickel rises up the food web—as copepods are eaten by small fish, which are in turn eaten by big fish, which are in turn eaten by birds—the pollution focuses to higher and higher concentrations. But parasites, which prey on even the top predators, concentrate even more pollution in their bodies. Tapeworms may carry hundreds of times more lead or cadmium than the fish they travel inside, and thousands more than the surrounding water.
Unlike free-living organisms, a parasite wanders through the many levels of its ecosystem, and it can report on the damage it comes across in its travels. Throughout its life cycle, a parasite may need to move through many hosts, each of which occupies its own niche in the habitat. Flukes in the Carpinteria salt marsh have to live in snails, which depend on the algae on the mud banks; from there they find a fish, which must eat zooplankton to survive; and finally the parasite must find the gut of a healthy bird in which it can mature. If any of those hosts should disappear, the parasite will suffer. In 1997, Kevin Lafferty found that in the degraded part of the Carpinteria salt marsh, there are only half the species of parasites as in the healthy part, and only half the number of individual parasites. Parts of the marsh are now getting restored, and by 1999, the snails there had regained the levels of parasites found in the pristine marsh.
This is why Brooks is cutting open frogs in Costa Rica. “You’ve got this guy walking around with nine or ten parasites, healthy and happy. Once you know all the parasites in the frogs, suddenly if something’s not there, something’s wrong with the frogs or with an intermediate host. If you’ve lost a parasite, you have lost something in the fabric of the ecosystem.” And once Brooks is done with his inventory, it may be possible to identify parasites by their eggs or larvae—and it won’t be necessary to sacrifice any more hosts.
Parasites may not only mark good ecological health; they may actually be vital for it. When ranchers overgraze their cattle and sheep on fragile grasslands, they can tip the ecology of the region over into a desert. As far as ecologists can tell, this move is pretty much irreversible, because the desert shrubs reorganize the soil in such a way that grasses can’t penetrate back in. It is a difficult and politically volatile matter to decide just how much grazing should be allowed on a given patch of land. Ranchers usually dope up their livestock with medicine to kill as many intestinal worms as they can, but the parasites might be able to keep the livestock in a careful balance with the grass they depend on. The larvae of some species of parasitic worms get into livestock by attaching to the grass they eat. When a worm gets into the gut of a sheep, it matures and starts siphoning off some of the sheep’s meals. Struggling with the effects of the worm, the sheep tends to live a shorter life and produce fewer lambs. In the end, the parasite shrinks the size of the herd.