April 4, 2006
A Weed, a Fly, a Mouse and a Chain of Unintended Consequences
By JIM ROBBINS
MISSOULA, Mont. — First came the knapweed. Then came the gall fly. And now the mice population is exploding — the mice that carry hantavirus. In a classic case of unintended ecological consequences, an attempt to control an unwanted plant has exacerbated a human health problem.
Spotted knapweed, a European plant, is a tough, spindly scourge that has spread across hills and mountainsides across the West. In Montana alone, one of the worst-hit states, it covers more than four million acres.
In the 1970's, biologists imported a native enemy of knapweed, the gall fly. The insect lays eggs inside the seed head, and the plant then forms a gall, or tumor, around the eggs. When the larva hatches, it eats the seeds.
Dean Pearson, who works at the
Rocky Mountain Research Station of the United States Forest Service, said the
fly had not halted the spread of knapweed. In a report in Ecology Letters,
however, Dr. Pearson reports that the introduced fly has changed the ecosystem's
dynamics. The fly larvae provide an abundant food source for deer mice in the winter, above the snow. Instead of dying out, as is often the case in cold and snowy weather, the deer mice climb the stalk of the plant above the snow to the seed head. They can eat as many as 1,200 larvae a night, at a time when there is normally no other food.
Mice numbers have tripled because of this food supply, said Dr. Pearson, and with them hantavirus, a viral infection is spread by urine and droppings. It is rare, but can cause a pneumonialike disease that can be fatal to humans.
"It illustrates the complexity of how these things play out in the system," Dr. Pearson said. "These kinds of things are not being considered" when exotics are released, he said, and there is every reason to suspect there are other, similar examples.
Since the 1970's, the importation of an exotic plant's natural enemies from its country of origin, something known as biological control, has been considered a safe and effective alternative to pesticides. Hundreds of enemy species have been released.
Biological control has worked well in some cases to tame serious problems and reduce the need for pesticides. The classic case was the release of a beetle to control a weed called St. John's wort in the 1940's. The beetle reduced the plant to less than 1 percent of its original range, and has kept it there.
To Dr. Pearson and other critics of biological control, however, the reason the process seems safe and effective is that it has not been well studied. Once an exotic is released, in other words, it isn't followed to see what happens.
"What Dean has found is the tip of the iceberg," said Svata Louda, a professor of biology at the University of Nebraska at Lincoln, referring to Dr. Pearson's study. "We don't know what we're doing when we mess up natural systems."
A major worry is that the introduced enemy will drift off target. Dr. Louda has studied the thistle flowerhead weevil that was imported from France and Italy in the 1970's to control a weed called the musk thistle, which spread across grazing land.
In Nebraska's Sand Hills, she found that the weevil switched to Platte thistle, a native plant, and, she says, has severely reduced it. She is worried the plant could disappear.
Dr. Pearson's work, however, raises a new kind of problem with biocontrol — that even if an exotic species stays on its intended target, it can disrupt ecosystems and cause environmental damage.
"This is the first time anyone has shown host specific organisms can impact nontarget species," he said. "And in this case the chain goes all the way to humans." There's no research on whether human cases of hantavirus have risen because of gall flies.
"It's good science," said Jim Story, a research entomologist at Montana State University in Bozeman who has studied and introduced biological control agents for knapweed for more than 30 years. "It helps us understand the whole system. But we've never be able to eliminate all risk. I don't think this is a huge black mark against biocontrol. Any time you bring in an exotic agent you have to assume there are going to be negative effects."
Critics, he said, should focus on the damage caused by knapweed. "We're not seeing the forest for the trees," he said. "Not too many people are focused on what knapweed is doing. We have land managers screaming at our door and wanting results." Problems with the gall fly larvae may be a moot point, he says. An exotic root weevil that has been introduced to control knapweed shows great promise.
Donald Strong, a professor of biological sciences at the University of California, Irvine, has studied the ethics of biological controls and says that, while this is a serious problem, no one could have foreseen it. On balance, he said, biocontrols are critical. "Invasive species are a huge problem," Dr. Strong said. "Biological controls are a very powerful arrow in our quiver for invasive species."
In Australia, authorities have introduced a virus, originally called rabbit hemorrhagic fever, to kill the rabbits that plague the countryside. The virus seems to be working, but the release has drawn criticism and prompted fears that the disease could spread to other animals, and is not controllable.
Problems with biological controls have emerged before in the United States. In 2001, researchers found that a parasitic fly brought from Europe to control the exotic gypsy moth had switched hosts and was attacking wild giant silk moths, a large and beautiful native moth, causing a precipitous decline in population.
Both Dr. Pearson and Dr. Louda think that exotics are too liberally used. Studies show that as many as three times as many exotics have been released as there are target species. They also say there are other things that can be done to control weeds. Rather than reduce grazing, Dr. Louda contends, the agriculture officials release beetles and the rancher can go on overgrazing.
Dr. Pearson questions a fundamental assumption of biocontrol — that pest enemies of the weeds are the factor that controls them. "The natural enemies hypothesis has dominated thinking in this field for a long time," he said. "But the reasons knapweed is not abundant in Europe may be what it competes with or climate. There could be a lot of reasons."
Dr. Pearson says the gall fly has changed the deer mouse ecosystem on a large scale. Pointing to knapweed-blanketed hills on the edge of town here, Dr. Pearson said that at one time the deer mice lived in island populations, widely separated, and kept in check by winter mortality. Now those populations are contiguous. Disruptions in ecological systems are often the cause of disease.
In the East, researchers suspect, building homes in forests has pushed out predators, which has elevated populations of deer and the white-footed mouse, which are hosts for Lyme disease. Heavy rains in the Southwest last year led to a profusion of wild flowers and food for mice, and scientists predict there could be an increase in hantavirus this year as a result.