THE migrating songbirds that brighten North America's forests each spring with dazzling colors and melodious strains have been regarded by many scientists as mere decorative frills that play a limited role, if any, in the functioning of their ecosystems. But a new study provides what researchers say is the first evidence that these birds, some of which are in decline, play a crucial role in maintaining the health and productivity of forest trees.
In the study, in the current issue of the journal Ecology, two researchers have shown that these voracious predators can protect trees by eating the bugs that chew the leaves. As a result, scientists are predicting that the decline of these birds could eventually threaten the productivity of forests in the United States as well as in the birds' wintering grounds in the tropical forests of Central and South America.
"It's really a spectacular result," said Dr. Donald R. Strong, an ecologist at the University of California at Davis, "and it's got to be generally true. The conservation importance and forest management importance is profound. We see that in order to protect our forests we've got to have these birds, some of which are becoming quite rare."
Dr. Richard T. Holmes, a bird ecologist at Dartmouth College in Hanover, N.H., said: "It's a very exciting paper. It's the first time that there have been data to show that birds feeding on insects have an impact on trees."
Working in the forest at the Tyson Research Center, a reserve near Eureka, Mo., the researchers enclosed white oak trees in netting cages. The cages were built with holes big enough to admit all the insects that might attack the trees but small enough to keep out the birds that would eat the insects.Continue reading the main story
For two years biologists monitored the insects flying in and out of the cages. Most were moths and butterflies laying eggs that would hatch into leaf-devouring caterpillars, the major insect enemies of these economically important timber trees.
What they found was that the plants that were caged to keep out birds were plagued by twice as many insects and had twice as much of their leafy green foliage eaten as uncaged plants. As a result, the damaged plants produced fewer leaves in following years than those trees cleaned of their pests by birds.
"There's been a lot of argument and controversy about the role that birds have in ecosystems," said Dr. Robert J. Marquis, a plant ecologist at the University of Missouri in St. Louis who was an author of the study. "Some say they do no more than provide weekend activity for bird-watchers. We've shown that the birds can have an effect."
Conservationists welcome the work as the first piece of evidence that the colorful dickey birds of spring do have a significant ecological and economic importance.
Dr. Tom Hinckley, Bloedel Professor of Forestry at the University of Washington in Seattle, said: "These birds in a sense enrich our environment, whether it's urban or rural, but it's sometimes difficult to put a value on them other than enrichment. Now here's a case where their value can perhaps be translated to a clear economic value."
Dr. Christopher J. Whelan, who holds the Emily Rodgers Davis Chair in Ecology at the Morton Arboretum in Lisle, Ill., and was the other author of the paper, said the migrants made up more than two-thirds of the nesting bird species in the forests.
During spring migration at the study site in the Ozarks, 40 to 50 bird species arrive to eat the insects on the fresh white oak leaves. Dr. Whelan said, some 30 of those species stay on to nest, including red-eyed vireos, American robins and scarlet tanagers. All appear to help the white oak.
Though many researchers agree that several species of these migrant songbirds are decreasing, the reasons remain controversial. Researchers have attributed the decline to forest destruction and other disruptive forces here and in their wintering grounds.
In addition to documenting beneficial effects for plants, researchers say the new study may provide clues to the birds' decline. Dr. Scott Robinson, an ornithologist with the Illinois Natural History Survey in Champaign, said that the migrant birds' ability to depress insect numbers so greatly on white oak suggests that competition for insect prey on breeding grounds in North America may be important to the birds' survival and reproduction, as well as a factor limiting their numbers.
Based on the new study, ecologists predict trouble ahead for forests, but they caution that for the moment such predictions are speculative. Whether any trees besides white oak will actually suffer remains to be seen.
Researchers say that even if there is an important effect on trees, the damage due to loss of birds may be difficult or impossible to tease out from that due to other causes. Although they cannot point to specific damage resulting from the loss of birds, some researchers say they fear damage to trees may be under way.
"My feeling is that it's going on as we speak," Dr. Whelan said. "In some of the small forest fragments that have very few birds in them, this may very well be happening right now."
Ecologists have known for some time that trickle-down effects along a food chain can be very important in aquatic systems. In these so-called trophic cascades, top-level predators, like fish that eat smaller fish, radically increase or decrease the numbers of creatures at the bottom of the cascade by eating more or fewer of those that act as predators further down the line. But scientists had remained skeptical about whether the terrestrial equivalent of these cascades -- involving birds, insects and plants -- were of much importance. Birds, it was supposed, would not be powerful enough predators to have an effect all the way down to the plants.
"The really exciting thing about this paper is, now we can believe that the birds are playing a big role, having a hand in this," said Dr. Strong.
The authors even propose that the birds constitute an ecologically correct form of pest management.
"Some people say insecticides are the way to go," said Dr. Whelan, "but, actually, what we show is that birds were as effective as insecticide. So if you have the birds, you don't need insecticide." Many ecologists say that anyone who is interested in reducing pests in forest tracts and increasing forest productivity should protect the beneficial birds.
Many options for increasing bird numbers are already well known. For example, many of the birds declining most quickly prefer forest interiors. One way to protect them is simply to prevent the fragmentation of forest tracts. Researchers say such fragmentation not only decreases the stretches of protected interior forest but provides habitat for species that like edges, like raccoons and blue jays, which often prey on the eggs and young of the declining bird species.
The researchers will continue their work by examining the effects of these migrant insect-eaters on tropical forest trees in their wintering grounds in Costa Rica.Continue reading the main story