Why Trees Need Birds

By Begley, Sharon | National Wildlife, August-September 1995 | Go to article overview
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Why Trees Need Birds

Begley, Sharon, National Wildlife

Scientists say the health of American forests may depend on songbirds, many of which are in decline

When Robert Marquis and Christopher Whelan had an idea six years ago that songbirds flitting about a woodland might be having a profound effect on the forest ecosystem, the two biologists took the next logical step: They wrapped net cages around trees to see what would happen to them without the birds.

With that simple experiment, the research team challenged a basic role of ecology and ended up with vital information for forest management. Scientists had long held that on land, the vigor of an ecosystem and the sizes of its plant and animal populations are determined at the base of the food chain, not at the top. That model would mean that for animals, the amount of food is most important, while their predators and parasites exert little population control. For trees and other plants low on the food chain, only rainfall, soil conditions and nutrients, sunshine and other such low-level inputs supposedly are important enough to affect populations. The world presumably works from the bottom up, in other words.

But Marquis (of the University of Missouri-St. Louis) and Whelan (of the Morton Arboretum in Lisle, Illinois) were not so sure. Other studies had shown that birds control insect populations. But, Marquis says, "No one had looked at it from the plant point of view." Previous research had focused on plants' release of toxic or bad-tasting compounds that can make insects beat a hasty retreat. "I was thinking," says Marquis, "that maybe predators of insects and other herbivores influence which plants are eaten."

Still, even the researchers were not prepared for their results, published last fall. Birds, it turns out, are nothing less than keepers of forest health, a finding with serious implications. For starters, populations of many species of migrant and resident birds have been decreasing since the 1980s. Not only that, evidence has been accumulating (two new studies were released just this past year) that the accelerating fragmentation of the nation's forests is hastening the decline of many bird species.

When Marquis and Whelan came up with their notion of top-down effects ("trophic cascades" in scientific jargon), they did have a model of sorts. Aquatic biologists had established that just such an effect exists in streams and lakes. Then a study released in 1992 strengthened the model: Researchers from the University of Washington had surrounded algae with wire-mesh cages to keep off the gulls and skuas that eat limpets (which graze on algae) and barnacles (which compete with algae for space). Without the birds, the limpets and barnacles rapidly reduced the algae population.

But on land, the effects of higher food-chain organisms on lower ones had always been questionable. Sure, birds eat insects, leaving fewer to munch on plants. But did that really affect the overall health or growth of plants? "Birds eating insects were thought not to depress the insects' numbers enough to affect plants," says biologist Richard Holmes of Dartmouth College.

And that's putting it politely. Deep into his research three years ago, Marquis found a review article in the journal Ecology titled "Trophic Cascades Are All Wet" - high humor for ecologists - meaning that the top-down effect only occurs underwater. Uh-oh, thought Marquis. Here was the pre-eminent journal in his field weighing in to say he and Whelan were chasing shadows. But the team was undaunted. "This review article just points out," Marquis said months later, "some of the big holes in our understanding of how natural systems work."

Give him points for understatement. Those "holes in our understanding" are big enough for flocks of wood thrushes, black-and-white warblers, scarlet tanagers, summer tanagers, grosbeaks, titmice, red-eyed vireos, yellow-billed cuckoos, chickadees and dozens of other species that are found in the Tyson Research Center forest, a 2,000-acre deciduous reserve in Eureka, Missouri.

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