The birds are falling: avian losses could hit ecosystems hard.

If many bird populations dip toward extinction in the coming century, as scientists predict, widespread harm could come to ecosystems that depend on these birds to pollinate plants, disperse seeds, scavenge carrion, and control insects.

Using a team of students to comb through the literature, Cagan H. Sekercioglu, Gretchen Daily, and Paul Ehrlich of Stanford University filled a database with information on diet, habitat, range, and other traits of all the nearly 10,000 known living and extinct species of modern birds.

Consideration of the recognized threats to avian survival--including alien predators, chemical contaminants, and fishing gear--led the scientists to forecast that 500 to 1,300 species will vanish by the end of this century, and that up to 1,050 others will become so depleted that they'll serve no significant ecological function. In contrast, only 129 bird species are known to have gone extinct in the past 500 years.

Scavengers, fish eaters, herbivores, fruit eaters, and nectar-drinking birds are particularly vulnerable, the investigators suggest in the Dec. 21 Proceedings of the National Academy of Sciences.

Many roles that endangered birds play in their ecosystems will go unfilled, the scientists predict. For instance, some birds that pollinate plants and disperse fruit seeds are so specialized that their loss will jeopardize the plants they serve, says Sekercioglu. Moreover, fish-eating seabirds fertilize remote islands with their droppings.

The new study is unprecedented in its global scope and sobering in its predictions, says ecologist Douglas J. Levey of the University of Florida in Gainesville. "Their best-case scenario is alarmingly bad," says Levey.

Nevertheless, he says, "projections of this kind are inherently dicey." Because the ecological role of one disappearing bird species may get taken over by different animals, he notes, "the impact on plant populations is not nearly as straightforward as [Sekercioglu and his colleagues] imply."

Although echoing that uncertainty, tropical ecologist Norbert J. Cordeiro of the University of Illinois at Chicago calls the study "pivotal" because it takes a first stab at identifying the ecological consequences of widely anticipated bird extinctions.

Thomas M. Brooks of Washington, D.C.-based Conservation International focuses on two groups of birds. "This study suggests that birds of prey may be encouragingly resilient in the face of extinction. On the other hand, one of [the study's] most disconcerting results is the catastrophe facing scavenging birds," he says.

South Asian vultures could provide a window on the future. Poisoning from a drug used in livestock whose carcasses they scavenge has severely depleted the vultures' numbers in the past decade (SN: 1/31/04, p. 69). With that decline, the birds' competitors, including disease-spreading feral dogs and perhaps rats, are proliferating in some locations, Sekercioglu says.