One Species’ Invasive Enemy May be Another’s Lunch

The emerald ash borer, an iridescent-green Asian beetle, has been munching its way through millions of North American ash trees since its discovery near Detroit in 2002. While this is bad news for the trees and the wildlife that depends on them, ornithologists wondered how insect-eating, cavity-nesting birds—white-breasted nuthatches and downy, hairy, and red-bellied woodpeckers—are responding to the invasion.

To find out, they used data collected from 167 sites between 2001 and 2011 by citizen scientists involved in Project FeederWatch. In the high-impact areas, the scientists report in Biological Invasions, red-bellied woodpeckers and white-breasted nuthatches began increasing in abundance when the beetles moved in, while downy and hairy woodpeckers at first drastically declined. But, at least in the past few years, downy and hairy populations seem to be on the mend and have even increased to outnumber those in control sites. The outcome could also be because the dead trees provide more nesting habitat.

“We didn’t get the results we probably would have predicted a priori if we had tried to,” says Walt Koenig, a study coauthor and a senior scientist at the Cornell Lab of Ornithology, which runs Project FeederWatch. “It’s just very difficult to predict what the effect of an invasive insect pest is going to be on native birds.”

This story originally ran in the November-December 2013 issue as "Exotic Dining."