Invasive insects damage bird habitat

Birds that specialize on eastern hemlock forests have been declining as a result of an invasive insect devastating the trees, according to new research.

Researchers knew from past studies that eastern hemlock is important for some bird communities. There had also been previous data looking at hemlock conditions and extrapolating the differences in habitat after insect infestation. But until recently, there hadn’t been any long-term research looking at these effects on the bird community before and after infestation of the hemlock woolly adelgid (Adelges tsugae).

“We were fortunate that previous researchers had collected baseline data in the year 2000 at the Delaware Water Gap National Recreation Area, which borders Pennsylvania and New Jersey,” said Matt Toenies, a former graduate student in the ecology program at Pennsylvania State University and lead author of a recent study published in The Condor: Ornithological Applications.

The East Asian insects had arrived when the data was collected, Toenies said, but they hadn’t yet affected the trees. Researchers were able to compare that data with what they found some 15 years later.

The team determined that birds specialized to hemlock — such as the Acadian flycatcher (Empidonax virescens), blue-headed vireo (Vireo solitarius), hermit thrush (Catharus guttatus), Blackburnian warbler (Setophaga fusca) and black-throated green warbler (Setophaga virens) — declined with the loss in habitat.

On the other hand, birds associated with non-hemlock habitats, including deciduous forest, edge habitat and shrubs, spread into the areas where eastern hemlocks had been. Shrub-layer-dwelling species such as the veery (Catharus fuscescens) and edge species such as the eastern wood-pewee (Contopus virens) and blue jay (Cyanocitta cristata) responded well.

“Some people think, ‘well we lost some species, but we gained others, so did we break even?’” Toenies said, but it’s not that simple. “We’re losing specialists and seeing more generalized species leading to less biodiversity on a global scale. We know this is a very common ecological response when people introduce non-native species to new areas.”

But Toenies said there may be some hope for the species that use eastern hemlock, where the hemlock woolly adelgid is managed on a small scale. The team found that where there was a higher density of trees, there was more devastation than in single small patches. To maintain hemlock associated birds, he said, “we don’t necessarily have to have all these hemlocks in perfectly healthy conditions. Even if we have just a few healthy hemlock trees, or many trees even if they’re in poor shape, we might be able to maintain the birds.”

Toenies said it’s also important to guard against future insect infestations. “There are more nonnatives and invasives coming through the door all the time,” he said. “We focus on each of these problem insects or other invasives — and I think we should. But we absolutely need to close the door to future introductions.”

Header Image: An Acadian flycatcher perches in a hemlock forest. This is one of the species that depend on eastern hemlocks, which have been devastated by infestations of invasive insects. ©D Williams