How One Invasive Plant Pest Degraded And Transformed An Entire Ecosystem

In an unusual role reversal, one tiny invasive insect is controlling the species composition and architecture of a large community of forest birds along the East Coast of the United States

Throughout the eastern United States, forests of eastern hemlock, Tsuga canadensis, are under attack by a tiny invasive insect with an insatiable appetite for these giant trees. This attack is causing the decline of eastern hemlock and as a consequence, it is driving the disappearance of a variety of birds that are specialized to them, according to a recent study. This study also found that birds that live in habitats not dominated by eastern hemlock are expanding into these dying hemlock forests, and this is diminishing avian biodiversity. Remarkably, in an unusual role reversal, this less diverse bird community is apparently due solely to just one invasive species of insect.

Who is this beastly bug? It is the hemlock woolly adelgid, Adelges tsugae. These insects are not large, but instead, their damage results from their vast numbers. To the unaided eye, a metropolis of hemlock woolly adelgids looks more like a series of white fuzzballs, or perhaps a downy white mould, on hemlock twigs from which its needles sprout.

The hemlock woolly adelgid feeds on needles and buds by sucking their juices, which leads to defoliation and limb death, a process that typically starts at the bottom of the tree and proceeds to the top. Although infested trees can die in as few as four years, most trees manage to postpone their eventual demise for a few years longer than that.

“The adelgids defoliate the lower branches of the hemlocks first — they do this in a pretty systematic way — and we often see a tight little clump of hemlock needles at the top of the tree, and then it dies,” said the lead author of the recently published study, Matt Toenies, who was a master’s degree student in the ecology program at Pennsylvania State University when he did this work.

This tiny aphid-like sap-sucking insect was accidentally introduced to Virginia from southern Japan by a gardener in the early 1950s. From this humble beginning, a massive infestation arose. Currently, things don’t look good for the eastern hemlock: as of 2015, the hemlock woolly adelgid had spread to 20 US states, and, according to the USDA Forest Service (ref), more than half of all US counties where the eastern hemlock is native were infested.

“So far, as far as stopping [the infestation] on a large scale, there is not anything promising that I know of,” Mr. Toenies said. And as the climate warms, this invasive insect is moving into the northern parts of the eastern hemlock’s range.

As trees go, the eastern hemlock is quite large, reaching up to 50 meters (165 feet) in height with trunks of more than 2 meters (6.5 feet) in diameter and a lifespan that exceeds 500 years. Hemlocks are slow-growing shade-tolerant trees that do best in moist forest environments. They are important to some specialized bird communities, as previous studies had established. Despite this tree’s importance to a variety of wildlife, it might surprise you to learn that, until now, there had not been any long-term studies that compared effects on the forest bird community from before and after infestation by the hemlock woolly adelgid.

“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,” Mr. Toenies said in a statement.

Although the insects were already on the scene when some of the early data were collected, they hadn’t yet seriously affected the trees. That original, prescient, survey was conducted by Robert Ross, a scientist employed by the U.S. Geological Survey, who was studying habitat use by breeding birds in hemlock forests threatened by the hemlock woolly adelgid. Dr. Ross looked at 80 locations across 22 forest stands — 11 deciduous and 11 hemlock.

Mr. Toenies and his colleagues followed up by returning to the same forest stands in 2015-2016 to conduct vegetation and bird abundance surveys to compare how individual bird species and groups of species there were doing some fifteen years later.

“Using the unique approach of examining the same hemlock stands both before and after degradation by the hemlock woolly adelgid, we found that hemlock decline drove changes in vegetation structure and bird communities,” Mr. Toenies said. “Hemlock stands declined significantly since the pre-infestation baseline, with the severity of decline varying across stands.”

But a tree is a tree is a tree, right? Wrong, especially if you are a bird. Mr. Toenies and his collaborators found significant changes in the forests, including decreased live hemlock foliage in the canopy, a greater number of standing dead hemlocks, and a denser deciduous understory.

Further, as the vegetation structure of the forest changed, so too, did the bird community. Gone were the “hemlock specialists”, 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.

“As a group those five species showed a pretty strong decline,” Mr. Toenies said.

These hemlock specialists were replaced by generalists: the species that showed the greatest increases were those associated with dense shrubby undergrowth, such as the veery, Catharus fuscescens, and forest edge species, particularly the brown-headed cowbird, Molothrus ater, and the blue jay, Cyanocitta cristata. This is significant because blue jays are nest predators and brown-headed cowbirds are nest parasites, so both present increased challenges for bird species that are already struggling to survive and to breed successfully in these rapidly changing forests.

“Because we observed the strongest trends in groups associated with the shrub layer, woodland edge and the declining forest type, these changing features in dying forests may be most influential in shaping bird communities as die-offs progress.”

Mr. Toenies and his collaborators proposed a hypothesis to explain the progression of events that was captured by their data (Figure 1). As the hemlock woolly adelgid invaded and fed upon eastern hemlocks, the trees lost their needles and died. This, in turn, opened up intact forest canopies and increased sunlight, which led to dense, shrubby undergrowth. Hardwood tree growth accelerated in the overstory following their release from competing hemlocks. This more open forest canopy also allowed avian nest predators and brood parasites to access these hemlock specialists’ nests, adding to the growing list of threats faced by the declining forest-interior bird community, according to Mr. Toenies.

In the long term, replacement of hemlock stands by other vegetation communities, such as deciduous forests or communities dominated by nonnative plants, may lead to less diverse, more homogeneous bird communities across the area.

“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.”

“Some people think, ‘well, we lost some species, but we gained others, so did we break even?’” Mr. Toenies said, and then pointed out that it’s not that simple.

“With anything related to ecology, you don’t really understand the potentially far-reaching effects of taking pieces out of the system,” Mr. Toenies explained. “The interactions between species and their environments is so complex that you can’t take something out, like an entire species, and fully understand the effects of it.”

But there are some faint flickers of hope. Mr. Toenies and his collaborators found that the devastation wrought by the hemlock woolly adelgid was most prominent in large, dense stands of eastern hemlock — individual trees in small patches fared better.

And surprisingly, as Mr. Toenies and his collaborators discovered, even dying hemlocks are better than none at all for most hemlock specialist birds.

“We don’t necessarily have to have all these hemlocks in perfectly healthy conditions,” Mr. Toenies cautioned. “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.”

That said, it’s important to guard against invasive species in the future.

“There are more nonnatives and invasives coming through the door all the time,” Mr. Toenies 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.”

Is there any hope for recovering the eastern hemlock forests? Probably not.

“We cannot turn back the clock — we cannot un-introduce the hemlock woolly adelgid; but we absolutely possess the power to prevent this story from repeating itself