Why invasive species matter

Every one of us is familiar with invasive species. They are the bright yellow dandelions dotting our lawns, the startlingly green honeysuckles spilling over our roadsides, and the swarming spongy moth caterpillars prematurely emptying our forest’s canopy last summer. But what is it that makes these organisms so invasive in the first place?

Being an invasive species requires two things. First, the species must be nonnative, or from so far away that it only got here because people moved it. Second, the species must be invading, or causing harm to the place it has been moved to. People often disagree about how distant or harmful a species must be to truly count as invasive, but from an ecological perspective, a few nefarious culprits clearly stand out.

A healthy, well-functioning ecosystem typically has many diverse species, each of which consumes and competes for resources like food, water and sunlight, while also providing resources for something else. This competition for survival ensures that different species can coexist without any one of them becoming so plentiful that it displaces the rest. The issue with invasive species is that they sidestep this system.

Many invasive plants, for example, are adapted to different climates than our own, so they avoid competition with native plants for sunlight by holding onto their leaves later into the fall. That’s why, right now, most of the green leaves in our woods belong to Norway maples, Russian olives or Japanese barberries.

They also have the advantage of being unpalatable to our herbivores. Many native insects will only eat the specific native species they’ve evolved to digest, while even our voracious white-tailed deer refuse to munch on most invasive plants until they are on the brink of starvation.

Because of these inherent advantages, nothing stops invasive populations from getting too big. While this can be a problem in and of itself, as an ecologist, I find the most troubling invasive species to be those that facilitate the success of others. This domino effect can be disastrous for entire ecosystems, but it also provides rare opportunities to handle a whole set of invasive problems by focusing on just one species.

The quintessential example in Litchfield County is an aggressive vine called bittersweet (Celastrus orbiculatus). While native vines cause some damage to trees over time, bittersweet is far more destructive because of the way it climbs: Instead of relying on minuscule roots to clamber up tree bark, it winds repeatedly around the trunk like a constricting snake, eventually choking the tree out by slicing into its bark as it grows.

Because bittersweet is so efficient at killing trees of all sizes, it quickly diminishes forest canopies and stops disturbed areas from regrowing. With the tree cover reduced, rapidly growing invasive shrubs with few predators take their place. On riverbanks, bittersweet invasions facilitate Japanese knotweed, a bamboo-like plant that shades out competitors and dismantles erosion control; deeper in the forest, they make space for Japanese barberries, European privets and multifloral roses, which create boggy thickets that are difficult to navigate and optimal for ticks.

Luckily, consistently removing bittersweet vines from our trees effectively slows down the other sun-loving invaders, protecting both the forest’s overstory and understory. Unluckily, there’s another invasive cascade headed our way and this time, the culprit is one of the trees.

The deceptively named tree-of-heaven is an Asiatic, colony-forming tree with long, fern-like leaves and abundant clusters of stinky, off-white flowers. Popular for landscaping due to its low cost and quick growth, it now persists abundantly along our yards, roads and waterways. While its aggressive spread was already pesky, the issue has been made much more pressing by the recent introduction of the spotted lanternfly.

Spotted lanternflies, which gained fame recently by infesting most of Pennsylvania, are a sap-sucking insect from China. Their eating habits extensively damage many of our agricultural and native plants, from grapes and apples to maples and birches. However, while they will happily eat any of the above, they have a lot of trouble reproducing without their preferred host plant: the tree-of-heaven.

Where invasive tree-of-heaven populations are not already established, spotted lanternfly invasions usually cannot establish, either. Though the bugs’ population has yet to explode in Connecticut, we likely have enough tree-of-heaven now for them to cause devastation akin to the spongy moths soon.

Because our native ecosystem has no way to check the population growth of the lanternflies, the task instead falls to us. Some management options will be available once they arrive, but in the meantime, we can take important, preemptive steps toward limiting the damage by restricting tree-of-heaven spread and instead promoting the growth of our native forests. Take a look at what’s growing in your yards today: Our trees are counting on us!

Alison Robey is a volunteer at the Sharon Audubon Center and a second-year PhD student in Ecology and Evolutionary Biology at Yale University.