Weed? Alien? Invasive? Malignant?

If you're trying to restore health to an ecosystem, such "alien species" as Japanese thicket parsley, oxeye daisy, and Queen Anne's lace are probably not worth your time. 

In contrast, "alien" teasel and reed canary grass in prairies and "native" maple in oak woods can destroy the entire ecosystem.

What is a Weed?

Weeds in nature are valuable plants that deserve respect. Of course, the ecological definition of “weed” is different from the traditional gardener’s definition.

A weed patch in the ecosystem often functions like a scab on a mild wound that you or I might suffer. Weeds are an ecosystem’s response to hurt, degradation, or disturbance. In healthy ecosystems, weeds make up the scab that helps the wound heal. They prevent erosion and start a succession process that historically ended up with the conservative plant species that were there before the wound.

Weeds are often annual or biennial. Classic prairie weeds include black-eyed Susan, ragweed, and common evening primrose. These days, the “healing scab” species may also include aliens like wild carrot and bull thistle. These “alien” species are long naturalized to North America and function like the native weeds. They should not be seen as problem species. A true weed goes away with time (and doesn’t even leave a scar if the disturbance is mild).

Many people worry needlessly (and sometimes counterproductively) about weeds in restoration areas. Relax. Welcome them. Don’t believe the people who say, “Oh, yes, carrot is a problem. I’ve seen it be very invasive.” No they haven’t. They’ve seen it become very common. That’s just a phase, and a step forward for the ecosystem. Relax. You have more important things to do. Weed species are easily outcompeted by more conservative species within a few years – if conservative plants are nearby (or have been inter-seeded).

Spraying herbicide on wild carrot or bull thistle may kill it. But that same herbicide will also likely kill the desirable young plants that would otherwise have out-competed the weeds. In response to herbicide, the same weeds – or possibly more damaging invasives – may well fill the herbicide-created void, instead of the young native plant that otherwise would have been thriving in a year or three. (Of course, in a demonstration area or wild garden, a person may choose to pull unsightly weeds (or allergy-provoking ragweeds) without waiting years for succession.)

What is Invasive?

In the early days of ecosystem restoration, we often used the word “alien” as the standard word to describe species that degraded ecosystems. But the word alien became problematic for two reasons. The first was that some people increasingly used it as an opportunity to argue politics. Attempts at scientific discussion were regularly hampered by unrelated Republican-Democrat arguments, especially as America divided over the “undocumented.”

More importantly, it became clear the “native” vs. “alien” wasn’t really the issue. Most non-native species were not a problem. And, in the modern context, many native species were very much so. Unburned prairies died in the shade of native gray dogwood or green ash as surely as they would have from the shade of alien buckthorn.

Many of us started using the words “invader” and “invasive” for what we once called “weeds” or “aliens.” This approach worked better, and, yet, while an improvement, “invasive” was not really quite the right word.

In savannas, although gray dogwood/box elder/sumac could indeed degrade the system in the absence of fire, they did not “invade” from outside. They were natural components that could become lethal to the basic ecosystem under some conditions. There’s a parallel with animals. Zoologists wondered why some species of turtles, snakes, and ground-nesting birds were disappearing from small preserves. It turned out that overabundant meso-predators (like raccoons and opossums) were seriously depleting the natural diversity of the ecosystems they’d long been part of – in the absence of large predators (like wolves, that keep raccoon numbers in balance).

Benign and Malignant

Some of us have begun to compare the health of the ecosystem to the health of the human body. It’s a fairly easy model for most people to understand.

In this metaphor, the problem species is like disease bacteria or cancer cell. Just as a perfectly respectable cell from my own body can start multiplying uncontrollably and kill me, just so, a formerly “in balance” member of an ecosystem can reproduce out-of-control and cause the loss of most species. That’s ill health, from a natural community perspective.

In the absence of fire, maples can invade a bur oak woods and wipe out most of the natural diversity of animals and plants. Okay – it seems fair to call that maple an invader or “invasive” in that scenario. But box elders, ash, and cherry can do it too. When “in balance,” they can be constituents of oak woodland. Natural fire keeps them in check. But in the absence of fire, those species can create a malignancy of shade. How should we refer to them in that case? They didn’t invade. Perhaps “out of balance” and “malignant” are the best words our language has.

Native white-tailed deer can erase most species of wildflowers, shrubs and saplings from a savanna or woodland; they can also seriously deplete many bird species in this process. They’re ‘non-invasive’ beautiful animals and have rights, but they also may have a malignant impact in the absence of equally admirable predators.

Conservation requires public support. Thus, we need clear language to describe problems and solutions. Most people care about nature but have only so much time (or inclination) to study intensively. Those few of us who enjoy the detail and the mission will want to use the words that will best communicate to the many who rightly take their cues from us.

Malignant Species we combat in the Somme Preserves

Asian honeysuckles

Bird's-foot trefoil

Common buckthorn

Common reed

Crown vetch

Gray dogwood (problematic in prairies but not so much in savannas)

Lesser celandine

Purple loosestrife

Sugar maple

White sweet clover