Tuesday, August 21, 2012

Weed? Alien? Invasive? Malignant?

Definitions can change the way we see the world – and how we act.

What is a Weed?

Weeds in nature are valuable plants that deserve respect. Be clear that the ecological definition of “weed” is different from the gardener’s definition.

In this photo by Mark Baldwin, the black-eyed Susan is a native weed, and the blue chicory is an alien weed. They’re both good.

A weed patch in the ecosystem often functions much like a scab on a mild wound that you or I might suffer. Weeds are an ecosystem’s response to degradation or disturbance. The scab helps the wound heal. Weeds help quality ecosystems heal. They prevent erosion and start a succession process that is likely to end with the conservative plant species that were there before the (mild and temporary) wound.

Weeds are often annual or biennial. Classic native prairie weeds include ragweed, black-eyed Susan, and common evening primrose. These days, the “healing scab” species may also include weedy aliens like wild carrot, bull thistle and chicory. 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 simple classic weed goes away with time and doesn’t leave a scar.

Many people worry needlessly (and sometimes counterproductively) about weeds in restoration areas. Relax. Welcome them. Some people say authoritatively, “Oh, yes, wild carrot is a problem. I’ve seen it be very invasive.” But they haven’t. They’ve seen it become very common. That’s just a phase, and a step forward for the ecosystem. All these species are easily outcompeted by more conservative prairie species within a few years.

Spraying herbicide on wild carrot or bull thistle may kill it. But that same herbicide will also likely kill young plants of more quality that would otherwise have soon out-competed the weed. In response to herbicide, the same weeds – or possibly more damaging invasives – may well fill the herbicide-created void, instead of the young prairie plant that otherwise would have been thriving in a year or two. Even pulling harmless weeds may slow down the restoration, because it takes time away from work that would actually do some good.

What is Invasive?

Most weeds do not lead to the damage that an invasive does. In the early days of ecosystem restoration, we often used the word “alien” as the standard word to describe species that degraded ecosystems. At first, that word seemed to communicate well, as many people quickly supported the need for remedies when it was used. “Alien” was partly accurate, since most of the problem species then recognized were from other countries. Many members of the general public (who were basically too busy or not interested enough in ecosystems to spend a lot of time learning about problem plants) seemed to feel like they understood what we were talking about through the metaphor that mixed the fun fear of space aliens with the now-politically-incorrect-in-progressive-circles concern about “outsiders.” 

But that word failed 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 totally unrelated Republican-Democrat arguments, especially as America became increasingly divided over the “undocumented.”

More important, it became increasingly clear the “native” and “alien” wasn’t really the issue. Most non-native species were no problem. And many native species were very much so, in the modern context. Unburned prairies died in the shade of native gray dogwood or green ash as surely as they did 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. It described the real problem better (but not good enough?).

I remember reviewing a draft policy of the Carter administration that would discourage non-natives and encourage natives in government projects. I commented that “invasive” was a better word, and explained. Apparently someone in that administration was convinced. When the official policy was published, “alien” was edited out and “invasive” replaced it. Does our system actually work!?! Perhaps many people made that recommendation. The new language also continued in some later administrations’ policies.

And, yet, while a great improvement, “invasive” was not itself 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. Yet they could be lethal to the basic ecosystem. Similarly with animals, overabundant meso-predators can seriously damage systems they were a part of, in the absence of large predators.

I began to recommend using “in balance” and “out of balance,” but many people found the “balance of nature” concept also “out of date” and misleading.

Benign and Malignant

Increasingly, many 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. The parallels are substantial.

In this metaphor, the “problem species” is the microbe or the cancer cell. Just as a formerly perfectly respectable cell from my own body can start multiplying uncontrollably and kill me, just so, a formerly “in balance” member of a community can reproduce out of control and cause the lost of most species. That’s ill health, from a 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 and wild black cherries are constituents of oak woodland. Natural fire keeps them in check. But in the absence of fire, those species 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 drive out most species of wildflowers, shrubs and saplings from a savanna or woodland; they can also seriously deplete many bird species. They’re native, beautiful animals and have rights, but they also may have a malignant impact in the absence of equally admirable predators. As Leopold’s Land Ethic makes clear, ecosystems deserve our respect and affection – and they also have rights.

Conservation requires public support. Thus, we need good words to describe problems and solutions to people who care about nature but will not have time to study intensively.  


orchidartist said...

Thank you for adding new helpful vocabulary to a work where words can do as much good as loppers!

Daniel Suarez said...

Thinking of the ecosystem as a human body with benign and malignant components is definitely helpful. Ecosystems should be thought of holistically, based on the immensely long scales of time that they fluctuate on. For me, "alien" sounds outdated and, in these times, politically charged. Rather than thinking of an ecosystem as a monolith with only good and bad parts, it makes sense to acknowledge the ever-changing relationship between the hundreds and thousands of elements constantly shaping a place. Just like eating fast-food, or smoking, or whatever else, might eventually kill one person, the very next person could show no adverse reactions. Likewise, in an ecosystem, one plant can become malignant in one setting but not in another. Is there any indication why this occurs?

On another note, I would like to learn more about the historical place of native and naturalized weeds in our natural areas. In the pre-European "virgin" prairie, when would one encounter ragweed, black-eyed susan, or common evening primrose? If areas were left undisturbed, how would one of these weeds take advantage? They must not have had a huge seed bank in the soil if they were only appearing in bits and pieces. Perhaps we don't fully know, but it is interesting to think about.

Stephen Packard said...

Daniel, let me second your thoughts on variation. A species that causes problems in one region might be benign in another.

Even for sites near each other, a plant or animal with malignant impact under an infrequent burn regime might be no threat at all with frequent burning -- and visa versa. The same could be said about high and low deer populations. I once caged a declining population of Michigan lily experimentally to see how it would do if the deer stopped eating it. Result: the cage filled entirely with gray dogwood (which the deer had also been eating), and the lily was lost entirely. As you write, these systems are very complex.

As for weeds in the original prairie and woods: they might be found around animal burrows, tree falls (especially where the roots tear up the soil), where logs burn, around landslides, where river ice scrapes away soil along river edges, and where animals concentrate for any reason.