Invasive Weeds: Passengers or Drivers?


In this fascinating New York Times Magazine article,  Tom Christopher goes behind the scenes at a lab doing research on the relationship between weeds and global warming. They found a vacant lot in Baltimore that already had the higher temperatures and higher CO2 levels that scientists are predicting for the future. This became a test plot for what weeds might do in those conditions.   The results were pretty startling — giant weeds producing massive amounts of pollen.

But what really interested me was this bit near the end about exotic invasives:

Popular opinion has treated the invasive plants as botanical illegal aliens…Major resources have been devoted to the spraying and
rooting-out of invasive plants in the belief that their removal would
enable an ecological revival….

research, however, suggests that invasive species, at least in some
instances, aren’t so much the causes of environmental degradation as
eco-opportunists taking advantage of disturbed habitats. Or, as the
biologist Andrew MacDougall
of the University of Guelph, Ontario, puts
it, the invasives may behave more as “passengers” than as “drivers.”

Field research, he went on to say, has demonstrated that when invasives are removed, the native plants don’t necessarily grow back.  In some cases, their decline only accelerates.  The researchers

concluded that rather than serving as drivers of change, the foreign
grasses were functioning more in the role of passengers, merely filling
in as the natives disappeared. In fact, the foreigners seemed to be
serving a stabilizing role.

Could it be that the spread of invasive weeds  does not cause a decline in native plant populations, but merely happens after native plants have declined for other reasons, like climate change or habitat disruption?

And if so, does that change anything?  Does it let invasive exotics off the hook?

Discuss amongst yourselves.


  1. I’m not really sure how I feel about invasives, as a budding gardener (a mid-20s gardener at that!). I do know that all the weeds I find creeping into my yard from my neighbors yard are extremely annoying, but those aren’t exotic invasives. I know that I find my tiny plot of grass that isn’t grass – it’s clovers, creeping charlie, and violets – drives me a little insane. But again, those aren’t exotic invasives.

    So, really, I guess I just don’t have enough knowledge on this topic to discuss it!

  2. Wow. A simple idea that really makes sense. Now possibly I’ll stop feeling so guilty about lacking the strength to eradicte the purple loosestrife around my pond.

  3. Not from my own experiences over the last 25 years.

    Natives almost always “roar right back” when exotics are removed. The reappearance of a young prairie in a very few years is an amazing process.

    Exceptions may be when ecological restoration doesn’t take original condition in consideration (trying to restore a forest where there was prairie, or hill prairie where there was mesic prairie)…

    Climate change may change some details, but not the overall rules…

    Exotic invaders are often quite aggressive even in their original habitat (they’re the “weeds”), and so may indeed have a “leg up” on native plants. (particularly since the native fabric has been virtually obliterated, and aliens are wide-spread!)

    It is expected that, if conditions have changed (for local or global reasons), that aliens would do well when their seed bank is already well-established.

    There certainly is no innate reason for alien plants to out compete or replace native plants (other than the lack of evolved predators)…there are native plants evolved for virtually all environments too. I think that we’ve simply done so much damage to the system that it will be harder to restore native communities during such climatic changes.

  4. In the desert Southwest, my experience tells me that the worst invasive exotics (mostly African grasses) are drivers rather than passengers.

    Here, it is not just that invasive grasses such as buffel grass and fountain grass displace native species, but rather that they grow in much higher densities than natives creating fire prone landscapes (essentially new savannas) where there were none. Our signature plants (like saguaro cactus) aren’t adapted to fire. If we want to keep our charismatic megaflora, the weeds (or at least the most aggressive African grasses) have to go.

    I also suspect that weed control is central to re-establishing prairies. To quote the prairie plant master Neil Diboll, the first step to establishing a native prairie is to get rid of the invasive plants in a three step process: “kill, kill, kill; mow, mow, mow; burn, burn, burn.”

  5. Interesting research but I remain cautious about making blanket statements and grouping all invasives vs natives situations together. There are too many variables at work – nothing operates in a bubble – that will impact results: the health and viability of native plant seed banks, existence and number of pollinators remaining, soil changes (Scotch broom reportedly changes the soil chemistry) and restoration knowledge and success rates (user error) are just a few that come to mind.

    I know of two examples that contradict their findings.

    Wherever English ivy has been removed from Forest Park, the largest wooded city park in the nation, the natives suppressed by the ivy have rebounded amazingly well. “Ivy deserts” have become trillim oases.

    In less than 3 years, a healthy, undisturbed, diverse ecosystem near Oak Ridge became a near monoculture of butterfly bush, impacting the butterflies who relied on numerous host plants now supplanted by BB.

    We’ve just barely begun to understand the numerous implications and factors at work. I hope research continues, providing us with useful tools to know what to do, when and how. I would hate to see one result be that of surrender.

  6. I read the article too and found it fascinating, but it left me up in the air. I have been a promoter of native plants (not exclusively) but articles like this point out that there is a lot we don’t know about how the interconnectedness of all things works. The article is provoking a lot of thought in my house!

  7. I had to read this article twice because some of what Mr. Christopher made sense and some of it did not. I think the theory of “all natives in the gardens all the time” has not been successful for many people because the highly modified urban jungle often does not provide the conditions that work well for a native garden. So for that hot urban jungle, with its lousy fill dirt, plant what will grow–almost any biomass is better than nothing.

    Restoring native habitat might take a little more effort whether we are in cities or not. Start by removing the non-native invasives, and then build up the soil. Plan and plant for the microclimates and welcome appropriate natives–eventually they will make themselves at home. It’s important for butterflies and birds that have become dependent upon native plants and their resulting habitat.

    Yes, we may be able to plant species that were native a little farther south from us, but we can still have good habitat in a slightly warmer environment. The USDA 1990 planting zone map shows the line between zones 8 and 9 running between the living room and bedroom of our house, but the 2006 Arbor Day Foundation map shows that we are solidly in the middle of zone 9.

    The Audubon Society has documented a 68% decease in some of our most beloved bird species. One of the main reasons for this decline is reduced habitat. Native plants play an important part in bird-friendly yards.

  8. In one of our local restorations (right across the street!:), we had carpets of euonomous (Winter Creeper) strangling more than an acre.

    A winter application of herbicide (when euonomous is green, but little else is) removed most of the plant…although we still have quite a bit “mopping up” to do.

    The next summer, hundreds of Solomon’s Seal came up over much of the area…some even bloomed. If there are any natives left in an area, they are often just waiting for the chance!

  9. RE: Letting Invasive Exotics ‘Off The Hook’— I think there are other influences for my decision rather than if they are competitive, under current CO2 or higher concentrations: They already were doing better, perhaps because no predators lived in the area. For me, I’m not a big fan of extremely fast invasive vines because they often escape into local parks where they do alot of damage by killing trees and all other plants. In my garden, I would spend all my time with it alone (I already spend a huge amount of time on removing sprouting invasive vines).

    Nice piece.

  10. Something Lisa Albert said struck a chord with me. We have no control over what happens in forests (unless we try to take that on in some way) but we do have control in our yards.

    It’s the garden centers and plant stores that force us into “cookie cutter” boxes. Need ground cover? Buy English Ivy! Like butterflies? By buddleia! But the real answers are more complex than that. We need to be asking ourselves what kind of ground cover is appropriate for the soil and light conditions–not a one size fits all. If we want an outcome such as increased butterfly, bird and bee activity, we should find out what natives provide those things.

    And if we would like to feature non-natives in our garden, for whatever reason, what are the responsible, non-invasive ways to do that?

    I don’t know about you, but over the weekend, I concluded that there were some non-natives I cannot live without. Neither could the colonists, as it turned out: namely, rosemary, oregano and basil!

  11. I reread the thought-provoking article.

    Pollen explosions and more toxic poison ivy, oh my! Are the weeds secretly plotting the overthrow of mankind since we’ve treated the earth so poorly? Gee, this sounds like the plot line for a Science Fiction movie from the 50’s and 60’s (a genre I love for their campy, intentional or otherwise, plots and bad special effects).

    I love the idea of finding a profitable incentive to weed control as was suggested by converting Kudzu to biofuel. In the 80’s, I heard about a proposal to develop a drug from Kudzu that would aid in the treatment of alcoholism. I’ve not heard anything since (but I haven’t paid much attention since I no longer work in that field) but the idea struck me as a fantastic win-win situation. When life gives you lemons…. I hope these opportunities are researched more.

    Glad my comments struck a chord with you, suzq.

  12. Invasives are not off the hook at all. Many natives can become invasive as well. Invasive is relative. Invasive in the lawn, hedgerow or cornfield?

    Purple loostrife chokes everything out.
    However some moron decided to call eunoymous alatus “burning bush” as invasive. HUH the flame red fall foliage that grows 1 inch a year.

    Mind you some self taught experts call Berberis Thunbergi agressive and assume all Berberis are culprits……….not the case at all.

    THE(protecting my space)TROLL

  13. I often remember driving through a Portugese area in a neighboring city and seeing a patch of someone’s front yard completely covered with the biggest brightest patch of dandelions I have ever seen. They were obviously very well cared for. Fertilized and “weeded” for sure. Of course, almost all of the dandelion is edible and it makes for good soup and salad ingredients which may account for the “crop”. The big bright yellow “blooms” rivalved many exotic plantings.
    One man’s invasive weed is another’s exotic staple.

  14. Quoting Lisa:

    “However some moron decided to call eunoymous alatus “burning bush” as invasive.

    Mind you some self taught experts call Berberis Thunbergi agressive and assume all Berberis are culprits……….not the case at all.”

    Those morons and self taught experts were probably finding Burning Bush and Barberry far from the landscapes they were originally planted in.

    I personally pull every baby Burning bush and Barberry plant I find. I am not talking about in folks back yards either, I am talking about fields, forests, on the fringes of old abandoned homesteads, anywhere the darn things should not be.

    Take a walk, look through “moron” and “self taught expert” eyes, and I am sure you will find these plants where they do not belong, and if left unchecked, they will very likely choke out the native plants.

    I understand folks frustration about losing a beautiful landscape plant like the burning bush, but replace it with a highbush blueberry. The leaves will turn red in the fall and you get the added bonus of getting fruit.

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