Research on the Impact of Invasives may Surprise You


PennState The importance of eliminating invasive plants, which has long been considered settled dogma, is unsettling itself before our very eyes. 

Harvard's Peter del Tredici is making waves among scientists and designers, though maybe not yet with ecologists, with his book Wild Urban Plants, briefly reviewed here.   He breaks the mold by looking not at a plant's history but at the ecological services it's currently providing in developed landscapes – like cites.

And more recently, Penn State's report on the research results of biologist Tomas Carlo – titled "Invasive Plants Can Create Positive Ecological Change" – is getting lots of attention.

"Among conservation biologists, ecologists, and managers, the default approach is to try to eliminate and root out non-native, invasive shrubs — anything that seems to change an ecosystem," Carlo said.  "But the problem is that most native communities already have been changed beyond recognition by humans, and many native species are now rare."

Carlo explained that his team wanted to test whether certain well-established, invasive fruiting species have negative or positive effects on bird and fruiting-plant communities.

When he compared test sites with and without an abundance of Japanese honeysuckle, the honeysuckle proved to be a boon to fruit-eating birds.

They determined that the abundance of honeysuckle predicted the numbers and diversity of birds within the region and even beyond the region. That is, the honeysuckle and bird communities had formed a relationship known as mutualism — a term that describes how two or more species interact by benefiting mutually from each other's existence.

Carlo argues that it's not only expensive to try to remove invasive plants like honeysuckle – and keep them removed – but it can actually harm the newly formed balance of an ecosystem. 

His big take-away? That instead of assuming that introduced species are inherently harmful we should ask: "Are we responding to real threats to nature or to our cultural perception and scientific bias?" 

I can just imagine the heated arguments going on in faculty lounges between biologists and ecologists.   

Hat tip to Tony Avent and his February Plant Delights newsletter. Photo credit: Tomas Carlo, Penn State.


  1. Hurray hurray! Sounds like real scientists – the ones still practicing old-fashioned skepticism- are looking at what all the invasives hysteria is doing to habitats and horticulture – more science please!

  2. Cyndi,

    “Real scientists” have been studying the impact of invasive plants for many years. That’s how we know just how bad they are, ecologically speaking.

    Tossing around loaded phrases like “dogma” and “hysteria” does no one any favors. Different people have different value systems, and I think that is just fine.

    But please don’t be fooled by anyone who is trying to convince you that the anti-invasive folks are uninformed or on shaky ground scientifically. Narrowly focused research, like that done by Carlo et al., is helpful but not particularly earth-shaking.

    Will some fruit-eating birds eat fruit from some invasive plants when nothing else is available? Yes, Carlo found, they will.

    That doesn’t mean that invasive plants are “good” or “beneficial” in any broad sense. It only means that habitat invaded by, say, honeysuckle is more productive than habitat invaded by, say, asphalt. Garden Rant may cheer that finding, but I say “duh”.

  3. I love this too. Finally, taking the whole ecosystem and all inhabitants and their homes (cities, too, yes!) into account. Here in Colorado I have long been skeptical that some weeds on the “noxious weed list” have much more to do with that plant invading grazing areas for cattle, and effecting that industry, than anything else. About money, not biology. And people are required to spray pesticides in these areas or have it done for them (and be billed).

  4. VV is correct, there’s a bias in such research, especially when the narrow focus is on big conspicous furry-feathery organisms that human like. In the prairie patches I study, silky bush clover forms dense clumps that crowd out many lower growing native plants. While dickey birds love nesting in the clover, all of the species, and in this case mostly invertebrates, that depend upon all those other species suffer. So it puzzles me why gardeners seem so delighted by such misleading research.

  5. I think this makes a critically important point: the difference between non-native plants in natural ecosystems, and those in urban areas. There is no such thing as a natural urban ecosystem — urban spaces are entirely artificial environments, and I think we really need to start thinking about how to shape those ecosystems to be as healthy as possible rather than getting in arguments about natives. NOTHING is native to cities.

  6. Years ago, I read that one of the reasons why Japanese honeysuckle shrubs are so bad is that they make songbirds more vulnerable. The shrubs provide plenty of food, but their branching habit make it easy for predators to catch birds. Songbirds eat or nest in the honeysuckle and the predators climb up and eat the birds and/or eggs.

    My point is that studying honeysuckle as a food provider might make it sound positive. But studying it in terms of predator-prey might make it negative.

  7. For those of you that seem to think plant species not native to the area you live in, aka USA, do not cause real ecological problems I suggest reading the following articles from Dave’s Garden this week which focused on the reality of certain plants being invasive period; and that non native invasive plants do cause serious issues. I am surprised that garden rant and it’s readers consider this to be a non reality and just a “rant” of some ecologically minded scientists and researchers.

  8. I feel that a plant should be valued on how it presents in the environment. Our climates are changing. However, just because a plant is beneficial to the birds, what about the rest of the wildlife? Birds are a part of it, but not the total of where we should be examining what we are creating.

  9. Quite a thought-provoking post, GardenRant! All the complexity and our lack of understanding of ecosystem interactions with the additional aspect of our biases and preconceptions thrown in to muddy the waters. We sometimes don’t know what we think we know and other times we don’t know what we know we know.

  10. Thanks Rant for bringing us to this obviously controversial research. I make a few assumptions when I read these new findings:
    1. The ecology is not a stable phenomenon; it is changing every year with weather, man’s influence, etc. What was “normal” 100 years ago is completely irrelevant now.
    2. Not only has our economy gone global, but our community has gone global, and guess what our ecological system has gone global. We cannot stop that process as much as we might like to.
    3. If a non-native plant is harmful then it should be controlled if possible. It is not harmful just because it is not native. Most of the most popular landscape plants in this country are not native; they are just not usually invasive and harmful.
    4. Our world is more complex than it was ten years ago and I’m very sure it will be far more complex 10 years from now; we need to accept that reality and live within its stresses. Nothing is simple in the ecosystem.

  11. Perhaps the lesson to be absorbed here is that nature hates an imbalance as much as a vacuum. Once we humans have messed things up, which is our nature, nature has the time and wherewithal to reset the balance far better than any human land managers with all their science know how. Nature doesn’t linger on the past. It moves forward. Have faith in nature’s abilities.

    We can use what we do know to create healthy diverse environments in the gardens we tend to give nature more choices on the menu. That includes native plants.

    But at a certain point it is futile to cry over centuries old spilt milk. We are not in charge and never were.

  12. Quite convincing statistics exist that explain why climate change is not the result of human impact on the planet. From what I can see, more statistics seem to exist that prove the opposite.

    I prefer to err on the side of caution and stay away from invasives, and I wish there were regulations that disallowed their sale. Sure, it’s all complicated, but why do we have to have water hyacinths clogging streams and lakes and pampas grass instead of California coastal chaparral, all in the name of free enterprise and free choice?

    This all reminds me a bit of the arguments of the benefits of smoking, too. There were many, and then we learned better and it was too late for those who died of the misguided science.

  13. I doubt any native plant lover seriously thinks that the goal is to remove all non-native plants. I think the goal is merely to help our native plants survive the fierce resource competition non-native invasives present long enough for balance to be restored to the ecosystem. In time, nature will notice the giant stands of Japanese Honeysuckle and some grub, worm, beetle or other predator will begin to help control the population. The invasive population will be weakened and that will once again give our natives a competitive toehold. My only goal in planting natives is to help them survive until such a time.

  14. Common sense will prevail………..All w hearare the doom and gloom of invasives, climate change etc etc. But now they have all run out of amunition and common sense has reurned to roost.

    And at Daisey: So what a guy named Dave writes ablog about dangeorous invasives. Are we to beieve him just because he wrote it?

    The TROLL

  15. For a totally different point of view and info about native insects and invasive plants, check out Doug Tallamy’s ‘Bringing Nature Home,” by Timber Press.

  16. Although I’d like to read a lot more on the subject, my gut feeling is that this makes a lot of sense and I don’t understand why anyone should be surprised. Darwin was right. Nature has an incredible ability to adapt and change is the mainstay of life, not its enemy. That’s not an excuse for rampant overdevelopment and habitat loss and the rate of species extinction should make us concerned, but I’m hoping that this kind of research will lead us to better decision-making and stop us from misguided efforts to turn back the clock.

  17. The genie is out of the bottle.
    Hardly anything is accomplished on principle alone.
    Balance is one thing.
    Taking advantage of an obvious imbalance is another.
    I hope we all find balance in our own gardens and leave the imbalances to the other side of the fence.

  18. I have spent the last year trying to understand biodiversity issues.The huge amount of research available is full of information regarding invasives and the way they limit biodiversity within ecosystems. From microorganisms and repilcating genes to plants, animals and the very habitats themselves diversity is of paramount importance to continued function over the long haul.
    An invasive takes temporary advantage of disturbance and does perform a temporary service of its own but may do much damage to the larger picture.

  19. This issue seems to be more and more complicated. I thought that one of the isssues is that even fruit eating birds need bugs to raise their young and that insects need native plants. Just as pollinators need larval and nectar plants, so do birds need certain foods at certain times.

  20. To tibs,

    Evidently you don’t know what “Dave’s Garden” is. It is NOT a blog. It is a wonderful website with incredible information from gardeners of all levels, from beginners to experts, from all over the world. It is an incredible resource about plants and everything to do with plants. Personal feedback about experiences gardeners have had with online plant and seed stores/nurseries to help other consumers make wise decisions about who they will deal with.

    There are articles written on all types of topics from members who either do the research or have first hand experience about what they are writing about. Other members respond with their experience. If you had taken time to read the links I gave, you would have found some first hand experiences of how non natives do change our USA environment and do kill out native species that wildlife depend on for food and shelter. It was an offer to show that there are real concrete realities about what invasive plants can do. Too bad some people are so closed minded to judge something without even knowing what it is.

  21. Hey, the planet would be unrecognizable to someone from 1900, and in 2100, the same thing would happen to someone of today. We area cruel, solpisitic species who never think before they leap. That’s ok for our adaptability–and I guess it seems ok for some birds’ adaptability, too.

  22. Whoa Daisy, Not sure how you took my agreeing with the statement “Are we responding to real threats to nature or to our cultural perception and scientific bias?” slams anything on Dave’s Garden or whether it is a blog or not. Humans apply bias to every decision we make.

  23. Daisy was responding to The Troll. Sometimes people accidentally take the above name for the comment below. It is a bit sad The Troll has never heard of Dave’s Garden. He should go there and see if his nursery has been reviewed.

  24. The Wisconsin DNR has come out with NR-40 listing many plants as invasive, and putting it out that they will get a warrant to remove them from private property and charge the property owner for doing so.

    They are proposing a second round of plants for inclusion on their hit list that includes burning bush, orange daylilies, ground cover euonymus, lysimachia nummeralia, willow herb, squill, Purple Robe locust, black locust, privet, barberry, etc. It is a pretty long list.

    Yes in the wrong place some plants are thugs, but the enforcement aspects of this rule have me shaken, not only the inclusion of some great plants.

    Can you image finding your state Dept. of Natural Resources at your door demanding you cut down your 100-year old burning bush (which now is more of a tree, or that you rip our your hedge that took five years to establish?

  25. Rachelle,

    Orange Daylilies are you serious? is it a “violation of color?” I love daylilies I planted all kinds this spring of many different colors. So are they saying you can have daylilies as long as there not orange colored?

  26. Rachelle,

    Wisconsin’s NR 40 regulations do not work precisely as you suggest. Invasive plants are categorized, according to commonality and impact. Some plants are “prohibited”, some are “restricted”, and others are merely labeled as “cautionary”.

    Plants that are already common in the state and/or are not especially harmful are not “prohibited” and will not prompt DNR to show up “at your door” with a warrant.

    The prohibited list currently has plants like Porcelain berry and Giant knotweed. Not exactly the same as your privet hedge.

  27. Oh, puh-leeeze. *Two* species of generalist birds can eat the berries, so suddenly we don’t need to worry about invasives and those of us whose spring gardening chores involve tearing out honeysuckle for days on end, lest we be buried, are being hysterical?

    Give me a break. Bet there aren’t any bugs living on it, and bugs do a lot of the heavy lifting in an ecosystem. Honeysuckle-covered areas may not be ecologically dead, but that sure doesn’t make it the healthy functioning ecosystem I want to live in.

    If the only birds in the garden are robins and mockingbirds, you’re doin’ in it wrong.

  28. Correction to my post of 3/9. It was a reply to a response by “the troll”, Greg Draiss, of 3/7 not to tibs. I just realized how they have the author of each post beneath the post.

  29. Non-native invasives limit biodiversity because native insects depend on native plants for the continuation of their lifecycles.Generally the invasives are useless to the insects. This is really the issue. (I guess everyone forgets them because they’re small, or just general aversion.)All species within an ecosystem are interdependent, but there are certain cornerstone species that support more than others. (Think of that really outgoing friend you have, the one that’s always helping people out.)These cornerstones vary by community, just like in people, but it’s often insects playing that role, because they are food for many birds, fish, and small mammals, and because they are the main pollinators of plants. Perhaps the simplest way to picture the problem is in terms of vast time. The plants and animals of a particular area have evolved together over long long periods of time, changing, of course, but maintaining a balance, and now, because human interference is moving at lightning speed, many species are not able to adapt quickly enough. Without one species, several others fall. Without a cornerstone, many others fall. Sure, life will balance itself once again,but at it’s preferred pace. Even though we get our food wrapped in plastic from the supermarket, we humans still depend on ecosystems for our existence. I think we should be alarmed, which means we should be informed. And learn to appreciate the insects for all they do.

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