Hi everyone! Is there a greater prevalence of invasive species in undisturbed landscapes or disturbed landscapes? I am thinking about the relationship between lawns/yards and why they seem to host so many more invasive species than elsewhere. Thank you!
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It seems to depend on where you are. Places like the desert seem to be able to âreclaimâ disturbed land given enough time. Here in coastal southern California that is definitely not always the case. Drive along the 1 through Malibu in the spring and on the hills you will see, alongside native coastal sage scrub habitat, lots of land covered in invasive fountain grass where the cliffs were disturbed to build the roads, or nonnative grasslands where a past wildfire was too intense, or entire fields full of highly invasive European mustards and wild radish that were previously cleared for some purpose.
We have so many invasive plants here that it seems like areas canât always recover on their own. It doesnât help that so much of the sage scrub and chaparral has been destroyed that there often isnât nearby habitat to repopulate a cleared lot unless youâre in the mountains. For example if you clear a lot here in urban Pasadena itâs likely no native plants will grow, I donât think Iâve found a wild-growing native plant in the city itself besides some very old engelmann oaks that probably predate the city.
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Lawns are a very unique environment in that the whole point is to have just the one grass growing. This makes it much easier to see anything else growing in a lawn as invasive.
Strictly speaking, many of the âinvasiveâ plants seen in lawns are just non-natives that do well in a well there, but wonât take over the lawn. A good example of a true invasive is Kudzu (Pueraria montana), which tends to overgrow everything (in Southern states in the US, anyway).
Disturbing soil does a few things:
- It opens up gaps in the soil for water to soak into instead of being a flat surface for water to run off of.
- It provides gaps for seeds to fall into where they can be out of the extreme temperature fluctuation of day and night.
- For some seeds, it breaks a protective outer layer allowing the seed to absorb water and sprout.
Therefore, disturbed soil is good for growing any plant. What makes invasives do well in disturbed soil is that they outcompete native plants in these conditions: they grow faster, prevent other plants from growing through allelopathy, reproduce faster, etc.
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Not talking about lawns, but of course disturbed landscapes will have more invasives, most of them are ruderals or near that state, so disturbed soil can often be the only place where they survive.
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I have an interesting âexperimentâ going on in my front yard. A couple of years ago, that area needed regrading to prevent water from flowing towards the foundation of the house, so it was disturbed by essentially removing the layer of top soil including the lawn that was one it. The contractors sowed grass seeds over it after they were done, but it didnât take very well (poor clay soil, sowed at beginning of summer, me being inconsistent with watering). I sowed a bag of white clover seeds over it when it got cooler/wetter during fall/spring, which did come up and covered it nicely for a year or two. And then I stopped mowing it except for once a year.
It has been fascinating to watch what comes up on its own. For the first couple of years, fleabane and hawkweed dominated in the spring and goldenrod and asters in the fall, so I tried to thin them out a little to make room for other species to have a chance to compete. This year Iâm getting a lot of evening primrose and wild carrot, so Iâve started pulling some of those as well. All of these have come in on their own from outside the yard rather than seeds from stuff I planted around the house.
By now it has formed a nice little ecosystem that I call my little frontyard prairie. Iâve gotten everything from invasives (which I try to hand-pull as much as possible) and a mix of plants producing seeds in my yard to all sorts of native volunteers and surprise appearances of native orchids, which I was particularly thrilled to see. Along with the flowers, there are mosses and native prairie grasses that just found their way in, which was nice to see. By now I get a few blackberries and tree saplings that I try to pull along with the more weedy stuff to keep the meadow nature. Other than that, I do basically no maintenance - no watering or fertilizer and this spring I didnât even mow. Itâs been eight years since I sowed the clover and there is very little, if any, evidence of the originally sown grass and clover seed left.
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The daunting invasives I see, are Australian wattles taking over the world after fire. Technically in relatively UNdisturbed land in Table Mountain National Park.
In the Swartland wheatfields, abandoned fields can be covered in Pattersonâs Curse.
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My understanding has always been that, generally speaking, disturbed areas will harbor more invasive species because removing or reducing native species takes away a lot of competition for resources. In that situation, many invasive species can establish & grow earlier or more rapidly than native species.
One particularly interesting example of invasives doing better in disturbed soils are the mustard plants (Brassicas) that have become ubiquitous in my area of Southern California. Unlike the vast majority of plants, which grow together with mycorrhizal fungi in the soil, they can thrive in extremely disturbed soils because they donât need the fungi (and may actively suppress it, making it harder for most native plants to establish later).
(https://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/flpmcar13137.pdf)
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Deserts can be quite varied, but the nature of them in Arizona, Utah, and New Mexico is such that disturbed sites fill quickly with Russian thistle (tumbleweeds) and that remains the dominant forb unless humans intervene.
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Kudzu seems to be a rather interesting case â according to Porcher & Raynerâs A Guide to the Wildflowers of South Carolina, the plant doesnât produce viable seeds in South Carolina. (I donât know about elsewhere.) I know that when I was down that way last year I expected to see a lot more of the stuff, based on extrapolating from what I remembered from when I was last in the Southeast. On the other hand, when it does become established on a siteâŚ
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In south Carolina there are forest that are covered with kudzu and it kills the trees.
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An article with some very good photographs of Kudzu taking over things.
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Probably disturbed land.
I get the impression that once you disturb land, youâre basically committed to continuing to disturb it. As Gheaton mentioned, probably different in each ecosystem, so donât put too much weight in my localized musings.
While many invasives can be out-shaded once a forest recovers, you would still want to pull those invaders up when you see them until it gets shady enough for the ecosystem to be more stable, as to not build up too many seeds in the seed bank for if a tree falls later. Donât want it coming back every time a tree falls. Mullein for example grows in clearcuts, but not so much in forest thatâs regrown.
Very few spready plants seem to do so in shaded, less disturbed areas in my region. Glossy buckthorn and false helleborine orchids, but not much else. Most seem to compete for sunny, disturbed areas moreso than the shady, less disturbed ones.
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Remember that not all nonnative species are invasives. There are plenty of nonnative plants that exist in gardens without really being present outside them, which are definitely not invasive. There are also plants that may become locally established, or even regionally established, without harming the local ecosystem. An organism is invasive if and only if it causes harm to native species. Otherwise itâs just a nonnative species, potentially a naturalized one.
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And sometimes native species can act invasively given right circumstances. We have that problem in our forest; there is no midstory because the former owner had horses & cattle, and even bushogged the forest floor so it looked all nice and open (we found photos from the 1990âs in the workshop showing it!). In his old age he started letting it go, so now there are about 10 yr of growth at oldest. But no midstory whatsoever. Muscadine vine is an issue, it covers the ground in some areas, choking out saplings and covering the young 2-5 year trees in a very kudzu-like fashion. We are trying to fight it back, cut it off the young trees, etc. In the little plots (10x10, etc) we got rid of it last summer, there are oak and hickory saplings this year. We will keep fighting it off until we get a healthier forest floor and a good midstory - as right now in a few areas it is literally preventing the forest from recovering. It isnât every where in the forest luckily, but the patches it is, itâs litterally acting like kudzu to the young trees.
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Would the cleared vines be something you could pass on to a goat farmer? Edible for browers?
Looks just like what happens here.:(
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Unless you go up to the high elevations and preserves closed to the public, Hawaiian rainforests are almost exclusively nonnative species â every level from the trees to the ground cover, as well as the fauna. With that said, they are biodiverse and seemingly self-sustaining ecosystems.
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Yes but no?
I suspect they would eat the saplings and young trees as well, knowing my friendâs goats xD
Oh wait do you mean like collect them and give them to goat farmer? maybe, but I donât know anyone who needs food for their goats - they move their goats around to keep their own areas trimmed haha. they are not the easiest to pull up because itâs spread like a groundcover, to be honest we are cutting them off the trees/saplings, and careful application of diluted roundup in the groundcover spots. It seems to work very well, the spots we did last year have healthy saplings this year, and the oaks and hickories we saved survived the careful application just fine. Itâs just slow and laborious.
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This was amazing to read about, you lived out the scenario Iâve been fantasizing about for a bit now. What if you removed all the plants from an area and just allowed natives to grow naturally, pulling up less-desirable plants like non-natives and the occasional overly weedy native? Not sowing seed, just seeing what nature does.
I would think the resulting ecosystem, once it has matured with a little human help, would be pretty resilient, and that seems to be true according to what youâve said. Such an unbelievably cool experiment! Letting nature itself plant your wild yard for you.
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The other issue in a lot of southern states (like mine) is privet. People plant it as a pretty evergreen bush that takes heavy trimming in their yard landscaping; but birds eat the berries and spread them far and wide. We loose a lot of nice forest to privet which chokes out the understory and groundcover. It is branching and herty, and once established, very hard to remove. A lot of invasives have a hard time on rocky soil and limestone outcroppings, but not privet. One of the last wonderful limestone cedar glades (very specific habitat with an array of rare & some endangered plants) the nature conservancy seems to have abandoned care for enough that it is almost all privet and very little cedar left. The privet is encroaching into the rest of the open glade as well. All it takes is a seed in a crack of the limestone.
Princess Trees and Persian Silk Trees are another pretty yard runaway which is less prolific than privet but still horrible; and it is choking out native understory trees.
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