by Dr. Ryan McEwan
April 5, 2022
Amur honeysuckle (Lonicera maackii) is a shrub that was brought to the United States from Asia. This species was originally envisioned as a useful species for wildlife and, in fact, there was a soil conservation service breeding program to improve this species for use in projects in North America! Unfortunately, this species moved into natural areas aggressively and became one of the most important invasive species in eastern North America. Amur honeysuckle invasion into forests is a serious problem for land managers; however, we noticed that streams meander through many of these forests and we found that Amur honeysuckle is extremely successful along these headwater streams. Here we wanted to provide a synopsis of a decade of research in the McEwan Lab on the ecology of Amur honeysuckle invasion along headwater streams in southwestern Ohio.
Amur honeysuckle fundamentally changes the structure of forests along stream banks.
In areas around Dayton, Ohio, the honeysuckle will create a very dense invasion and sometimes arch over the stream and create almost a “cave” effect.
Here is a picture of a water engineer doing research on honeysuckle and you can see the arching canopy of honeysuckle spanning the stream above him. This invasion creates a dense physical structure which casts a lot of shade on the stream and we found that this structure may also be trapping materials like branches and trees and preventing woody materials, that are important habitat for aquatic organisms, from entering the stream (LINK). There are no native plants, at least in Ohio, that create a structure like this and so the system is changed due to the invasion of honeysuckle.
Here is a picture taken by Dr. Rae McNeish, who did a lot of research on honeysuckle (she is now a professor in California) that shows an Ohio stream in winter with the over-arching branches of honeysuckle creating a dense structure.
Amur honeysuckle contributes materials to streams that are toxic to some organisms.
The dense growth of honeysuckle over the stream is also associated with deposition of a LOT of materials into the stream. Flowers, fruits and leaves are deposited into the water. We discovered that these materials may be toxic to some plants (LINK) and that led us to wonder what is happening in the streams. After many experiments, we discovered that some of these materials are toxic to some aquatic organisms. For example, we put experimental leaf packs in the stream and checked what organisms colonize the leaves and found a unique collection of organisms colonize the honeysuckle leaf packs (LINK). We also put aquatic organisms in little experimental jars (“microcosms) with tea made from honeysuckle leaves and found that at higher concentrations of honeysuckle leaves, these organisms died (LINK). We also put little jars into the stream so that water could flow through, and then put organisms in the jars with honeysuckle fruits and flowers and found evidence of toxic effects (LINK). In an ironic twist, we also found that honeysuckle materials might encourage mosquito larvae (LINK)!?!
Here is a picture of honeysuckle fruit in the bottom of a stream, they may create toxic conditions for some aquatic organisms.
Here is a picture of a stream where, very late in the fall or early winter, honeysuckle has dropped its leaves. This is a lot of leaf material, and especially when the streams are running low and there are stagnant places, our evidence suggest this could create toxic conditions for some aquatic organisms.
Amur honeysuckle invasion changes the aquatic community in fundamental ways
When you put it all together, we have relatively strong evidence that honeysuckle influences the aquatic community in headwater streams, and specifically that more sensitive organisms are less successful where honeysuckle is growing in dense stands along the stream (LINK).
Here is what we think is happening overall. Amur honeysuckle is creating a unique structure on the stream banks and over the stream, and depositing a lot of unique (and potentially toxic) materials into the stream. These materials are really important to aquatic organisms and the composition of that community is changed by the ecological invasion of honeysuckle. This change in aquatic organisms has strong potential to change other aspects of the aquatic (and terrestrial!) food web.
One thing to be careful about – our evidence does not indicate that honeysuckle is a stronger effect than intense human disturbances. For example, our data do not support the idea that honeysuckle is having a worse effect than having a lot of paved areas next to the stream, or deposition from agricultural fields, or salt from roadways, or any number of other insults to nature brought about by human commerce! Please understand: our research really focused on how honeysuckle influenced streams so we were really careful to minimize variation in other aspects of the stream biology. We tried very hard to find streams that were similar in every other way except honeysuckle invasion. In that situation, we have good scientific evidence that honeysuckle invasion causes important changes to the aquatic biology.
Restoration efforts, even at smaller scales, seem to create a positive response in stream organisms
Some good news…in an experiment in which we removed honeysuckle from a stream, we found that even removal along a relatively short portion of a stream can made a difference! Honeysuckle removal created a measurable reaction in the aquatic organisms even when the removal occurred along a relatively short portion of a stream that still had honeysuckle upstream (LINK).
If you are thinking about doing honeysuckle removal to improve the stream in your local natural area, one thing we really want to emphasize is that honeysuckle prevents native plants from growing well. This means that under the honeysuckle what you often have is mud!
Here is a picture of a stream under a dense invasion of honeysuckle- even in the middle of summer it is a mud mess because honeysuckle has suppressed all the native plants.
If you remove honeysuckle and simply leave the stream bank exposed with no vegetation present there is a strong possibility of causing significant soil erosion into the stream. Therefore, we recommend that you think very carefully about what plants you want to have in place prior to ever starting any kind of honeysuckle removal effort. Ask yourself or your team: “What plants are going to be there to hold the niche once we remove the honeysuckle?” If you and/or your team cannot answer that question, then you might be better off not removing honeysuckle at all!
Figure out what plants you will use to hold the niche before you do any removal!
If you are wondering what plants you might want to replace honeysuckle, two really good choices in central Ohio are:
pawpaw (Asimina triloba) – LINK
spicebush (Lindera benzoin) – LINK
We also recommend contacting your local conservation service or metropark to inquire as to their recommendations for removal techniques and also replacement plants. In Ohio, the Ohio Invasive Plants Council has information about removal and replacement that might be a good place to start: LINK
On March 25, 2022 as one of the keynote talks during the Ohio Rivers Symposium, I gave an overview of our research. A recording of the talk is here:
A list of publications from the McEwan Lab (papers, presentations, and other materials) focused on the effect of Amur honeysuckle on headwater streams can be found here:
An archive of all the data from our projects is found here:
A scientific review article about Amur honeysuckle
A link to download a comprehensive scientific review on the biology of Amur honeysuckle that our lab produced is found here:
work was funded by the National Science Foundation (DEB: 1352995). An abstract of the grant is found here: (Project Abstract).
Funding was also provided by the University of Dayton Graduate School- through Graduate Student Summer Fellowships to multiple students, and through the University of Dayton Honors Program through funding of multiple undergraduates who worked in the lab through the years.
Funding was also provided by a Sigma Xi Grant-in-aid of Research (GIAR) to Rae McNeish.
Funding was also provided by the Ohio Invasive Plants Council research grant to Rae McNeish.
This work would not have been possible without wonderful partnerships with many local and regional land management agencies including Aullwood Audubon Center and Farm (Aullwood Reference Stream), the Miami County Park District (Charleston Fall Moderate Site!) and many other who allowed us to visit or work in their natural areas. We specifically want to offer our thanks to the Centerville-Washington Park District. Specifically, for allowing us to conduct research in Black Oak Park, where Dr. Rae McNeish conducted many studies.
We also are extremely grateful to the Five Rivers Metroparks who allowed us to conduct research in many of their parks over the years.
We look forward to continuing to partner with these, and other, agencies into the future!
We hope you found this research synopsis useful!