Kudzu vs. Honeysuckle: Helping the "little guy" win

Competition, it’s something that many of us have experienced in one way or another in our lives. Whether it was for a little league tournament, video games, or a family game night, competition is something that everyone has experienced in one way or another in their lives. But competition is not a solely human experience. This phenomenon is something we see everywhere in the natural world. Lions fighting hyenas for food, gorillas competing for territory, or plants battling it out for water and light. These are just a tiny few in a seemingly endless list of competitive interactions we see in nature.

However, by this point, I think it’s clear that I will not be discussing lions or gorillas in this blog, but rather the intricate world of competition amongst plants. One characteristic of invasive species of any flora or fauna is its competitive advantage over native inhabitants. So, in this blog I will dive into the competitive advantages of the widely spread invasives, kudzu and honeysuckle, as well as the consequences of this competition on the ecosystem as a whole.

Competitive advantages

Both honeysuckle and kudzu are known to be highly successful invasives in the US, as is evident from their widespread range. This invasiveness can be caused by numerous factors, however one of the key differences between native species and invasives is the ability to out-compete the other. Honeysuckle, for instance, has been shown to bud earlier, hold onto its leaves longer in the fall, and hold-up better during freeze-snaps in springtime than its potential native competitors (McEwan et al, 2009). The three factors basically mean that honeysuckle has a longer period of time with leaves present than its neighbors and therefore can out-compete them to higher amounts of sunlight.

Similarly to honeysuckle, the battle for light is won by kudzu over native plants. Kudzu, we all know, is a vine that can cover pretty much anything in its path, once it gets a foothold (as is seen in Figure 1). With this ability to spread and cover entire landscapes, any native plant that would have previously grown there is no longer able to succeed. Additionally, kudzu has been shown to be able to withstand biomass removal (and even come back stronger afterwards), and kudzu seedlings can survive several weeks with no light (Frye et al, 2012).

Figure 1: Photo of kudzu entirely covering a house. (credit: Jonathon Van Buren)

Impacts of this competition on biodiversity

As you can imagine, this ability to over-grow anything below it, has dire consequences for the overall health and biodiversity of the effected ecosystem. For example, a 2008 study showed that a more mature (and therefore larger and more established) honeysuckle plant resulted in a significant decrease in the both density and species richness of herbaceous plants, seedlings and saplings growing under and around these invasive shrubs (Hartman & McCarthy, 2008). This pattern can be seen in Figure 2, however the bottom-most graph in that figure depicts a difference between honeysuckle and kudzu.

As was discussed earlier, kudzu grows over anything in its path, including full grown trees. We see that honeysuckle does not have this direct impact on overstory, or treetops in Figure 2. So, while kudzu indiscriminately takes over, on a surface level, honeysuckle does not impact the more mature trees.

Figure 2: From Hartman & McCarthy, 2008. Without diving into the details, we can see from this figure that the understory density (seen in the top three graphs) significantly decreased as the honeysuckle bushes matured and grew. This is because the amount of light reaching the understory and the overall accessibility of the understory is prevented by the thick clustering of honeysuckle branches above it. We also see that the overstory (tree tops) is not impacted by the honeysuckle, because this invasive is a shrub and does not reach heights to effect trees.

You may be asking, why does this biodiversity matter? Honeysuckle is a beautiful, sweet-smelling shrub and kudzu does create quite a lush, green blanket. Well, this biodiversity goes beyond the direct impact to native plants. There are other organisms that rely on natural, native ecosystems to survive. Once this balance is thrown off, animals that prefer native plants to survive on will be displaced. What once may have been an intricate collage of plants and animals, will become a monotonous landscape of invasives.

So, what now?

I know that these posts can seem a bit daunting and negative, but I always want to leave things off on a hopeful note! Because these plants have been studied for decades now, we have a pretty good understanding of how to try to control them. So not all hope is lost!

For honeysuckle, we know that they grow in both forested and open areas. However, they prefer higher light areas. This being said, avoiding the thinning of the forest canopy and forest fragmentation will lessen the likelihood of honeysuckle taking over that area (Luken et al, 1997). Clipping honeysuckle has been shown to have an impact on growth of the shrub. In forests, this technique may be all it takes to control honeysuckle. However, in more open areas, where honeysuckle prefers, clipping plus the use of herbicide have been shown to be a decent control (Luken & Mattimiro, 1991).

Kudzu is a bit trickier. It’s thought that the only plants that can truly compete with this vine are other invasives (such as Chinese privet and our beloved honeysuckle)(Blaustein, 2001). However, it has also been shown that planting certain grasses in the fall can help control kudzu once it is weakened (Everest et al, 1999). Biocontrol (aka bringing in things to eat the vine) has also proven to be a useful mechanism for controlling the spread of kudzu (Frye et al, 2013).

All of these control methods require persistence. Honeysuckle has been shown to come back with a vengeance if cut and then abandoned (Luken & Mattimiro, 1991). So, if clipping is the solution, it is suggested that this happens at least once a year, but more often if possible. Kudzu requires more than 2 or 3 years of biocontrol (Frye et al, 2012) but this is known to be a very effective method, if done correctly. All of these controls disrupt the competitive advantage these invasives have and give the native plants a fighting chance to survive.

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So, while competition is a natural part of the world, whether that be our family game night becoming heated, or a gorilla defending its territory, it can be a destructive force is left unchecked. The competitive advantage of invasive plants can be compared to an older and younger sibling wrestling, the little one doesn’t stand a chance. But we can at least try our best to give the little guy (in this case our native plants) any advantage we can.

References:

Blaustein, R. J. (2001). Kudzu’s invasion into Southern United States life and culture. The Great Reshuffling: Human dimensions of invasive species, 55-62.

Everest, J. W., Miller, J. H., Ball, D. M., & Patterson, M. (1999). Kudzu in Alabama history, uses, and control. Alabama A&M and Auburn Universities, Alabama Cooperative Extension System ANR-65.

Frye, M., Hough-Goldstein, J., & Kidd, K. (2012). Response of Kudzu (Pueraria montana var. lobata) Seedlings and Naturalized Plants to Simulated Herbivory. Invasive Plant Science and Management, 5(4), 417–426. https://doi.org/10.1614/IPSM-D-12-00001.1

Frye, M., & Hough-Goldstein, J. (2013). Plant Architecture and Growth Response of Kudzu (Fabales: Fabaceae) to Simulated Insect Herbivory. Environmental Entomology, 42(5), 936–941. https://doi.org/10.1603/EN12270

Hartman, K., & McCarthy, B. (2008). Changes in forest structure and species composition following invasion by a non-indigenous shrub, Amur honeysuckle (Lonicera maackii). The Journal of the Torrey Botanical Society, 135(2), 245–259. https://doi.org/10.3159/07-RA-036.1

Luken, J., & Mattimiro, D. (1991). Habitat-Specific Resilience of the Invasive Shrub Amur Honeysuckle (Lonicera Maackii) During Repeated Clipping. Ecological Applications, 1(1), 104–109. https://doi.org/10.2307/1941852

Luken, J., Kuddes, L., Tholemeier, T., & Haller, D. (1997). Comparative Responses of Lonicera maackii (Amur Honeysuckle) and Lindera benzoin (Spicebush) to Increased Light. The American Midland Naturalist, 138(2), 331-343. doi:10.2307/2426826

McEwan, R., Birchfield, M., Schoergendorfer, A., & Arthur, M. (2009). Leaf phenology and freeze tolerance of the invasive shrub Amur honeysuckle and potential native competitors. The Journal of the Torrey Botanical Society, 136(2), 212–220. https://doi.org/10.3159/08-RA-109.1

Kudzu vs. Honeysuckle: Changing Populations

Kudzu and honeysuckle are two of the most prevalent and well-known invasive plants in the US. We know some key things about how these plants impact the natural ecosystem and their overall range. However, these plants are changing their patterns and population dynamics, evolving to expand ranges, and impacting other species along the way. New studies and information allow for control and management techniques to improve. In this post I will discuss how kudzu and honeysuckle populations are changing and how these changes impact both the environment they invade and those in charge on trying to manage said invasions.

How they’re changing as a population

Kudzu is clearly a highly invasive plant. We see it across the southeast; along highways, in neighborhoods, etc. This invasiveness is due to many different characteristics. One of these is that there is little to no natural predator to this plant in the US. In a 2014 study in China, a team of researchers took kudzu specimens from the invaded Southeast US and specimens that were from the native populations of China. In this study they found that the invasive plants had much lower resistance to herbivory in China than the native Chinese plants (Yang et al., 2014). This supports the idea that the invasive species has evolved in its new range without the need to be resistant to any predators. We all only have so much energy in a day to do the things we need to do. Plants are no different; because invasive kudzu does not give energy to herbivory resistance, it can spend this energy in areas like growth and reproduction.

Figure 1: This figure was taken from Yang et al. 2014. It demonstrates that invasive kudzu in the US were less resistant to herbivory than the native plants in China. This shows that the invasive species have potentially evolved to lack this resistance due to the lack of natural predator in the US.

Figure 2: This figure is pulled from Yang et al. 2014.  It demonstrates the higher overall biomass of the invasive kudzu from the US. In comparison to the native Chinese individuals, the invasive plants showed significantly higher biomass, both aboveground in the stems and leaves, and belowground in the roots.

In addition to this evolution to live without predators, kudzu is able to grow amongst and with those kudzu plants that are genetically unique (Kartzinel et al., 2015). This ability to intermix with genetically variable plants allows kudzu to remain genetically variable while continually growing as a population. According to Kartzinel et al. 2015, this information means that some of the most common management tools for kudzu (mowing, cutting, etc.) may inadvertently be increasing genotypic diversity.

As for our other species of interest, studies have found that invasive honeysuckle growing near the margin of its habitat (rather than at the core of its invaded area) show signs of higher fitness and survival (Figure 2 & 3) (Kilkenny & Galloway, 2015). In terms of invasiveness and population success, this is a key finding. If a plant can spread to the borders of its habitat and have even greater success than at the heart of the population, this will inherently allow for more expansion. For invasive species like kudzu and honeysuckle, this ability allows for further invasion and species success.

Figure 3: This figure is pulled from Kilkenny & Galloway, 2015. It demonstrates the improved fitness of honeysuckle individuals at the margins of the invaded area. This figure gets at the idea that invasive species could potentially benefit from spreading into new habitat, and even show higher survival rates at these outer barriers.

How these changes are affecting the ecosystem

With the spread of these invasive plants due to their continued evolution to better succeed in their environments, the impacts of these plants also become more widespread. For instance, kudzu has been shown to create less diverse communities in the areas it invades. It is an over-arching vine that shades and negatively impacts most other plants in its path. Similarly, both honeysuckle and kudzu have been shown to have negative impacts on trillium plants (a genus of plant native to much of eastern North America) (Heckel, 2004). Trillium species are a commonly studied plants because the genus has both wide-spread and rare species. This dichotomy allows researchers to compare closely related organisms with very different habitat expanses. Regardless of this variation, it has been shown that both of these invasive plants negatively impact any trillium population.

Not only do these plants impact other plants negatively, there are impacts to those animals that use resources from said invasives. For example, there was a fly species discovered that has genetically evolved from a hybridization of two species in response to a new ecosystem invaded by honeysuckle (Carroll, 2007). These flies fed on the honeysuckle to the point that they became genetically distinct from related flies and have now been deemed the Lonicera fly (after the scientific name of honeysuckle: Lonicera japonica).

On to a more charismatic fauna, white-tailed deer have been shown to be a seed disperser for honeysuckle (Guiden et al., 2015). Many of us have seen white-tailed deer munching on vegetation around our backyards or along a roadside. These deer do not discriminate as to what they eat, especially when that food source has such sweet, sugary nutrients like honeysuckle. Because of its attractiveness to deer populations, these deer eat the fruit of the honeysuckle bush and therefore ingest the seeds along with the fruit. On down the road, the seeds pass through and the deer become a disperser for this invasive honeysuckle plant. The form of dispersal is effective and yet another reason why honeysuckle is such a highly invasive species.

How we can manage these changes and impacts

So, we know how invasive these plants are. We know that they are able to change their population dynamics and evolve to better survive in new territories. Now what do we do with this knowledge and how can we implement better management? Researchers are always on the lookout for better tools for management. An example of this innovation is found in an article published that introduced a new model that simulates how invasive plants can be managed best by looking at particular species characteristics. The researcher’s example work was done with kudzu and demonstrated the ability of the model to assist with management and control methods (Aurambout & Endress, 2018). With knowing things about the species biology and dispersal along with local management practices, the model finds solutions at local and regional scales and these solutions “can be compared to identify those most effective given the level of available resources” (Aurambout & Endress, 2018). This is just one example of work being done to better understand and control these invasives.

As for ourselves, how should we think about these invasive plants in our lives? A 2011 article from the US Department of Agriculture nicely summarizes how human activities can increase invasive success (Li et al., 2011). Invasive species are known to take advantage of newly disturbed habitat. This can be related to new residential developments, infrastructure, and industries. Kudzu is a great example of this invasive tactic. Keeping this in mind, it is always a good thing to disturb the natural ecosystem as little as possible. While we may not be able to control the building of new roads or apartment complexes, we can at least keep in mind that invasive preference for these disturbed habitats is just another reason to avoid human disturbance.

References:

Aurambout, J., & Endress, A. (2018). A model to simulate the spread and management cost of kudzu (Pueraria montana var. lobata) at landscape scale. Ecological Informatics., 43, 146–156.

Carroll, S. (2007). Natives adapting to invasive species: ecology, genes, and the sustainability of conservation. Ecological Research, 22(6), 892–901. 

Guiden, P., Gorchov, D., Nielsen, C., & Schauber, E. (2015). Seed dispersal of an invasive shrub, Amur honeysuckle (Lonicera maackii), by white-tailed deer in a fragmented agricultural-forest matrix. Plant Ecology, 216(7), 939–950.

Heckel, C.D., "Impacts of Exotic Invasive Vines on the Ecology and Reproduction of the Endangered Trillium Reliquum" (2004). Georgia Southern University: Electronic Theses and Dissertations. 692.

Kartzinel, T.R., Hamrick , J.L., et al. (2015). Heterogeneity of clonal patterns among patches of kudzu, Pueraria montana var. lobata, an invasive plant. Annals of Botany 116(5), 739–750.

Kilkenny, F., & Galloway, L. (2016). Evolution of marginal populations of an invasive vine increases the likelihood of future spread. The New Phytologist., 209(4), 1773–1780.

Li, Z., Dong, Q., Albright, T., & Guo, Q. (2011). Natural and human dimensions of a quasi-wild species: the case of kudzu. Biological Invasions 13(10), 2167–2179.

Yang, X., Huang, W., Tian, B. et al. (2014) Differences in growth and herbivory damage of native and invasive kudzu (Peuraria montana var. lobata) populations grown in the native range. Plant Ecology, 215, 339–346.