Invasive Japanese Knotweed
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Does invasive Japanese knotweed hurt the aquatic insects and bacteria of the Delaware River?
By. Amy King, Dr. Jeanne Kagle, and Dr. Gregory Moyer
Alliance for Watershed Education, Pocono Environmental Education Center, and Mansfield University of Pennsylvania
The Intense Invader
Have you seen tall, bamboo-like plants growing on roadsides and by the banks of a river near you and wondered what that weird plant was? That is most likely Japanese knotweed. Sad to say, but this plant is extremely invasive, growing up to 12 feet tall (Dyer at al. 2009). Japanese knotweed is virtually unstoppable, taking over what ever is in its path, not even to be stopped by asphalt. So far, no 100% effective solution has been found (Gerber et al. 2007). People have tried chemicals, burning, and cutting it. If the plant roots are moved and placed on the ground in any other location, it can grow (Pysek and Prach 1993; Weston et al. 2005). This invasive plant is extremely determined to grow and as a result it does not allow other plants to thrive, reducing the diversity of plants that can grow in their original, native habitat. If Japanese knotweed is not stopped, it can take over banks along rivers and streams and even our roadsides where they tend to grow (Dyer at al. 2009). Does it affect anything besides other plants? What about when the leaves fall into the streams nearby? That is one question that has not been readily explored. In a recent study done in the Susquehanna River, it was found that native American sycamore leaves decomposed at a significantly faster rate than Japanese knotweed (Fogelman et al. 2018). This may be an indication that the organisms responsible for the decomposition of the organic leaf matter, may be able to better decompose native species than non-native species. Aquatic organisms such as insects and bacteria play an important role in such decomposition (Webster and Benfield 1986; Cummin et al. 1989; Fogelman et al. 2018). In this research, we explored how much these insects and bacteria are actually affected by the presence of Japanese knotweed in the Delaware River, a river with invaded banks. Three different kinds of leaf packs were submerged in the Delaware River, within the Upper Delaware Scenic and Recreational River, for 29 days. Leaf packs were made of mesh produce bags that contained dried leaves within them, simulating a natural gathering of leaves against a rock in the river (Benfield 1996). The three different types of leaf packs consisted of only Japanese knotweed leaves or only native tree leaves, or a mixture of both. The leaf packs were randomly placed in two locations of the Delaware River. After the 29 days, the leaf packs were collected and from them, we gathered bacteria samples and counted the different types of macroinvertebrates (aquatic insects) found in them.
Bacteria
Some microbes, particularly bacteria, have the ability to break down cellulose, a compound often found in plant material. When they have that ability, they are given the term cellulolytic (Beguin 1993). By growing bacteria on a substance that contains cellulose, we are able to see what bacteria we collected that are cellulolytic (Mohanta 2014; Meddeb- Mouelhi et al. 2014). This tells us if the bacteria found in the leaf packs were taking part in the decomposition. We discovered that there was not a significant difference in the number of cellulolytic bacteria per different type of leaf pack (p-value= 0.53). This allows us to believe that the bacteria’s ability to degrade cellulose was not affected by Japanese knotweed and can therefore still utilize it as a source of nutrients.
Aquatic insects
The aquatic insects, also known as macroinvertebrates, were identified to the taxonomic order they belong in and counted per order. We used that information to calculate the diversity or variety of insects per leaf pack. The larger the diversity, the better the aquatic insects are faring. We found that there was not a significant difference in diversity per different type of leaf pack (p-value= 0.70). This allows us to believe that the aquatic insects have the ability to utilize Japanese knotweed as a possible shelter or even as a food resource.
What does this mean?
According to our results, it appears that bacteria and aquatic insects have to ability to adjust to the invasion of Japanese knotweed. As much as a nuisance as it is to the native plants in the area of invasion, the nearby bacteria and insects in the streams may be perfectly fine. This information may be important when considering future action against Japanese knotweed. The removal of Japanese knotweed might not be a priority over the removal of other invasive plants we are currently dealing with. Through this research, we were reassured that bacteria and aquatic insects have not been affected negatively by the Japanese knotweed that grows along the Delaware River.
Made Possible By:
This research was fully funded and supported by the Alliance for Watershed Education and the Pocono Environmental Education Center, approved by the National Park Service, and supported by Dr. Kagle and Dr. Moyer of Mansfield University of Pennsylvania.
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