Predicting the Japanese stiltgrass

Japanese stiltgrass (Microstegium vimineum)

article written by Karl Reiman, TVERC volunteer

Japanese stiltgrass (Microstegium vimineum) is considered to be among the highest-risk potential invasive species for Great Britain and Europe. Originally from Southeast Asia, the species has become an aggressive invasive plant in eastern United States since its introduction in 1919, altering a wide variety of ecosystems ranging from floodplains to forests. Worryingly, a recent study by Miller et al. (2020) found the Japanese stiltgrass to be the most common invasive in the national parks of eastern US.

To estimate its potential to spread in Great Britain, and more particularly in Oxfordshire and Berkshire, I created a Maxent species distribution model based on recorded observations of the species in eleven states across the East Coast of US, using five environmental variables (maximum temperature, minimum temperature, distance from developed areas, canopy cover percentage, and elevation) to assess its niche requirements. To carry out the analysis, I used openly available species occurrence records from the Global Biodiversity Information Facility (GBIF) and environmental information available on government and research groups’ websites. I did most of my work on ArcGIS and R, and used the Maxent software by Philips et al. to run the model.

Although the output of the model is continuous, I divided the results into suitable and unsuitable habitat based on a 10th percentile function (essentially omitting a proportion of the least representative areas for the species). According to the predictions, about 4.7% of Great Britain’s land cover seems to be susceptible to invasion, particularly areas in Southeast England. This percentage was slightly higher for Oxfordshire and Berkshire at 6.1%. More problematically, however, Sites of Special Scientific Interest (SSSIs) in the two counties seem to be much more vulnerable than the nation- or county-wide average, with approximately 39.6% of the total protected habitat being susceptible to a potential invasion. Some SSSIs, such as Bisham Woods in Berkshire or Chinnor Hill in Oxfordshire, were shown to be particularly vulnerable, providing a suitable habitat for the Japanese stiltgrass in about 93% of their range. Overall, 96 of the 186 SSSIs (51.6%) in Oxfordshire and Berkshire were shown to contain at least some habitat susceptible to invasion by the Japanese stiltgrass. Canopy cover percentage was by far the most significant predictor of habitat suitability, followed by elevation and distance from developed areas. This supports previous suggestions that the species may have a competitive advantage in shaded habitats due to its C4 photosynthesis (although the claim has been contested), and its reliance on humans for rapid spread.

While some have argued that the Japanese stiltgrass may also bring some benefits in the form of providing habitat for frogs or certain arthropods, the species has been shown to have profound impacts on ecosystems by changing food webs and nutrient cycling, thus posing a serious threat to already fragile habitats. While I do not think that the results of my short modelling exercise were conclusive, I would consider them to be another warning that the Japanese stiltgrass might not struggle with making itself at home in some of the most valuable natural sites in Oxfordshire and Berkshire.

Posted: May 11, 2021