Himalayan balsam’s damaging impact on rivers revealed in new Stirling study

Pioneering University of Stirling led research has revealed the diverse and damaging impact Himalayan balsam has on river ecosystems.

A three-year study led by Dr James Hardwick, a Lecturer in Earth Sciences at the University’s Faculty of Natural Sciences, has shown that the invasive species is doing more than just replacing native plants.

The new research, carried out in partnership with Newcastle University, has shown that the species can disrupt the physical stability of ecosystems in ways that can have far-reaching consequences. Including significantly weakening riverbanks over winter.

By outcompeting native plants and then dying back completely each year, Himalayan balsam makes invaded riverbanks more susceptible to erosion during high-flow periods.

When riverbanks erode, more fine sediment enters their waters. This damages habitats, harms wildlife, and even changes the shape and structure of rivers.

This can then have a negative impact on water quality, biodiversity, and the benefits that healthy rivers provide.

Lead researcher Dr James Hardwick explained: “Our study reveals a previously overlooked seasonal process that illustrates how this plant weakens riverbanks. Himalayan balsam is often assumed to increase riverbank erosion simply because it dies-back leaving riverbanks bare over winter, but very little research has tested this relationship.

“Our work provides the first evidence that the impact of Himalayan balsam on the riverbank is not just about dieback. It’s about the way it suppresses native vegetation during summer, creating a loop that weakens riverbanks in winter. This seasonal mechanism has important implications for both river restoration and the management of invasive species.

“Understanding how invasive plants influence riverbank stability is essential for designing effective strategies that reduce erosion risks and protect river habitats.”

The research team hope that the findings will help river managers, conservation charities, local authorities, and environmental regulators better understand the risks associated with Himalayan balsam. Allowing more informed decisions to be made, particularly on intervention strategies.

Himalayan balsam was first introduced to the UK in 1839 as an ornamental garden plant. It was planted in places like Kew Gardens before escaping and spreading rapidly into the wild, especially along riverbanks and damp areas.

Researchers combined three years of field measurements with advanced statistical modelling. Over this period, they surveyed vegetation and measured riverbank shear strength - an indicator of how resistant the bank is to erosion - at multiple sites with and without Himalayan balsam.

To uncover these processes, researchers used Piecewise Structural Equation Modelling - a statistical method that breaks complex cause-and-effect links into smaller models - to explore how Himalayan balsam and native vegetation interact across seasons and the impact of these changes on riverbank stability.

Together, the field data and modelling approach enabled the team to quantify, for the first time, the indirect effects of Himalayan balsam on winter riverbank strength.

Invasive non-native plants indirectly destabilise riverbanks was published in the journal Biological Invasions as a cross-institution project between the University of Stirling and Newcastle University.

It was funded by Newcastle University as an internal PhD studentship with support from the Forth Rivers Trust and Tees River Trust who helped obtain site access.