Effective Management of the Invasive Aquatic Plant Australian Swamp Stonecrop (Crassula helmsii) in Wetlands

Student thesis: Doctoral Thesis


Small-unmanned aircraft systems (sUAS), commonly referred to as drones, present a novel and rapidly evolving approach to spatial data collection and research. One application for this technology is the detection and management of invasive species. Australian swamp stonecrop (Crassula helmsii) is an invasive aquatic plant capable of dominating still or slow-flowing waterbodies such as ditches, ponds, and canals. It is spreading throughout the United Kingdom (UK) and Europe, where it can form dense vegetative mats that out-compete native flora. At present, there is no successful method of control for this species. However, it is estimated that the cost of effectively managing C. helmsii in UK waterways could be as much as £3 million. The aims of this project were to improve the scientific understanding of C. helmsii, specifically focusing on its ecological impacts and the effectiveness of novel monitoring techniques using sUAS.

The study was undertaken on the Pevensey Levels wetland in East Sussex, UK. This site represents one of the largest wetland landscapes in South East England and is of global nature conservation significance. The extensive network of ditches on the Pevensey Levels are highly valuable and support a diverse range of aquatic flora and invertebrate species. However, the area is invaded with C. helmsii to such an extent that the site could lose its conservation designations if this invasive species is not controlled.

A fixed-wing sUAS (SenseFly eBee Plus) equipped with multispectral sensors was used to monitor ditch networks colonised by C. helmsii. Ditches containing the invasive plant were surveyed across both spatial and temporal scales during the main growing season (April-October) and took place over three fieldwork seasons between 2017 and 2019. Acquired imagery was subsequently processed and analysed using vegetation indices, in order to determine the effectiveness of this approach as a tool for detecting and monitoring the invasive plant species. This analysis allowed C. helmsiito be rapidly identified and subsequently monitored within the field. Furthermore, sUAS surveys repeated at monthly intervals across a year-long period were used to determine the optimum timeframe for monitoring C. helmsii via this approach. Additional analysis demonstrated the successful application of this technology for post treatment monitoring of C. helmsii control.

In addition to sUAS investigations, ground-based field vegetation surveys were undertaken to monitor the ecological impacts of the invasive species on native ditch vegetation communities. The results of this analysis showed that the presence of C. helmsii within ditch habitats has a negative impact on particular groups of native plant species. The focus of this study on C. helmsii within ditch habitats and the application of sUAS represents novel research that will aid researchers and practitioners concerned with the ecology and management of C. helmsii across its invaded range. Recommendations from the project include a combined treatment approach integrating both mechanical and biological control, with additional outcomes comprising of a plan of eradication for C. helmsii on the Pevensey Levels wetland and a control brief for contractors.
Date of Award2020
Original languageEnglish
Awarding Institution
  • University of Brighton
SupervisorChristopher Joyce (Supervisor) & Niall Burnside (Supervisor)

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