Impacts of non-native Hydrocotyle ranunculoides on native macrophyte communities, and the effects of management, nutrients, and temperature

  • Jane Birch

Student thesis: Doctoral Thesis


Hydrocotyle ranunculoides L.f. (Floating Pennywort) is native to South America and is an aggressively competitive aquatic plant species in the family Araliaceae and native to South America. It is invasive and non-native in the United Kingdom, Europe, and many countries worldwide. It has a large capacity for vegetative spread and dominance of waterways causing considerable problems for their ecology and costing over £25 million annually in management costs and recreation losses in the UK and Europe. The study site, Pevensey Levels in southern England, is a globally important wetland and is particularly species rich in aquatic macrophytes but 10% (45km) of the watercourses are infested by H. ranunculoides. This research investigates H. ranunculoides in relation to its impacts on native macrophyte communities and the effectiveness and sustainability of management methods, the effects of nutrients on its growth, and the potential influence of climate change through increasing temperatures. Macrophyte communities on 60 sample ditches on the Levels, both infested and un-infested, managed, and un-managed, were surveyed and nutrient levels and environmental variables measured in the sediment and water over a period of four years. Greenhouse experiments were run to investigate the influence of nutrients and temperature on growth, and temperature on seed germination and production. Results show that H. ranunculoides has a negative impact on native macrophyte species diversity (when cover >50%), may alter rare plant communities (P=0.001 for one rare species) and threaten international site designations. Traditional mechanical removal alone is not effective and reduces native macrophyte diversity. H. ranunculoides abundance was lower in sites receiving mechanical removal followed by chemical spraying (P=0.031) and this management treatment did not impact on native
macrophyte diversity. Whilst no specific nutrient seemed to drive the growth of H. ranunculoides, it appeared to preferentially use nitrogen from water and phosphate from sediment. There were interactions between H. ranunculoides nutrient uptake and pH values (P=0.002). There were positive correlations between rising temperature and increased leaf abundance (P<0.001), biomass, and seed abundance (P=0.01). In addition, H. ranunculoides was shown to be capable of reproducing using sexual means in its invaded regions in Europe under current climate conditions. These findings have implications for the conservation of aquatic and riparian communities worldwide, challenge the effectiveness of management methods, and illustrate the potential for the increasing invasive capability of H. ranunculoides because of global climate warming.
Date of AwardSept 2021
Original languageEnglish
Awarding Institution
  • University of Brighton
SupervisorChris Joyce (Supervisor) & Niall Burnside (Supervisor)

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