Pevensey Levels wetland: Invasive aquatic plant species in wetlands

Invasive non-native aquatic plants are a major global threat to wetland ecosystem services, including agriculture, flood control and biodiversity. They can infest water bodies such as ditches, ponds, and canals to form dense vegetative mats that out-compete native flora and reduce oxygen levels. Non-native aquatic species are expanding rapidly as a consequence of international travel and trade, climate change, and land use impacts.

Despite increasing efforts and many millions of pounds spent trying to control invasive species in the UK, many non-native aquatic species are proving resilient to effective management. This research programme incorporated two of the most problematic species, Australian Swamp Stonecrop (Crassula helmsii) and Floating Pennywort (Hydrocotle ranunculoides), in order to develop effective management and monitoring methods.

In partnership with the Environment Agency, Natural England and 

CABI (Centre for Agriculture and Biosciences International), this programme focused upon the control of C. helmsii and H. ranunculoides on the Pevensey Levels wetland, East Sussex, UK. The Levels represent one of the largest wetland landscapes in south east England and are of global nature conservation significance. However, the area is infested with invasive aquatic plants to such an extent that the site could lose its conservation designations if they are not controlled. Projects are funded by the Environment Agency with additional support from Natural England.

The programme includes two complementary projects with aims to:

> map the extent of C. helmsii infestation on the Pevensey Levels using a small-Unmanned Aircraft System (sUAS) (i.e. drone)
experimentally test C. helmsii control methods in the field, including biological and mechanical techniques
> identify the impact of H. ranunculoides on native macrophyte communities
describe which nutrient(s) might limit the growth of H. ranunculoides
determine the impact of climate warming on the reproductive capacity of H. ranunculoides.

This programme improved the scientific basis of invasive aquatic plant management by focusing upon the control and monitoring of C. helmsii and H. ranunculoides on the Pevensey Levels wetland.

The data collected and analysed provided new evidence about changes to plant diversity, species assemblage and community dynamics as a result of invasion by non-native species. The research provided evidence for the effectiveness of different control methods for these two invasive species. The effects of nutrients and climate change on the growth and expansion capability of H. ranunculoides was also determined for the very first time.

The application of a sUAS to facilitate assessment of control methods represented a novel contribution to invasive species management. This study will monitor the effects of a biological control agent developed by CABI (Centre for Agriculture and Biosciences International). The Pevensey Levels may be the first site worldwide in which a biological agent is released to control C. helmsii.

Anticipated outcomes from the project included a plan of eradication of invasive C. helmsii and H. ranunculoides from the Pevensey Levels wetland using the preferred option(s) of treatment and a control brief for contractors to be used nationally. Scientific publications relating to the monitoring and control of the two invasive species were also produced.