The ecological impacts of an aggressive aquatic invader: is Crassula helmsii the biggest threat to European freshwater biodiversity?

Crassula image courtesy of Saxifraga Foundation/Jan van der Straaten.

Primary Supervisor: Professor David Bilton (University of Plymouth)

Secondary Supervisor: Dr Andy Foggo (University of Plymouth)

Additional Supervisor: Mr Gavin Measures (Natural England)

Research background

Biodiversity loss through biological invasion and subsequent biotic homogenisation represents one of the greatest threats to ecosystems worldwide (1). 

Freshwaters are disproportionately biodiverse and provide crucial ecosystem services, but are also disproportionately threatened by invasions (2-3). New Zealand pygmy weed, Crassula helmsii, is the most aggressive freshwater invader in Europe, causing widespread habitat degradation and threatening aquatic plant life (4-7) in the most biodiverse freshwater habitats in the landscape (8). 

However, limited research has been conducted on Crassula’s impacts, with no published data on invertebrates, which comprise the bulk of biodiversity in the standing waterbodies Crassula colonizes. 

Our pilot studies suggest that Crassula stands are devoid of invertebrates (despite their structural complexity), that it is toxic even to non-herbivores, and that biotic release (9) may underlie its spread. 

This project aims to provide much-needed insight into Crassula’s impact and so inform conservation policy and practice.


  • To quantify the impact of Crassula on macroinvertebrate communities in the field.
  • To determine Crassula’s direct impact on macroinvertebrates in the laboratory and whether this differs between trophic levels.
  • To establish whether Crassula’s impact on herbivores depends on timescale of ecological/evolutionary exposure.
  • To explore the potential biochemical basis of Crassula’s impacts.

Research methodology

The impact of C. helmsii will be assessed through fieldwork in three regions of the UK (Dartmoor Commons; Pevensey Levels; Midland Meres) all invaded within the last 30 years. 

In each, ca. 10 sites with and without Crassula will be selected, their macroinvertebrates sampled, physicochemistry of the waterbody and macrophyte species composition recorded, and vegetation fractal complexity characterised (10). Macroinvertebrate assemblage composition will be analysed with various univariate and multivariate approaches assessing the influences of physicochemistry, vegetation composition/complexity and Crassula invasion. 

Crassulaceae contain two groups of secondary metabolites; piperidine alkaloids and phenolics (11). Biochemical investigations including FT-IR fingerprinting, GCMS and spectrophotometric assays will be used to screen whole plants, solvent-extracts and aqueous leachates for secondary metabolites. 

A series of laboratory mesocosm trials will examine putative acute and chronic toxicities via exposure and, where relevant, ingestion, of C. helmsii to macroinvertebrates spanning the taxonomic/ecological range in invaded waters (e.g. Radix (grazer), Notonecta & Agabus (predators), Chironomus (detritivore)) and including Potamopyrgus antipodarum, an abundant invasive mollusc originating from Crassula’s native range, to determine whether a long history of ecological/evolutionary co-occurrence has resulted in greater ability to utilise/coexist with Crassula.

Student experience

The student will acquire a broad range of doctoral-level transferrable, employment-related skills, typically through one-to-one training, including: experimental design; macroinvertebrate identification and husbandry; advanced multivariate and univariate statistics; toxicology and bioassay techniques; spectrometric assays; science communication. 

This multidisciplinary project provides opportunities for the student to expand, contract or modify aspects in line with their expertise or interests. 

The student will join a team of ca. 45 PhD students and 22 research scientists and have access to a range of research skills training courses (including writing for publication) and a postgraduate teaching course. 

Presentation at international conferences and Natural England meetings (to translate science into policy) will further promote the student and their work.


(1) Clavero, M. & García-Berthou, E. 2005. TREE 20: 110.

(2) Dudgeon, D. et al. 2006. Biol. Rev. 81: 163–182.

(3) Strayer, D.L. & Dudgeon, D. 2010. J. N. Am. Benthol. Soc. 29:344–358.

(4) Dawson, F.H. & Warman, E.A. 1987. Biol. Conserv. 42: 247–272.

(5) Lockton, A.J. 2009. Species account: Crassula helmsii. Botanical Society of the British Isles. [online]

Available from

(6) Euwald, N.C. 2014. Crassula helmsii in the New Forest – a report on the status, spread and impact of this non-native invasive plant, and the efficacy of control techniques following a two-year trial. Freshwater habitats Trust Oxford. [online] Available from

(7) Langdon, S.J. et al. 2004. Weed Technology 18: S1349-1352.

(8) Williams, P. et al. 2003. Biol. Conserv. 115: 329-341

(9) Keane, R.M. & Crawley, M.J. 2002. TREE 17:164-170

(10) McAbendroth, L….& Bilton, D.T. 2005. Oikos 111: 279-290.

(11) Stevens, J.F. et al. 1995. Biochem. Syst. Ecol. 23: 157-165


This project has been shortlisted for funding by the ARIES NERC Doctoral Training Partnership. Undertaking a PhD with ARIES will involve attendance at training events.

ARIES is committed to equality & diversity, and inclusion of students of any and all backgrounds. All ARIES Universities have Athena Swan Bronze status as a minimum.  

Applicants from quantitative disciplines who may have limited environmental science experience may be considered for an additional 3-month stipend to take appropriate advanced-level courses.

Usually, only UK and EU nationals who have been resident in the UK for three years are eligible for a stipend. Shortlisted applicants will be interviewed on 26/27 February 2019.

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