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

Scientific background
Biological invasions represent one of the most significant threats to biodiversity on the planet. Freshwaters are both disproportionately affected by such invasions, and home to a disproportionately large proportion of biodiversity, especially invertebrates. They also provide crucial ecosystem services. Crassula helmsii, a native Australasian aquatic plant, has been aggressively invading European freshwaters for over 30 years, with drastic consequences for their biodiversity although, to date, detailed studies have been restricted to impacts on the flora. Preliminary data suggest that Crassula stands are relatively devoid of invertebrate life and that, with its CAM metabolism, Crassula may be exerting a toxic influence upon invertebrates in the freshwaters it invades. Given Crassula’s rapid ongoing spread, there is an urgent need to better understand the species impacts on aquatic invertebrates, particularly from the perspective of informing the management and maintenance of freshwater biodiversity in the landscape.

Research methodology and training
This project will examine the effects of Crassula upon freshwater invertebrates via 3 approaches: field survey, laboratory mesocosm trials, and biochemical assays. The student will receive training in the conduct and analysis of all 3; from freshwater invertebrate taxonomy through experimental design for toxicity trials, to advanced univariate and multivariate analyses of ecological and spectrometric data. They will gain experience of fieldwork alongside experienced scientists and government officers, develop knowledge of cutting-edge approaches in biostatistics using R, break new ground in the application of rapid analytical techniques such as FTIR
spectrometry to secondary chemistry in plants and test fundamental ecological hypotheses about the mechanistic bases for the spread and control of invasive species. They will have the opportunity to develop skills and gain qualifications as a university demonstrator, disseminate their work and its results via both written and oral outputs, and will build a library of skills and competences that will elevate their employability to a level commensurate with doctoral status, applicable across a wide range of employment opportunities.

Person specification
We are seeking a candidate prepared for long days in the field and laboratory; you must be numerate and literate; some demonstrable experience in invertebrate taxonomy and identification is desirable.

Research background 

Biodiversity loss through biological invasion and subsequent biotic homogenisation represents one of the greatest threats to ecosystems worldwide. Freshwaters are disproportionately biodiverse and provide crucial ecosystem services, but are also disproportionately threatened by invasions. New Zealand pygmy weed, Crassula helmsii, is the most aggressive freshwater invader in Europe, causing widespread habitat degradation and threatening aquatic plant life in the most biodiverse freshwater habitats in the landscape. 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 may underlie its spread. This project aims to provide much-needed insight into Crassula’s impact and so inform conservation policy and practice. 


1. To quantify the impact of Crassula on macroinvertebrate communities in the field. 
2. To determine Crassula’s direct impact on macroinvertebrates in the laboratory and whether this differs between trophic levels.
3. To establish whether Crassula’s impact on herbivores depends on timescale of ecological/evolutionary exposure.
4. 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. 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. 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 utilize/coexist with Crassula. 

Student experience: 

The student will acquire a broad range of doctoral-level transferrable, employmentrelated 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. 


Dudgeon, D., Arthington, A.H., Gessner, M.O., Kawabata, Z.-I., Knowler, D.J.,Lévêque, C., Naiman, R.J., Prieur-Richard, A.-H., Soto, D., Stiassny, M.L.J.,Sullivan, C.A., 2006. Freshwater biodiversity: importance, threats, status andconservation challenges. Biological Reviews 81: 163–182 

Euwald, N. 2014. Crassula helmsii in the New Forest – a report on the status, spreadand impact of this non-native invasive plant, and the efficacy of control techniquesfollowing a 2 year trial. Freshwater habitats Trust. 

Bilton, D.T., McAbendroth, L.C., Nicolet, P., Bedford, A., Rundle, S.D., Foggo, A. &Ramsay, P.M. 2009. Ecology and conservation status of temporary and fluctuatingponds in two areas of southern England. Aquatic Conservation Marine andFreshwater Ecosystems 19: 134-146.

Bilton, D.T., McAbendroth, L., Bedford, A. & Ramsay, P.M. 2006. How wide to castthe net? Cross-taxon congruence of species richness, community similarity andindicator taxa in ponds. Freshwater Biology 51: 578-590. 

Borell E.M., Foggo A., Coleman R.A. 2004. Induced resistance in intertidalmacroalgae modifies feeding behaviour of herbivorous snails. Oecologia 140: 328–334.