The Ecology, Impacts, and Control of Crassula Helmsii Clare E. Dean
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The Ecology, Impacts, and Control of Crassula helmsii Clare E. Dean This thesis has been submitted in partial fulfilment of the requirements of the degree of Doctor of Philosophy Bournemouth University in collaboration with the National Trust and with the Royal Society for the Protection of Birds June 2015 This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with its author and due acknowledgement must always be made of the use of any material contained in, or derived from, this thesis. 2 Abstract A major focus of research regarding non-native invasive plants is to understand what ecological variables (abiotic and biotic) increase or decrease the likelihood that a non-native plant species will become invasive, and to determine whether invasion has a negative impact on native plant species. Crassula helmsii is a plant indigenous to Australasia, which has invaded wetland habitats across much of Britain. It is a low growing, clonally spreading plant, which occupies the margins and shallows of freshwater and brackish waterbodies. Crassula helmsii is considered invasive due to its ability to produce vigorous aboveground growth, which in some locations spreads to form dense monospecific mats of vegetation. There are concerns that C. helmsii can exclude native plant species, and therefore that its invasion may have a negative impact on biodiversity. The research presented in this thesis was conducted with the aim of better understanding the mechanisms which allow C. helmsii to develop dense, dominating vegetative mats. This aim was addressed by investigating how C. helmsii abundance was related to variation in the abiotic environment, how C. helmsii abundance was affected by generalist herbivores, and whether C. helmsii abundance was related to the presence or absence of co- occurring plant competitors. To investigate how C. helmsii abundance is related to the abiotic environment, firstly field data were collected of pH, nutrient levels, water depth, and shade levels along C. helmsii invaded margins of waterbodies. Multiple linear regression was used to address the question: How well does abiotic variation predict variation in C. helmsii abundance in the field? Finding that C. helmsii cover was significantly greater at a higher pH and at lower shade levels. Secondly C. helmsii was grown in mesocosms at different strengths of salinity (0, 2, 4, 8 ppt) to address the question: What is the estimated minimum salinity level for the control of C. helmsii in brackish water habitats? Relative growth rate decreased significantly with increasing salinity and C. helmsii died in the highest salinity treatments (8 ppt). These two experiments demonstrated that C. helmsii abundance can be affected by the abiotic environment, and specifically by pH, shade levels, and salinity. Dense, dominating mats of C. helmsii are more likely to occur in freshwater conditions, which have a neutral to alkaline pH, and with minimal overhead shading. More effective control against C. helmsii dominance may be achieved by preventing C. helmsii from colonising sites with such abiotic conditions, or by focusing more effort on removal of C. helmsii vegetation which has colonised such sites. To investigate the effect of disturbance from generalist herbivores, a field-based experiment was set up, in which 4 m2 fenced plots which excluded large vertebrates were erected along a C. helmsii invaded drawdown zone. The abundances of all plants species within the fenced plots and within adjacent unfenced plots were recorded repeatedly over a year, and compared, in order to explore the question: What is the effect of grazing on the abundance of C. helmsii 3 within a wetland plant community? Crassula helmsii declined in abundance in the fenced plots compared to the unfenced plots, and the abundance of co-occurring vegetation was consistently higher in the fenced plots than in the unfenced plots. Crassula helmsii appeared to benefit from grazing disturbance, attaining a higher abundance in the unfenced plots, where competitor plant species had been grazed out. Crassula helmsii could have a negative impact on native plant species diversity, if it is able to fill a niche usually occupied by native plant species of open, grazed drawdown zones (e.g. Teucrium scordium). The relationship between C. helmsii abundance and the presence or absence of co-occurring plants was investigated in order to ascertain the effect of competition on the abundance of C. helmsii, and also to ascertain the effect of competition from C. helmsii on the abundance and diversity of co-occurring plants, therefore giving an indication of whether the spread of C. helmsii can have a negative impact on native species. Firstly a common garden experiment and a field based experiment were separately conducted, both assessing how well C. helmsii could spread into available bare ground compared to native competitors, and addressing the question: Is available bare ground an important pre-requisite to high C. helmsii abundance? These experiments found that C. helmsii can rapidly respond to available bare ground, but that simultaneous spread of native competitors might limit C. helmsii community dominance. The effect of C. helmsii on native plants was species specific; the common garden experiment showed that C. helmsii limited the spread of Anagallis tenella, but not Hypericum elodes or Hydrocotyle vulgaris. Secondly field survey data of plant species abundance and diversity were collected to explore the question: Can the invasion of C. helmsii negatively affect the plant community in a Phragmites australis dominated fen habitat? Survey data were analysed to determine whether C. helmsii was negatively correlated with variables of the plant community, a pattern indicating that C. helmsii could increase in abundance by displacing native plants. The variables ‘total community abundance’ and ‘species diversity’ were not significantly correlated with C. helmsii. At the single species level, C. helmsii and the dominant plant species P. australis showed a high capacity to coexist, although C. helmsii was most abundant where P. australis was absent. Based on these three experiments, evidence was not found to suggest that C. helmsii has a negative impact through direct displacement of native plants. However, because dominating C. helmsii growth may be more likely to occur in an area with ample bare ground into which this species can spread, and where tall competitive plant species (e.g. P. australis) are in low abundance, management activities designed to increase species diversity by creating patches of open vegetation structure, might instead just encourage a higher abundance and dominance of C. helmsii. 4 Contents Heading Sub-heading Page Abstract 3 List of figures 8 List of tables 12 Acknowledgements 14 Author’s declaration 15 1. Main introduction 16 1.1. Background 16 1.2. Study species: Crassula helmsii (T. Kirk) 25 Cockayne 1.3. Research aim and objectives 34 2. General methods and materials 36 2.1. Field surveys 36 2.1.1. Sampling technique 36 2.1.2. Survey sites 37 2.2. Data analysis 41 2.2.1. Arcsine transformation 41 2.2.2. Shannon-Weiner diversity and evenness 41 3. How well does abiotic variation predict variation in C. helmsii abundance in the field? 42 3.1 Introduction 42 3.2 Methods and materials 44 3.2.1. Field survey 44 3.2.2. Laboratory analysis 45 3.2.3. Regional data 45 3.2.4. Data analysis 47 3.3. Results 49 3.4. Discussion 55 5 4. What is the estimated minimum salinity level for the control of Crassula helmsii in 59 brackish water habitats? 4.1. Background 59 4.2. Action 61 4.3. Consequences 63 4.4. Discussion 65 5. What is the effect of vertebrate grazing disturbance on the abundance of C. helmsii 67 within a wetland plant community? 5.1. Introduction 67 5.2. Methods and materials 69 5.2.1. Experimental design 69 5.2.2. Statistical analysis 70 5.3. Results 72 5.4. Discussion 79 6. Is available bare ground an important pre-requisite to high C. helmsii abundance? 82 6.1. Introduction 82 6.2. Methods and materials: Common garden 85 experiment 6.2.1. Experimental design 85 6.2.2. Data collection 88 6.2.3. Statistical analysis 89 6.3. Results: Common garden experiment 92 6.4. Methods and materials: Field experiment 99 6.4.1. Experimental design 99 6.4.2. Data collection 101 6.4.3. Statistical analysis 102 6.5. Results: Field experiment 103 6.6. Discussion 110 6.6.1. Common garden experiment 110 6.6.2. Field experiment 112 6.6.3. Is available bare ground an important pre- 113 requisite to high C. helmsii abundance? 6 7. Can the invasion of C. helmsii negatively affect the plant community in a Phragmites 114 australis dominated fen habitat? 7.1. Introduction 114 7.2. Methods and materials 117 7.2.1. Data collection: plant species 117 7.2.2. Data collection: plant traits 117 7.2.3. Statistical analysis 119 7.3. Results 122 7.4. Discussion 135 8. Main discussion 139 9. References 152 10. Appendices 173 I Data volunteered from C. helmsii invaded 173 sites II Is there an effect of pH on the growth rate 175 of C. helmsii? III Plant trait matrix 178 IV Insect visitors 182 7 List of figures Figure Description Page Main thesis 1. The appearance of the leaves of Crassula helmsii when the plant is growing 26 terrestrially along the margins of waterbodies (top), compared to the appearance of the leaves when C. helmsii is growing submerged within water (bottom). 2. The distribution of Crassula helmsii in Britain, shown at a resolution of 10 km squares, 29 and containing data from records up to 2014. Data courtesy of the NBN Gateway with thanks to all the data contributors: https://data.nbn.org.uk/Taxa/NBNSYS0000004639/Grid_Map (accessed 11th March 2014).