The Ecology, impacts and control of Crassula helmsii.
Authors: Dean and Dean, C.
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 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.