Date of Award
Master of Science (Biological Sciences)
School of Science
Professor Glenn Hyndes
Professor Kenneth L. Heck Jr.
Associate Professor Adriana Vergès
Professor Fiona Tomas
Rising sea temperatures through climate change produce shifts in the distribution of tropical species to temperate regions, a process termed “tropicalisation”. The poleward expansion of tropical herbivores into temperate seagrass meadows is predicted to increase grazing pressure and alter ecosystem services and processes in these seagrass systems. This study attempted to examine the effects of tropicalisation on temperate seagrass meadows along the western coast of Australia, where the increasing abundance of tropical consumers such as the herbivorous Siganus fuscescens has already been documented. Through the assessment of fish assemblages in seagrass meadows and the grazing levels on seagrass in 2001 and 2016/17, as well as in situ and mesocosm feeding preference experiments, this study attempted to estimate the grazing rates and impact that the growing abundance of S. fuscescens may have in temperate seagrass meadows.
Shifts in the grazing rates on seagrass between 2001 and 2016/17 were inconsistent, varying between seagrass species and location. Based on observational data on the bites on seagrass leaves, rates of consumption increased for Posidonia sinuosa while no similar pattern was found for Posidonia australis. This was despite an apparent greater consumption on P. australis compared to P. sinuosa in 2001, and the minimal amount of grazing on tethered seagrass. The higher observed level of P. sinuosa consumption in 2016/17 is likely explained by the changed herbivorous fish species composition, even though no fish were clearly observed feeding on seagrass in the current study. The tropical herbivore S. fuscescens was more abundant in 2016/17 that 2001, although abundances were patchy and no fish was observed feeding on seagrass. The higher level of P. sinuosa consumption in 2016/17, compared to 2001, supports the prediction that with increasing abundances in temperate seagrass ecosystems, tropical herbivores will enhance the consumption of seagrass. However, seagrass consumption is likely to be strongly influenced by the availability of macroalgae which were shown as the preferred food sources.
Feeding trials in mesocosms were compromised by the large number of deaths and the limited grazing on natural food sources by S. fuscescens, suggesting that the population in the Perth region is susceptible to adverse handling and husbandry effects. To maximise the survival rate of captured fish, the fishing and handling procedures were altered to adapt to the ongoing observations in the response of fish to handling in the field or in the mesocosm facilities. Lesson learned from the capture, handling and husbandry of S. fuscescens in feeding trials in the current study will hopefully provide greater success for feeding preference experiments in the future. The sampling program initiated in 2001 and repeated in 2016/17 provides base-line data and the opportunity to monitor and track the shift in abundances of tropical herbivores and resultant increases in grazing rates to test the above predictions. The consequences of tropicalisation will depend on the variety of abiotic and biotic factors, including the fish assemblages in the area, the abundance of tropical species, the availability of food, and the feeding preferences that invading species will develop in response to the changed environmental conditions.
Samsonova, M. (2020). Tropicalisation of temperate seagrass meadows in Western Australia: Predicting the impact of tropical herbivorous fishes on temperate seagrass meadows. https://ro.ecu.edu.au/theses/2294
Available for download on Wednesday, March 30, 2022