Climate change impacts on macro and micro scale interactions between a tropical herbivore and temperate kelp forests

Marine ecosystems globally are being directly impacted by multiple climate-mediated stressors, including: gradual ocean warming, an increase in the frequency and intensity of marine heatwaves (MHWs) and changes to ocean circulation such as the strengthening of western boundary currents. These stressors not only directly affect individual species but also indirectly impact ecosystem structure and function by altering species interactions. Changes to species interactions are partly driven by shifts in species distributions. For example, warming and strengthened currents are facilitating the poleward movement of tropical herbivores into temperate reefs by opening up thermally suitable areas poleward and increasing poleward larval dispersal. In Australia’s temperate Great Southern Reef, this has already strengthened some herbivore- kelp interactions in some regions and contributed to the loss of kelp, the reef’s main foundation species. Warming stress can also alter the algal microbiome, which in turn influences algal health, and there is the potential that this may affect changes in kelp-herbivore interactions by altering kelp palatability. This thesis aimed to gain further insight into the indirect effects of climate change stressors on the dominant kelp, Ecklonia radiata, by investigating the complex interplay of kelp-herbivore-microbe interactions. Firstly, I investigated the effects of ocean warming and MHWs on the kelp microbiome and on kelp susceptibility to herbivory. I aimed to understand if changes in the kelp microbiome influence interactions between kelp and a potentially range-shifting tropical herbivore, the sea urchin Tripneustes gratilla. Secondly, I used a bio-physical model to investigate climate-driven changes in dispersal of T. gratilla, to determine potential impacts on the urchin’s biogeographical range and on its interactions with kelp. I discovered that warming and MHWs increase kelp palatability and shift the microbial community of kelp towards larger abundances of microbes associated with disease and macroalgal degradation. Modelling forecasts indicate that T. gratilla is likely to expand its range poleward and create strengthened interactions with kelp. Overall, I discovered indirect future threats to Australia’s dominant kelp by forecasting strengthened kelp-tropical herbivore interactions and decreased resilience of kelp to grazing under warming and MHW conditions