The role of plant and soil microbial interactions in plant invasion in Australian temperate grasslands

Abstract

In this thesis I aimed to determine the role of plant-soil interactions in plant invasion in Australian temperate grasslands. This aim was tackled by determining whether plant-soil interactions explained variation in the invasion success of exotic species through assessing exotic species impact on native species via competition and interactions with the soil microbial community. To achieve this I chose several high-impact exotic, low-impact exotic and native species that commonly occur in Australian temperate grasslands and examined plant-soil interactions during the process of establishment, when exotic species first encounter native soil biota, and the subsequent stages of invasion, when exotic species have established and can alter the microbial community. Firstly, I established a robust way to report the error associated with plant responses to different soil communities, by assessing different approaches to calculate 95% conference intervals around the log response ratio. Second, I examined whether, at establishment, invasion success and impact of exotic grass species could be explained by 1) competitive superiority over resident native species, 2) advantages gained from the positive effects of native soil communities, or 3) both. To test these hypotheses, I inoculated glasshouse pots with soil taken from three native-dominated grassy ecosystems in Australia and compared the response of high-impact and low-impact exotic species and native species to the native soil biota. I found that exotic species were stronger competitors than natives and that the effects of native soil biota on plant performance were weak and did not predict variation in invasion success of exotic species. Differences in species relative competitive abilities demonstrated a way by which some exotic species could outcompete natives and suppress their growth during the invasion process. Third, I assessed how exotic species altered native soil communities. This was achieved by characterising the diversity and composition of bacterial and fungal DNA in soils after growing exotic and native plant species separately under glasshouse conditions for 3 months. High- impact exotic species decreased biodiversity in soil communities relative to native species while low impact exotic species did not. Changes in biodiversity induced by high-impact exotics could have an effect on the function of native soil communities and the performance of both native and exotic plant species. These findings open the ‘black box’ and to demonstrate that exotic species can affect Australian native soil communities. Fourth, I quantified plant responses to soil microbial communities associated with conspecific and heterospecific plant species to quantify species-specific plant-soil feedbacks and competitive interactions. I conducted a glasshouse experiment to quantify changes in plant biomass due to plant-soil feedback and competitive effects of high-impact, low-impact and native grasses and integrated these values into the plant-soil feedback framework (Bever, Westover and Antonovics, 1997; Bever, 2003; Revilla et al., 2013). High-impact exotics performed better on conspecific soil communities compared to low-impact and native species. Exotic species were often predicted to dominate natives. This was because the performance of exotics was often better, and outcompeted natives in both their own soil and native cultivated soil. In three out of 14 cases, plant-soil feedbacks were sufficiently strong to alter competitive relationships between native and exotic species which favoured natives over exotics and only occurred when competitive difference where small. Overall, I found that exotic species did impact native soil communities and that their competitive advantage allowed exotic to invade, supress and replace native plant species. For the grass species used here, plant-soil interactions did not appear to be important in allowing exotic species to invade native communities. Importantly, I found that considering the effects of plant-soil interactions in conjunction with competitive ability gave a greater understanding into the role of plant-soil feedbacks in invasion ecology.

Date of Award	2023
Original language	English
Supervisor	Richard DUNCAN (Supervisor), Lizzie WANDRAG (Supervisor), Peter H. Thrall (Supervisor) & Luke G. Barrett (Supervisor)