Exploring the mechanisms and impacts of plant invasions in grasslands

Abstract

Non-native plant species are being introduced to ecosystems worldwide at an unprecedented rate. Many of these non-native species become invasive by spreading across their introduced ranges and impacting native biota, ecosystems and human societies. However, the impacts of the majority of non-native plant species remain unquantified. Moreover, the mechanisms that underpin the spread and impact of non-native plants are unclear, which hinders our ability to predict and mitigate impacts. The aim of this thesis is to improve our understanding of these mechanisms. To achieve this, experiments were conducted in the lab, greenhouse and field; across local, regional and global scales. The findings of this thesis add to mounting empirical evidence that contradicts prevalent assumptions in invasion ecology. Results showed that invader density is rarely proportional to impact and that changes to soil properties resulting from plant invasion reduced litter decomposition rates, while changes in litter quality had no effect. There was no evidence for belowground enemy release in driving the spread of three widespread invasive grasses in New Zealand, although biogeographic differences in soil biota influenced invasive species responses to nutrient enrichment. There was strong evidence that belowground competition underpinned the impact of a widespread invasive grass on native species, regardless of nitrogen availability. Most surprisingly, dominant native and non-native species did not differ in specific leaf area or leaf %N across natural or nutrient enriched grasslands worldwide. Since these traits are indicative of plant growth strategies, this suggests successful native and non-native grassland species exhibit similar growth strategies. However, non-native species showed higher leaf %P and %K, along with lower leaf %C than native species. These findings have contributed to unpicking some of the complex mechanisms that underlie the spread and impact of invasive plants. They also demonstrate that a more nuanced understanding of plant invasions is needed to protect biodiversity and ecosystem functions in an era of rapid global change.