<p>Recent biological invasions provide a unique opportunity to examine how species may adapt to novel conditions over relatively short time frames. Introduced species may respond to novel environmental conditions in the new range via rapid evolution, phenotypic plasticity, or the rapid evolution of phenotypic plasticity. However, the prevalence of these different mechanisms in introduced species remains unclear. In this thesis, I explore how introduced plant species may adjust their phenotype when introduced to a new range. First, I tested for evidence of phenotypic change through time in key morphological traits (plant height, leaf area, leaf shape, and leaf mass per unit area), using historic herbarium records for 34 plants introduced to Australia and New Zealand. Thirty-two out of 94 trait-species combinations showed evidence for change through time. The rate and direction of trait change was variable across species and the local climate. One possibility is that species introduced to a new range exhibit different trait responses depending on the relative difference in environment between the native and introduced range. To investigate this, I quantified climatic niche shifts in introduced species relative to their native range. I then predicted trait change through time from the magnitude and direction of climate niche shift in a meta-regression. This is the first study to simultaneously assess trait change in multiple introduced species in relation to a shift in their realised niche. Overall, climate niche shifts did not predict trait change through time, suggesting that climate may not be the predominant driver of trait change in plants introduced to Australia and New Zealand. Alternatively, the combined uncertainty and the mismatch in spatial scales that may arise when combining these two methods could mask any underlying patterns in plant trait responses to the new environment. It has been hypothesised that introduced species may respond to a sudden change in environment, by rapidly selecting for an increase in phenotypic plasticity. I tested for a difference in phenotypic plasticity between the native and introduced ranges of a beach daisy, Arctotheca populifolia. Contrary to my expectations, A. populifolia has shown a loss of phenotypic plasticity in as little as 80 years since its introduction to Australia. When using a meta-analysis to test for an overall difference in plasticity across multiple traits, I found that the current practice of calculating an effect size of an effect size (Hedges’ d) can lead to misleading results. I demonstrate how this issue arises when calculating a difference in Hedges’ d between two populations with different standard deviations. I propose an alternative way to calculate Hedges’ d to give a more accurate reflection of the difference in plasticity between ranges. Finally, I combine different lines of evidence from the previous chapters in a case study to explore how A. populifolia has changed since its introduction to Australia, and examine any discrepancies between the results. A glasshouse experiment revealed distinct trait differences between native and introduced populations of A. populifolia, which were not reflected in trait change through time inferred from herbarium specimens. Additionally, measured trait differences between ranges in the glasshouse experiment better reflected a niche shift into wetter climate, than the predicted trait change through time from herbarium specimens. This suggests that trait differences determined in glasshouse or common garden experiments, may be a more suitable approach to assess trait change in relation to a realised niche shift than using herbarium specimens.</p>
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