Humans appropriate ecosystem goods and services to maintain their quality of life. Consequently, anthropogenic activities have modified 40% of terrestrial ecosystems in a relatively short time. A major component of these modified systems are invasive species. Invasive species can drive a decline in native species richness and alter community structure, influence resource use and availability in their community, and in turn alter ecosystem functioning. Approaches analysing the similarity of traits between invasive and native species have become an increasingly common means to infer the processes underlying successful invasions and to predict future invaders. These
approaches comprise part of a larger research context of understanding how species’ traits link their response to environment, influence on ecosystem functioning, and interactions with
other species.

In this thesis I used two overarching research questions to investigate whether environmental selection for plant functional traits influences species’ population trajectories, which bears relevance for both native species decline and spread of invasives. These two questions were: a) How do environmental drivers of change influence species and trait composition (and can these influences be detected over the influence of spatial processes)? and b) how does the trade-off between competitive release (associated with novel traits) and environmental filters on community traits determine the success of species?

I addressed these questions by testing a number of hypotheses pertaining to each. To do this, I used data on plant communities over 30 years from grasslands across the South Island of NZ and sourced trait data to describe these communities from online databases. I used a multivariate analysis framework to test for trait—environment relationships and describe how these relationships varied across multiple spatial scales. Additionally, I used measures of trait novelty and fit to environment, within a mixed modelling framework, to test how differences in these measures related to change in each species’ abundance over time.

In doing so, I did not find support for my hypothesis that it is advantageous for species to have a novel (unique or original) set of traits, and that this advantage would be reflected in novel species increasing in abundance over time. Instead, I found that the environment shaped species traits, and that species with traits that deviated from those determined by environmental filters decreased in abundance. My findings suggest that searching for traits of ’invasiveness’, or even comparing invader traits to those of the community, may have limited value in predicting the success of invaders unless these are considered within the context of environmental trait selection.