Aim  There is debate over whether alien plants necessarily alter the communities they invade or can coexist with native species without discernable impacts. We followed the fate of montane plant communities in response to the experimental sowing of the alien weed Hieracium lepidulum, looking for changes in plant community composition and structure over 6 years. Location  Craigieburn Range, New Zealand. Methods  We used a replicated randomised block design, with 30 × 30 cm plots (n = 756) subdivided into 5 × 5 cm cells to examine and compare the effects of H. lepidulum at 0.09 m2 (plot) and 0.0025 m2 (cell) scales. Plots were sown with between 0 and 15,625 H. lepidulum seeds in 2003, forming gradients of invader density and cover. Measurements comprised community richness, evenness and diversity along with H. lepidulum density and cover at both scales. The relationships between the invader and local community attributes were modelled using hierarchical mixed‐effect models. Results  Plant communities differed in the extent to which they became invaded, with H. lepidulum cover in the plots ranging from 0% to 52%, with a mean of only 1.89%. Plot species richness increased from 2003 to 2009, with a component of this increase (+0.002 species per year) associated with increasing H. lepidulum density. Other relationships between the plant community and H. lepidulum were generally non‐significant. Main conclusions  In these montane plant communities, it appears H. lepidulum coexists with the native community with no measurable negative effects after 6 years on species richness, evenness or diversity, even where density and cover of the invader are highest. We suggest H. lepidulum has persisted preferentially at those sites with abiotic conditions sufficient to support a species‐rich assemblage.

Abstract. Invasive species distributions tend to be biased towards some habitats compared to others due to the combined effects of habitat-specific resistance to invasion and non-uniform propagule pressure. These two factors may also interact, with habitat resistance varying as a function of propagule supply rate. Recruitment experiments, in which the number of individuals recruiting into a population is measured under different propagule supply rates, can help us understand these interactions and quantify habitat resistance to invasion while controlling for variation in propagule supply rate. Here, we constructed recruitment functions for the invasive herb Hieracium lepidulum by sowing seeds at five different densities into six different habitat types in New Zealand's Southern Alps repeated over two successive years, and monitored seedling recruitment and survival over a four year period. We fitted recruitment functions that allowed us to estimate the total number of safe sites available for plants to occupy, which we used as a measure of invasion resistance, and tested several hypotheses concerning how invasion resistance differed among habitats and over time. We found significant differences in levels of H. lepidulum recruitment among habitats, which did not match the species' current distribution in the landscape. Local biotic and abiotic characteristics helped explain some of the between-habitat variation, with vascular plant species richness, vascular plant cover, and light availability, all positively correlated with the number of safe sites for recruitment. Resistance also varied over time however, with cohorts sown in successive years showing different levels of recruitment in some habitats but not others. These results show that recruitment functions can be used to quantify habitat resistance to invasion and to identify potential mechanisms of invasion resistance.

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Risk assessments of alien species are usually conducted at species level, assuming that all individuals of a given species pose similar risks. However, this may not be the case if there is substantial within‐species variation that could influence invasion success. We used a seed addition experiment, comprising 25 taxonomically stratified varieties of three Brassica species introduced to roadside habitats in Canterbury, New Zealand, to quantify variation in performance among species, subspecies and varieties. We aimed to assess if species was the most appropriate taxonomic level at which to evaluate invasion risk. Differences among varieties within species explained approximately 30 times more of the variation in performance (number of individuals/quadrat) than differences among species. Some of the variation among varieties was attributable to differences in seed viability. Nevertheless, differences among taxonomic groups explained only 7% of the total variation in performance; 28% was attributable to differences among plots, reflecting broad‐scale environmental variation, while 65% was attributable to differences among quadrats nested within plots, highlighting the importance of fine‐scale variation in the availability of suitable microsites. Policy Implications. Our seed addition experiment quantified variation in performance of 25 taxonomically stratified Brassica taxa introduced to roadside habitats. Varieties (nested within species) differed in performance far more than did species. This suggests risk assessments carried out at species level may overlook important subspecific variation in invasion risk. This is particularly true for conventionally bred and genetically modified species, which may contain taxa posing risks different to that at which the species is assessed. Consideration should be given to subjecting unassessed subspecies and varieties of plants to risk assessments similar to those applied to species.

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