Testing weed risk assessment paradigms: intraspecific differences in performance and naturalisation risk outweigh interspecific differences in alien Brassica

1.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.

2.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.

3.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.

4.Nevertheless, differences among taxonomic groups explained only 7% of the total variation in perf...