Alien Brassica: variation in performance among and within species and locations

Abstract

Our ability to consistently predict which taxa will become invasive and where, and understanding of the drivers of this variation is limited. A better understanding of how variation in traits within species and biotic and abiotic conditions within regions influence patterns of invasion is needed to improve our understanding of the processes underlying invasions and our ability to manage alien species. The taxonomic unit used to assess alien invasiveness is usually the species. There is considerable intraspecific variation in plant traits, but the extent to which these differences can affect invasiveness is unknown. I tested the hypothesis that most of the variation in plant performance among taxa would reside at the species level (relative to subspecies and varieties). I quantified variation in performance among taxa of alien Brassica with data from a seed sowing experiment comprising 24 taxonomically stratified varieties (six subspecies, three species) used to parameterise hierarchical models. Around 30 times more variation resided among varieties within species than among species. Differences in the viability rates of the sown seeds explained some of the differences in performance. My results show that intraspecific taxa of alien species can vary significantly in performance and suggest novel genotypes may pose a different invasion risk to that currently established for the species. Weed risk assessment protocols could be modified to include varietal traits such as seed viability in assessments.

The availability of seeds and bare ground both influence whether plant populations establish, but the degree to which they influence persistence is less well known. The taxonomic level at which most variation in alien performance resides may shift under different regimes of seed and site availability. I quantified the effects of relaxing seed and site limitation on the performance of a taxonomically stratified selection of 12 varieties of alien Brassica, and looked for shifts in the taxonomic level at which most variation in performance resided. Seeds were sown at two densities (400 seeds/m⁻² and 4000 seeds/m⁻²), and with and without pre-sowing soil disturbance. Both a higher sowing density and pre-sowing disturbance increased Brassica emergence, but only disturbance increased survivorship, and neither resulted in significantly higher population growth rates. Most variation in performance among taxonomic levels resided at the varietal level regardless of seed density or disturbance. While high seed density and disturbance can increase seedling recruitment, unless aliens can pass subsequent ecological filters persistence is unlikely.

Alien plant populations often become extinct due to demographic and environmental stochasticity, unsuitable traits, and Allee affects. Such aliens may appear to be naturalised if new populations are founded by dispersed or immigrant propagules. Differentiating between transient and persistent populations is crucial to our understanding and management of invasions. I recorded feral alien Brassica populations in three annual surveys, along with measures of seed sources, site and management characteristics that may influence the probability of presence and survival. I identified correlates of population presence and survival, and quantified the probability of population survival from year to year. Brassica would likely not persist in the landscape beyond ten years without anthropogenic seed input. Only around 40% of populations survived two or more years after foundation, but new populations focused around transportation routes and seed companies compensated for extinctions. Stochasticity and trait-site mismatch were causes of population extinction. Adventive aliens may be controlled by managing propagule sources; accounting for population extinctions is important to accurately assess distributions from cumulative presence data.

Further research is required to identify which traits drive variation in plant performance and invasiveness within species. In addition we need to be able to identify taxa which are most likely to contain substantial intraspecific variation in these traits. Applying the precautionary principle would suggest that the same restrictions should apply to the importation of new plant varieties, subspecies and genotypes as are applied to the importation of new plant species.