Responses of native and invasive Brassicaceae species to slug herbivory

It has been proposed that invasive plants are often less palatable or better able to compensate for biomass losses by herbivory than related, non-invasive species growing in the same area. We hypothesised that low palatability to slugs and/or an ability to compensate for grazing damage are traits contributing to the invasiveness of perennial Brassicaceae forb species introduced to northwestern and central Europe. In common garden and glasshouse experiments we compared life-history and fitness parameters of three native and three invasive Brassicaceae species of central European provenance that were subjected to herbivory by two slug species. Using the same species we performed leaf disc preference assays and investigated the effects of slug herbivory on small plants regenerated from root fragments and seedlings in field and glasshouse experiments. We found high between-species variation in susceptibility to slug herbivory but these were not related to the native or invasive status of the species. While the proportions of seedlings damaged or killed by slug herbivory did not differ between the two groups of species, the survival of damaged root regenerates was higher than that of seedlings. Consistent with our hypothesis, the invasive species, particularly those with clonal reproduction, showed higher compensation growth after slug herbivory. Our results suggest that a high ability for compensation growth in invasive Brassicaceae species makes them more tolerant to slug damage than native congeners. The potential to regenerate from root fragments, which are less vulnerable than seedlings to herbivory, appears to be another important factor contributing to the invasiveness of some clonal species. Since many invasive plant species share these traits (though regeneration may be from plant parts other than roots), we suggest that tolerance of herbivory may be one of the characteristics of many successful invaders.