Prioritizing weed species based on their threat and ability to impact on biodiversity: A case study from New South Wales

Weeds pose a significant threat to biodiversity, but information on which species pose the greatest threat and the level of their impact is missing (i.e. only available for a few of the 3100+ naturalized plant species in Australia). Here we assessed the 1665 naturalized plant species in NSW to ascertain their level of threat as well as their ability to impact on native species. First we excluded those species which were (i) not known to be invasive, (ii) known from only a few locations, or (iii) not environmental weeds. The remaining 340 species were then modelled to establish a prioritized list. While we did not assess the actual impacts, the five attributes used in the model (i.e. spatial threat, species impact, invasive ability, number of species at risk and habitat type), when combined, provide a reasonable assessment given the lack of robust data available on impacts. The modelling process identified three extreme and 19 very high priority species with respect to their ability to have negative impacts on biodiversity. Of these many have been identified in other weed lists (e.g. the three extreme species, Anredera cordifolia (Madeira vine), Lantana camara (lantana) and Chrysanthemoides monilifera subsp. rotundata (bitou bush) were ranked as forty-first, fourth, and sixth, respectively, in the determination of the Weeds of National Significance). Our prioritized list of weed species based on their ability to impact on biodiversity will help to aid management decisions, especially for those weed species posing a landscape scale impact, in the absence of impact data.