How can knowledge of the climate niche inform the weed risk assessment process? A case study of chrysanthemoides monilifera in Australia

Aim: Climate change and the ability of alien populations to realize different climatic niches compared to native populations pose challenges for pre-empting invasion risk. These issues are not addressed in Weed Risk Assessments (WRAs), which have been developed to identify potentially invasive species and prevent their importation. Chrysanthemoides monilifera, native to Southern Africa, has two subspecies invasive in Australia, which has led to an importation ban on all six subspecies. We assess whether the two invasive subspecies occupy different realized climatic niches, compared with native populations, and the climatic suitability of Australia for all subspecies under current and future climate scenarios. Location: Southern Africa and Australia Methods: Realized climatic niches of native and alien populations of two invasive subspecies (Bitou Bush and Boneseed) were compared using niche identity tests. The distribution of climatically suitable habitat within Australia for all subspecies was modelled using MaxEnt, under current and future climate scenarios. For invasive subspecies, models were calibrated using (1) native or (2) alien range data. Results: Realized climatic niches of native and alien populations are not identical, with some alien populations of Boneseed occupying climatic niches absent from Southern Africa. As such, MaxEnt models for Boneseed based on native range data failed to classify one-third of Australian populations as inhabiting suitable climate. Main Conclusions: We validate the Australian decision to ban all subspecies by showing that climatically suitable habitat in Australia for non-introduced subspecies exceeds that of introduced subspecies, under current and future climates. Niche shifts and climate change alter estimates of invasion risks, and this may reduce efficacy of current WRAs. We call for greater dialogue to identify and standardize a comprehensive system for incorporating these challenging issues into WRA systems to ensure that they remain effective in reducing the weed risk into the future.