Predicting the Australian Weed Status of Southern African Plants

Abstract

A method of predicting weed status was developed for southern African plants naturalized in Australia, based upon information on extra-Australian weed status, distribution and taxonomy. Weed status in Australia was associated with being geographically widespread in southern Africa, being found in a wide range of climates in southern Africa, being described as a weed or targeted by herbicides in southern Africa, with early introduction and establishment in Australia, and with weediness in regions other than southern Africa. Multiple logistic regressions were used to identify the variables that best predicted weed status. The best fitting regressions were for weeds present for a long time in Australia (more than 140 years). They utilized three variables, namely weed status, climatic range in southern Africa and the existence of congeneric weeds in southern Africa. The highest level of variation explained (43%) was obtained for agricultural weeds using a single variable, weed status in southern Africa. Being recorded as a weed in Australia was related to climatic range and the existence of congeneric weeds in southern Africa (40% of variation explained). No variables were suitable predictors of non-agricultural (environmental) weeds. The regressions were used to predict future weed status of plants either not introduced or recently arrived in Australia. Recently-arrived species which were predicted to become weeds are Acacia karroo Hayne (Mimosaceae), Arctotis venustra T. Norl. (Asteraceae), Sisymbrium thellungii O.E. Schulz (Brassicaceae) and Solanum retroflexum Dun. (Solanaceae). Twenty species not yet arrived in Australia were predicted to have a high likelihood of becoming weeds. Analysis of the residuals of the regressions indicated two long-established species which might prove to be good targets for biological control: Mesembryanthemum crystallinum L. (Aizoaceae) and Watsonia meriana (L.) Mill. (Iridaceae).