Predicting Acacia invasive success in South Africa on the basis of functional traits, native climatic niche and human use

Australian Acacia species have been widely planted worldwide for different purposes. Some of them have spread and altered the native ecosystem functions to the extent of being considered economic and ecologic threats. Understanding factors that allow these species to become invasive is an important step for mitigating or preventing the damaging effects of invasive species. We aimed to test the importance of native niche climatic width and average, plant functional traits (plant height, leaf area, seed mass and length of flowering season) and anthropogenic factors (number of uses, time since introduction) for predicting invasive success, in terms of abundance and range, of 16 Australian Acacia species in South Africa. By using multiple regression analysis, we constructed one different model for each type of predicting factors. When more than two predicting variables were available in a category, they were reduced to a maximum of two predictors by means of principal component analysis. Acacia spp. abundance and range in South Africa were highly correlated. The anthropogenic model (using number of human uses as predictor) was the best to explain both abundance and range of acacias in South Africa. This may be attributed to the importance of humans as dispersal vectors and to the relatively recent introduction of these species (circa 150 years). The functional traits model was the next best model explaining Acacia range, but not abundance, acacias with higher height and leaf area being more widespread in South Africa. Taller plants may disperse their seeds more efficiently by attracting dispersal agents, such as birds. The climatic affinities model was the following in the ranking explaining both range and abundance, acacias coming from moister, cooler and less seasonal regions in Australia being more successful in South Africa. This pattern may be attributed to the fast growth genotype generally selected for under low climatic stress conditions. Acacias with wide climatic niche in the native region were also more widespread and abundant in South Africa, probably because the same traits that allow them to be widespread in Australia, also contribute to overcome the climatic filters to establish throughout South Africa. This study provides managers with tools to identify those exotic Acacia ssp. having more chances to become successful invaders in South Africa.