The global distribution of acacia

Abstract

A global assessment of the distribution of the Australian genus Acacia (hereafter Acacia or ‘wattles’) in 2011 suggested that the worldwide expansion of this group was far from complete. Capitalizing on the recent explosion of crowdsourcing data, we undertook a global reassessment, more exhaustive and reproducible than the 2011 effort, to highlight new knowledge obtained from improved data integration and to assess changes in distribution of alien wattles over the past decade. We gathered data from 105 sources to assemble a global database listing species introductions and naturalizations at the country level; and make this available online to facilitate future reassessments. We explore patterns of introduction and naturalization and modelled the relationships between naturalizations of wattles, macroclimate, sampling effort, and various variables of introduction pressures across countries. Wattles have been introduced to 172 countries. However, only 28% of the 959 introduction events have (until now) led to naturalization, and even fewer species naturalized in the five countries that received most introductions (between 9 and 27%). We found evidence of 417 Acacia species (41% of species in the genus) introduced outside Australia so far; this adds 31 species to the 2011 assessment. At least 75 species have become naturalized. Two-thirds of reported introduction events only involved the 30 most widely exported species. Only 22% of the species ever exported have been reported over the past 70 years outside Australia, of which 20% have naturalized. Naturalization richness was well explained by our model, which showed the number of uses of introduced species to be a strong determinant of naturalization success. Our results provide insights into the consequences of the ongoing momentum of the human-mediated dissemination of wattles. They show that most wattle introductions have not (yet) resulted in naturalization, but that species that are intensively planted during their introduction, notably those used in forestry, often naturalize. We discuss how recent trends point to a globalization of wattle transfers, especially of forestry species, and suggest that many more naturalizations lie ahead. We also discuss requirements for future reassessments and highlight some challenges.