Even well-studied groups of alien species might be poorly inventoried: Australian Acacia species in South Africa as a case study

Understanding the status and extent of spread of alien plants is crucial for effective management. We explore this issue using Australian Acacia species (wattles) in South Africa (a global hotspot for wattle introductions and tree invasions). The last detailed inventory of wattles in South Africa was based on data collated forty years ago. This paper aimed to determine: 1) how many Australian Acacia species have been introduced to South Africa; 2) which species are still present; and 3) the status of naturalised taxa that might be viable targets for eradication. All herbaria in South Africa with specimens of introduced Australian Acacia species were visited and locality records were compared with records from literature sources, various databases, and expert knowledge. For taxa not already known to be widespread invaders, field surveys were conducted to determine whether plants are still present, and detailed surveys were undertaken of all naturalised populations. To confirm the putative identities of the naturalised taxa, we also sequenced one nuclear and one chloroplast gene. We found evidence that 141 Australian Acacia species have been introduced to South Africa (approximately double the estimate from previous work), but we could only confirm the current presence of 33 species. Fifteen wattle species are invasive (13 are in category E and two in category D2 in the Unified Framework for Biological Invasions); five have naturalised (C3); and 13 are present but there was no evidence that they had produced reproductive offspring (B2 or C1). DNA barcoding provided strong support for only 23 taxa (including two species not previously recorded from South Africa), the current name ascribed was not supported for three species and, for a further three species, there was no voucher specimen on GenBank against which their identity could be checked. Given the omissions and errors found during this systematic re-evaluation of historical records, it is clear that analyses of the type conducted here are crucial if the status of even well-studied groups of alien taxa is to be accurately determined