Distribution and management of Acacia implexa (Benth.) in South Africa: A suitable target for eradication?

Abstract

This study is the first detailed assessment of an invasion by Acacia implexa (Benth.) (screw-pod wattle) anywhere in the world. Approximately 30 000 A. implexa individuals were found spread over about 600 ha (a condensed canopy area of ~96 ha) in three geographically distinct populations, all in the Western Cape, South Africa. Population structures indicate rapidly increasing populations at all sites, with vegetative suckering dominating over reproduction by seed. Populations appear capable of rapidly densifying if given the opportunity, creating monocultures and even out-competing other invasive acacias. Although seed viability is high (~60%), there is relatively low recruitment from seed, likely as a result of high seed predation (with some previous estimates of up to 100% seed loss). While high seed mortality may suggest limited rates of spread (populations are at most 1 km from initial plantings at two of the sites), seeds appear to be able to disperse along roads and watercourses (in particular plants are now established about 5 km down the Eerste River from the putative initial planting site in Stellenbosch). This suggests that the extent of all three populations could increase much more rapidly in the future. Formal risk assessment and bioclimatic niche modelling indicate that this species has the potential to invade large parts of South Africa, particularly in coastal regions. Given the current limited distribution, the potential threats posed and the success of control to date, we consider eradication a feasible and desirable management goal. The estimated cost of clearing established stands was ~ZAR 700 000, but given the strong ability of A. implexa to resprout, proper control and follow-up would be essential to prevent re-establishment of dense stands and further spread. A systematic eradication programme over the next decade will cost an estimated ZAR 1.5 million, giving a total eradication cost of ZAR 2.2 million. We support the proposed listing of the species as category 1a under the National Environmental Management: Biodiversity Act, and suggest that the species should not be introduced to other countries without clear and comprehensive contingency plans.