Abstract
This study is the first detailed assessment of an invasion by Acacia implexa (Benth.) (screw-pod wattle) anywhere in the world. Approximately 30000 A. implexa individuals were found spread over about 600ha (a condensed canopy area of ~96ha) 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 1km 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 5km 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 700000, 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 1.5million, giving a total eradication cost of ZAR 2.2million. 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.
Highlights
IntroductionCultural purposes (Kull and Rangan, 2008; Kull et al, 2011; Richardson et al, 2003; Stirton, 1978)
Australian Acacia species have a long history of introductions to South Africa for forestry, dune stabilization, horticultural and⁎ Corresponding author at: Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Matieland 7602, South Africa
We identified three populations of A. implexa in South Africa with population extents of 100–200 ha (Table 1) — (1) at Tokai on the eastern slopes of Table Mountain, (2) at Wolseley near the Kluitjieskraal forestry station ~ 4 km west of the town and (3) in Stellenbosch where the main infestation is on Papegaaiberg though several plants are spread along the Eerste River, and there are a few individuals presumably planted for ornamental purposes in the J.S
Summary
Cultural purposes (Kull and Rangan, 2008; Kull et al, 2011; Richardson et al, 2003; Stirton, 1978) Characteristics such as rapid growth rates (Witkowski, 1991) and prolific seed production (Milton, 1980; Richardson and Kluge, 2008) have led to some Australian Acacia species becoming the country's worst invaders, with roughly a third of the total cost of alien plant clearing spent on this group (van Wilgen, et al, 2012). B Southern African Plant Invaders Atlas (SAPIA) is a mapping project started in 1994 to gather information on invasive and naturalized plant species in South Africa, Lesotho and Swaziland (Henderson, 2001, 2007).
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