A review of two decades of government support for managing alien plant invasions in South Africa

Large sums of money are spent globally on invasive alien plant control projects, but their effectiveness in the medium to long term is seldom reported. Here we review the cost, extent and effectiveness of the management of plant invasions by South Africa's “Working for Water” program between 1998 and 2020. We use a broad framework of indicators for assessing the inputs, outputs, and outcomes of alien plant control interventions at a national level. Our study is based on (1) spatially explicit data on efforts that targeted selected sites and species for control; (2) surveys of the extent of invasion; and (3) case studies of control effectiveness. An average of ZAR310 million (adjusted to 2020 values) was spent annually, creating the equivalent of 8113 full-time jobs, with expenditure rising rapidly between 1998 and 2003 and again between 2009 and 2015, dropping more recently. Control efforts were directed at 178 species, with 15 taxa receiving two thirds of the total expenditure. Control was conducted on ~76,000 sites covering 2.7 million ha, which is ~14 % of the estimated invaded area. Over a quarter of the control was not in priority areas for biodiversity and/or water conservation. The effectiveness of control operations has not been monitored regularly, but a few studies at local scales have found reductions in alien plant cover, and it is likely that “Working for Water” has had a role in limiting invasion. Nonetheless, national surveys suggest that plant invasions have continued to grow—the problem is too large to expect that control can be achieved everywhere. We recommend that, to bring plant invasions under control at priority sites, a national strategy should be based on conservation triage, focussing on clearly defined priority sites, improving planning and monitoring, and increasing operational efficiency.