An assessment of changes in the abundance of two Australian <i>Acacia</i> species under biological control in the Southwestern Cape region, South Africa

Considerable investments have been made in the biological control of invasive Australian Acacia species in the Fynbos biome of South Africa. The biological control agents introduced for the most invasive Acacia species do not kill standing trees but rather reduce the reproductive output, thereby reducing the rate of spread. Here, we consider the changes in the abundance of Acacia saligna (Labill.) H. L. Wendl. and Acacia cyclops A. Cunn. ex G. Don., in the presence of biological control agents for the purpose of informing long-term decision-making on the costs of manual control. We used remote sensing of permanent monitoring plots to assess the impacts of biological control in combination with fire and firewood removal. The cover of these two species where the plots had been left to biological control, informal firewood harvesting, and fire with no other forms of prescribed control for a 20-year period was assessed. The cover decreased by up to 85%, with smaller changes over periods of less than 10 years, and no impacts over a 6-year period. Biological control resulted in a marked slowing down in the rate of increase of the Acacia spp. cover. The reduction in the rate of spread of these two highly invasive species due to biological control should reduce the cost of clearing and, therefore, the financial burden on landowners and natural resource management entities involved in Acacia spp. control.