Invasive trees remain a threat despite substantial control efforts

Triacnthella madiba

Invasive alien pine trees now cover about 60 000 condensed ha in the fynbos biome. Formal control operations have treated 7400 condensed ha in the fynbos at a cost of R70 million, yet the pines continue to spread and dominate over large areas (photo: B.W. van Wilgen).

C·I·B Core team member Dr Brian van Wilgen of the CSIR recently led an assessment of progress in alien plant control in South Africa over the past 15 years. The team comprised researchers from the CSIR as well as the Agricultural Research Council and the Working for Water programme. The study, published in the journal Biological Conservation, reviewed data from three national-level estimates of the extent of invasion, records of the costs and spatial extent of invasive species control operations, assessments of the effectiveness of biological control, and smaller-scale studies. The team identified the most important invasive species in terrestrial biomes, and assessed how control efforts have impacted on the extent of invasion of these species. The study focussed on 18 alien species, mainly trees, that were prominent invaders. Control costs over 15 years amounted to R3.2 billion, almost half of which was spent on 10 species, the most prominent being invasive trees in the genera Acacia, Prosopis, Pinus and Eucalyptus. Despite significant spending, control operations were in many cases applied to a relatively small proportion of the estimated invaded area, and invasions appear to have increased in many biomes, where they remain a serious threat.

The findings of this assessment suggest that South Africa’s national-scale strategy to clear invasive alien plants should be substantially modified if impacts are to be effectively mitigated. This is a sobering finding, as significant amounts have been invested without apparently stemming the tide. While there has been some progress in some areas, overall the situation has deteriorated. The study makes several clear recommendations that, if implemented, would improve the effectiveness of the control operations. These include the need to:

  • Invest in, and significantly improve, planning, monitoring and evaluation, within a framework of adaptive management that will allow the program to adjust to realities;
  • Prioritize areas and species, and to focus on these priority areas and species;
  • Increase spending on biological control (which has by far brought about most successes), and improve the integration of biological control into planning and management in a better way;
  • Improve efficiency and professionalism;
  • Encourage, or even require, much more widespread use of payment for ecosystem services (e.g., through levies on water);
  • Find better ways of dealing with privately-owned land;
  • Face up to conflict species (for example pines, wattles and mesquite, where there are both benefits and costs), assess the costs and benefits properly, and take appropriate decisions at a political level on how to deal with them; and
  • Re-affirm the primary goal of the flagship Working for Water programme — to control invasive alien plants so that people can continue to benefit from ecosystem services, instead of allowing them to be systematically eroded. While job creation and other benefits will and should arise from the program, they should not detract resources from the core purpose.

If these recommendations are adopted and implemented at a national scale, the available funds could achieve much more.

To read the paper, click here.