A frame-based model of acacia invasion and clearing in headwater streams of the southwestern Cape (South Africa)

Restoration of indigenous vegetation following invasive species removal is prone to failure, especially in riparian corridors where disturbances and propagule transport favor rapid reinvasion. Effective restoration strategies need to be based on more than an understanding of how to clear unwanted species from a single location and also take into account spatial context and community dynamics. Models capturing key ecosystem components and their responses to restoration decisions are needed to complement field experiments. I developed a frame-based model for headwater streams in the Southwestern Cape of South Africa to compare the effectiveness of various clearing strategies for promoting the recolonization of indigenous vegetation. Riparian forest and scrub has been replaced by invasive aliens, such as Acacia mearnsii, along many watercourses in this region. Alien removal has been ongoing since 1995 and involves felling, usually combined with herbicide application and follow-up hand removal. The model takes advantage of the knowledge of Acacia growth and removal costs from South African agencies responsible for these restorations. Model predictions indicate that increasing the labor available for clearing has the primary benefit of limiting the portion of the corridor in dense Acacia but is less important for restricting its extent. Clearing upstream-to-downstream, regardless of Acacia density, should sustain a greater portion of invasive-free stream reaches than other schemes. The model also suggests that even with a low invasion threshold to trigger management, frequent scouting, and follow-up clearing, reducing the extent of mixed Acacia-scrub below 25% of the corridor may be problematic using common clearing approaches.