Plant population ecology and biological control: Acacia nilotica as a case study

The conversion of grasslands and savannas to shrublands and woodlands by invasive weeds is a global problem confounding scientists and land managers. Since the 1960s, prickly acacia, Acacia nilotica ssp. indica, has substantially increased in density and range to become one of Australia's worst tropical rangeland weeds. Although the plant has been classified as a noxious weed for more than 30 years, relatively little research on its biology or ecology occurred until the late 1980s. A program of introducing biological control agents commenced in 1980 and resulted in the release of two insects: one failed to establish and the other has failed to exert any control. We suggest that studies of the population dynamics of a weed should occur prior to implementation of biological control programs. These can be valuable to identify weaknesses that can be exploited and to focus the search for effective agents, thereby enhancing the success rate of biological control efforts. We examine the life history and dispersal mechanisms of A. nilotica with a view to understanding the relative sensitivity of the population to reductions in each lifestage, understanding the vulnerability of each lifestage to biological control, and prioritizing potential control points in the lifecycle. Seedlings and juveniles appear to be the most susceptible lifestages to target for control. They are relatively easy to kill, and significant reductions in their numbers will reduce the overall weed status of the plant