Costs, benefits and management options for an invasive alien tree species: the case of mesquite in the Northern Cape, South Africa

Abstract

Mesquite (Prosopis species) were introduced to South Africa to provide fodder and shade for livestock, but some have become invasive, impacting on water and grazing resources. Mesquite’s net economic effects are unclear and their unequal distribution leads to conflict. We estimated the value of mesquite invasions in the Northern Cape Province for different scenarios, differentiating between productive floodplains and upland areas. The estimated net economic value of mesquite in 2009, covering 1.47 million ha, was US$3.5–15.3 million. The value will become negative within 4–22 years, assuming annual rates of spread of 30 and 15%, respectively. The estimated 30-year present value (3% discount rate) of the benefits of control in the floodplains exceeded that of costs but the opposite was true in the uplands. Control efforts should therefore focus on floodplains while preventing spread from uplands into cleared or uninvaded floodplains. More efficient control methods are needed as estimated control costs (>US$9.5 million yr-1) exceed financial capabilities of Public Works programmes. Control in the floodplains was not economically justifiable using an 8% discount rate, because this substantially discounted future costs. We conclude that more effective control methods, such as biological control, are needed to prevent substantial economic losses.