Costs and benefits of biological control of invasive alien plants: case studies from South Africa

Abstract

Invasive alien species can have significant negative environmental and economic impacts. Such species are often controlled biologically by means of introducing host-specific insects or pathogens that can reduce the species' invasive potential. In many cases, plants are brought under complete control. In this paper, we describe an attempt to estimate the costs and benefits of the biological control of 6 weed species (Opuntia, aurantiaca, Sesbania punicea, Lantana camara, Acacia longifolia, A. pycnantha and Hakea sericea) in South Africa. We estimated the costs of biological control research that was conducted on the plants, the rate at which each of these species spreads in the absence of biological control, and the degree to which spread has been arrested or reversed by biological control. This, in turn, was used to estimate the extent to which the species would have spread had biological control not been introduced. We then estimated the most likely levels of consequences associated with uncontrolled spread. The effects were expressed in the form of 3 categories of benefits associated with the prevention of invasion: the loss of water due to excessive transpiration by invasive plants; reductions in the values of land that became invaded; and reductions in value added by biodiversity to ecosystem services. We compared these benefits with the costs of biological control research to derive cost: benefit ratios. The economic benefits of preventing invasion ranged from R300 ha-1 year-1 for jointed cactus (O. aurantiaca) to R3600 ha-1 year-1 for golden wattle (A. longifolia) (values are discounted to the year 2000). The economic value of water accounted for 70% of the combined benefits. Benefit: cost ratios for the historical analysis (from the release of the biological control agent to the year 2000) ranged from 8:1 for lantana (L. camara) to 709:1 for jointed cactus. When future estimates of benefits were considered, benefit: cost ratios were greater, and ranged from 34:1 for lantana to 4333:1 for golden wattle. These large differences can be attributed to the length of time that the biological control agents have been released (this ranged from 13 to 65 years for different weed species) as well as to the 30-fold differences in the potential area that different weed species would eventually invade. A sensitivity analysis revealed that the model was sensitive to changes in the estimated rate of spread. The sensitivity analysis also showed that the returns on investment in biological control research generally remain positive with some variations between species.