Current and potential economic costs of Opuntia stricta invasions in Africa
摘要
Cacti have been introduced to many parts of the world for food, fodder, and ornamental purposes. Many of these have become invasive, impacting negatively on human and animal health, biodiversity, and pasture production. Opuntia stricta is invasive in several countries in Africa. Studies in Laikipia County, Kenya, have shown that this invasive plant has significant negative impacts on livelihoods and biodiversity. We estimated the impact of current invasions on the value of fodder for livestock production in Laikipia and regionally in eastern Africa. Using an eco-climatic model, we extrapolated potential costs to sub-Saharan Africa on the assumption that O. stricta is likely to at least partially invade all areas that are climatically suitable. Areas invaded by O. stricta in Laikipia prevented access by livestock and wildlife to an average of 408 g/m2 of forage, currently valued at USD 0.25/m2. Based on this finding, we estimate that the value of forage inaccessible to livestock due to current O. stricta invasions is over USD 8.4 million and 246.6 million in Laikipia and in three eastern African countries respectively. A conservatively estimated area of 682,000 km2 (2.3% of sub-Saharan Africa) is at different levels of risk of invasion. We estimate that the present value of lost forage in sub-Saharan Africa over the next 50 years is USD 307 billion, assuming an annual spread rate of 10% and a discount rate of 5%. A sensitivity analysis that used a range of values for input variables predicted present values between USD 115 and 562 billion at a spread rate of 10%, with the outputs being most with sensitive to the assumed rate of spread and the assumed current extent of invasions. The introduction of the biological control agent, Dactylopius opuntiae ‘stricta’ biotype, has already reduced these impacts significantly in South Africa and Kenya, and could substantially offset these costs elsewhere.