Current and potential economic costs of Opuntia stricta invasions in Africa

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 costs of current invasions to 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/m ² of forage, valued at USD 0.12/m ² . Based on this finding, we estimate that the current cost of O. stricta invasions is over USD 5.67 and USD 168 million in Laikipia and in eastern Africa respectively. A conservatively estimated area of 682,000 km ² (2.3% of sub-Saharan Africa) is at different levels of risk of invasion. Using plausible spread rates of 5–15% annually and scenarios of area at risk of invasion, we estimate that costs associated with the loss of forage in sub-Saharan Africa could grow to a mean present value of USD 77 billion over 50 years (range 10–300 billion assuming discount rates of 3 and 5%). The introduction of the biological control agent, Dactylopius opuntiae ‘stricta’ biotype, has already reduced these potential impacts significantly in South Africa and Kenya.