Invasive species may be generally described as biotic agents that occupy new environments or habitats and cause harm to the habitats or human interests. They include all categories of life such as from land plants to microorganisms. Invasive species are introduced intentionally or unintentionally to new environments. Invasive species are recognized as a major threat to the world’s biodiversity. They are also of significant socio-economic impact around the world.
Global map the number of species native to a country but considered invasive alien species (IAS) in other countries
Major invasives on the African continent include plants such as Prosopis, species which were introduced for agroforestry and landscape restoration in East Africa; paper mulberry (Broussonetia papyrifera) introduced to Ghana and Uganda for pulp paper production, and Leuceana (Leucaena leucocephala), introduced to many countries in Africa and around the world as multipurpose species for agroforestry. Aside from introduced plants, invasive insects and pathogens have also had a significant impact on Africa’s forests.
For example, the fall-armyworm [Spodoptera frugiperda (J.E. Smith); FAW] invasion has exacerbated maize (Zea mays L.) crop yield losses in sub-Saharan Africa (SSA), already threatened by other stresses, especially those that are climate-change induced. Other invasive insects include the blue gum chalcid (Leptocybe invasa) on Eucalyptus in eastern Africa, and the Sirex wasp (Sirex noctilio) and bronze bug (Thaumastocoris peregrinus) which have emerged in South Africa (Wingfield et al. 2008) as major threats to plantations. In South Africa, the pathogen Phytophthora cinnamomi infects native fynbos areas, as well as forests in the Tsitsikamma region and kills Ocotea bullata trees.
In eastern and southern Africa, invasive alien plant species are known to impact negatively on the conservation of biodiversity as well as the livelihoods of rural people that depend heavily on natural resources
A very problematic invasive plant, Opuntia stricta was initially introduced to many regions for ornamental and hedging (live fencing) purposes, but has escaped cultivation and has spread, including into conservation areas, rangelands and agricultural areas. Though it is considered responsible for a range of negative impacts, a systematic quantification of them is still lacking. Long-term monitoring of the impacts of invasive species on protected areas biodiversity and ecosystem services is lacking over large scale.