Though the scale of the socio-economic costs associated with IAS is poorly understood, it is estimated that the direct impacts of IAS and their management cost the global economy billions of US$ annually. It is estimated that IAS cost the EU at least €12.5 billion/year, and Australia at least AUS$13.6 billion/year. Invasive alien insects alone, due to their impacts on agriculture and forestry, cost at least US$70 billion/year globally. The global cost of controlling invasive freshwater biofouling animals, such as the zebra and quagga mussels which accumulate on wetted surfaces in electric power generation and water treatment facilities, is estimated at more than US$277 million/year.
The water hyacinth (Eichhornia crassipes), for example, which is native to South America, has been intentionally introduced around the world for ornamental purposes and as animal food, and has rapidly invaded water ways, irrigation channels, lakes and rice paddies. Under the right conditions it can double in biomass within two weeks forming dense mats, with potentially disastrous consequences. In Africa’s Lake Victoria, water hyacinth infestations covering 12,000 hectares have blocked shipping trade and access to ports, and halted fishing activities, impacting 40 million people.
Agriculture and fisheries are particularly vulnerable to the impacts of IAS, placing food security at risk and jeopardising livelihoods. For example, the fall armyworm (Spodoptera frugiperda) was introduced to sub-Saharan Africa most likely from the USA in 2016, and is spreading rapidly across the continent causing yield losses of over 40% for smallholder maize farms in some countries. Mesquite (Prosopis), a thorny drought-tolerant shrub native to the Americas, was widely introduced to tackle the impacts of desertification but became invasive, forming impenetrable thickets in many countries. In Ethiopia it has seriously affected the ability of pastoralist communities to rear their livestock.
IAS also pose a threat to human health, directly by exposing people to injuries and wounds e.g. through bites and stings, or indirectly by transmitting diseases through infectious pathogens, or providing suitable habitat for disease spreading organisms. The Asian tiger mosquito (Aedes albopictus), native to South-East Asia, is a vector of a number of human diseases such as Dengue fever and West Nile virus. It has spread to many countries around the world via shipping, surviving in standing water (e.g. used tires). The mosquito facilitates disease outbreaks such as the Chickunguya outbreak in the Indian Ocean islands of Réunion and Mauritius (2005-2006) where more than 272,000 people were infected.
The impacts from IAS can be compounded by climate change which can facilitate the spread and establishment of alien species. For instance, climatic events such as floods can bring invasive species into new areas. The resilience of natural habitats can also be reduced by IAS, making them more vulnerable to the impacts of climate change. For example, introduced grasses and trees may alter fire regimes, particularly in areas that are becoming warmer and drier due to climate change, putting habitats and human life at risk.