The invasive ant Linepithema humile disrupts seed dispersal in indigenous South African plants. Credit: Mark Moffett/Minden Pictures/Alamy

South Africa is losing its battle against biological invaders, according to the first attempt by the government to comprehensively assess the status of the country’s alien species.

The invaders, including forest-munching wasps, hardy North American bass and trees attractive to mosquitoes, cost the country approximately 6.5 billion rand (US$450 million) a year and are responsible for about a quarter of its biodiversity loss. That’s the conclusion of a pioneering report that the South African National Biodiversity Institute in Pretoria released on 2 November.

Invasive species also guzzle a substantial amount of South Africa’s water, a serious problem in a country suffering from a prolonged and catastrophic drought that is expected to worsen as the climate changes.

The report, which the institute compiled in response to 2014 regulations that mandate a review of invasive species every three years, examines the pathways by which these species enter the country and the effectiveness of interventions. It also weighs the toll they take on the nation’s finances and biodiversity.

This achievement constitutes a “significant advance” compared with efforts by most other countries, says Piero Genovesi, who chairs the invasive species specialist group of the International Union for Conservation of Nature in Rome. He says that other reports have looked at the impact of biological invasions, or at measures to address the problem, but they have not considered all aspects of invasions.

Helen Roy, an ecologist at the Centre for Ecology and Hydrology near Oxford, UK, says that, to her knowledge, this is the first comprehensive synthesis of the state of invasive species by any country. The report provides “an incredible basis” on which to build predictive approaches to invasive species that could be used to inform prevention strategies in South Africa, she says.

Climate change

Across the world, invasive alien species — organisms that have been introduced into ecosystems beyond their natural habitats, and that spread over large distances on their own — are considered a major threat to biodiversity, human health and economies. Climate change is expected to further their spread around the world, in part by reducing the resilience of native ecosystems. In 2015, 37 researchers from 14 national organizations, led by the National Biodiversity Institute and the Centre of Excellence for Invasion Biology at Stellenbosch University, began compiling the South African report. The researchers collated data from agencies and institutions around the country to measure the different aspects of biological invasion.

They report that 7 new species are introduced into South Africa each year, and that about 775 invasive species have been identified so far. This contrasts with the 556 invasive taxa listed in the government’s 2014 regulations on invasive species. Most of the species identified by the latest report are plants, with insects the next most common. (For comparison, the United Kingdom reports that it has 184 non-native invasive species). The report’s authors consider 107 of these invaders to have major impacts on the country’s biodiversity or on human well-being.

Invaders of note include trees in the Prosopis genus, such as honey mesquite (P. glandulosa), which was introduced throughout Africa for animal fodder. The shrub damages animal grazing areas, outcompetes local plants and, according to a 2017 study in Mali, seems to encourage the growth of populations of the malaria-carrying Anopheles mosquito1, among other things.

The water hyacinth Eichhornia crassipes, originally from South America, can choke South Africa's waterways. Credit: John Robinson/Africa Media Online/Alamy

Others invasive species include the Sirex wasp (Sirex noctilio), first detected in the country in 1962, which seriously threatens South Africa’s 16-billion-rand forestry industry; the ant Linepithema humile, which comes from Argentina and disrupts seed dispersal in indigenous plants; the North American small-mouth bass (Micropterus dolomieu), which has outcompeted indigenous fish species; and the water hyacinth (Eichhornia crassipes), originally from South America, which chokes the country’s dams and waterways.

Thirsty invaders

As well as their significant financial toll, the report holds invasive species responsible for a quarter of the country’s biodiversity losses. The researchers also find that invasive species in South Africa take a shocking toll on the water supply.

This year, Cape Town almost became the first major city in the world to run out of water. (It was saved at the last minute by stringent water restrictions). In May, researchers argued that alien plants, which often use more water than do indigenous ones, consumed more than 100 million litres of water a day — about a fifth of the city’s daily usage. They warned that water losses due to invasive species could triple by 2050 because trees including black wattle and cluster pines are spreading. The latest report estimates that invasive trees and shrubs, if left unchecked, could threaten up to a third of the water supply to cities such as Cape Town, and consume up to 5% of the country’s mean annual rainfall runoff.

Despite enacting the 2014 regulations and spending at least 1.5 billion rand a year to curb invasive species, the country is not keeping up, says the report. “The most concerning finding was how ineffective we have been,” says report co-author Brian van Wilgen, an applied ecologist at Stellenbosch University.

But the authors also note that their confidence in almost all their estimates is low, because of poor monitoring and evaluation data — a problem that can be mitigated in future reports through increased research into impacts and monitoring techniques.

Jasper Slingsby, an ecologist with the South African Environmental Observation Network in Cape Town, agrees that researchers in South Africa right now are limited by the available data. “We need better funding and concerted research effort in this space as a national priority,” he says.