Invasive species drive extinctions
A major new report warns that invasive alien species are wreaking havoc on nature – and thus on nature’s contribution to people
In the 1950s, the Nile perch was introduced into Lake Victoria, Africa’s largest lake, to boost local fisheries. Instead, the fierce predatory fish has caused the global extinction of numerous endemic fish species.
It is one of thousands of harmful invasive alien species that are driving local and global species extinct, wreaking havoc on ecosystems and threatening human health and livelihoods, according to a landmark scientific assessment by the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES).
The report by the world’s leading scientific authority for biodiversity draws from more than 13000 references and is the most comprehensive assessment ever carried out on invasive alien species around the world. It was produced over four and a half years by 86 experts from 49 countries.
The “conservative estimate” is that 37 000 established alien species have been introduced by human activities — such as increased global trade and human travel — across all regions and biomes. This figure is now rising at unprecedented rates, with 200 new alien species being recorded annually, the report said.
More than 3500 of these are categorised as harmful invasive alien species. About 6% of alien plants, 22% of alien invertebrates, 14% of alien vertebrates and 11% of alien microbes are known to be invasive, posing major risks to nature and to people. But this global threat is “underappreciated, underestimated, and often unacknowledged”, the report said.
The global economic cost of invasive alien species exceeded $423billion annually in 2019, but this figure is likely to be a “gross underestimate”. The documented global economic cost of biological invasions has increased fourfold every 10 years since 1970 and is anticipated to continue rising.
According to the report, the “vast majority (92%) of global costs accrue from the negative impact of invasive alien species on nature’s contribution to people or on good quality of life, while only 8% … is related to management expenditures of biological invasions”.
Anibal Pauchard, of the University of Concepcion in Chile and a co-chair of the assessment, said in a statement that 85% of the harm caused by biological invasions on native species are negative. This includes the ways that North American beavers and Pacific oysters change ecosystems by transforming habitats, often with severe consequences for native species.
Almost 80% of the documented effects of invasive alien species on nature’s contributions to people are also negative — particularly through damage to food supplies — such as that caused by the Caribbean false mussel to fishery resources in India.
Diseases such as malaria and the Zika and West Nile viruses are being spread into new regions by invasive alien mosquito species such as Aedes albopictus and Aedes aegypti.
In Lake Victoria, fisheries have declined from the depletion of tilapia caused by the spread of water hyacinth, the world’s most widespread terrestrial invasive alien species. Lantana, a flowering shrub, and the black rat are the second- and thirdmost widespread globally, with “farreaching impacts on people and nature”, the IPBES said.
“It would be an extremely costly mistake to regard biological invasions only as someone else’s problem,” Pauchard said. “Although the specific species that inflict damages vary from place to place, these are risks and challenges with global roots but very local impacts, facing people in every country, from all backgrounds and in every community — even Antarctica — is being affected.”
Invasive alien species have been a major factor in 60% of recorded global plant and animal extinctions — and the only driver in 16% of them. At least 218 invasive alien species have been responsible for more than 1200 local extinctions.
In tropical Africa, the invasion of the little fire ant has caused the local extinctions of forest floor and leaf-chewing invertebrates, while in Gaum the invasion of the brown tree snake has led to local extinctions of most resident populations of its 25 bird species.
Sebataolo Rahlao, a lead author of the report and a scientific manager of conservation research and assessment at Ezemvelo KZN Wildlife, said there were numerous examples of extinctions caused by invasive alien species across Africa.
“Classic examples are the Nile perch in the dams and lakes in East Africa, which caused many of the indigenous species to disappear completely in those parts,” Rahlao said.
“The brown tree snake is almost everywhere, and in many parts of South Africa, which has caused extinctions of many indigenous bird species because they are not used to it and don’t know what it is.”
The report shows that 34% of the impacts of biological invasions were reported from the Americas, 31% from Europe and Central Asia, 25% from Asia and the Pacific and about 7% from Africa.
Most negative effects are reported on land (about 75%) — especially in forests, woodlands and cultivated areas — with considerably fewer reported in freshwater (14%) and marine (10%) habitats. Invasive alien species are most damaging on islands, with numbers of alien plants now exceeding the number of native plants on more than 25% of all islands, the report said.
“In Africa alone, when you actually go deep into the numbers, you will realise that it’s actually massive,” Rahlao explained.
“We found more than 1 700 impact reports of individual species at a particular point of time and they came from across more than 50 countries so we almost covered the entire continent.”
Citing the case of the Nile perch, he said: “The impacts of that are actually massive, if you were to look at … where it has impacts on food security, on livelihoods, on human health … But because we had to put this in a global perspective, you think that 7% is actually low. If we were to do an African only assessment, you would see the numbers would be a lot higher.”
As for some examples of invasive alien species proliferating in South Africa, Rahlao noted how water hyacinth has invaded most all dams, while the fall armyworm had devastated maize crops and the invasion of the polyphagous shot hole borer is “likely to cause problems to indigenous trees”.
Another is the invasive parthenium weed. “It has so many impacts on agriculture, tourism, livestock and human health. If we were to quantify all those impacts you would find that the number is staggering.”
Its pernicious spread has become a health issue in areas such as Hluhluwe-imfolozi Park in Kwazulu-natal.
“A lot of animals, wherever they’ve eaten this species, become unproductive,” he said, adding that parthenium causes rashes when people walk through the park’s bushes. “When tourists want to come to these areas, they just see all these invasive alien species and they don’t get excited.”
Livelihoods, too, are being harmed by invasive alien species. “A lot of people in Africa, in the deep rural areas, like in the Eastern Cape, depend on biodiversity for harvesting fuel wood and wild crops, for example. In some instances, they may not be able to do that because of invasive alien species that are now becoming a problem.”
The report describes how, in the past 50 years, the number of people in the world has more than doubled, consumption has tripled, and global trade has grown nearly tenfold, with shifting patterns across regions.
“This acceleration of the world economy is increasing the rate and magnitude of many direct and indirect drivers [of invasive alien species], particularly those related to trade, travel and land and sea-use change, leading to further biological invasions,” it says.
Although 80% of countries have targets for managing invasive alien species in their national biodiversity plans, only 17% have national laws or regulations specifically addressing them. And nearly half of all countries do not invest in the management of biological invasions.
Pauchard added the good news is that, for almost every context and situation, there are management tools, governance options and targeted actions that really work.
“Prevention is absolutely the best, most cost-effective option — but eradication, containment and control are also effective in specific contexts,” he said.
‘The impacts of that [the Nile perch] are massive ... impacts on food security, on livelihoods, on human health’