Invasive alien species are increasing the threat of emerging infectious diseases, a new study from a global research team has warned.
Alien species – animals, plants and microbes – are those introduced by people in regions where they do not naturally occur, either intentionally or by accident. Many thrive in their new locations and can cause harmful impacts on the environment, human health and the economy.
The Independent’s Stop The Wildlife Trade campaign was launched by its proprietor Evgeny Lebedev to call for an end to high-risk wildlife markets and for an international effort to regulate the illegal trade in wild animals to reduce our risk of future pandemics.
The new study, conducted by researchers from 13 countries across six continents, found that numbers of invasive alien species are rapidly increasing, with more than 18,000 currently listed around the world. The study was published last week in the journal Biological Reviews.
Professor Laura Meyerson, who researches invasion biology and restoration ecology at the University of Rhode Island and was part of the global study, said that it’s highly likely the number is much greater.
“These are the ones that we’ve detected and recorded,” Prof. Meyerson told The Independent. “But there are a lot of species that are introduced and become established that we don’t even notice, the most obvious being bacteria, viruses and fungi, which can be quite harmful.
“We take a long time to figure out that they are actually there. Some we don’t even have names for yet because biodiversity is so huge. There are also species that look a lot like native species that, if you’re not a taxonomist, you might not realise have been introduced.”
The coronavirus pandemic has made us acutely aware of the risks of zoonotic diseases – viruses which “jump” between animals and humans.
Alien species are a significant source of “pathogen pollution” such as viruses and bacteria to new hosts or regions, according to the findings.
The study warns that “little is known about the impacts of alien pathogens… and associated emerging infectious diseases on biodiversity and ecosystems”.
Professor Meyerson said: “When you bring in an animal, either through the illegal wildlife trade or as a pet, there’s a great possibility that it’s going to be carrying some pathogens that could be zoonotic or spread to other animals.
“It’s not just the animal-to-human transmission but also the animal-to-animal transmission which is an issue both for wildlife and agriculture.”
“It’s all a biosecurity issue,” she added.
One example noted in the study is the tiger mosquito. It is a known disease vector – an epidemiology term for an agent which carries and transmits an infectious pathogen.
In 2019, the mosquitoes, which carry Zika, chikungunya and dengue viruses, were identified for the first time in Vermont. (Public health officials said the mosquitoes found were not carrying these viruses and as Vermont’s climate was inhospitable, it would likely kill them off.)
Biological invasions have broad impacts on human well‐being. In Eastern Africa the invasion of water hyacinth, which covered Lake Victoria, led to fishing being abandoned and emigration.
The invasive invertebrate comb jelly caused anchovy fisheries to be abandoned in parts of the Black Sea after it clogged the nets. Invasion of Tamarix shrubs in the US southwest degraded agricultural land and led to abandonment in some areas.
There is also the concern for human safety from invasive species, such as venomous fish which injured fishermen in the eastern Mediterranean.
Alien species have contributed to one-third of animal extinctions and 25 per cent of plant extinctions globally, according to 2017 analysis.
Each year, environmental losses from introduced species are estimated at more than $100bn in the US, UK, Australia, South Africa, India and Brazil.
Islands and coastal areas are invasion hotspots. The most affected regions are the Hawaiian Islands, New Zealand’s North Island, and the Lesser Sunda Islands of Indonesia.
In New Zealand, predatory mammals have impacted numbers of the naïve, native bird species which evolved without native mammal predators.
Islands and coastlines are not the only affected areas, the report notes, as “even the most isolated and well‐managed reserves are experiencing pressure from invasive alien species”.
Urban areas are hotspots of alien plant species which spread, in part, through trade and horticulture.
Wealthy regions with dense human populations, spread across large areas, support the most established alien species. Florida, for example, is the top hotspot for alien species among continental regions, including the Burmese python. The most-established alien fish species are in the Colorado (100 species) and Mississippi (73) river basins.
A variety of pathways has escalated the numbers of invasive alien species in new regions with global trade, travel and the climate crisis playing significant roles.
There has been deliberate release of some species, for example game animals and sport fish, for hunting.
Enthusiasm for rare and exotic pets is another growing threat. Europe has an estimated 54 million individual ornamental birds, 28 million small mammals, 14 million aquaria fishes, and nine million reptiles owned as pets.
“Many of these species can establish outside of captivity, especially under future climate scenarios,” the researchers found.
These pets may also be vectors of diseases, particularly wild animals that are kept in captivity.
But despite the risks, online sales of living species, which can carry parasites, remains largely unregulated and notoriously difficult to track.
Trade has also facilitated the movement of invasive species, leading to an "explosion of tree‐killing insects and pathogens introduced to new regions which have large impacts on forests".
Other invasive species have “hitched” rides on freight aircraft and container ships (as is suspected to have been the case with the recent “murder hornets”, native to Asia, that were found in Washington state).
The dramatic rise of non-biodegradable plastics in recent decades also play a part, providing “rafts” for organisms to cross oceans. Plastics are swept into the ocean by tsunamis and hurricanes, monsoons, typhoons which are increasing in size and ferocity due to climate impacts.
Compared to biodegradable trees and roots, which naturally dispersed species across oceans for millions of years, “plastics create rafts that can last for decades, permitting more species to be transported as passengers far longer and further”.
The climate emergency is exacerbating the problem as higher temperatures can allow species to thrive in regions they might previously have found inhospitable.
With the warming of the Arctic Ocean, it means an increasing flow of species between the Atlantic and Pacific Oceans.
Antarctica, previously described as the “final frontier for marine biological invasions”, is also seeing an acceleration, caused by new facilities and tourism, accompanied by increased ship traffic, the study found.
Some conservationists have also advocated for moving certain species threatened by the climate crisis to new regions for their survival. The study found that this strategy needs to be “carefully assessed” as it could potentially launch invasions of alien species.
It is a monumental problem to tackle but one that requires ramping up national biosecurity programmes and international cooperation, the study found.
More stringent border controls including X-ray machines and detector dogs can have an impact. Such measures has led to a decline in invasive fungal plant pathogens entering New Zealand.
Hiring and training more border agents to focus on wildlife and deploying new technologies would also have an effect.
Some countries have had more success than others. In Australia and New Zealand, for example, biosecurity is a national priority and has had success with the eradication of rats and cats on increasingly large islands and biological control of weeds across continental areas, the researchers indicated.
However, in some regions, invasions receive little attention or funding.
Prof Meyerson pointed out that last year, the Trump administration cut the budget of the National Invasive Species Council (NISC) by 50 per cent. Their annual budget, reportedly $1m, Prof Meyerson said, is a small price to pay “when we know invasions cause $100bn a year in damage”.
She said that the threat of invasive species, in terms of policy and financial resources, needed to be elevated to “a biosecurity issue”.
“It’s costing us hundreds of millions of dollars and it’s harming our health,” she said. “Covid-19 is an invasive species issue. We knew what to do and we didn’t do it.”
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