Famine weed | Picture: Thembeka Thwala
Famine weed | Picture: Thembeka Thwala

Alien species threaten human, climate, economic health

By sheree bega Time of article published Jun 30, 2020

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Invasive alien species are hitching rides on rafts of plastic waste strewn across oceans, slipping into the Arctic and Antarctica as the climate warms and spreading into new territories through the online pet trade.

“Invasions are pervasive - thousands of alien species have arrived and more are arriving almost everywhere - and require much bolder actions,” warned a team of international researchers in a new global overview of environmental change from invasive alien species, yesterday. 

Invasive alien species refer to plants, animals and microbes introduced by people, accidentally or intentionally, to areas they don’t naturally occur. Many thrive, spread widely and have harmful effects on the environment, economy, or human health.

But while they have emerged as one of the top five threats to biodiversity and ecosystems globally, only a handful of countries, including Australia and New Zealand, view biosecurity measures as a priority.

The paper, Scientists’ Warning on Invasive Alien Species, which was published in the journal, Biological Reviews, forms part of the World Scientists’ Warning to Humanity: a Second Notice Initiative calling for an urgent change in humanity’s approach to the stewardship of the earth and life on it.

The scientists describe how the number of national lists of harmful alien organisms has risen “exponentially” in the past 50 years, with more than 18000 species listed. Other drivers of global change, such as climate change, land-use change, coupled with international trade, are worsening the impacts of biological invasions.

The rapid increase of global shipping in coming decades could see many thousands of species be transported around the world as stowaways in ballast water” and as contaminants of transported goods to regions becoming increasingly susceptible to new invasions owing to climate warming.

Marine invasions are being exacerbated by the dramatic increase in the use of non-biodegradable plastics, depositing billions of tons of plastics globally at the land-sea interface.

“A new mechanism for ocean rafting is created when these plastics are swept into the ocean by tsunamis or by the increasing (owing to climate change) number and size of cyclonic storms (hurricanes, monsoons, typhoons).”

Plastics create rafts that can last for decades, permitting more species to be transported as passengers far longer and further.

The permanent opening of the Arctic Ocean is allowing the flow of species between the Atlantic and Pacific oceans and “new fleets of exploratory, cargo, fishing, and tourist vessels will inadvertently transport marine and terrestrial species”.

Antarctica, described as the “final frontier for marine biological invasions”, is available for colonisation by poleward-moving species.

“Invasions are being accelerated by new facilities and new forms of tourism accompanied by increased ship traffic, such that more alien species have already been observed and even more are predicted.”

Terrestrial species are accidentally transported in trade as wood packaging material often harbours bark and wood-boring insects and microbes.

“Increases in trade have produced an explosion of tree-killing insects and pathogens introduced to new regions with large impacts on forests.”

The dramatic recent growth in the trade of unusual pets is another threat.

“Europe alone contains an estimated 54million individual ornamental birds, 28million small mammals, 14million aquaria fishes, and 9million reptiles owned as pets; many of these species can establish outside of captivity, especially under future climate scenarios.” 

They may be important vectors of animal and human diseases, particularly when sourced from the wild.

“Despite such threats, movement of contaminants remains largely unregulated and becomes even more difficult to manage effectively in an era of a rapidly growing ‘bioweb’ of online commerce of living species.”

Harlequin beetle | Picture: Ingrid Minnaar


Another new challenge comes from conservation itself - the translocation of individuals of certain species threatened by climate change to new regions predicted to favour population persistence.

“This strategy often involves moving species to sites where they are not currently found and may never have been native. This approach can potentially launch invasions.”

For South Africa, a recent local assessment listed 1422 alien species that have become naturalised or invasive, with some having serious negative impacts on ecosystems, including “thirsty” alien tree species.

Professor David Richardson, director of the Centre for Invasion Biology at Stellenbosch University and a lead author, says South Africa has invested heavily in a national programme to reduce the negative impacts of widespread invaders on ecosystem services but far more urgent action is needed at national and international levels.

“There have been long-term successes, such as eradication of rats and cats on increasingly large islands and biological control of weeds across continental areas,” states the paper.

“However, in many countries, invasions receive little attention. Improved international cooperation is crucial to reduce the impacts of invasive alien species on biodiversity, ecosystem services, and human livelihoods.”

The researchers cite numerous examples of human impacts of invasions, such as the cover of water hyacinth Lake Victoria, in East Africa, which made fishing grounds inaccessible, and the abandonment of anchovy fisheries in parts of the Black Sea because clogging of nets by the comb jelly made fishing impractical. 

Data has shown that for all groups of organisms on all continents, “the numbers of alien species have increased continuously over the last 200 years.

“For most taxonomic groups, rates of first recorded introductions are higher now than other time, no signs of a slow-down are evident, and many new invasions will be discovered in the near future given the typical time lags between introductions, establishment and spread.”

Growing human population and a greatly expanded global network of commerce, combined with environmental changes and their uncertainties, “result in often surprising appearances and subsequent establishment of species all around the world. 

“In the absence of concerted political and social action, expanding global trade will continue to transport many species with no history of invasion, some of which are likely to feature on future ‘worst invaders’ lists.

“Potentially, thousands of species, including many with no known history of invasiveness, could become as damaging as current poster-child invaders such as the zebra mussel, chestnut blight, Dutch elm disease fungus, kudzu, Nile perch, harlequin ladybird, muskrat, varroa mite, and the amphibian chytrid fungus.”

Invasive regions

-South Africa, India, California, Cuba, Florida, Queensland and Japan are regions with the highest numbers of reported invasive plant species.

-Ornamental horticulture continues to be a major driver of alien plant invasions.

-Invasive alien species are listed as one driver of extinction for 261 of 782 animal species and in 39 of 153 plant species worldwide. 

-Islands and coastal mainland areas are hot spots of invasions, but ecosystems in all biomes throughout the world are increasingly affected.

The Saturday Star

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