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Scientists warn of increasing threats posed by invasive alien species
Author: Media & Communication, Faculty of Science
Published: 26/06/2020

Invasive alien species have emerged as one of the top five threats to biodiversity and ecosystems globally, yet only a handful of countries regard biosecurity measures as a priority.

So warns a team of international researchers in a new global overview of environmental change due to invasive alien species. The article, published in the journal Biological Reviews on 26 June 2020, forms part of the World scientists' warning to humanity: a second notice initiative* which calls for an urgent change in our approach to stewardship of the earth and life on it. 

Professor David Richardson, Director of the Centre for Invasion Biology at Stellenbosch University (SU), South Africa, and one of the lead authors of the article, says: “South Africa has invested heavily in a national programme to reduce the negative impacts of widespread invaders on ecosystem services, but much more action is needed. Urgent interventions are needed at both national and international levels to tackle the challenges more effectively."

Alien species are plants, animals and microbes that are introduced by people, accidentally or intentionally, to areas where they do not occur naturally. Many of them thrive, spread widely and have harmful effects on the environment, economy, or human health.

The study which was carried out by an international team of researchers from 13 countries across Africa, Asia, Australasia, Europe, North and South America, states that the number of invasive alien species is increasing rapidly, with over 18,000 currently listed around the world.

In South Africa, a recent assessment listed 1 422 alien species that have become naturalised or invasive, with some having serious negative impacts on South African ecosystems, for example, 'thirsty' alien tree species that extract large quantities of water from catchments.

The team of researchers attribute the escalation in biological invasions to the increase in the number and variety of pathways along which species spread, and to the increasing volume of traffic associated with those pathways. They highlight the role of emerging pathways such as the online trade in unusual pets and plants for ornamental horticulture, and the transport of species across oceans on rafts of plastic pollution.

The study also shows how other drivers of global change, such as climate change, land-use change, alongside international trade are exacerbating the impacts of biological invasions. For example, species transported through shipping can now thrive in new regions, due to climate warming; and the permanent opening of the Arctic Ocean with global warming is allowing marine species to move between the Atlantic and Pacific Oceans.

The authors stress that biological invasions can be managed and their impacts mitigated. They point to approaches that are working around the world and make specific recommendations for improved management. For example, the introduction of more stringent border controls, including X-ray machines and detector dogs, has led to a progressive decline in the rate of fungal plant pathogens entering New Zealand.  

Professor Petr Pyšek of the Czech Academy of Sciences and Charles University in Prague and a research associate at the Centre for Invasion Biology at SU, first author of the paper, says: “As our knowledge about invasive alien species increases, the problems associated with biological invasions are becoming clearer. The threats posed by invasive alien species to our environment, our economies and our health are very serious, and are getting worse. Policy makers and the public need to prioritize actions to stem invasions and their impacts."

On the photos, above:

Pine invasions in the mountains of South Africa's Cape Floristic Region is dramatically reducing streamflow from water catchments. Photo: Andrew Turner

The Harlequin ladybird, an invasive species native to Asia, cause damage to ecosystems by reducing the population sizes of native ladybird species. They are also a great nuisance causing economic losses by tainting wine with their bitter secretions, and by damaging fruit crops. Photo: Ingrid Minnaar

Read the paper in Biological reviews

Pyšek P., Hulme P. E., Simberloff D., Bacher S., Blackburn T. M. Carlton J. T., Dawson W., Essl F., Foxcroft L. C., Genovesi P., Jeschke J. M., Kühn I., Liebhold A. M., Mandrak N. E., Meyerson L. A., Pauchard A., Pergl J., Roy H. E., Seebens H., van Kleunen M., Vilà M., Wingfield M. J. & Richardson D.M.: Scientists' warning on invasive alien species. Biological Reviews doi: 10.1111/brv.12627 -

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Professor Dave Richardson

Director, DSI-NRF Centre of Excellence for Invasion Biology, hosted at Stellenbosch University

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* World scientists' warning to humanity

The paper is a part of the World scientists' warning to humanity: a second notice, an initiative calling for urgent change in the management of the natural world.

In 1992, a community of eminent scientists from around the globe put their names to a document warning that humanity was on a collision course with the rest of the natural world (Union of Concerned Scientists, 1992). Twenty-five years later, Ripple et al. (2017) evaluated the human response and in a 'second warning' concluded that humanity had failed to make sufficient progress in dealing with the environmental challenges. Indeed, they found that most of these problems had worsened.

The original 1992 call was supported by more than 1,700 scientists, while in 2017 over 15,000 scientists added their signatures to the declaration.

What is the scale of the problem?

A recent analysis of global extinctions in the IUCN Red List database (IUCN, 2017) revealed that alien species contributed to 25 per cent of plant extinctions and 33 per cent of terrestrial and freshwater animal extinctions. Meanwhile, annual environmental losses caused by introduced species in the United States, United Kingdom, Australia, South Africa, India and Brazil have been calculated at over US$100 billion.

Action against invasive alien species

The importance of taking action against invasive alien species globally has been widely recognized (Millennium Ecosystem Assessment, 2005). The recent global assessment report on biodiversity and ecosystem services by the Intergovernmental Science Policy Platform on Biodiversity and Ecosystem Services (IPBES) ranked invasive alien species fifth among direct drivers of change in nature with the largest relative global impacts, after changes in land and sea use, direct exploitation of organisms, climate change, and pollution.

About the Centre for Invasion Biology (C·I·B)

The C·I·B is an inter-institutional research centre with its headquarters at Stellenbosch University. Its members undertake research on the consequences of biological invasions in South Africa's freshwater, marine and terrestrial ecosystems. The aims of the Centre's work are to reduce the rates and impacts of biological invasions by furthering scientific understanding and predictive capability, by developing research and management capacity through postgraduate student training.

In 2020 the produced an encyclopaedic assessment of the status of biological invasions in South Africa. The open-access book is available at:

For information about the Centre for Invasion Biology, visit