Invasive Species, Including Weeds

Unique identifier / Notation

BI0403

Synonyms

Alien species Alien invasive species Invasive alien species

‘Invasive species’, also known as ‘alien invasive species’, are species whose introduction, establishment and spread into new areas threaten ecosystems, habitats or other species and cause social, economic or environmental harm, or harm to human health (FAO, 2007:82).

Primary reference(s)

FAO, 2007. Invasive species. In: State of the World’s Forests. Food and Agriculture Organization of the United Nations (FAO). Accessed 10 October 2020.

Annotations

Additional scientific description

Invasive species include species, subspecies or lower taxa of invertebrates, plants, microorganisms and vertebrates non-native (alien or exotic or non-indigenous) to a particular ecosystem and whose human-mediated or unintentional introduction and spread causes, or are likely to cause, socio-cultural, economic or environmental harm or harm to human health (FAO, 2007). In this description, plants encompass all types of weeds including parasitic. An alien species is a species, subspecies or lower taxon, introduced outside its natural past or present distribution; it includes any part, gametes, seeds, eggs, or propagules of such species that might survive and subsequently reproduce (UNEP, 2002).

Increasing international trade and human mobility, exacerbated by the impacts of climate change, have increased the introduction of plant and animal species into new areas where they have become invasive (FAO, 2020). These invasive species can have both direct nd indirect impacts on crop production and also damage natural habitats. Certain weeds pose additional threats to forests, water resources (such as aquatic weeds) or even the health of livestock and humans (poisonous weeds).

Estimates of the full costs of biological invasions are rare because of the difficulty in assessing the costs of impacts on biodiversity, ecosystem functions and human health, or other indirect costs such as the impact of control measures. The costs of invasive species to the forest sector have not been studied on a global scale. However, based on a study of six countries (Australia, Brazil, India, South Africa, the United Kingdom and the United States of America), it was estimated that as many as 480,000 alien species have been introduced in agriculture and forestry worldwide, with an annual cost of more than USD 1.4 trillion (FAO, 2007). Invasive plant and animal species are now considered one of the most important causes of biodiversity loss, especially in many island countries (CBD, 2009).

Concerning invasive weeds, the Convention on Biodiversity highlights that invasive plant species have been reported among inter alia seaweeds, trees, shrubs, vines, forbs and grasses. The indicative economic impacts of some invasive alien species are significant. For example, the direct impact of knapweed (Centaurea spp.) and leafy spurge (Euphorbia esula) on the economy of three US states is estimated at USD 40.5 million per year (direct costs) and USD 89 million as indirect costs (UNEP, 2001). Native to North America, common ragweed arrived in Europe in the 19th century and have since affected agriculture and human health. Large populations of this weed currently exist in many European countries being present in open spaces, waste grounds, and along roadsides. It is resistant to herbicides, and its long seed germination capacity (over 30 years) makes control difficult. It is known to reduce yields in the cultivation of maize, wheat, sunflowers, millet, peanuts, soy, beans and potatoes. It also plays a role as a secondary host for organisms that are harmful to cultivated plants (e.g., fungal pathogens that are harmful to sunflowers). Furthermore, its pollens are allergens for humans estimated to affect between 1 and 3.5 million people (Anses, 2024).

Preventing and reducing the harmful effects of invasive species requires an approach that incorporates biological, ecological and social sciences, economics, policy analysis and engineering. National efforts should include early warning systems, eradication and control, as well as increased awareness and political leadership. Global, regional and bilateral efforts include standards and guidelines, monitoring and assessment, and information and action networks (FAO, 2007).

Numerous international and regional programmes and instruments, binding and non-binding, have been developed to address the problem of invasive species (FAO, 2007).

Different countries have different approaches - two of note are the Invasive Species Definition Clarification and Guidance prepared by the United States Department of Agriculture (USDA, 2006) and the Australian Weed Strategy (Australian Government, 2016).

Metrics and numeric limits

Not available.

Key relevant UN convention / multilateral treaty

The Convention on Biological Diversity (1992) has three main objectives: the conservation of biological diversity, the sustainable use of the components of biological diversity, and the fair and equitable sharing of the benefits arising out of the utilisation of genetic resources (United Nations, 1992). At the time of writing, there were 196 parties to the Convention on Biological Diversity.

The Convention on Biological Diversity (CBD) calls on its parties to ‘prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats, or species’ (Article 8[h]). The parties have adopted a series of 15 guiding principles to lead governments and organisations in developing effective strategies for minimising the spread and impact of invasive alien species (UNEP, 2002). The UN CBD introduced a commitment to endorse these principles in the 2011–2020 Global Biodiversity Strategy by adopting Aichi Target 9 (CBD, 2020).

European Commission (Environment) EU Regulation 1143/2014 on Invasive Alien Species (2015). Invasive alien species are animals and plants that are introduced accidentally or deliberately into a natural environment where they are not normally found, with serious negative consequences for their new environment. They represent a major threat to native plants and animals in Europe, causing damage worth billions of Euros to the European economy every year. As invasive alien species do not respect borders, coordinated action at the European level will be more effective than individual actions at the

The International Plant Protection Convention (1997) is an intergovernmental treaty signed by over 180 countries, aiming to protect the world’s plant resources from the spread and introduction of pests and promote safe trade (FAO, 1999). The Convention introduced International Standards for Phytosanitary Measures (FAO, 2021) as its main tool to achieve its goals, making it the sole global standard-setting organisation for plant health. The International Plant Protection Convention (IPPC) is one of the ‘Three Sisters’ recognised by the World Trade Organization’s (WTO) Sanitary and Phytosanitary

The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES). In the IPBES Regional Assessments (IPBES, 2018a) and Global Assessment Report (IPBES, 2019), invasive alien species were identified as one of the main direct drivers of biodiversity loss worldwide. The rapidly growing threat that invasive alien species pose to biodiversity, ecosystem services, sustainable development and human wellbeing is, however, generally poorly quantified and little understood by decision makers (IPBES, 2018b).

The International Union for Conservation of Nature (IUCN). The IUCN Species Survival Commission (SSC) Invasive Species Specialist Group maintains two global databases: the Global Invasive Species Database (GISD), which contains profiles of key invasive alien species, and the Global Register of Introduced and Invasive Alien Species (Pagad et al., 2018), which was developed with a mandate of the CBD and collates data on alien species in all taxonomic groups for all nations.

Drivers

An increase in the introduction and spread of invasive species or invasive alien species (insect pests, pathogens, microorganisms, vertebrates, plants and weeds) can result from increasing international trade, travel and the impacts of climate change.

Impacts

Economic, environmental, social and cultural impacts due to the loss of host trees, loss of crops and loss of biodiversity.

Insect pest infestations including invasive species may cause food security challenges especially if the staple food crops are affected. They can also result in market or price instability. The best-known example of such great impact was experienced in and around the 1840s when a deadly disease called potato late blight devastated the potato productions in Ireland causing the well-known great famine resulting in the death of over a million people (FAO-IPPC, 2017). Many outbreaks at smaller scales have been recorded in history. One recent example has been the rapid expansion of fall armyworms in Africa and Asia putting many countries in a difficult position to protect crops, particularly of smallholder farmers.

In addition to impact on plant production and food security, invasive species can also devastate natural landscapes. These can also include effects on water resources (aquatic weeds) and the potential health of animals and humans (poisonous weeds). Other implications on local populations can include local conflicts or migration due to shortage in severe locust upsurges. Furthermore, in certain cases, insecticide sprays can have implications concerning the health of the local habitants.

Multi-hazard context

The figure below summarises common interactions between invasive species & weeds and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.

Multi-hazard diagram

Risk Management

This includes the establishment of early warning and early action activities; the implementation of phytosanitary standards in the agriculture and forestry sectors; and increased public awareness and community participation in early warning systems.

The Global Invasive Species Programme (GISP) was established in 1997 to address global threats caused by invasive species and to provide support to the implementation of Article 8(h) of the CBD. To increase awareness and provide policy advice, GISP has prepared the Global strategy on invasive alien species, which outlines ten strategic responses to the invasive species issue (FAO, 2007).

The GISD is a free, online searchable source of information about alien and invasive species that negatively impact biodiversity. It aims to increase public awareness about invasive species and to facilitate effective prevention and management activities by disseminating specialist knowledge and experience to a broad global audience. The GISD focuses on invasive alien species that threaten native biodiversity and natural areas and covers all taxonomic groups from microorganisms to animals and plants (GISD, no date).

The GISD is managed by the Invasive Species Specialist Group (ISSG) of the IUCN SSC. It was developed between 1998 and 2000 as part of the global initiative on invasive species led by the GISP (GISD, no date).

Because invasive species are addressed in different agreements and conventions (e.g., the Convention on International Trade in Endangered Species, IPPC and the World Trade Organization Agreement on the Application of Sanitary and Phytosanitary Measures), many countries have difficulty keeping up with the reporting requirements (FAO, 2007) In response, the UNEP World Conservation Monitoring Centre (UNEP-WCMC) has recently developed a set of issue-based modules summarizing country obligations under the CBD and other conventions, and in 2020 published a report on Building a Multidimensional Biodiversity Index (Soto-Navarro et al., 2020).

Early warning systems are most practical and effective at local and regional levels. The early warning systems targeting plant pests cover also invasive species. There are some early warning systems focusing on plant pests and diseases, particularly at regional levels, for example for North America (Meissner et al., 2015). International knowledge sharing and collaboration is critical in establishing early warning communications. At global level, the pest outbreak alert and reporting system (POARS) of IPPC can be considered the most official resource for reports of the occurrence of quarantine plant pests including invasive species, weeds and diseases (IPPC, 2025). Although such reports are deposited after the establishment of a pest, these can be considered as early warning resources for other concerned countries.

Monitoring

The section and the table below offer an overview of monitoring for invasive species and weeds. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.

Which institution(s) produce(s) Disaster Risk Data/Information?

At global level, the pest reporting system of IPPC can be considered the most official resource for reports of the occurrence of quarantine plant pests including diseases (IPPC, 2025). Although in most cases such reports are deposited well after the establishment of a pest, these can be considered as early warning resources for other concerned countries. In this respect, the most efficiently functioning monitoring and early warning system is the desert locust monitoring system operated by FAO. The system is run by the Desert Locust Information Service (DLIS) and supported by three commissions (Western Region, Central Region and Southwest Asia) as well as the desert locust control committee (DLCC) (FAO, 2025a). In addition, there are numerous privately-run digital crowd-sourcing platforms such as Plant Village (Plant village, 2025).

How is the Hazard Observed/Monitored Forecast?

The IPPC pest outbreak and alert system (POARS) is based on official reporting of events by countries, while the desert locust monitoring system is based on reports of a network of concerned countries that run surveys and share data over satellite tools with the Desert Locust Information Service (DLIS), operating since 1978 (FAO, 2025b) and housed at the headquarters of FAO, where the data is analysed together with geospatial and meteorological data. Analysis helps monitoring the overall situation and production of forecasts and early warning for potential outbreaks to be shared with the concerned countries publicly. Other insects are monitored through official reports of countries, network communications, direct communications and public news.