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Invasive Alien Species - A Growing Global Threat

  1. The Global Invasive Alien Species Issue
  2. Environmental /Ecosystem costs 
  3. Economic costs
  4. Human health costs
  5. Addressing the IAS issue
  6. Further information about IAS

1. The Global Invasive Alien Species Issue

The spread of invasive alien species (IAS) is now recognised as one of the greatest threats to the ecological and economic well-being of the planet. These species are causing enormous damage to biodiversity and the valuable natural agricultural systems upon which we depend. Direct and indirect health effects are increasingly serious and the damage to nature is often irreversible. The effects are exacerbated by global change and chemical and physical disturbance to species and ecosystems.

Continuing globalisation, with increasing trade, travel, and transport of goods across borders, has brought tremendous benefits to many people. It has, however, also facilitated the spread of IAS with increasing negative impacts. The problem is global in scope and requires international cooperation to supplement the actions of governments, economic and public sectors and organisation at national and local levels.

Invasive species occur in all major taxonomic groups, including viruses, fungi, algae, mosses, ferns, higher plants, invertebrates, fish, amphibians, reptiles, birds and mammals. Even though only a small percentage of species that are moved across borders become invasive, these may have extensive impacts.

2. Environmental /Ecosystem costs 

Invasive alien species can transform the structure and species composition of ecosystems by repressing or excluding native species, either directly by out-competing them for resources or indirectly by changing the way nutrients are cycled through the system. IAS can affect entire systems; for example, when invasive insects threaten native species of insects, they can also have cascading effects on insect-eating birds and on plants that rely on insects for pollination or seed dispersal.

Increasing global domination by a relatively few invasive species threatens to create a relatively homogeneous world rather than one characterised by great biological diversity and local distinctiveness.

No criteria have yet been agreed upon for the minimum damage, spread or size of population needed for an alien species to be considered invasive. However, it is clear that a very small number of individuals, representing a small fraction of the genetic variation of the species in its native range, can be enough to generate, through its reproduction and spread, massive environmental damage in a new environment.

3. Economic costs

Invasive alien species have many negative impacts on human economic interests. Weeds reduce crop yields, increase control costs, and decrease water supply by degrading water catchment areas and freshwater ecosystems. Tourists unwittingly introduce alien plants into national parks, where they degrade protected ecosystems and drive up management costs. Pests and pathogens of crops, livestock and trees destroy plants outright, or reduce yields and increase pest control costs. The discharge of ballast water introduces harmful aquatic organisms, including diseases, bacteria and viruses, to both marine and freshwater ecosystems, thereby degrading commercially important fisheries. And recently spread disease organisms continue to kill or disable millions of people each year, with profound social and economic implications. GISP has not sought to estimate an aggregated economic cost of invasions globally, but one study for the USA estimates costs of $137 billion per year from an array of invasive species

Considerable uncertainty remains about the total economic costs of invasions; however, estimates of the economic impacts on particular sectors indicate the seriousness of the problem. The varroa mite, a serious pest in honeybee hives, has recently invaded New Zealand and is expected to have an economic cost of US$267-602 million, forcing beekeepers to alter the way they manage hives. Beekeepers argue that had border rules been followed or had surveillance detected the mite earlier, the problem could have been avoided entirely. It now appears too late to eradicate the mite, requiring a mitigation plan that is expected to cost $1.3 million in its first stage.

A 1992 report by the Weed Science Society of America estimated that the total cost of invasive weeds was between $4.5 billion and $6.3 billion. While the range of these figures indicates their uncertainty, they also indicate the order of magnitude of impact and argue for significant investments to prevent the spread and proliferation of these species.

In addition to the direct costs of management of invasives, the economic costs also include their indirect environmental consequences and other non-market values. For example, invasives may cause changes in ecological services by disturbing the operation of the hydrological cycle, including flood control and water supply, waste assimilation, recycling of nutrients, conservation and regeneration of soils, pollination of crops, and seed dispersal. Such services have both current use value and option value (the potential value of such services in the future). In the South African Cape Floral Kingdom, the establishment of invasive tree species decreases water supplies for nearby communities, increases fire hazards, and threatens native biodiversity justifying government expenditures of US$40 million per year for manual and chemical control.

Although the loss of crops due to weeds or other alien pests may be reflected in the market prices of agricultural commodities, such costs are seldom paid by the source of the introductions. Rather, these costs are negative "externalities", i.e., costs that an activity unintentionally imposes on another activity, without the latter being able to extract compensation for the damage received. One special feature of biological invasions, as externalities, is that the costs of invasions are largely self-perpetuating, once they are set in motion. Even if introduction ceases, damage from the invasives already established continues and may increase.

Most evidence of economic impact of IAS comes from the developed world. However, there are strong indications that the developing world is experiencing similar, if not proportionally greater, losses.

Invasive alien insect pests, such as the white cassava mealybug and larger grain borer in Africa, pose direct threats to food security. Invasive weeds constrain efforts to restore degraded land, regenerate forests and improve utilisation of water for irrigation and fisheries. Water hyacinth and other alien water weeds affecting water use currently cost developing countries over US$100 million annually.

Further, many introductions are unintentional, including most invertebrates and pathogens. Prices or markets cannot readily reflect the costs of these introductions. But even in the case of introductions involving deliberate imports to support agriculture, horticulture, forestry, and fisheries, market prices for seeds, plants, or foods, do not generally reflect the environmental risks associated with their use. Thus producers have little financial incentive to take account of the potential cost of the loss of native species or disturbance to ecosystem functions. The policies developed to deal with conventional externalities involved in the general problem of biodiversity loss - such economic tools as taxes, subsidies, permits, and so forth - may not always be well suited to deal with the problem caused by invasions. This point highlights the urgent need for new economic approaches to deal with IAS.

4. Human health costs

The dynamism among invasive pathogens, human behaviour, and economic development are complex and depend on interactions between the virulence of the disease, infected and susceptible populations, the pattern of human settlements, and their level of development. Large development projects, such as dams, irrigation schemes, land reclamation, road construction and population resettlement programmes, have contributed to the invasion of diseases such as malaria, dengue, schistosomiasis and trypanosomiasis.

The clearing of forests in tropical regions to extend agricultural land has opened up new possibilities for wider transmission of viruses that carry haemorrhagic fevers that previously circulated benignly in wild animal hosts. Examples include Argentine haemorrhage fever, "Guaranito" virus, Machupo virus, and Basia virus. Some pathways for the biotic invasion are complicated. For example, the prevalence of lymphatic filariasis in the southern Nile Delta has increased 20-fold since the building of the Aswan dam in the 1960s. This increase has been due primarily to the increase in breeding sites for the mosquito vector of the disease following the rise in the water table caused by the extension of irrigation. The problem has been exacerbated by increased pesticide resistance in the mosquitoes, due to heavy agricultural pesticide use and by rural-to-urban commuting among farm workers. Thus invasive species combined with variations in inter-annual rainfall, temperature, human population density, population mobility and pesticide use all contribute to one of the most profound challenges of invasive species: the threat to human health.

Infectious disease agents often, and perhaps typically, are invasive alien species. Unfamiliar types of infectious agents, either acquired by humans from domesticated or other animals, or imported inadvertently by travellers, can have devastating impacts on human populations. Pests and pathogens can also undermine local food and livestock production, thereby causing hunger and famine.

Indirect health affects associated with IAS include the use of broad spectrum pesticides against invasive pests and weeds. Free from their natural controlling factors, these organisms often reach sustained outbreak levels that encourage widespread and chronic pesticide use.

5. Addressing the IAS issue

The spread of invasive alien species is creating complex and far-reaching challenges that threaten both the natural biological riches of the earth and the well-being of our people. While the problem is global, the nature and severity of the impacts on society, economic life, health, and natural heritage are distributed unevenly across nations and regions.

Some aspects of the global IAS problem require solutions tailored to the specific values, needs, and priorities of nations while others call for consolidated action by the larger world community. Preventing the international movement of invasive alien species and co-ordinating a timely and effective response to invasions requires cooperation and collaboration among governments, economic sectors, non-governmental organisations, and international treaty organisations.

At the national level, consolidated and coordinated action is required. This could be part of a national biodiversity strategy and action plan, with close involvement of the economic sectors and identifying people responsible for operative actions involving potential IAS as a key prerequisite. Clear responsibilities for each relevant sector would need to be identified.

Insurance mechanisms and liability regulations for the spread of IAS are almost non-existent, presenting a major deficiency for controlling the problem. Governments should therefore be encouraged to cooperate with the insurance sector to find solutions, beginning with feasibility studies.

Capacity and expertise to deal with IAS are not yet sufficient in many countries. Further research on and capacity building around the biology and control of IAS and biosecurity issues therefore need to be given attention and priority. This also relates to financial institutions and other organisations responsible for environment and development co-operation, at national and international levels.

A global information system regarding the biology and control of IAS is also required. Tools, mechanisms, best management practices, control techniques and resources need to be provided and exchanged. Such a proposed system is currently developed as part of the Global Invasive Species Information Network (GISIN) and is intended to link to the Clearing House Mechanism of the Convention on Biological Diversity.

Awareness raising and education regarding IAS should be given high priority in action plans, and development of economic tools and incentives for prevention are urgently needed.

The Global Invasive Species Programme and its Partnership Network have been created to meet these needs.