Invasive Alien Species - A Growing Global Threat
1. The Global Invasive Alien Species Issue
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
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
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
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.