Food and Fertilizer Technology Center - publications

Nov. 01, 2003

The Problem of Invasive Species

Under the World Trade Organization, Asian and Pacific countries have become part of a global economy in which most barriers to trade have been removed. As food crops are imported and exported all over the world, many of their diseases and pests are traveling with them. There has been a marked increase in the number of accidentally introduced pests over the last few decades. Non-indigenous species of all kinds - animals, plants, and microorganisms - are flowing into the Asian and Pacific region at a remarkable rate. Unchecked by natural enemies, many of these are becoming invasive pests that threaten human, animal, and plant health.

Asian farmers are already under pressure from free trade. The situation will become worse if they are also faced with new pest and disease problems. Careful monitoring and speedy control are essential if Asian farmers are to be protected from accidentally introduced plant pests and diseases.

The losses caused by invasive species to Asian agriculture have been enormous. Exotic weeds have invaded pastures and crops, while exotic pathogens and pests have caused devastating crop losses. To the cost of these crop losses must be added the large sums needed to control the invasive species which caused them.

An important output of the meeting was the development of a database, the Asian-Pacific Alien Species Database (APASD). As a background to the database, the seventeen papers presented and the discussions provided a wealth of information about recent research on invasive pests in the Asian and Pacific Region. Some of the important points are summarized below.

The Introduction of Alien Species

Not all alien species are harmful. In fact, the agriculture of all countries, including those of Asia, incorporates a number of major crops that originated elsewhere. Other alien species have been deliberately introduced as biological controls for agricultural pests. For example, since 1963 Thailand has introduced a total of 39 species of exotic insects for the biological control of various weeds and insect pests.

As was emphasized in several papers presented at the meeting, most exotic species are useful and enrich human life. However, the impact of an invasive species on a new environment is difficult to predict. There may be an unforeseen impact which damages the economy, the environment, or both. When this occurs, it may be difficult to initiate suitable monitoring or control programs. Decisions about how to manage exotic species, and what countermeasures to adopt, are sometimes made too late. Furthermore, many of the decisions are political rather than technical. There are always conflicts among the stakeholders engaged in world trade, including the international trade in agricultural products and agricultural planting materials. Conflicts also occur between different domestic sectors in the same country, for example between those who advocate importing the large-mouth bass as a game fish, and those who fear its impact on indigenous aquatic species.

Most harmful invasive species are thought of as accidental introductions. However, many of the invasive species which are now doing most damage in Asia were introduced intentionally by people who saw some advantage in doing so.

Intentional Releases

In most cases, it was intended that these intentionally introduced species should become established. However, invasive species often have a much higher capacity to spread and dominate in their new habitat than in their old one, because there are no natural enemies or other controls to limit their population build-up. The result may be a decrease in the environmental quality or biodiversity.

Accidental Introductions

Accidental introductions often enter a country in association with some other product. Nearly all plant pathogens introduced from overseas are accidental introductions. In Thailand, two important bacterial plant pathogens are fruit blotch of watermelon, caused by Acidovorax avenae, and bacterial wilt caused by Ralstonia solanacearum. A. avenae was probably brought into Thailand in infected seeds as part of the international seed trade. Bacterial wilt, which infects many important crops such as potato, tomato and ginger, was probably imported in infected potato tubers which were imported as planting materials.

While exotic pests which are introduced accidentally are not necessarily more damaging than those which are introduced intentionally, they tend to arouse more concern. This is because they are less susceptible to systems of management. Whereas risk assessment can be carried out in advance for intentional introductions, with accidental introductions action has to be retroactive. It was generally agreed that risk assessment and risk management are central to the proper management of invasive alien species.

Risk Assessment

Risk assessment refers to the process by which risk is measured. Risk management is the process of addressing the risk.

Advance planning is not possible with accidental introductions, since we do not know what species is involved or where it is, until after it has already arrived. The model for accidental releases must focus on the dispersal pathway, and also on any history of damage caused by the same species in other countries.

In practice, it is difficult to predict what species are likely to become a problem. One indicator is that the species has already caused problems elsewhere. If a species has a history of invasive behavior, it is more likely to be dangerous. Other general principles are that a species is more likely to be invasive when it has a large natural range, a high rate of increase and a large founding population. A habitat is more vulnerable to invasion by alien species if it has undergone a high level of disturbance, if it does not contain any similar species, and if it has low species diversity.

Detection and Monitoring

Since the impact of alien species can be so damaging, it is very important to develop the best possible detection and monitoring system, to keep the entry of invasive species to a minimum. All countries have national systems in place to counter the threat of invasive alien species, and are constantly striving to improve these. However, the increased flow of imported agricultural products and international tourism mean that quarantine inspectors find it very difficult to monitor the flow of trade products.

Another problem is the identification of invasive species. They include a wide range of plants, insects and microorganisms, many of which are unfamiliar to quarantine staff and farmers.

Identification Software

Rapid and correct identification of new invasive species is critical. However, it may be difficult for quarantine inspectors and extension staff, few of whom have advanced taxonomic training, to identify a particular pest or weed species, especially if it is a new one which has not been encountered before. Fortunately, we now have identification software such as Lucid, and on-line taxonomic keys, to help carry out this function.

Molecular Studies of Genetic Relationships

New molecular methods of analyzing genetic relationships are a valuable aid in identification, and also help to show the origin(s) of particular exotic pests. They are able to characterize different strains of bacterial and fungal pathogens. In this way, it is possible to define the relationships between different types in different countries, thus showing the pathway of invasion.

One paper presentation discussed the results of amplified PCR testing of bacterial wilt of ginger and other crops in Japan. Two types were identified, Type 1 and Type 2, probably introduced independently in contaminated seeds imported from overseas. Type 1 probably originated in Thailand, while Type 2 had probably originated in Australia and had invaded Japan via China. Thus, the disease seems to have originated in two different places, and spread into Japan through two separate routes.

Dispersal Pathways

The dispersal pathway is of central important in assessing and managing the risk of exotic species. Information required should include the origin of the pathway, the pathway itself (in terms of the vector, and the size and frequency of invasions), and the point of landing.

Managing the dispersal pathway is often the best way of controlling the invasive species. These may be natural mechanisms, such as wind or ocean currents. More often, the means of accidental introductions are ships, planes or packages sent by post. A survey in New Zealand found that around 400,000 containers are unloaded from ships each year, of which around 39% are contaminated by unwanted organisms! The growth of containerized shipping all over the world in recent years may well have increased the risk of invasion by unwanted species.

Control of Invasive Species

However good the precautions, some introductions of alien pests and weeds are inevitable. The problem then becomes one of management and control.

Many alien species do not become established. Of those which do become established, not all are harmful. The "Tens Rule" suggests that only about 10% of the species that arrive will become established, and of these, only about 10% will have an ecological effect.

For those which do damage the agricultural or natural environment, the question then is whether an eradication program is feasible, and also economically viable. Successful programs to eradicate established exotic species are relatively rare. Some examples are the eradication of Melon fly and Oriental fruitfly from Okinawa by the Sterile Insect technique (SIT). For most other pest species in most other areas, eradication is not usually possible, either technically or economically. It has not been possible to eradicate Polynesia rats in Taiwan, for example. However, poison baits have been able to suppress the population to a low level and limit it to a relatively small area, so it does not spread throughout the island.

The Database

The database APASD (Asian Pacific Alien Species Database) is being developed by the National Institute for Agro-Environmental Sciences (NIAES) of Japan, and contains detailed information on invasive species. Information on invasive species will include a history of when they invaded, whether they have become established, whether they are expanding, the economic damage they cause, and some available control measures. The database will also include references to current publications about the species concerned.


The indigenous floral communities of Southeast Asian countries are the product of a long process of competition and succession. Even on a single landmass such as Eurasia, there are distinctive regional and local ecosystems. Plants and animals cannot extend their distribution indefinitely. All encounter restrictions to their ability to migrate and disperse, leading to the development of local varieties and species. However, with international trade and international tourism, geographical barriers are no longer as effective as they used to be.

Not all introduced species become established. Of those which do become established, only a small percentage cause major problems. Risk assessment is an invaluable guide as to whether a species is likely to become a problem, and whether the cost of an eradication program will be justified. These systems are improving all the time, and it seems that with further research, we may develop them even more. Public education is also important, so that farmers and extension staff, as well as members of the public, can recognize exotic species, and know what they should do about them.

A global regional distribution map of exotic species would be very valuable, while the international sharing of biological information would greatly improve the quality of regulatory decisions. Immediate information on detection and eradication measures is particularly critical, and such information is often not available within a country where the pest has not yet become established. A shared international databases such as APASD should offer common ground for the scientific information that can be used as a basis for such decisions.

If a species has already caused problems as an invasive species, it is likely to cause similar problems wherever it is introduced. Historical information is thus of great importance. Also important in the accidental introductions is the dispersal pathway, although this type of information may involve sensitive political aspects.

International Seminar on Biological Invasions: Their Environmental Impact, and the Development of a Database for the Asian and Pacific Region

Held in Tsukuba, Japan on November 13-15

No. of countries participating: 10 (Australia, Japan, Indonesia, Korea, Malaysia, Philippines, Taiwan ROC, Thailand, United States, Vietnam)

No. of papers presented: 18

No. of participants: 160

Co-sponsor: National Institute for Agro-Environmental Sciences (NIAES), Japan

Index of Images

Figure 1 Siam Weed, a Native of the Americas Which Has Become a Major Weed Problem in Asia. It's Numerous Windborne Seeds Enable It to Spread Rapidly Once It Is Established.

Figure 1 Siam Weed, a Native of the Americas Which Has Become a Major Weed Problem in Asia. It's Numerous Windborne Seeds Enable It to Spread Rapidly Once It Is Established.

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