Two sets of complementary strategies for dealing with alien invasive species are: prevention and early detection; and response, which includes eradication (rarely achieved), containment, control and mitigation. Many agencies have been advocating the concept of biosecurity which is a strategic and integrated approach that encompasses the policy and regulatory frameworks (including instruments and activities) that analyse and manage risks in the sectors of food safety, animal life and health, and plant life and health, including associated environmental risk (FAO, 2003). It covers the introduction of insect pests and diseases, the introduction and release of genetically modified organisms and their products, and the introduction and management of alien invasive species and genotypes (FAO, 2003).
Unless otherwise noted, the information presented in this section is taken from Wittenberg and Cock (2001).
Prevention and early detection
Prevention is the first line of defence against biological invasions and is also the most cost effective since once an alien invasive species becomes established, it is extremely difficult and hence costly to eradicate it. An important first step in prevention is identification of the species capable of becoming invasive, the possible susceptible sites and more importantly, the pathways in which they can be introduced. The more comprehensive approach of identifying pathways rather than individual species results in a greater concentration of effort where pests are more likely to enter a country which not only avoids wasting resources elsewhere but also helps in the identification of more species, vectors and pathways. Once pathways are identified then potential prevention tools and methods can be more specifically developed.
The International Plant Protection Convention (IPPC) is an international treaty with a purpose to secure action to prevent the spread and introduction of pests of plants and plant products, and to promote appropriate measures for their control. This convention specifically governs transboundary movement of all plants and plant products (including forest products) and the scope is not limited to agricultural plants. It is governed by the Interim Commission on Phytosanitary Measures (ICPM) which adopts International Standards for Phytosanitary Measures (ISPMs). All of these ISPMs have direct relevance to the forest sector including guidelines for the export, shipment, import and release of biological control agents and other beneficial organisms (ISPM No. 3), guidelines on pest risk analysis (ISPM No. 2, 11 and 21), guidelines on pest eradication programmes (ISPM No. 9), guidelines on pest status and reporting (ISPM No. 8 and 17), and guidelines for regulating wood packaging materials in international trade (ISPM No. 15) (IPPC, 2005).
Some of the important tools used to prevent the entry and establishment of alien invasive species include:
• public information and education;
• risk assessments and environmental impact assessments for intentional introductions;
• national and international regulations on prevention and quarantine measures and their enforcement with inspections and fees;
• treatment of imported commodities, including through fumigation, immersion, spraying, temperature treatment, ultraviolet sterilization, and pressure;
• technically justified trade restrictions;
• emergency measures or actions if a pest should be detected before establishment (McNeely et al., 2001).
Early detection of alien species should be based on a system of regular surveys – general, site-specific or species-specific – to identify newly established species. Although not all alien species become invasive, the costs of those that do become invasive suggest that a precautionary approach to the issue is best. If alien species are identified early, the chances for eradication will be high in particular because for some invasive species there can be a long lag period between initial introduction and subsequent population explosion (Crooks and Soulé, 1999; Parker, 2004). The longer species go undetected the fewer the options for its control or eradication and the more expensive any intervention will become. The IPPC’s ISPM No. 5 provides the international standard to which general surveillance should be conducted for plants (IPPC, 2005).
Early detection is highly dependent on the capacity of individuals to recognize both native and alien species (See Box 1). As a result, a large component of this step is training, not only of national professionals responsible for surveying but also for any persons that spends time in the natural environment such as farmers, gardeners, forest workers, ecologists, tourism workers, photographers and hikers. Trained professional national workers should be able to not only recognize native and alien species and the ecological effects of alien species but they also should be able to use databases, keys, manuals and other identification sources. Early warning systems which include lists and datasets of recorded or potentially invasive species, in given countries, time sets and conditions are important tools in this regard.
Finally a contingency plan outlining the actions that should be taken once an alien invasive species has been identified or an invasion is suspected, should be developed.
Box 1. Attributes of potentially invasive species and susceptible ecosystems
Though much is known about the factors that contribute to the introduction and spread of alien invasive species, little is known about the attributes of alien invasive species and susceptible habitats.
Isolated ecosystems, evolutionarily and geographically, are particularly vulnerable to biological invasions while deserts, semi-deserts, tropical dry forests and woodlands, and arctic systems appear to be the least susceptible (McNeely et al., 2001; Perrings et al., 2002). Ecosystems with low species diversity, especially if predators and competitors are absent, seem to be more susceptible than species-rich systems with well-established species interactions (McNeely et al., 2001; Perrings et al., 2002). However, such species-rich ecosystems may be at risk to a greater range of invaders because of their greater diversity of habitats (McNeely et al., 2001). Frequent disturbance, slow recovery rate and fragmentation of communities promote plant invasions (Rejmánek, 1999).
Some ecological factors that may allow introduced species to spread include the following (Pimentel et al., 2000).
• Lack of predators, competitors and parasites.
• Ability of an alien parasite to switch to a new host.
• Ability to be an effective predator.
• Availability of artificial or disturbed habitats.
• High adaptability to novel ecosystems.
• Efficient dispersal.
While the success of alien invasive species is often explained by simply placing an introduced species in a favourable environment, some have suggested that the lack of enemy pressure actually results in a reallocation of resources from defence, as it is no longer needed in enemy-free habitats, to growth and reproduction (Petit et al., 2004; Withgott, 2004). Others have suggested that this may not be the case for all circumstances and there is likely not a simple trade-off between defence mechanisms and growth (Withgott, 2004).
Identification of the life-history traits of potentially invasive species has been attempted with some success, primarily for woody species. For conifer species, invasiveness is associated with: small mean seed mass (<50 mg); short juvenile period (<10 years); and short intervals between large seed crops (Richardson and Rejmánek, 2004). A short juvenile period and short intervals between large seed crops results in early and consistent reproduction. A short juvenile period may also be related to fast growth in general and small mean seed mass is associated with a greater number of seeds produced which are better dispersed, a high initial germinability and a shorter chilling period to overcome dormancy. Examples of alien invasive species with such life-history characteristics include Cryptomeria japonica, Larix deciduas, Picea sitchensis, Pinus contorta, Pinus strobus and Pseudotsuga menziesii.
Another important consideration is the fact that some alien invasive species explode quickly while others may have a long lag period between initial introduction and subsequent population growth and expansion. Some reasons for such time lags include the following (Crooks and Soulé, 1999).
• The area in which the species has been introduced may become more susceptible to invasion.
• The species may have been undergoing rapid yet undetected expansion and some environmental change may trigger invasion.
• Several decades may be needed to build up large enough populations to have significant reproductive potential.
• Newly introduced species may be confined to restricted habitats until mutations favourable for further colonization became available.
Knowing more about which species invade and which ecosystems are more susceptible will improve and refine capacities to prevent, or detect in a timely manner, biological invasions.
When the preventative and early detection measures have failed to stop the introduction of alien invasive species, eradication is the preferred next method of action. Eradication involves the elimination of the entire population of an alien species, including resting stages, in managed areas. As a rapid response to early detection of an alien species, eradication is often the key to a successful and cost-effective solution. Careful analysis of the costs involved and the likelihood of success must be made before any eradication attempts are made. Some groups of organisms are more suitable for eradication such as plants, terrestrial vertebrates and some terrestrial invertebrates and in some cases insects. Also, well established populations and large areas of infestation are unsuitable for such programmes.
Successful eradication programmes must:
• be scientifically based;
• ensure that all individuals of the target population are susceptible to the technique being used thus ensuring that eradication of all individuals is achievable;
• build support from the public and all relevant stakeholders;
• ensure that the legal and institutional framework is sufficient for dealing with the issue;
• secure sufficient funding;
• ensure through prevention measures that there is no immigration of the target species into the area;
• put in place a method to detect the last survivors;
• include a subsequent monitoring phase to ensure that eradication has been achieved, and to prevent re-invasion;
• ensure that techniques are environmentally, socially and ethically acceptable;
• include any necessary measures to restore ecosystems after eradication.
Eradication programmes involve several control methods as outlined in the control section below. The IPPC’s ISPM No. 09 specifically provides general guidance for eradication programmes targeting plant pests and alien invasive species (IPPC, 2005).
Containment is a special form of control aimed at restricting the spread of an alien invasive species and to contain the population in a defined geographical range. The methods used are the same as those described for prevention, eradication and control. An important component of containment programmes is the ability to rapidly detect new infestations of the alien species spreading from the defined containment area or into new areas, so that control measures can be implemented quickly. In cases where eradication is not possible, containment of the alien invasive species into a defined area can be very effective in saving other regions of a country.
The long-term reduction in density and abundance of an alien invasive species to below a pre-set acceptable threshold is the aim of control programmes. Suppression of invasive populations below such thresholds can favour native species. All control programmes need long-term funding and commitment. Control methods, including mechanical control, chemical control, biological control, habitat management, and hunting or a combination of these, have all been used successfully in controlling alien invasive species.
Not all options for the control of alien invasive species are practical, effective, economically justifiable, or environmentally sound for application in forests. In the forest sector, control measures should be integrated to maximize yield and profit while minimizing negative environmental impacts. Some other options that are suitable include cultural techniques, such as seedling management, planting patterns, buffer zones, ecoclimatic matching, and pest and pathogen resistance; mechanical methods, such as cutting, bulldozing and shading; biological control; and integrated pest management.
If eradication, containment and control methods have failed in managing an alien invasive species, the only option is to accept the species while mitigating the impacts to other species and the environment. The focus here is on protecting native species rather than harming the alien invasive species. Mitigation methods can include translocation of a population of species to an ecosystem that has not been invaded and alterations in the behaviour of desired species such as conditioning species to use breeding areas inaccessible to the invaders or artificial feeding sites.
Central to any effective alien invasive species management programme is rapid and transparent communication with all stakeholders which helps ensure maximum stakeholder participation and increased awareness of the issues. Such communication is vital during the risk assessment, prevention and management phases.
For plants and plant products, governments have specific information exchange obligations in this regard (IPPC, 2005) to create awareness in neighbouring countries and among trading partners. Such communication can greatly facilitate trade by increasing transparency and trust between trading partners.