Ecology and Management of Invasive Alien Species: Management

The four main strategies for dealing with established invasive alien species are:

• Eradication

• Containment

• Control

• and Mitigation.

1.  Eradication

Eradication is the elimination of the entire population of an alien species, including any resting stages, in the managed area. When prevention has failed to stop the introduction of an alien species, an eradication programme is the preferred method of action. Eradication as a rapid response to an early detection of a non-indigenous species is often the key to a successful and cost-effective solution. However, eradication should only be attempted if it is feasible. Eradication is the type of clear-cut decisive intervention that appeals to politicians and the public, but beware of the temptations of attempting an eradication programme that is unlikely to succeed. A careful analysis of the costs (including indirect costs) and likelihood of success must be made (rapidly) and adequate resources mobilized before eradication is attempted. However, if eradication of the invasive species is achieved it is more cost-effective than any other measure of long-term control (Case Studies 4.2 "Early Detection and Eradication of White-Spotted Tussock Moth in New Zealand" and 5.5 "Eradication of a Deliberately Introduced Plant Found to be Invasive").

Eradication programmes can involve several control methods on their own or a combination of these. There are few situations where a single method is a proven eradicator of an invasive species. Therefore it is wise to plan for and use all possible methods. The methods vary depending on the invasive species. Successful eradication programmes in the past have been based on:

• mechanical control, e.g. hand-picking of snails and hand-pulling of weeds;

• chemical control, e.g. using toxic baits against vertebrates and spraying insecticides against insect pests;

• biopesticides, e.g. Bacillus thuringiensis (BT) sprayed against insect pests;

• sterile male releases, usually combined with chemical control;

• habitat management, e.g. grazing and prescribed burning;

• hunting of invasive vertebrates.

Some groups of organisms are more suitable for eradication efforts than others. However, it has to be borne in mind that each single situation needs to be evaluated to find the best method in that area under the given circumstances.

2.  Containment

Containment of non-indigenous invasive species is a special form of control. The aim is to restrict the spread of an alien species and to contain the population in a defined geographical range. The methods used for containment are the same as those described for prevention, eradication and control and are therefore not presented here in detail. Monitoring and public involvement will again be a critical feature

Containment programmes also need to be designed with clearly defined goals: barriers beyond which the invasive species should not spread, habitats that are not to be colonized and invaded, etc. (Case Study 5.14 "Containment of the Spread of Chromolaena Weed in Australia"). In order to establish these parameters there needs to be clear understanding of why the containment is being done in the first place, e.g. to protect particular areas or habitats from invasion, to allow time to mobilize other control or eradication measures etc.

An important component of a containment programme is the ability to rapidly detect new infestations of the invasive species both spreading from the margins of its distribution, or in completely new areas, so that control measures can be implemented in as timely a manner as possible. These new infestations will initially be at very low densities, so early detection will be challenging.

The invasive species’ population is suppressed using a variety of methods along the border of the defined area of containment, individuals and colonies spreading beyond this are eradicated, and introductions into areas outside the defined containment area are prevented. The distinction between containment and eradication is not a l ways clear-cut depending upon the scale of operations considered (Case Study 5.15 "Containment vs. Eradication: Miconia calvescens in Hawaii").

A species most likely to be successfully contained in a defined area is a species spreading slowly over short distances. The nearest suitable habitat for the species should be preferably separated by a natural barrier, or an effective artificial barrier. The most suitable cases for containment are habitat islands without suitable connections that would allow the easy spread of invasive species. The spread of alien freshwater species between different parts of watersheds is a good example where containment may be possible.

Containing a species in a defined area will, however, need constant attention and control of the species at the border and prevention measures against spread of the species (Case Study 5.16 "Seed Movement on Vehicles: a Study from Kakadu National Park, Australia"). Thus, successful containment is difficult to achieve and involves several different costly methods.

3.  Control

Control of non-indigenous invasive species aims for the long-term reduction in density and abundance to below a pre-set acceptable threshold. The harm caused by the species under this threshold is considered acceptable with regard to damage to biodiversity and economy. It is not always clear what this level should be set at in order to achieve the management objective. Research to establish what indigenous biodiversity is at risk and how much of the invasive species’ impact can be tolerated may need to be carried out.

Suppression of the invasive population below that threshold can tip the balance in favour of native competing species. The weakened state of the invasive species allows native species to regain ground and even further diminish the abundance of the alien species. In rare cases this might even lead to extinction of the non-indigenous species (especially combined with habitat restoration efforts to support native species and put intact natural systems back in place), but this is clearly not the principle goal of control efforts.

If prevention methods have failed and eradication is not feasible managers will have to live with the introduced species and can only try to mitigate the negative impacts on biodiversity and ecosystems. All control methods, with the exception of classical biological control, which is self-sustaining, need long-term funding and commitment. If the funding ceases, the population and the corresponding negative impacts will normally increase, perhaps leading to irreversible damage.

Since, in the short-term, control seems to be a cheaper option than eradication, it is often the preferred method. Funding and commitment do not need to be at such high levels as for eradication programmes, and funding can be varied between the years depending on the perceived importance of the problem, political pressure, and public awareness. However, the lower recurring costs are deceptive, because in the long run effective control is more expensive in total than a successful eradication campaign.

4.  Mitigation

If eradication, containment, and control are not options or have failed in managing an invasive alien species, the last resort is to "live with" this species in the best achievable way and mitigate impacts on biodiversity and endangered species. Mitigation as used in this context differs from containment and control in that the activity undertaken does not directly affect the invasive species in question but rather focuses on affected native species. Mitigation is most commonly used in the conservation of endangered species and can be approached at various levels. At its simplest and perhaps most extreme form it could mean the translocation of a viable population of the endangered species to an ecosystem where the invasive species of concern does not occur or, in the case of a rehabilitated system, no longer occurs.