Control
Initial Assessment
The objectives for any control program need to be clearly thought out before any control can begin. The GISP site discusses the need for clear management goals in the context of a conducting an initial assessment of the problem. Read Section 5.1 of the GISP Toolkit (Wittenberg & Cock 2001) and consider the clear link between initial management assessments and risk assessments.
Prioritising Effort
In many cases managers are faced with a number of species that may need some control effort. Resources for control (staff, time, money) are always limited so it is necessary to rank the species so that you can priorities your effort.
The GISP Toolkit section 5.2 (priorities for management) describes four main categories to consider when attempting to priorities control efforts. Reflect on the links between these four categories and the issues covered in Topics 1 - Globalisation of Ecology and Topic 2 - The impacts of invaders.
Also consider how easy/difficult it would be to assign rankings to these categories given that some are based on technical and scientific information (which may be lacking) and others are based on value judgements.
Management Strategies
When dealing with invaders there are a number of possible management strategies that can be adopted depending on how well established the invasive species is. Eradication may be an option if the invader is detected early enough. If not, then it may be necessary to contain and limit the further spread or to attempt control to reduce the population below an acceptable limit. If the invasive species is beyond control then the only course of action is to attempt to mitigate negative impacts.
The GISP Toolkit gives a short overview on each of these different management approaches. As you read the material in Chapter 5 of the GISP Toolkit (details below), consider how easy or difficult it is to draw clear distinctions between these different management strategies.
Eradication
Complete eradication is sometimes feasible but usually only if the invader is detected early and resources to control it are applied quickly. The GISP Toolkit (section 5.3.1) contains numerous interesting examples of eradication programs. Read the brief examples of the eradication of the Black striped mussel from Darwin (case study 5.19) and of the eradication of Siam weed from Queensland (case study 5.14). Also read the section on Fire ant control in the US (case study 5.10) as its particularly relevant in tropical Australia, given the ongoing outbreaks in SE Queensland.
Additional reading:
Mack et al. (2000) suggests that three key factors contribute to successful eradication. Read this paper to discover what these factors are.
Containment
If eradication is not an option, then it may be possible to still attempt to contain a weed within a particular area and reduce the risk of its further spread. The GISP Toolkit (section 5.3.2) discusses the issue of containment as a management strategy.
Read the example of the current Siam weed (Chromolena odorata) containment project in Queensland (case study 5.14). The difficulties of containment at the borders of a management area are clearly illustrated by the example of weed seed movement into Kakadu National Park Case (case study 5.16).
Maintenance Control
If eradication fails or if the species is further down the invasion process and eradication is simply no longer an option, then the management gaol becomes "maintenance control" of a species at an acceptable level. Mack et al (2000: Reading 4.4) provide a summary of the three main approaches that have been used in maintenance control: chemical, mechanical, and biological control. These authors consider the pros and cons of each of these approaches.
Mechanical
Mechanical controls include such methods as hand pulling, bulldozing and burning of weeds and shooting, trapping and hand removal of invasive animals. Read The GISP Toolkit section 5.4.1 and the case study 5.19 which describes hand picking of Black-striped mussels from Darwin harbour.
Chemical
Chemical control including herbicides, pesticides, and fungicides is still a widely used tool for controlling many invasive species. Read the GISP Toolkit section 5.4.2 to find out more.
Biological
The problems associated with chemical and mechanical controls have led people to focus attention on biological control. Biological control is essentially a planned invasion which involves introducing the natural enemy of an invasive species (5.4.3). It should be noted that biological control itself is unlikely to result in eradication. Why not? Many tropical pests and invaders have been effectively controlled using biological control. For some examples read the GISP Toolkit case study 5.26.
Habitat Management
Some invasive species can be controlled by manipulating habitats through strategic use of fire, grazing, hunting and other forms of management (read the GISP Toolkit case study 5.4.4)
Integrated Control
Given that each of the approaches described above have their own advantages and disadvantages, and given that no single approach may be able to solve a control problem, there has recently been a greater focus on the use of combinations of control methods to provide an integrated control program (the GISP Toolkit section 5.4.5). The control of Water Hyacinth provides a good example of this (The GISP Toolkit case study 5.30).
Mitigation
When eradication is not an option and other management strategies fail to reduce the populations below an acceptable level, then we may have accept that we will have to live with the species. This is best achieved by reducing its impact on biodiversity. These "last resort" mitigation strategies are discussed in the GISP Toolkit (section 5.3.4) and a tropical example is the attempts to mitigate the effects of rat predation on an endangered Seychelles Black Parrot (The GISP Toolkit case study 5.41).
As the volume of trade continues to increase and the means of transporting plants, animals and their products improves, and the balanced risk approach is utilised, this reactive rather than preventive approach is likely to become more common.
Mitigating the impact of an invasive species may, like prevention, involve risk assessment. The Higgins & Richardson (1996) model that you examined in Topic 3: Pests and Population Ecology (Reading 3.10) is an example of a risk assessment tool that aims to identify high-risk habitats by explicitly incorporate both species attributes and habitat/community vulnerability to invasion.