6 The production of transformation knowledge – Research that addresses the options for managing biotic invasions

Management of invasive species can be divided into three different realms: i. prevention, ii. early detection and eradication, and iii. control of established alien species (Lodge et al., 2006*; Wittenberg and Cock, 2001*). Each of these three phases has different demands for research (Table 3).

6.1 Prevention

Prevention of introductions of potentially problematic alien species to a new area, or secondary releases of already introduced alien species within an area, builds on risk assessment systems and the regulation of the transportation of the identified problematic species (Kowarik, 2003*; Simberloff, 2005*; Wittenberg and Cock, 2001*). Prevention is widely considered the most cost-effective way for managing invasive species (Baker et al., 2005*; Lodge et al., 2006*; Wittenberg and Cock, 2001*). The application of risk assessment systems (Table 2) is confronted with a number of practical problems (Andersen et al., 2004a; Baker et al., 2005*; Simberloff, 2005*). In particular, the efficacy of expert-based risk assessments is very low. These assessments currently cover only a very small proportion of newly introduced species (Hulme, 2006*; Lodge et al., 2006*), because the assessment of a single species typically lasts for many months to several years (Lodge et al., 2006*; Simberloff, 2005*). Such assessments are also said to be highly vulnerable to political pressure (Simberloff, 2005*). As an improvement, a precautionary approach has been proposed that treats all new alien species as problematic until proven otherwise (Simberloff, 2005*; Wittenberg and Cock, 2001*).

Traditionally, risk assessments have been implemented at a national or transnational level. However, the uncertainties of risk assessments may be reduced when they are performed at smaller local scales, e.g. on a regional (Radosevich et al., 2005) or habitat scale (cf. Lodge et al., 2006*), or specifically address particular introduction pathways (Hulme, 2006*). Pathway risk analyses have for instance been developed for the trade of untreated wood (cf. Simberloff, 2005). Context specific risk assessments allow for a comprehensive risk management, i.e., different management measures are combined to optimize the cost-benefit balance (Hallman, 2007*). The risk management of Mediterranean fruit fly introductions through trade of pink tomatoes from Northern Africa to the U.S. combines regulations about the origin of the product, restrictions on the seasons when trade is allowed, and a number of measures during production and transport (Hallman, 2007). Increased context specificity allows also to better shape research according to the needs of particular actors, such as in the cases of marine shipping (Minton et al., 2005), or the horticulture and pet industries (e.g. Perrings et al., 2005*; Reichard and White, 2001).

Besides risk assessment systems, market mechanisms such as tradeable risk permits (Horan and Lupi, 2005), taxes (Knowler and Barbier, 2005; Perrings et al., 2005*), or the implementation of the polluters pay principle (Perrings et al., 2005), have been discussed as alternative policy instruments for prevention.

6.2 Early detection and eradication

Eradication is very difficult, because literally all reproductive individuals have to be removed, and the risk of re-invasion has to be zero (Genovesi, 2007*; Myers et al., 2000*). Cost-effective eradication of an invasive species is therefore only possible in a very early phase of an invasion when the population size and infested areas are still small. It is generally assumed that after prevention early detection of newly introduced species and immediate action to eradicate the species before it spreads should be the second priority in invasive species management (Genovesi, 2007*; Lodge et al., 2006*; Wittenberg and Cock, 2001*). However, the detection of small populations in an early stage of an invasion is particularly challenging and potentially costly (Hulme, 2006*; Lodge et al., 2006*). Research that combines statistical and biological considerations can optimize surveying methods (Baker et al., 2005*; Hulme, 2006*; Rew et al., 2005), e.g. by focusing surveys on the areas where the appearance of new invasive species is most likely. Both environmental factors and human activities can help to predict the likelihood of new appearances (Buchan and Padilla, 2000). Search theory, a sophisticated mathematical approach, can help to define the most efficient search strategy (Cacho et al., 2006; Mehta et al., 2007). Early detection may also profit from new technologies such as remote sensing (Hulme, 2006*; Lodge et al., 2006*) or molecular biology techniques (Baker et al., 2005*; Lodge et al., 2006*). Finally, successful early detection depends on socioeconomic factors, i.e., the necessary awareness and financial and institutional capacity needs to be established so that professionals from all relevant agencies as well as volunteers can be effectively involved (Genovesi, 2007*; Hegamyer et al., 2003; Hulme, 2006*; Lodge et al., 2006*; Wittenberg and Cock, 2001*).

6.3 The control of invasive species

Planning

When eradication of an invasive species is not possible, an appropriate strategy to contain the spread of the species is defined (control of an invasive species) (Genovesi, 2007*; Lodge et al., 2006*; Myers et al., 2000*; Wittenberg and Cock, 2001*). Typically, priority areas are identified where invasive species are kept below a certain threshold through continuous management (Mack and Lonsdale, 2002; Myers et al., 2000*; Thomas and Reid, 2007*). Every successful control programme has to agree on clear objectives (Anderson et al., 2003*; Baker et al., 2005*; Genovesi, 2007*; Wittenberg and Cock, 2001*). A number of decision aid tools have been developed to support the setting of appropriate objectives for control programmes. These tools may for instance help to define priority species and areas (Hiebert, 1996; Tassin et al., 2006; Wittenberg and Cock, 2001*), or acceptable levels for the population density of an invasive species – a concept that has a long tradition in agricultural weed management and biological control (Hulme, 2006; Smith et al., 2006; Thomas and Reid, 2007*). Often a detailed consideration of the stakeholders’ valuation of a management attempt is necessary. The socioeconomic valuation of management measures, including prevention (Finnoff et al., 2007), may compare the costs and benefits of different management scenarios (cost-benefit analysis), or the costs and effectiveness to achieve a certain goal through different strategies (cost-effectiveness analysis) (Binimelis et al., 2007*; Born et al., 2005*). Thereby the valuation of the positive and negative aspects of a management strategy may vary considerably between stakeholder groups (Bremner and Park, 2007; Veitch and Clout, 2001).

A major recent research effort has focused on a detailed modelling of the costs and benefits of different management strategies by combining socioeconomic valuation with information on the biology and management options of a particular species. Such bioeconomic modelling helps to identify the optimal control technique (Baker et al., 2005*), and to define the conditions under which it is still economic to eradicate a species (Regan et al., 2006) or the optimal threshold population density of a species that is contained (Anderson et al., 2003*). Bioeconomic models are often based on sophisticated biological models, that include, for instance, multitrophic biotic interactions (Gutierrez and Regev, 2005) or spatially explicit spread modelling (Olson, 2007*). Similarly, the economic models may vary from simple economic to complex socioeconomic valuation of management strategies (see above). A range of control techniques from mechanical, chemical or biological control to habitat management approaches may be considered (see below) (Olson, 2007*), as well as different management contexts and scales from local natural area management to nationwide efforts (Born et al., 2005; Olson, 2007*).

Control techniques

Control techniques can be classified into mechanical, chemical and biological control and habitat management (Lodge et al., 2006*; Myers and Bazely, 2003*; Wittenberg and Cock, 2001*). Successful control programs depend often on the experience and knowledge of practitioners (Bossard et al., 2000; Tu et al., 2001). Mechanical control encompasses all actions that physically destroy an invasive species, e.g. the pulling out of a plant by hand or shooting of an animal. Chemical control measures target invasive species through the application of a pesticide. Major obstacles are the risk of an environmental contamination of natural areas with the chemical, non-target effects on native species, or the evolution of pesticide-resistance by the invasive species (Baker et al., 2005; Lodge et al., 2006*; Myers and Bazely, 2003*; Wittenberg and Cock, 2001*). In the case of biological control a natural enemy such as a herbivore or pathogen is introduced with the aim that the introduced organism controls the invasive species (Babendreier, 2007; Messing and Wright, 2006; Mack and Barrett, 2002; Thomas and Reid, 2007). Major risks of the introduction of an (alien) organism in biological control programs are non-target effects on the native biota. Finally, habitat management targets invasive species through the manipulation of environmental characteristics of the invaded habitat, e.g. through prescribed fire, planting of native species, or the alteration of soil conditions (Myers and Bazely, 2003; Wittenberg and Cock, 2001*). Habitat management strategies are closely linked to habitat restoration (D’Antonio and Meyerson, 2002; Zavaleta et al., 2001*). A habitat perspective in invasive species removal is important to avoid negative impacts of invasive species control programmes, e.g. because of habitat disturbance or of the removal of alien species that play an important role for the native biota or ecosystem (Anderson et al., 2003*; Myers et al., 2000*; Zavaleta et al., 2001).

The wider socio-political context

Successful control programmes depend on the concerted long-term action of many agencies and the support of many stakeholders (Anderson et al., 2003; Genovesi, 2007; Myers et al., 2000; Wittenberg and Cock, 2001*). A holistic approach to form such an alliance is social marketing (Wittenberg and Cock, 2001). Social marketing is based on an analysis of the social, cultural, political and economic context of the invasion, and aims at forming a partnership among all relevant agencies and stakeholders. Based on a thorough understanding of the conflicts of interests and values (target knowledge) and the options and constraints of the actors (transformation knowledge) in a particular management context, control strategies are defined that are thought to be effective and acceptable to all stakeholders.