5 The production of target knowledge – Valuation of the impacts of invasive species

Five different research approaches can be distinguished that have been or are used in invasion research to clarify target knowledge. These approaches differ in their research questions and methods, and have different implications for problem solving (Table 2). They are in the following shortly introduced in the order in which they appeared in time.

5.1 Biological impact research

The negative impacts of invasive species on native biota and ecosystems were discussed by Charles Darwin and other naturalists as early as the 19th century (Cadotte, 2006), and a wealth of case examples have been documented (e.g. Drake et al., 1989*; Vitousek et al., 1987). However, only recently have frameworks for a more systematic research on impacts been suggested (Levine et al., 2003*; Parker et al., 1999; Vitousek, 1990). Invasive species have been shown to substantially change ecosystem processes such as nutrient cycling, fire and other disturbances regimes, or hydrology (Levine et al., 2003; Mack et al., 2000*). Besides impacts on ecosystem processes, the role of invasive species in native species decline and extinction is a major concern (MEA, 2005*). Invasive species have contributed to many species extinctions on oceanic islands (Reaser et al., 2007), but it is still debated to what extent this pattern can be generalised to continents (Gurevitch and Padilla, 2004). A major open question is whether and how impacts of alien species persist in the long-term (Hobbs et al., 2006; Strayer et al., 2006). Invasions may lead to habitat deterioration (Simberloff, 2006), or support ecosystem recovery after major habitat destruction (Kueffer and Daehler, 2008; Kueffer et al., 2007b; Safford and Jones, 1998). Recently, positive ecological effects of alien species on native biota (Rodriguez, 2006), and the usefulness of alien species for natural area management (D’Antonio and Meyerson, 2002*; Zavaleta et al., 2001*) have been emphasised.

5.2 The native/alien debate

A central tenant of invasion research is that the alien provenance of a species is relevant to predict and value their impacts. However, many authors have argued that the use of the native/alien dichotomy for judging a species is problematic because such an argumentation has close affinities to xenophobia or racism (Larson, 2005; Sagoff, 2005; Simberloff, 2003; Theodoropoulos, 2003; Warren, 2007). Nevertheless, given the weak predictability of the impacts of individual alien species, a precautionary approach has been proposed for invasive species risk assessment systems, which is based on the assumption that an alien species is problematic until proven otherwise (Simberloff, 2005*; Wittenberg and Cock, 2001*). Thus, the ongoing discussion among biologists, social scientists and philosophers on an appropriate use of the native/alien distinction as a proxy for understanding and predicting impacts of invasive species (Brown and Sax, 2004; Colautti and McIsaac, 2004*; Kendle and Rose, 2000; Lodge and Shrader-Frechette, 2003; Shrader-Frechette, 2001) cannot easily be dismissed.

5.3 Ecological risk assessment

Ecological risk assessment is a procedure that evaluates the likelihood of negative ecological effects from exposure to a stressor (Simberloff, 2005*). It consists of an assessment of the probability that a component of an ecosystem is exposed to a stressor and the characterisation of the ecological effects of the stressor on this ecosystem component (Andow and Hilbeck, 2004*; Simberloff, 2005*). In invasive species risk assessment systems, the risk of exposure is defined as the risk that a species is introduced, establishes in a new range and spreads into natural areas. For this component, risk assessment systems build on biological research on the invasiveness of alien species (Daehler et al., 2004; Pheloung et al., 1999; Wittenberg and Cock, 2001*). In contrast, the formal characterisation of ecological effects has so far proven to be difficult, and explicit assessments of ecological effects are done only rarely or only in a qualitative manner through expert judgments (Andow and Hilbeck, 2004; Lodge et al., 2006*; Mack and Barrett, 2002*; Simberloff, 2005*). The difficulties in characterising the impacts of biotic invasions in formalised systems are seen in the complexity of their impacts – they are often indirect, affect populations or communities rather than individuals, depend on the context of the invaded ecosystem, and show unpredictable temporal dynamics (Andersen et al., 2004b*; Simberloff, 2005*). Recently, spatially explicit modelling of biotic invasions has been proposed for a more quantitative assessment of exposure risks and corresponding ecological effects (Allen et al., 2006; Andersen et al., 2004a*). An open question regarding current risk assessment systems is if and how economic or socioeconomic valuation (see below) should be included in the procedures (Andersen et al., 2004b).

5.4 Economic valuation

The monetary equivalent of the impacts of a biotic invasion is calculated through economic valuation. Estimates of the total costs stemming from biotic invasions are based on the current or projected future costs of all damages caused by alien species (Born et al., 2005*; Olson, 2007*; Pimentel et al., 2005*). Sometimes the expenditure for controlling invasive species or other mitigation costs, e.g. treatment of allergic reactions to pollen of an invasive plant, are also included (Born et al., 2005*; Pimentel et al., 2005*). The monetary value of the ecological damages caused by alien species are usually calculated based on the concept of ecosystem services (Binimelis et al., 2007*; Born et al., 2005*; Charles and Dukes, 2007*), i.e., the loss of ecosystem services through biotic invasions is derived from biological impact research (see above) (Charles and Dukes, 2007*). Estimates of total costs stemming from biotic invasions for countries such as the U.S.A., Canada, Australia or Germany are typically of the order of more than US$ 1 billion per year (Olson, 2007*; Pimentel et al., 2005). However, to date such economic assessments are mainly based on expert knowledge and extrapolations from only a few well-documented aspects such as the annual expenditures to control a species in a particular area.

5.5 Socioeconomic valuation

Because many valuable things do not have a market price, or things are valuable for more than only economic reasons (Brun and Hirsch Hadorn, 2008*), broader socioeconomic approaches to assess damages by biotic invasions are being developed (Binimelis et al., 2007*). For instance, the damages to landscape aesthetics or the recreation value of nature have been estimated (Binimelis et al., 2007*; Charles and Dukes, 2007), or the role of invasive species for poor people and developing countries is considered (Drake and Keller, 2004; Perrings, 2005). In socioeconomic valuation it is particularly important to recognise that alien species can have both positive and negative roles for people, and that the valuation of an alien species is dynamic and may change with time and context (Binimelis et al., 2007*). Social sciences, ethics and historical research have provided a deeper understanding of socioeconomic valuation of invasive species issues in different contexts and in a historical time frame (e.g. Coates, 2007; Foster and Sandberg, 2004; Hall, 2003).