integrating natural and social sciences for understanding and

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Chapter 4

Integrating Natural and Social Sciences for Understanding and Managing Plant Invasions

Christoph KUEFFER1 Institute of Integrative Biology

Abstract

Research on biological invasions has only recently become interdisciplinary. However, despite the growing contribution of social sciences and humanities to invasive species research, integration of natural and social sciences remains weak. In this article a framework that helps to clarify when and how such integration is important is presented and illustrated with examples mostly from oceanic islands and the Pacific. Three different types of knowledge are relevant for assisting environmental problem solving, and producing each of them requires interdisciplinarity: Solving a problem involves understanding causes (‘systems knowledge’), defining management targets and clarifying conflicts of interests and values (‘target knowledge’), and developing management tools to achieve these targets (‘transformation knowledge’).

Much theory on invasive species is based on analyses of past invasions, but such analysis is only reliable if human factors are taken into account: which species were introduced to an area and why (‘species selection’), where and how were they intro-duced (‘introduction effort’), and what kind of habitat disturbances and land use did they encounter (‘invasion opportunities’).

People differ in their perceptions of invasive species and willingness to support management action. Innovation of effective management strategies will most likely happen when the management problem – not the biology of invasions – is taken as the starting point for scientific enquiry (‘problem orientation’). Focusing on species introduction pathways or habitat management challenges – rather than decontex-tualised risks of individual species – represents increased problem orientation.

Key Words: alien, Hawaii, interdisciplinarity, invasive species, novel ecosystem, oceanic island.

1 Institute of Integrative Biology – Plant Ecology, Swiss Federal Institute of Technology (ETH) Zurich, CH-8092 Zurich, Switzerland.

CdS Larrue c3g.indb 71 18/03/13 13:31

In: S. Larrue (ed.) 2013. Biodiversity and Society in the Pacific Islands.Presses Universitaires de Provence, Marseille, France & ANU ePress, Canberra, Australia.

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Résumé

La recherche sur les invasions biologiques a récemment développé des tendances interdisciplinaires. Cependant, les approches fédératives, intégrant les sciences naturelles et sociales, restent encore peu courantes. Cet article tente de définir un cadre montrant où et pourquoi de telles approches sont importantes. Trois types de savoir sont pertinents afin de résoudre les problèmes environnementaux, et chacun de ces types nécessitent une approche interdisciplinaire : la connaissance des causes; l’identification des conflits, des intérêts et des valeurs; et le développement d’outils de gestion permettant d’appréhender les deux premiers points.

La plupart des théories sur les espèces invasives se fondent sur des analyses biologiques des invasions passées, mais de telles analyses ne sont fiables que si elles tiennent compte des facteurs humains: quelles espèces ont été introduites (« sélec-tion des espèces »), où et comment elles furent introduites (« moyen des intro-ductions »), et quels types d’habitat ou d’utilisation des terres ont-elles rencontrés (« opportunité des invasions »).

Appréhender les représentations sociétales des populations à l’égard des espèces invasives, est une nécessité permettant de mieux prévenir les risques d’invasions biologiques. La gestion des invasions biologiques sera plus efficace si les études scien-tifiques se focalisent dès le départ sur les origines sociétales du problème (« entrée du problème ») et pas uniquement sur la question biologique des invasions.

Mots-clés : Hawaï, interdisciplinarité, espèces invasives, nouveaux écosystèmes, îles océaniques.

1. Introduction

Biological invasions are considered one of the five major causes of biodiversity loss, alongside habitat destruction, over-exploitation, climate change and pollution (Millennium Ecosystem Assessment 2005). The massive increase in global human travel and transportation over the past few centuries led to the introduction of large numbers of diseases, animal and plant species to new geographic areas where they would not be present without human assistance (Brunel et al. 2013; Mack et al. 2000; Mooney et al. 2005). It has for instance been estimated that over 10,000 non-native plant taxa have been cultivated in Hawaii, and more than 1,000 of them are natura-lized (Kiehn 2012; Woodcock 2003), and internationally transportation rates of species to new areas are still rapidly increasing (Hulme 2009). Only a small propor-tion of these non-native species become established, some spread, and a small subset produces major negative ecological, economic or social effects (Mack et al. 2000; Millennium Ecosystem Assessment 2005; Mooney et al. 2005; Reaser et al. 2007; Vila et al. 2010). Those non-native species that lead to ecological or economic costs are generally termed ‘invasive species’. The ecological and economic costs of invasions by non-native species are widely recognized in national and international policies (Brunel et al. 2013; Riley 2005). The Convention on Biological Diversity (CBD) recognizes the need to address invasive non-native species in its article 8(h), which requires its Parties to “prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats and species”



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Biological invasions on oceanic islands have attracted the attention of ecologists and conservationists for a long time. Already Charles Darwin considered that invasive non-native species are a particular threat to native species on oceanic islands and speculated whether island ecosystems are more vulnerable to invasions than conti-nental ones due to their impoverished biota (Denslow 2003). Ever since oceanic islands have been an important model system for invasion biology (e.g. Elton 1958; Kueffer et al. 2010a; Mooney and Drake 1989; Mooney et al. 2005; Vitousek et al. 1987), and ecological and economic impacts of invasive species on islands are widely recognized (e.g. Caujapé-Castells et al. 2010; Millennium Ecosystem Assessment 2005; Reaser et al. 2007).

Invasive species research builds on a 50-years history and has rapidly grown over the past few decades (Richardson 2011). But the field has only in recent years become more interdisciplinary (Kueffer and Hirsch Hadorn 2008). Increasingly, social scientists, historians, philosophers, psychologists or economists address aspects of the invasive species problem (Atlan and Darrot 2012; Barbault and Atramentowicz 2010; Born et al. 2005; Fall and Matthey 2011; Hall 2003; Kueffer and Hirsch Hadorn 2008; McNeely 2001; Mooney et al. 2005; Richardson 2011; Rotherham and Lambert 2011; Ruiz and Carlton 2003). However, while the research field has become more diverse, integration of natural and social sciences research has not progressed to the same degree.

In this article a conceptual framework that helps to clarify different roles of interdisciplinary knowledge for understanding and managing plant invasions is developed and illustrated mostly with examples from plant invasions on oceanic islands, and especially from the Pacific and the Hawaiian islands. Oceanic islands are a particular good example for discussing an interdisciplinary understanding of biological invasions because most ecosystems, species distributions and ecological processes on islands are strongly influenced by humans; experts and stakeholders with contrasting opinions, value systems, and cultural backgrounds are involved in invasive species management; and invasive species management is very prominent and different management strategies have been widely discussed and tested.

2. A framework: understanding causes, clarifying interests and values, and developing tools for action

Solving a problem requires that the causes of the problem are understood, manage-ment targets are defined, and means for action are developed to reach these targets. In environmental problem solving it is assumed that science can significantly assist problem solvers in achieving each of these three steps (Kueffer and Hirsch Hadorn 2008; ProClim 1997). Sometimes the terms ‘systems knowledge’, ‘target knowledge’, and ‘transformation knowledge’ are used to denote the contribution of science to problem analysis, clarification of targets, and development of appropriate means for action, respectively (Kueffer and Hirsch Hadorn 2008; ProClim 1997, Fig. 1).

‘Systems knowledge’ assists problem solving by reducing the uncertainties regar-ding the understanding of the causal relationships relevant for the genesis and possible further development of a problem. In the case of biotic invasions this entails understanding and predicting the distribution and ecological effects of non-native


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– or more generally spreading – species in natural areas. ‘Target knowledge’ assists in clarifying conflicts of interests and values entangled within the problem solving process. Research on target knowledge helps to clarify the economic and ecological benefits and costs associated with biological invasions and different management options (including no action, prevention, and different ways of containment and control). ‘Transformation knowledge’ assists in developing effective management tools that take existing practices and institutions into consideration and help adapt them to the problem. This entails developing tools for prevention, containment, and control of invasive species.

In the case of complex problems, such as environmental problems, the situa-tion is further complicated because many factors related to different scientific disci-plines can be relevant for understanding and addressing an environmental problem. Thus, how a problem is approached (‘problem framing’) influences the production of systems, target, and transformation knowledge (Brewer 1999; Kueffer and Hirsch Hadorn 2008; Rosenhead and Mingers 2001). The different forms of knowledge are interrelated and can change with time due to scientific advancement, new manage-ment experiences, changing stakeholder views, and the evolution of the problem itself (due to ecological and/or social change) (Fig. 1). Consequently, asking the right questions becomes in itself a problem for research and participatory deliberation (Kueffer and Hirsch Hadorn 2008).

Evolving problem framings

Advancement of science

Rapid ecological and social change

New management experiences

Changing stakeholder views

Systems knowledge What explains the presence and ecological effects of non-native

species in natural areas?

Target knowledge Which species are a problem?

What are appropriate management targets?

Transformation knowledge What are effective management tools

to tackle the invasive species problem?

Current problem framing

Deliberations about problem framings

Figure 1. A framework that helps to clarify different roles of interdisciplinary knowledge for understanding and managing plant invasions. Solving a problem involves understanding causes (‘systems knowledge’), defining management targets and clarifying conflicts of interests and values (‘target knowledge’), and developing management tools to achieve these targets (‘transformation knowledge’). These three forms of knowledge are interrelated, and how they and their relationships are framed (‘problem framing’) can change in time due to the changing nature of the problem, innovations triggered through new research or management experiences, or evolving stakeholder perceptions. How problem framings evolve is not evident but can strongly affect policies and interests of stakeholders, and therefore structured and inclusive deliberation processes about appropriate problem framings are needed.



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In the next four sections I will illustrate the importance of integrating natural and social sciences information for producing systems, target, and transformation knowledge, and guiding deliberations about problem framings, respectively.

3. Systems knowledge: understanding the causes of plant invasions

Invasion biology aims to explain why certain non-native species have become abundant in natural areas and how they might affect colonized ecosystems. Much effort has been invested in identifying biological factors that drive invasions. However, invasions are strongly influenced by human behaviour. Therefore understanding the causes of past invasions and predicting future invasion risks requires the integration of biological and social sciences research (Kueffer 2010).

A lack of integration of social sciences in invasive species research is particu-larly problematic when the aim is to determine which traits make some non-native plant species invasive (invasiveness, Richardson 2011). Scientific information on the invasiveness of species is often based on meta-analysis that searches for charac-teristics common to documented invasive species, i.e. non-native species that have become invasive through processes in the past. This approach assumes that biological traits of invasive species alone make some non-native species invasive. If however human behaviour significantly influenced past invasion processes, then such an approach can lead to spurious results.

An understanding of the role of human factors as drivers of past invasions is thus important to improve invasiveness studies because it allows accounting for diffe-rences in human introduction efforts that favoured some species, and other anthro-pogenic factors that enabled invasions by particular types of non-native species (Kueffer 2012). For instance, those species that are introduced at a site that is ecolo-gically and geographically distant from the site of invasion depend on high ecolo-gical plasticity and efficient dispersal abilities for spreading across a heterogeneous landscape. Unintentionally introduced species often depend on an ability to establish from small founder populations. In contrast, other species became invasive thanks to large-scale deliberate planting in the areas where they became invasive. These invasive species do not depend on an efficient dispersal mechanism or an ability to establish after a genetic bottleneck. Each of these scenarios is influenced by different human behaviours and enables the spread of non-native species with different traits.

An understanding of social factors as drivers of plant invasions is also impor-tant as a basis for robust predictions of future invasion risks in a time of rapid social and ecological change (Kueffer 2010). Invasions are strongly influenced by the way humans move species, manage the land, and value nature and non-native species. All of these processes will be affected by global change, with important implications for predicting and managing invasion risks.


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3.1 Past human activities explain invasion patterns

3.1.1 Size of introduced and invasive floras is correlated with econo-mic development

The number of introduced and invasive plant species that are present on a particular island or in an island group is accurately predicted by the level of human activities on this island measured by factors such as the gross domestic product (GDP) or the number of airports (Denslow et al. 2009; Kueffer et al. 2010a). The more species get introduced to a place, the more naturalize and become invasive. Human activity is a much more important predictor of the size of introduced and invasive floras on islands than classical island biogeography / invasion biology variables such as size of native flora, island area, island age, or isolation from the closest continental landmass (Kueffer et al. 2010a).

The importance of human activity for determining introduced and invasive floras on islands can also be observed in time series of non-native and invasive species records. The number of introduced plant species on an island increases in parallel with an increase of economic activities. Data from the Galapagos islands illustrate this dynamics particularly well. The number of recorded naturalized plant species increased from 77 in 1971, to 600 in 2001, and 870 species in 2010 (Kiehn 2012; Trueman et al. 2010; Tye 2006), and this increase happened in parallel to rapid human population growth (Trueman et al. 2010; Tye 2006). The pattern can partly be explained as a result of more intensive floristic work in recent years, but it is likely mainly the result of a rapid increase in plant introductions to the Galapagos islands over the past few decades.

3.1.2 Human selection determines composition of introduced and invasive floras

The imprint of human activities can also be seen in the composition of non-native island floras. Kueffer et al. (2010a) for instance showed that human selection of non-native species mostly explains biotic homogenisation of island floras, i.e. the pattern that different island floras become increasingly more similar due to invasions and extinctions. The authors compared the number of species shared between pairs of island archipelagos for non-native species introduced to the islands (but that are not necessarily invasive) and for those non-native species that became invasive on the islands. While any two islands shared on average about one third of their intro-duced plant species, only about 5% of invasive species were shared among pairs of islands in average. It appears that humans introduced the same plant species to many different islands, but among the introduced species different species became invasive on different islands. In other words, biotic homogenization is – at least in the case of island floras – the result of the recurrent human introduction of the same species to different islands rather than the recurrent invasiveness of the same species in places where introduced.



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3.1.3 Past human activity determined spatial distribution of non-na-tive plants within islands

Past human introductions of non-native plants also shaped distribution patterns of non-native plants within islands. Larrue et al. (2010) for instance showed that the patchy spatial distribution of the Polynesian bamboo, Schizostachyum glaucifolium [Ruprecht] Munro (Poaceae) in Tahiti is closely related to the location of ancient sites of Polynesian occupation. More generally, many non-native species are present or invasive in those areas to which they were deliberately introduced by humans (e.g. pasture or forestry species) (Kueffer 2010, 2012).

Past human introduction efforts can often also help to explain why some habitats are more invaded than others (‘invasibility’). For instance, high elevation habitat above the treeline is both on continents (Pauchard et al. 2009) and on islands little affected by plant invasions (e.g. in Hawaii, Daehler 2005; Jakobs et al. 2010; Juvik et al. 2011). The apparent resistance of high-elevation habitat to invasions is partly explained by the fact that in the past few non-native plants were introduced to islands that are pre-adapted to abiotic conditions of high-elevation habitat (cold and dry climate, young soils, wind exposure). Indeed, non-native plants are more likely to become invasive in a habitat characterized by a climate that matches the climate of the native distribution of the species (Kueffer et al. 2010a; Petitpierre et al. 2012). In some cases the low degree of invasion of a particular habitat may thus have less to do with an inherent resistance of the habitat to invasions (low invasibility), but more with the fact that non-native species pre-adapted to the growth conditions of the habitat have not (yet) been introduced to a place.

3.2 Understanding how humans shaped past plant invasions

Given the importance of human behaviour for shaping plant invasion patterns on islands, knowledge about how and why humans introduced non-native plants to islands is a prerequisite for understanding past invasions. For instance, non-native plants were introduced to Pacific islands in several waves (Daehler 2008; Kiehn 2012; Whistler 2009; Woodcock 2003). Already several thousand years ago, Polynesians brought c. 50-100 non-native plants to the Pacific islands (Florence 2003; Whistler 2009). Since the 18th century, European settlers imported non-native plants for home gardens and new forms of more intensive and large-scale agriculture (e.g. pastures, sugar cane, pineapple plantations) (Daehler 2005; Daehler 2008). In the early 20th century, forestry trees were introduced for large-scale afforestation and resto-ration of watersheds (Woodcock 2003). More recently, ornamental plants became a particularly important group of deliberately imported plants (Guézou et al. 2010; Larrue 2008; Meyer and Lavergne 2004; Meyer et al. 2008).

A thorough historical analysis of these different phases of plant introductions to Pacific islands is largely missing but would likely represent a major advancement in the understanding of why plant invasions happened (Kueffer et al. 2010a). For each phase it would be valuable to know which plants were introduced and why (‘species selection’), where and how they were introduced (‘introduction effort’), and what the habitat conditions – e.g. habitat degradation and already established invasive flora – were at the time of introduction and invasion (‘invasion opportunities’).


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3.2.1 Species selection

Most non-native plants are deliberately imported to islands, for instance 90% of invasive plants in Hawaii are considered deliberate imports (Daehler 2008), and for c. 80% of non-native plants in the Galapagos a plant use is known (Guézou et al. 2010). In different phases, however, ecologically and taxonomically different plants were introduced.

Only one Polynesian introduction – Aleurites moluccana [L.] Willdenow (Euphorbiaceae) – is considered invasive in Hawaii (Daehler 2008). One possible explanation for the low invasiveness of these early introductions might be the plant characteristics for which they were selected: these canoe plants had to be adapted to long-distance transport across oceans and were grown in coastal and lowland areas, and many have large fruits or seeds (e.g. Inocarpus fagifer [Parkinson] Fosberg (Fabaceae), Cocos nucifera L. (Arecaceae), Spondias dulcis [Solander] Forster (Anacardiaceae)), tuber roots (e.g. Colocasia esculenta [L.] Schott (Araceae), Dioscorea spp. (Dioscoreaceae), Ipomea batatas [L.] Lam. (Convolvulaceae)), or reproduce only vegetatively (e.g. Artocarpus altilis [Parkinson] Fosberg (Moraceae)). Such traits might have restricted dispersal to and invasion of inland habitat (Larrue 2007).

In contrast, for afforestation and restoration of watersheds in the early 20th century tree species were selected that were thought to grow well in the targeted inland areas, and up to 10,000 different taxa (including herbs) were tried (Woodcock 2003). The taxonomic and ecological diversity of introduced plants has likely further increased in recent decades; again partly due to human preferences. Many recent introductions are ornamentals, and because an important human motivation for selecting ornamentals is a desire for novelty (Daehler 2008), it might be expected that these recent introductions are characterised by high taxonomic diversity and a wide variety of traits. At least some of them will likely become invasive after some time lag (Meyer and Lavergne 2004; Meyer et al. 2008).

Species that originate from a climate similar to the one in the area of introduction generally grow best. It might thus be expected that people generally imported plants that were best adapted to the climate of a particular island, but that was not always the case in the past. Settlers imported many plant species that they knew from home for nostalgic or practical reasons. This meant that European and North American settlers often brought climatically poorly pre-adapted temperate plant species – many of European or North American origin – to subtropical or tropical islands for home gardens (Daehler 2008), pastures (Daehler 2005), and sometimes reforesta-tion (Woodcock 2003). The history of human settlement can thus strongly influence whether introduced species are climatically pre-adapted to the new area, with important implications for their potential to become invasive (‘climate matching’) (Daehler et al. 2004; Kueffer et al. 2010a; Petitpierre et al. 2012).

Reconstructing past species introductions is also important for a better unders-tanding of failed invasions: most introduced non-native species do not become invasive. For instance, foresters tested in the early 20th century many species of Eucalyptus, Ficus, and Pinus for reforestation of Hawaiian landscapes (Woodcock 2003), but despite major introduction efforts these genera generally showed no important tendency to become invasive (Kueffer et al. 2010a); with the exception of Pinus radiata D. Don (Pinaceae) in some high-elevation habitat (Daehler 2005).



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3.2.2 Introduction effort

As important as knowing which species were introduced is understanding how and where they were introduced. Polynesians brought probably relatively small amounts of propagules per species to a new island because space on boats was limited, and some of these species were sterile, and therefore genetic diversity of the introduced plants was likely low. These canoe plants were planted in confined lowland agricul-tural areas. In contrast, during reforestation programmes in the early 20th century forestry trees were introduced to Hawaii at a much larger scale (Woodcock 2003). Seed of some species, e.g. Ficus spp. or Spathodea campanulata, was sown across large areas from the air with the assistance of the U.S. Army Air Service, and millions of trees were planted (Little and Skolmen 1989; Woodcock 2003). For many of these trees different varieties were introduced and tested, and much effort was invested in ensuring their acclimatisation in nurseries in Hawaii. These tree species profited not only from the massive scale of introduction, but also from being introduced directly to the area of invasion (Kueffer 2012). It is thus no surprise that more than 50% of the non-native woody plant species considered dominant invaders in Hawaii today (Kueffer et al. 2010a) were introduced through forestry activities in the early 20th century (Woodcock 2003). Similarly, many of the dominant herbaceous invaders in Hawaii were deliberately or as seed contaminants introduced at a large scale to Hawaii for improvement of pastures (Daehler 2005). Again, many of these non-native pasture species never became major invaders, and it would be interesting to unders-tand why they failed, but it is not surprising that some of them did succeed.

It appears that only very few dominant invasive plant species in Hawaii did not profit from major human introduction efforts at some point in the past (Kueffer et al. 2010a). Examples of invasive plants that apparently established from very small founder populations in Hawaii are Clidemia hirta [L.] D. Don (Melastomataceae), Miconia calvescens A.P. de Candolle (Melastomataceae), and Senecio madagasca-riensis Poir. (Asteraceae) (Kueffer 2012). For most other invaders, understanding their invasion success means first of all documenting major past human efforts that promoted their establishment and spread.

3.2.3 Invasion opportunities

Windows of opportunity for invasion are influenced by land use, habitat disturbances (e.g. Merlin and Juvik 1992), and the composition of the already established invasive flora (Kueffer et al. 2010a). These factors and thus invasion opportunities change in time. An illustrative example is the invasion of the non-native tree, Cinnamomum verum J.S. Presl (Lauraceae) in the Seychelles (Western Indian Ocean) (Kueffer et al. 2013). Cinnamon is today by far the most abundant tree in most habitats of the inner granitic islands of the Seychelles; often making up 80-90% of canopy trees (Kueffer et al. 2007). This high abundance can likely be explained by a short historic window of opportunity in the early 19th century when the islands were rapidly deforested and Cinnamon was one of only few native or introduced species able to invaded degraded land (Kueffer et al. 2013). A few years later other highly invasive species such as Psidium cattleianum J. Sabine (Myrtaceae) or Syzygium jambos [L.] Alston (Myrtaceae) were introduced to the Seychelles (Kueffer and Vos 2004) but by that time Cinnamon already colonized much of the deforested land. In colonized areas


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it then hindered the establishment of other non-native species through strong belowground root competition (Kueffer et al. 2007). The sequence of introduction of non-native plants might often have played an important role in determining invasion patterns (Kueffer et al. 2010a): the species that arrives first occupies the land and stops invasions by latecomers (compare Waters et al. 2012).

Invasion opportunities also change due to changing human perception of and responses to non-native species. For instance, since World War II many different biological control agents were introduced to Hawaii to target over 20 different non-native plants (McFayden 1998; Woodcock 2003). Some control attempts were successful, while others failed. While past biological control programmes are often rather well documented, mechanical and chemical control attempts are mostly not documented; but these might often also have strongly influenced invasion success of particular species. It has for instance been argued that the recent and rapid spread of invasive creepers in the Seychelles might be explained by the abandonment of agriculture after the opening of the international airport and the rise of tourism in the early 1970s (Kueffer and Vos 2004). Before, livestock was possibly involved, alongside frequent mechanical removal, in controlling agricultural weeds (including invasive creepers and herbaceous plants).

3.3 Predicting future invasion risks

It often takes time before a non-native plant becomes invasive (‘time lag’) (e.g. Kueffer et al. 2010a; Trueman et al. 2010). Most of today’s invasive plant species on islands were introduced at least 50 years ago (Kueffer et al. 2010a). However, new records of non-native plants on oceanic islands have continued to rise over the past decades (e.g. Greimler et al. 2002; Guézou et al. 2010; Sax and Gaines 2008; Trueman et al. 2010), and some of these new introductions will likely become invasive in the future. It must be expected that the rate of new introductions of non-native plants will not slow down any time soon, and many of the future introductions might represent new types of non-native species, which might pose novel invasion risks (Kueffer 2010). Understanding recent and possible future species introductions is therefore critical for predicting future invasion risks.

3.3.1 Imports of new non-native plants

The past few decades have seen a surge of introductions of new horticultural plants, and some of them start to show a potential to become invasive on islands (Meyer and Lavergne 2004; Meyer et al. 2008). The horticultural sector is responding strongly to globalisation and its structure is rapidly changing (Dehnen-Schmutz et al. 2010). New emerging exporters of horticultural plants include China, India, Africa, and South Africa (Dehnen-Schmutz et al. 2010); and distribution channels and consumer demands also become more diverse (Drew et al. 2010). Anticipating future invasion risks requires understanding changing horticultural markets and consumer demands. More generally, island societies are increasingly more tightly integrated into global trade transportation networks. New analysis techniques are available to extract information from trade data to predict emerging connections between geographic areas (e.g. Kaluza et al. 2010; Tatem 2009).



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3.3.2 Land use changes

Anticipating future invasion risks requires also understanding land use changes. Small island economies are forced to opportunistically respond to changing market opportunities. Indeed, historic land use on many islands is characterised by frequent shifts in land use priorities (e.g. Juvik and Juvik 1998; Kueffer et al. 2004); and with each land use change new types of non-native species were introduced and new invasion opportunities emerged because land was disturbed in new ways. Future land use changes will affect invasion patterns.

For instance, 40 different potential biofuel plants have been proposed for Hawaii and 70% have a high risk of becoming invasive versus one-quarter of non-biofuel non-native plant species (Buddenhagen et al. 2009). It has frequently been proposed to grow biofuel crops on abandoned and marginal agricultural land (e.g. Campbell et al. 2008), and for this purpose new types of non-native plants might be selected and bred that tolerate drought or nutrient-poor soils and these might pose new invasion risks (especially with climate change). Also ecotourism is still advancing to less developed areas and might bring non-native species into areas that are today less affected by invasions (Kueffer 2011). Or, sea-level rise will in the next decades severely affect island societies because settlements and developed areas are mostly located in coastal areas. Island people might be forced to retreat to inland areas that are today less affected by human activities (Wetzel et al. 2012).

Invasions are strongly influenced by the way humans move species, manage the land, and value nature and non-native species. All of these processes will be affected by global change, with important implications for predicting and managing invasion risks. Anticipating future invasions risk thus requires foremost understan-ding changing socioeconomic activities on islands, and trade relationships between islands and the world.

4. Target knowledge: clarifying interests and values

Traditionally, invasive species policies built on the assumption that all non-native species are problematic ‘until proven innocent’, meaning until it can be shown that a species is very unlikely to spread and establish in natural areas (Mack et al. 2000; Wittenberg and Cock 2001). And once a non-native species has established, the preferred strategy is to eradicate it when this is possible, and otherwise to contain its further spread (Mack et al. 2000; Wittenberg and Cock 2001). However, this approach makes no allowance for the potentially high economic and ecological costs of these measures. These include implementation costs resulting from impacts of control measures upon ecosystems (e.g. Zavaleta et al. 2001), and opportunity costs resul-ting from the economic loss of industries such as forestry and horticulture that are prevented from importing potentially valuable species. And in the case of non-native species that can be used to restore natural areas and support native biodiversity, there may also be ecological opportunity costs (Ewel et al. 1999; Ewel and Putz 2004; Kueffer and Daehler 2009). Both implementation and opportunity costs can be parti-cularly high on oceanic islands. Ecological processes in many island ecosystems are strongly shaped by non-native species and control of some of these keystone invasive species might negatively affect ecosystem properties; and often native biodiversity


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depends on interactions with non-native species (Kueffer and Daehler 2009; Kueffer et al. 2010b). Island economies and societies are highly dependent on non-native plants for agriculture, forestry, and other purposes.

Once the universal rule ‘no spreading aliens anywhere’ is abandoned, however, a broader range of management options become available (Kueffer and Hirsch Hadorn 2008), including no action, prevention, mitigation of negative impacts, and different control strategies. Deliberation about these alternatives can profit from the inputs of social sciences. A calculation of economic costs and benefits is one option for valua-ting the impacts of invasive species or the cost-efficiency of alternative management strategies (Born et al. 2005; Vila et al. 2010). But economic assessments will often not be able to account sufficiently for all relevant aspects that affect how people evaluate invasive species and their management. Thus, besides economists, the involvement of psychologists, social and cultural scientists, philosophers, communi-cation experts, ethnologists and historians is needed to clarify why people differ in the perception of invasive species.

4.1 What shapes human perceptions of invasive species?

There has recently been some work on human perceptions of risks and effects related to biotic invasions, and the appropriate management options to be taken (e.g. Andreu et al. 2009; Bardsley and Edwards-Jones 2006; Bremner and Park 2007; Fischer and van der Wal 2007; Garcia-Llorente et al. 2008; Humair et al. 2012; Javelle et al. 2006; Rotherham and Lambert 2011; Selge and Fischer 2010). These studies suggest that different people and social groups (e.g. experts, decision-makers, stake-holders, practitioners, or the general public) often differ strongly in their valuation of invasive species and willingness to participate in management actions (e.g. Andreu et al. 2009; Bardsley and Edwards-Jones 2006; Javelle et al. 2006). Figure 2 presents a heuristic overview of factors that can be involved in shaping human perceptions of invasive species: it is important to consider what is being valuated, and people refer for their valuation to different types of knowledge and values. Social relationships also matter.

4.1.1 What is being valuated?

Effects of an invasive species on an invaded ecosystem are an important considera-tion in people’s valuation (Selge and Fischer 2010; Selge et al. 2011). A complication is that an invasive species exerts usually multiple effects on an ecosystem, which requires ways to evaluate the net benefit or loss across all effects (Humair et al. 2012; Kueffer and Hirsch Hadorn 2008). On oceanic islands, for instance, honey bees can be pollinators of rare native plants (a positive effect on native biodiversity) and at the same time competitors of native pollinators (a negative effect on native biodiversity); or non-native predators might negatively affect some native prey species but keep other non-native predators at bay (Kueffer 2012; Kueffer and Daehler 2009).

Besides effects, the perception of the invasive species, the invaded ecosystem, available management options, and involved actors constitute part of the valuation of an invasion problem (Fig. 2). For instance, acceptance of control efforts is often low when large, attractive or companion animals or organisms of cultural value are



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targeted (Bremner and Park 2007; Fitzgerald et al. 2007). On islands, controlling valued species such as wild pigs can be faced with opposition even if the immediate interests of stakeholders, e.g. hunters, are not affected. People that know and cherish an invaded habitat tend to support invasive species management more readily. Some people are opposed to particular management options, e.g. killing animals to restore island ecosystems (Boyle 2011), and more generally type and effectiveness of management options influence views of practitioners and stakeholders (Andreu et al. 2009; Fischer and van der Wal 2007). For others the social relations with actors that promote management action matter. For instance, some opponents of biolo-gical control of strawberry guava (P. cattleianum) in Hawaii are concerned about how the government “attacks their property rights” by releasing an insect that might affect strawberry guava on private land (http://www.savetheguava.com/, accessed on September 17 2012).

InvasiveSpecies

InvadedEcosystem

ManagementOptions

InvolvedActors

MultipleEffects

Uncertain knowledgeand ignorance

Rapid social andcultural change

Figure 2. A heuristic overview of factors that can be involved in shaping human perceptions of invasive species. It is important to consider what is being valuated: multiple effects of an invasive species, the invasive species, the invaded ecosystem, available management options, and involved actors. People refer for their valuation to different types of knowledge (scientific facts, general knowledge, tacit knowledge based on personal experiences) and value-related factors (e.g. emotions, interests, cultural and historical aspects, and worldviews), and social relationships also matter. Valuation is further complicated because facts are often highly uncertain or entirely unknown (‘ignorance’), and rapid social and cultural change can render opinions unstable.


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4.1.2 The role of knowledge in valuation

Knowing about invasive species, their negative impacts and available management options is important (Garcia-Llorente et al. 2011; Shine and Doody 2011). People are generally in favour of control measures against invasive species for which detrimental effects on ecosystems have been demonstrated (Selge and Fischer 2010; Selge et al. 2011), and support is higher among people with direct personal experience of the overabundant presence or negative impacts of an invasive species, or a successful management project (Andreu et al. 2009; Bremner and Park 2007; Fitzgerald et al. 2007; Johnson et al. 2011). But also more general knowledge (‘competence’) about invasive species, nature, and nature conservation can influence perception of invasive species (Garcia-Llorente et al. 2011).

However, knowing about effects is only one relevant factor among others and considering effects can in itself be a complicated matter. Discerning an effect and attributing it to an invasive species can in complex situations often not be fully substantiated by facts and therefore is partly value-laden; for instance it is often not certain whether invasive species are drivers or passengers of ecosystem change (Didham et al. 2005). Further, especially when knowledge is uncertain or incomplete different people may evaluate the validity and significance of facts differently (e.g. Fitzgerald et al. 2007; Shine and Doody 2011).

4.1.3 The role of personal connectedness

Valuation is affected by the personal connectedness of people with aspects of an invasion. Emotional connectedness to the invasive species or affected species/ecosystem influences valuation (Fischer and van der Wal 2007), and so do specific values that particular persons assign to an invasive or affected species/ecosystem, e.g. aesthetic attributes of a species (Fischer and van der Wal 2007). Interests involved in invasive species management are evidently also important (e.g. Binimelis et al. 2007), e.g. the control of wild pigs on many islands faces opposition by hunters (Culliney 2006), and the regulation of plant imports requires that conflicts of interest with the horticultural industry are resolved (e.g. Kueffer and Loope 2009).

4.1.4 The role of social factors

Social factors have recurrently been shown to affect invasive species valuation. The socioeconomic or demographic background of a person matters (Bremner and Park 2007; Fitzgerald et al. 2007), and so does to which social or interest group a person belongs. For instance, tourists, nature conservation managers, or local residents can differ in their perception (Garcia-Llorente et al. 2008). Social relationships between actors and trust in implementing organisations are also essential, and can be strengthened through participatory processes (Veitch et al. 2011; Wittenberg and Cock 2001). In Hawaii, for instance, a weed risk assessment system with the aim of predicting potentially invasive non-native species that should be prevented – the Hawaii-Pacific Weed Risk Assessment system (HP-WRA) – was at first developed relatively independently of stakeholders, especially the plant industry, and conse-quently acceptance by stakeholders was low. Thanks to committed representa-tives of the plant industry a more participatory process was later initiated, which



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significantly increased the acceptance of weed risk assessments and preventative measures among industry partners (Kueffer and Loope 2009). In Australia, rivalry between community groups affected support of control measures against cane toads (Shine and Doody 2011).

4.1.5 The role of history and culture

Values related to invasive species management are often rooted in the history or culture of a society. Many non-native species that are today considered major invasive species were in the past introduced as beneficial species to an area. Valuations of particular non-native species changed often within decades from positive to negative, and many local residents experienced earlier times when an invasive species was promoted as a beneficial species (Stromberg et al. 2009). The presence of non-native species might disturb a feeling of sense of place, but often after a short timespan non-native species are adopted as integral parts of local identi-ties (Humair et al. 2012; Marris 2011); which relates to what in the nature conser-vation literature is called ‘shifting baseline syndrome’: humans tend to adapt quickly to a new situation, and often they forget within a human life the nature of the past (e.g. Turvey et al. 2010). Some non-native species also play, or played in the past, an important role in the culture of island societies (Atlan and Darrot 2012); for instance the Polynesian introductions, candlenut tree A. moluccana and Crateva religiosa Forster (Capparidaceae) in French Polynesia (Larrue 2007).

4.1.6 The role of worldviews, value-based principles, and metaphors

Valuations can also be strongly influenced by generic value-laden concepts such as worldviews or value-based principles; and these are often transported through metaphors. Widely discussed in the literature are the cultural and social connotations of terms such as non-native, alien, exotic, or invasive, and how they can influence how people perceive invasions (Hall 2003; Humair et al. 2012; Kueffer and Hirsch Hadorn 2008; Larson 2005; Rotherham and Lambert 2011); and the use of certain metaphors in communication about invasions has been criticized (Larson 2005). Value-based principles related to how people think about nature, e.g. ‘balance’ or ‘naturalness’ have been shown to influence perception of invasive species (Fischer and van der Wal 2007). How people see the role that humans play and should play in nature can also be relevant (Selge and Fischer 2010; Rotherham and Lambert 2011; Selge et al. 2011).

Another important example are conflicting worldviews of those that emphasise precaution and prefer to prevent introductions of non-native species until proven ‘innocent’ (Simberloff and et al. 2011), and those that mainly see the high imple-mentation and opportunity costs of strict prevention and counsel action only when a clear threat can be demonstrated (Davis et al. 2011). These alternative worldviews of a preventative precautionary approach (e.g. Andow and Hilbeck 2004) versus a willingness to allow for experiments in nature (Gross 2010) are characteristic of a broad range of debates about environmental problems. Producing more scientific facts about the risks of invasions will often not help to resolve such conflicts, because uncertainties might remain high and leave space for alternative interpretations. But


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social sciences can help to make underlying value-based assumptions of conflicting views transparent.

5. Transformation knowledge: developing effective manage-ment tools through problem orientation

‘Transformation knowledge’ assists problem-solving by developing effective manage-ment tools that take existing practices and institutions into consideration and help adapt them to the problem. Past research on invasive species relied on first gaining a biological understanding of drivers of invasions and then communicating such a biological problem understanding to actors in the hope that they would find ways to implement appropriate management action (Kueffer and Hirsch Hadorn 2008). However, such an approach proved often to be ineffective. Indeed, few countries and island societies have succeeded in establishing effective measures against the intro-duction of new non-native species in a highly globalised world (Brunel et al. 2013; Kueffer 2011; McGeoch et al. 2010), while control of established non-native plants has also often proved ineffective (Reid et al. 2009); including in the Galapagos islands (Gardener et al. 2010; Vince 2011).

To overcome such shortcomings it will be necessary to take the management problem – not the biology of invasions – as the starting point for scientific enquiry; and this will inevitably lead to more interdisciplinary research. The term ‘problem orientation’ has been used to refer to such attempts to orient research with the aim of providing the answers needed for formulating policy (Kueffer and Hirsch Hadorn 2008; Wuelser et al. 2012). Interestingly, recently new conceptual perspec-tives developed in invasive species research that, by allowing for the concerns of managers, represent greater problem orientation.

5.1 Pathway management

The first innovation represents a shift away from preventing the spread of particular non-native species to managing risks associated with introduction pathways (e.g. horticulture, forestry, tourism, marine ballast water), including the human activities that create, shape and sustain such pathways (Brunel et al. 2013; Kueffer and Hirsch Hadorn 2008). A new line of research has become established that integrates natural and human sciences and is referred to as ‘pathway or vector science’ (Mooney et al. 2005; Richardson 2011; Ruiz and Carlton 2003). This research recognises human activities as the central cause of movement of species, and allows developing preven-tive tools that are tailored to the specific needs of particular groups of actors and their institutional, economic and social constraints.

On islands in particular, it is important that preventative measures take the limited financial and human resources of different actors into consideration. How prevention can work on islands in an increasingly globalised world is not evident. A solution may lie in developing multi-layered biosecurity systems based on shared responsibilities among many agencies and citizens, with post-introduction detection as a second important filter after border control (Kueffer 2011). Possible invasive non-native species that pass through border control must be detected as early as



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possible after introduction while they are still localized and eradication is still feasible (Kueffer and Loope 2009). This will require regular systematic early detection surveys as illustrated by the Hawaiian example (Kueffer and Loope 2009) but depends also on the collaboration and awareness of other agencies – e.g. those involved in road maintenance, landscaping, forestry and agriculture – and the general public. Such approaches depend on social and institutional innovations. In Hawaii it was for instance effective to setup a separate Invasive Species Committee (ISC) on each of the different islands (Kueffer and Loope 2009). These island-specific ISCs allow preventa-tive measures, early detection, monitoring, awareness building and outreach to be tailored to the specificities of the individual islands.

5.2 Habitat focus

The second innovation addresses the need to take the local habitat context into account when valuing and managing invasive species (Kueffer and Daehler 2009). Although it makes sense to prevent non-native species from entering remnants of undisturbed native habitat, a different strategy is needed for disturbed ecosystems, especially in a time of global change. Indeed, accepting mixed non-native/native communities may be the only realistic management target in many of these ecosys-tems, while in other situations the active introduction of non-native species may be necessary to restore lost ecological functions (Hansen 2010; Kaiser-Bunbury et al. 2010; Kueffer and Kaiser-Bunbury 2012). Emerging research on ‘novel ecosystems’ is acknowledging that the specific ecological, social, cultural, and institutional context of particular ecosystems must be taken into account to devise appropriate invasive species management strategies (Cole and Yung 2010; Ewel et al. 2013; Hobbs et al. 2009); and this requires interdisciplinary research from the onset of research and management projects.

6. Structured deliberations about problem framings

Recent years have been characterised by increased pluralism of opinions, and critique of scientific uncertainties and limitations of invasive species management approaches (Davis et al. 2011; Simberloff and et al. 2011; Vince 2011). It has become apparent that invasion biologists, managers and the public do not always agree on a common terminology and problem understanding (Humair et al. 2012; Young and Larson 2011). When problems are complex such pluralism of opinions and interpre-tations of facts is the rule (Kueffer et al. 2012; Rosenhead and Mingers 2001; Stirling 2012), and even fundamental assumptions and problem framings might be regularly exposed to scrutiny (Fig. 1). In other words, problem framings evolve in time, and with them research priorities and management approaches. Rapid ecological and social change affect the nature of an environmental problem. For instance, due to climate change the distinction between non-native and native species becomes increasingly less evident (Humair et al. 2012; Walther et al. 2009). New scientific insights and management experiences trigger innovation, e.g. new concepts such as ‘pathway management’ or ‘novel ecosystems’ (see above). And stakeholder perspec-tives evolve.


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Such adaptations of problem framings cannot be legitimated by (often uncer-tain) facts alone, while they have important implications for policies and thus affect interests of stakeholders (Elzinga 2008; Kueffer and Hirsch Hadorn 2008). Structured and inclusive participatory processes are needed for deliberation about problem framings and fundamental but value-based assumptions that guide research and management (Harremoës et al. 2002; Kueffer and Hirsch Hadorn 2008; Stirling 2012). Social sciences can contribute to the quality of such delibera-tions by making underlying value-based assumptions of alternative positions trans-parent and developing methodologies and institutional frameworks that facilitate participatory processes.

Conclusions

Research on biological invasions has a long tradition. But for most of the time invasive species research was largely a biological science with little contribution from the social sciences and humanities. However, it is evident that invasions are strongly influenced by human behavior, and management responses are embedded in economic, political, social, and cultural contexts. In the past decade social scien-tists have gained a stronger interest in biological invasions, but natural and social sciences research have remained largely separated or even confrontational endea-vours (e.g. Raffles 2011).

Integrating natural and social sciences for understanding and managing plant invasions is necessary and possible. I have in this article discussed how and where such interdisciplinarity is particularly important, and why. Understand past invasions requires a thorough understanding of the human factors that shaped those invasions; and anticipating future invasion risks depends on an understanding of possible future plant introductions and land use changes. People differ in their perceptions of invasive species, their impacts and their management, and often divergent opinions affect not only the valuation of particular management cases but also the funda-mental underlying assumptions and problem framings guiding research and manage-ment. Understanding these different perceptions and making reasons for different opinions transparent is essential for effective management. Ultimately, the aim is to develop effective management strategies for addressing negative impacts of invasive species. And this requires that invasive species research and management involves natural and social sciences at all stages from problem analysis to the identification of management targets and the development of tools for action.

Acknowledgements

Many ideas in this paper and in particular in the section on “Target knowledge: clarifying interests and values” profited from inputs from Franziska Humair (ETH Zurich).



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