Biol Invasions (2019) 21:587–602 https://doi.org/10.1007/s10530-018-1846-5 (0123456789().,-volV) (0123456789().,-volV) ORIGINAL PAPER A social-ecological system framework to assess biological invasions: Corbicula fluminea in Galicia (NW Iberian Peninsula) Noé Ferreira-Rodrı́guez . Omar Defeo . Gonzalo Macho . Isabel Pardo Received: 16 January 2018 / Accepted: 12 September 2018 / Published online: 17 September 2018 Springer Nature Switzerland AG 2018 Abstract In this study we hypothesize that Ostrom’s social-ecological system (SES) framework can be useful to address the introduction pathways and dispersal vectors of non-native species. We have applied this framework to the introduction of the Asian clam Corbicula fluminea in freshwaters in Galicia (Spain). We reviewed scientific and grey literature and performed a series of interviews with key stakeholders. This information, coupled with an extensive survey campaign, provided an updated C. fluminea distribution map. Using a Principal Component Analysis and pair-wise correlations we analyzed a set of 18 social, ecological, economic and governance variables and related them to the total number of water bodies invaded by C. fluminea in Galicia. Our field data on C. fluminea distribution indicated a mean upstream spread of 3.6 km yr-1. We suggest that the total number of water bodies invaded by C. fluminea in Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10530-018-1846-5) contains supplementary material, which is available to authorized users. N. Ferreira-Rodrı́guez (&) G. Macho I. Pardo Departamento de Ecoloxı́a e Bioloxı́a Animal, Facultade de Bioloxı́a, Campus As Lagoas – Marcosende, Universidade de Vigo, 36310 Vigo, Spain e-mail: [email protected] O. Defeo Faculty of Sciences, UNDECIMAR, Iguá 4225, 11400 Montevideo, Uruguay Galician freshwaters is mainly linked to the following variables of the social (higher education level and mass media news), ecological (endangered freshwater species, scientific publications, dams, wastewater treatment plants, livestock, and agricultural lands), economic (gross domestic product, gross imports, and industrial productivity index) and governance (surveillance, legislative instruments and non-governmental organizations) dimensions. The SES framework has been useful in identifying introduction pathways and dispersal vectors of non-native species. We encourage decision-makers to get involved in real implementation of legislative instruments and management plans that will be benefitted by collaborating efforts between stakeholders. Keywords Non-native invasive species Management Recreational freshwater use Invasion vectors SES framework Introduction Biological exchange has increased dramatically with global trade since the 19th century (Cambray 2003; Jeschke and Strayer 2005). In Europe, approximately 750 freshwater species reported as non-native are estimated to have been introduced (EASIN 2018). Europe’s Iberian Peninsula, specifically its Northern 123 588 region whose freshwater habitats are particularly hospitable to new invaders, constitutes a ‘‘hot spot’’ for non-native invasive species (Maceda-Veiga et al. 2010). In this regard, rivers can act as transportation networks for new introductions and subsequent spread of non-native invasive species (Galil et al. 2007; Leuven et al. 2009). Scientific evidence of the broad geographic and taxonomic extent of species invasion in freshwater environments is robust. Nevertheless, the process for early detection of a potential nonnative invader to the publication of the first scientific evidence and implementation of management strategies could take years or even decades. Non-native invasive species are causing important changes in communities and ecosystems (Gurevitch and Padilla 2004). The threat of non-native species clearly requires the implementation of preventive strategies, because once a species is detected in the recipient environment and the exponential growth phase starts, little or nothing can be done to prevent subsequent ecological, economic and social damage (Sakai et al. 2001). Hence, the involvement of different stakeholders to avoid the introduction of non-native invasive species has been invoked as one of the most effective tools in an early warning system (Dehnen-Schmutz et al. 2010). Traditionally, research efforts on non-native invasive species are focused on the ecological dimension (i.e., relationship between non-native invasive species and the environment). To prevent new species introductions, management policies should consider both ecological and social dimensions. In practice, management policies involve the identification of the problem based on the invasive potential of each species (i.e., ecological dimension) (Maguire 2004), and are intended to avoid new introductions and eradicate or control existing ones. Some examples provide evidence of the effectiveness of institutional actions to address new introductions. For example, the chemical elimination (i.e., chlorine and copper sulfate) of the marine bivalve black striped mussel [Mytilopsis sallei (Récluz 1849)] from Australian tropical waters was one of the few successful aquatic pest eradications undertaken in the world (Ferguson 2000). Similarly, after scientific concerns were raised about initial efforts to combat the seaweed Caulerpa taxifolia on the American Pacific coast, governmental agencies began an immediate eradication program (Williams and Schroeder 2004). More recently, in Europe, managers have 123 N. Ferreira-Rodrı́guez et al. begun controlling the yellow-legged hornet Vespa velutina Lepeletier, 1836, through collaborative efforts among different stakeholders due to its ecological, economic and social impacts (Monceau et al. 2014). However, the social dimension remains understudied and many factors are not taken into consideration (e.g., public perception, stakeholders knowledge) to increase our understanding of nonnative invasive species dynamics (Larson et al. 2011). Insufficient consideration of social processes (the dynamic interactions between individuals, institutions, social organizations and cultural norms) contributes substantially to failure of management policies (Ban et al. 2013). In order to analyze ecosystem resilience and/or resistance in the face of perturbations (e.g., invasive species introduction) a number of diagnostic approaches are emerging. In this context, the socialecological system (SES) framework is an interdisciplinary and integrative approach proposed to analyze and compare SES through a common set of variables identified in theoretical and empirical research (Ostrom 2007, 2009; McGinnis and Ostrom 2014). Ostrom (2009) defined a SES as a complex system composed of both biophysical and social components embedded in a network of relationships among them. The four core subsystems of the focal SES (i.e., resource units, resource system, governance system, and stakeholders) interact to produce outcomes at the SES level. These subsystems are then decomposed into second-tier variables, which are further composed of deeper-level variables that could be used for delineating the SES. The framework also considers external variables from related ecological ecosystems or social-economic-political settings that could affect the SES (Ostrom 2009). The SES framework is increasingly used in different countries and disciplines (e.g., fishery, forestry, freshwater ecosystems, dryland areas) and still remains in progress (McGinnis and Ostrom 2014). The Asian clam Corbicula fluminea (Müller 1774) is one of the most successful invasive species in European freshwater environments (Leff et al. 1990; Sousa et al. 2008a). This species began its worldwide dispersion at the beginning of the 20th century associated with human activities such as global trade, fishing and recreational activities (Counts 1981; Araujo et al. 1993; McMahon 1999; Darrigran 2002). It colonizes both lotic and lentic habitats (Vidal A social-ecological system framework to assess biological invasions: Corbicula fluminea… et al. 2002) and its distribution in invaded rivers ranges from fresh to brackish waters (Pérez-Quintero 2008; Sousa et al. 2008b; Vohmann et al. 2010; FerreiraRodrı́guez and Pardo 2014). In addition to its native range in Asia, the species is currently found in North and South America, North Africa and in almost all European river basins (Crespo et al. 2015). C. fluminea is also included in the Spanish Catalogue of non-native invasive species (Royal Legislative Decree 630/2013). Despite the potential threat that C. fluminea represents to freshwater ecosystems and their biodiversity, the social and economic factors contributing to its successful establishment are understudied. Here we applied the SES framework to understand the invasion process of C. fluminea in Galicia. To achieve our objective, we identified and characterized the following SES subsystems: resource system (Galician rivers network), resource unit (C. fluminea), stakeholders (professional and recreational users) and governance (organizations and rules that govern nonnative species management). Main interactions among these core subsystems were also characterized to provide management actions (SES outcomes) for addressing this biological invasion (Fig. 1). In an ecological dimension, we aim to test if loss of habitat quality and native biodiversity provides an empty niche that can be easily occupied by C. fluminea resulting in invasions. In social and economic dimensions, since people often recreate in freshwater systems, we propose that recreational uses linked to economic development may be important vectors for C. fluminea dispersal. In addition, we aim to analyze if increasing social awareness could reduce the rate of new invaded water bodies. Hence, educational attainment among the population (university or college studies), and knowledge dissemination through the mass media were included as explanatory variables. We also analyze if the implementation of legal instruments (e.g., specific laws regulating the pet trade, abolition of border checks and increased surveillance) would promote or delay C. fluminea dispersal associated to human activities. Materials and methods This study characterized the invasion process of C. fluminea in the autonomous Community of Galicia (NW of the Iberian Peninsula, Spain, 29,434 km2) 589 whose population is approximately 2.7 million. Galicia has an extensive hydrographic network; its topography determines the existence of short watercourses draining to the North into the Cantabrian Sea and rivers with steep slopes draining to the West into the Atlantic Ocean. The fishing sector (professional and recreational users) shapes both the economy and social identity in Galicia (Gottschalk 1970; Boutureira 1999; Macho et al. 2013). An integrative approach was used to map C. fluminea distribution and understand its dispersal vectors in Galicia. Historical patterns of distribution and rates of spread (i.e., new records in invaded waterbodies) were traced with peer-reviewed publications and non-peer reviewed ‘grey literature’. In addition, updated distribution data were obtained with field surveys. Local stakeholder knowledge was also included to identify dispersal vectors and new invasions. We performed a bibliographic search to trace the distribution of C. fluminea in Galicia by searching titles, abstracts, and keywords of articles characterizing C. fluminea distribution using the Institute for Scientific Information (ISI) Web of Science search engine. We scanned peer-reviewed publications for different research terms (exotic, invasive species, Corbicula fluminea, Asian clam, Galicia, freshwater, river) and their combinations. To carry out the most thorough literature search possible in the non-peer reviewed ‘grey literature’ (e.g., personal websites, technical reports, theses and newspapers) that could provide relevant information on C. fluminea distribution in Galicia, we searched for similar terms on the Google.com search engine (Stansfield et al. 2016). The aim was to gain as much information as possible to update the C. fluminea distribution map and to understand its main dispersal vectors in Galicia. These data were collected from key stakeholders through semi-structured interviews (n = 20). A semistructured interview is a meeting where open-ended questions are made by simulating a casual conversation (i.e., without completing a questionnaire) following an interview guide (i.e., general topics to guide the conversation). Participants were asked about the distribution and human use of C. fluminea in Galicia (Online Resource 1). Both, C. fluminea and C. fluminalis (Müller 1774) occur in Europe, but only C. fluminea was known to occur in Galicia (Sousa et al. 2007b; Ferreira-Rodrı́guez and Pardo 2016). Hence, we assumed that all stakeholders referred to the same 123 590 N. Ferreira-Rodrı́guez et al. Fig. 1 The Galician social-ecological system (SES) for Corbicula fluminea invasion. The four core subsystems of the focal SES (Resource system, Resource unit, Governance and Stakeholders), their interactions and outcomes, are presented. Second-tier variables within these subsystems are identified: resource system (Galician river network), resource units (C. fluminea), governance (organizations and rules that govern non- native species management), and stakeholders (fishers, recreational users, local population, NGO’s). Interactions among variables can be used to identify the most important introduction vectors and dispersal pathways for C. fluminea. SES outcomes reveal the capacity of governance to address species introduction, and are related to possible management actions in each invasion phase. Adapted from McGinnis and Ostrom (2014) species. Stakeholders were identified at or near our field survey sites (in the Tambre, Ulla and Miño-Sil basins). Key stakeholders were local NGO members and users of the resource system (i.e., divers and professional and recreational fishers) with extensive proven knowledge of the area (i.e., visiting the resource system on a weekly basis). Following Greenland-Smith et al. (2016), coded responses were tabulated by the number (n) and percentage (x) of stakeholders who reported a new location or mentioned C. fluminea use in their interviews. We included survey data corresponding to fifteen water bodies in 62 sampling locations. Samples were collected in spring 2014 adhering to European Water Framework Directive standards for Spanish invertebrate sampling (Pardo et al. 2014). C. fluminea was sampled in littoral zones with a kick-net (1 mm mesh size), and each sample was composed by 20 subsamples (sample area of 0.5 m 9 0.25 m) distributed proportionally based on the most representative littoral habitats (e.g., sand, gravel, vegetal detritus), representing [ 5% of the sampling area in a 100-m fixed river stretch length (total sampled area for 20 kick samples of 2.5 m2). To characterize the SES, we identified a quantifiable set of variables based on four primary dimensions (social, ecological, economic and governance). We adapted the second-tier variables proposed by Basurto et al. (2013) with respect to introduction pathways and dispersal vectors for non-native species. Based on previous works (Ruiz et al. 1997; Dextrase and Mandrak 2006), the most common pathways for non-native species introduction in estuaries and freshwaters were included as variables (i.e., gross imports, exotic pet trade, and sport fishing licenses). Other key variables affecting non-native species introduction (population of a given country, loss of native biodiversity, habitat quality, and warming) were adapted from case studies (see Garcı́a-Berthou et al. et al. 2005; Dextrase and Mandrak 2006; Rahel and Olden 2008). Furthermore, increased scientific knowledge on non-native invasive species in a region 123 A social-ecological system framework to assess biological invasions: Corbicula fluminea… (represented by the number of peer-review scientific papers published) was included in our study. Similarly, we have included variables of the education attainment among the population (number of university students) and non-native invasive species portrayed in mass media (number of news about nonnative invasive species). Following Basurto et al. (2013), some governance variables remained constant over time and were therefore not a source of variance and thus not included in the analysis (e.g., policy area, geographic range, regime type). On this basis, we selected a total of 18 second-tier variables nested within the four core subsystems of the focal SES. Information on these variables was gathered from 1980 to 2014. The final matrix dimension was 35 years by 18 variables. Detailed information about these variables is given in Table 1. We used a principal component analysis (PCA) to reduce the dimensionality of second-tier variables to five components. The most explanatory components explaining 70–80% of the variance were selected following the Cattell’s scree test (Cattell 1966), which consists of plotting the eigenvalues (y-axis) against the components (x-axis), and inspecting the shape of the resulting curve for a large change in the slope. This point on the curve indicates the maximum number of components to retain for the analysis. Pairwise Spearman’s correlation coefficients were calculated for all combinations of PC axes scores and the number of water bodies invaded by C. fluminea in Galicia. In addition, differences between the number of water bodies invaded by C. fluminea pre- and post-implementation of legislative instruments (Schengen Agreement and the Spanish Nature Conservation Law on Natural Heritage and Biodiversity, hereinafter Law 42/2007) were computed and compared using the Wilcoxon test. SPSS v.15.0 software was used for all data processing and statistical analyses. Results The invasion history of C. fluminea in Galician water bodies was reconstructed following Sakai et al. (2001). Before 1920, C. fluminea was confined in its native range. Then, the species was introduced in North America in the 1920s and Europe in the 1980s, surpassing geographical barriers (i.e., Pacific and Atlantic oceans). In 1989 the species was first recorded 591 in Galicia (River Miño; Araujo et al. 1993) derived from the River Tagus probably associated to human activities (e.g., leisure craft, sport fishing, or pet trade). After a lag phase where the species surpassed the local environmental barriers the exponential growth phase started—from 1996 to 2005. In 2005, maximum population densities were recorded in the River Miño (up to 4000 ind. m-2). Hereafter, new water bodies were invaded all over the region (Table 2). In the last 15 years, our long-term information gathered from several sources showed new records in the Centeáns ponds (Ayres 2008), and the Deva (empty shells), Mero and Sil rivers (Lois 2010). Additionally, the grey literature signaled its presence in the Pontiñas stream in 1997 and in the Antela pond in 2002 (de la Cigoña and Oujo 2002). While nearly all the stakeholders interviewed (n = 18, x = 90%) were aware of the presence of C. fluminea in nearby rivers, only two reported new locations, with empty shells in the Tambre river in 2009 and in the Tea river in 2013. When discussing C. fluminea uses, responses indicating no use of this species (n = 15, x = 75%) were more common than positive responses (n = 5, x = 25%). The most common use was for food for humans (n = 3, x = 15%), followed by its use as bait in sport fishing (n = 2, x = 10%). Our field sampling data reported the presence of C. fluminea in 16 of 62 sampled localities (Fig. 2) along Galician water bodies with a density (mean ± SD) of 544 ± 790 ind. m-2. C. fluminea was found in Miño, Sil, Ulla and Mero rivers, but not in Tea or Tambre rivers where it had previously been reported. In addition, no live or dead individuals were found in Anllóns, Eume, Eo, Lérez, Louro, Masma, Mandeo, Sor and Umia rivers. The highest densities were found in the River Ulla (845 ± 1330 ind. m-2) and in the River Miño (702 ± 914 ind. m-2). In the River Mero, C. fluminea density was much lower (360 ± 241 ind. m-2). Low densities were recorded at the limit of the invaded area in the Miño-Sil basin (River Sil: 5 ± 1 ind. m-2). Detailed information is given in Online Resource 2. Spatial data from field surveys together with historical records from peer-reviewed publications and non-peer reviewed ‘grey literature’ indicated a mean upstream spread along the Miño, Ulla and Mero rivers of 3.6 km yr-1. In the Miño-Sil basin, C. fluminea reaches 197 km upstream from the river 123 592 N. Ferreira-Rodrı́guez et al. Table 1 Social, ecological, economic and governance variables of the SES framework for Corbicula fluminea dispersal in Galicia Dimension Variable Description Justification Source of data Social Population General census of population (number of persons) living in Galicia Growth of human population and per capita resource use as the main drivers of global change by the industrial and agricultural development 1 Fishing licenses Recreational fishing licenses in force and expedited by the competent authority in Galicia Transport and release of non-native aquatic organisms used for sport fishing activities 4a Higher education level University students enrolled in any of the three Galician universities General and active interest in nature and knowledge of non-native invasive species 1, 2 Mass media news Number of news about non-native invasive species yearly portrayed in mass media Public learning about original scientific research, new records and impact of invasive species 5 Gross domestic product (GDP) Millions of euros of all the finished goods and services Dispersal of non-native invasive species facilitated by the development of transport networks and human mobility 1 Gross imports Imports of goods and services in millions of euros Direct introduction of non-native species as ‘stowaways’ 2, 6 Exotic pet trade Thousands of individuals imported alive with commercial objectives (to Spain) from captive born or wild species Accidental and/or intentional release from the pet industry and aquariums 7 Industrial productivity index (IPI) Indicator which reflects the development of value added in the different branches of industry Loss of water quality through industrial discharges 1, 2 Endangered freshwater species Number of species placed into one of four categories of conservation concern (Near Threatened, Vulnerable, Endangered, and Critically Endangered) Average annual temperature Biotic resistance of native species in freshwaters to limit the invasion of nonnative species (ecosystem resistance) 3, 9 Increment in the number and distribution range of warm-temperate non-native freshwater species, and reduced ecological resistance of native environments to invasions 10 Scientific publications Peer-reviewed publications queried under search terms freshwater AND Galicia, river AND Galicia, invasive species AND Galicia Increased knowledge of non-native invasive species and their distribution and management 11 Dams Impoundments in Galicia Costa and MiñoSil hydrographic demarcations Habitat modification and disruption of upstream river continuity due to transverse structures altering flow regimes, blocking fish migration and facilitating non-native species establishment 4b Wastewater treatment plants Cumulative number of wastewater treatment plants that release treated effluent to adjacent water bodies Improvement of water quality to achieving the over-arching objectives set by the European Water Framework Directive 4b Livestock General census of farm animals, with the exception of poultry; encompasses cattle, sheep, pigs, goats, horses (mostly in semiferal conditions), and donkeys Loss of water quality through increased nutrient loading from animal waste 1, 2 Economic Ecological Temperature 123 A social-ecological system framework to assess biological invasions: Corbicula fluminea… 593 Table 1 continued Dimension Governance Variable Description Justification Source of data Agricultural land Extent of total cultivated land (rainfed and irrigated) in hectares, with the exception of forest plantations Loss of water quality through increased concentrations of nutrients, pesticides and sediment loads 1, 2 Surveillance Cumulative number of environmental guard corps responsible for the condition of the environment and natural resources Increasing environmental vigilance through the creation of the Nature Protection Service (SEPRONA, unit of the Spanish Civil Guard, 1985) and the Environmental guards (Civil environmental guard crop, 2000) responsible for nature conservation and management of the hunting and fishing industries 4c Legislative instruments Cumulative number of legislative instruments related to non-native invasive species introduction and dispersal The Schengen Agreement, implemented in 1995, supposed the gradual abolishment of the internal borders (and controls) between European countries. National Legislation Law 42/2007, further developed by Royal Legislative Decree 630/2013, created the first Spanish Catalogue of non-native invasive species and set a preliminary framework for taking management measures for controlling and even eradicating nonnative invasive species. Regulation (UE) 1143/2014 on the prevention and management of the introduction and spread of invasive non-native species 14 NGOs NGOs involved in environmental governance, including local and regional groups* Groups dedicated to environmental protection, sustainable development, surveillance, invasive species eradication, and other proenvironmental issues 4 Source of data: 1INE (Instituto Nacional de Estadı́stica), 2IGE (Instituto Galego de Estatistica), 3DOGA (Diario Oficial de Galicia) Decreto 88/2007, 4Xunta de Galicia, aConsellerı́a do Medio Rural, bAugas de Galicia, cServizo de Conservación da Natureza, 5 Autonomic newspaper (Faro de Vigo), 6Tizón (2002), 7CITES, 9IUCN, 10Meteogalicia, weather stations network, 11ISI Web of Knowledge, 14Boletı́n Oficial del Estado (BOE). *Data corresponding to 82 environmental NGOs from Lugo (one of the four provinces in the Galician region) have been excluded from the analysis due to their unreliability mouth, indicating maximum spread rates of 9.2 km yr-1 (Table 3). The PCA showed that the first two components explained 76.74% of the variance (Table 4). The first component (57.91% of total variance) was linked to variables of the social (higher education level and mass media news), ecological (endangered freshwater species, scientific publications, dams, wastewater treatment plants, livestock, and agricultural lands), economic (GDP, gross imports, and IPI) and governance (surveillance, legislative instruments, and NGOs) dimensions. The second component (18.83% of the variance) made reference to the social (population, sport fishing licenses, and higher education level), ecologic (livestock) and governance (legislative instruments) dimensions. Spearman’s correlation between the 2 PC scores and the total number of water bodies invaded by C. fluminea in Galician freshwaters showed a positive correlation with PC1 (r = 0.982, P = 0.0001). A non-significant correlation was found with PC2 (r = - 0.066, P = 0.71). Our results indicated that the number of new water bodies invaded by C. fluminea remained stable postimplementation of the legislative instruments (Wilcoxon Z = - 1.604, P = 0.109). No differences were found in the number of new water bodies invaded by C. fluminea (Wilcoxon Z = - 1.289, P = 0.197) preand post-implementation of the Schengen Agreement. 123 594 N. Ferreira-Rodrı́guez et al. Table 2 Conceptualization of the Corbicula fluminea invasion process in Galicia (NW Iberian Peninsula). The status of C. fluminea is assessed year-to-year in each invasion stage and barriers overcome between each one, with special emphasis on the possible management action in each phase. Source: adapted from Richardson et al. (2000) and Sakai et al. (2001), own compilation from interviews and field data Year Status Invasion phase Management options Comments Before 1980 Native species Native range Prevention Native from the eastern Mediterranean to southwestern Asia; parts of Africa and Australia. Introduced in North America in the 1920s and Europe in the 1980s Transport Prevention Transport between Iberian Atlantic basins associated to commercial fishing gear, leisure craft and use as sport fishing bait for cyprinids, pet trade and deliberate or accidental introduction Establishment Eradication The species establishes a self-sustaining population by surviving environmental conditions and biotic interactions and successfully reproduce. First record in the Miño river in 1989, derived from a unique introductory event of the species in the River Tagus (1980) Lag phase Eradication Low population size, evolution and adaptation to the new habitat characterized by low spread rates. Observed spread rate: 1.5 km yr-1 Exponential growth Control/ Restoration Expansion phase marked by increasing spread rates and unregulated exponential growth. Maximum population densities surpassed 4000 ind. m-2. Population decreases to 2000 ind. m-2 in 2005 summer heatwave (Sousa et al. 2008b). Observed spread rate: 2.9 km yr-1 Dispersal Control/ Restoration Population density stabilizes (ca. 3000 ind. m-2; FerreiraRodrı́guez and Pardo 2018). Secondary dispersal resulting from biological features in conjunction with human activities. The Centeáns ponds and River Ulla (2008); River Tambre (2009); Deva, Sil and Mero rivers (2010), River Tea (2013). Possible records at the Antela pond and Pontiñas stream. Observed spread rate: 9.2 km yr-1 Geographical barriers 1980–1989 Introduced species Environmental (local) barriers 1989 Casual species Reproductive barriers 1989–1996 Naturalized species Reproductive barriers 1996–2005 Invasive species Barriers to dispersal 2008–the present Invasive species Environmental and biotic barriers (disturbed habitats) At present Invasive species Ecological and human impacts Control/ Restoration Discussion The SES framework was a useful tool in identifying variables related to non-native species in freshwater environments. Our case study from Galicia supports the idea that the increasing distribution range of C. fluminea is closely associated with the social, ecological, economic and governance dimensions of the SES. 123 Changes in sediment biogeochemical composition, negative effect on native bivalve abundance and diversity, substrata for invertebrate settlement, facilitation of native and non-native fauna (Richardson et al. 2000, Sakai et al. 2001; Vander Zanden and Olden 2008) Applying the SES framework to C. fluminea invasion had several strengths: (1) it facilitated the integration of data from different disciplines (Janssen and Anderies 2013), including diverse quantitative and qualitative data; (2) it allowed the explicit consideration of the governance as tool to prevent the introduction into new waterbodies; and (3) as none of four multi-linked core subsystems (resource units, A social-ecological system framework to assess biological invasions: Corbicula fluminea… Fig. 2 Current known distribution of Corbicula fluminea in Galicia (NW Iberian Peninsula). Closed circles—C. fluminea presence, stars—empty shells, diamonds—presence reported in 595 grey literature. Only main tributary rivers are depicted Source: literature review, own compilation from interviews and field data (n = 62 sampling locations) Table 3 Summary of data related to the mean annual upstream spread (km from the river mouth) rate by Corbicula fluminea, either actively or by means of passive transport with natural or anthropogenic vectors Basin River Year Miño-Sil Miño 1989 Ulla Mero Years post-invasion 0 Distance (km) Spread (km yr-1) Source of data 13 – Araujo et al. (1993)a Miño 2004 15 44 2.1 Pintos and Fernández (2004) Miño 2006 17 76.2 3.7 Lombardero (2006) (Grey literature) Sil 2009 20 197 9.2 Lois (2010) Ulla 2008 0 2.8 – Angueira (2013) (Grey literature)a Ulla 2014 6 13.9 1.9 Present results Mero 2009 0 8.6 – Lois (2010)a Mero 2014 5 14.3 1.1 Present results a Denotes first record in the basin resource system, governance system and stakeholders) can be completely understood in isolation, the SES framework was well-suited to incorporate the complexity of a biological invasion in the analyzed system. In addition, the SES framework had an explicit focus on stakeholders in the system. In this regard, our study highlights the importance of collaborating efforts between stakeholders and academic institutions for early detection of new invasions and to develop management strategies jointly with decision-makers. Corbicula fluminea invasion The origin of C. fluminea invasion in Galicia remains highly uncertain. Recent genetic studies have suggested that C. fluminea in the region resulted from a 123 596 Table 4 Explained variance of the first two components and variables loading in the Principal Components Analysis of the social, ecological, economic, and governance dimensions analyzed from 1980 to 2014. Highest variables loadings are indicated in bold for each axis N. Ferreira-Rodrı́guez et al. Dimension Social dimension Ecological dimension Economic dimension Governance dimension unique introduction event of the species in the River Tagus (Portugal), which subsequently dispersed to other freshwater ecosystems (Gomes et al. 2016). Taking the population dynamics of invasive species into consideration (Sakai et al. 2001), successful establishment is usually followed by an exponential growth phase, where the population starts to produce sufficient number of larvae, and dispersal can drive the invasive species to colonize new areas (Arim et al. 2006). Assuming that expansion progressed upstream gradually, our results (mean upstream spread of 3.6 km yr-1) are similar to those previously reported for other European watercourses (e.g., the Elbe and Rhine rivers: 2.4 km yr-1; Beran 2006; Schmidlin and Baur 2006). Despite the existence of great barriers to dispersal in the Miño river catchment (twenty-three larger dams; more than 25 m high), we found a maximum spread rate of 9.2 km yr-1, which may be related to human activities (e.g., use as fishing bait) or to aquatic birds (i.e., attached to feet or feathers through mucous threads secreted by small individuals of C. fluminea; Prezant and Chalermwat 1984). In fact, early works have identified flotation as a means of downstream dispersal in small individuals of C. fluminea (Yavelow et al. 1979). In contrast, without human intervention, upstream dispersal in freshwaters may be slower and limited to crawling of juveniles and 123 Variable PC#1 PC#2 Population - 0.250 0.808 Sport fishing licenses - 0.035 - 0.948 Higher education level 0.602 - 0.754 Mass media news 0.657 0.485 Endangered freshwater species 0.966 0.026 - 0.355 Temperature 0.068 Scientific publications 0.901 0.315 Dams Wastewater treatment plants 0.984 0.964 - 0.055 0.214 Livestock - 0.590 - 0.522 Agricultural lands - 0.894 0.010 Gross domestic product (GDP) 0.976 0.116 Gross imports 0.960 0.179 Exotic pet trade 0.179 - 0.041 Industrial productivity index (IPI) 0.771 - 0.368 Surveillance 0.926 - 0.138 Legislative instruments 0.663 0.533 NGOs 0.988 - 0.035 adults (mean upstream spread of 1.2 km yr-1) and tidal transport of larvae and floating juveniles during rising tides in estuaries, especially during spring tides (Voelz et al. 1998; Rosa et al. 2014). Social and economic dimensions Since the beginning of the 21th century, news in mass media about non-native invasive species greatly increased. Together with the higher education level attained by the population, the knowledge dissemination through the mass media should increase public awareness about the impact of non-native invasive species. Nevertheless, field results suggest that human activities seem to be the main drivers of its introduction in new water bodies. This assumption is supported by the presence of empty shells in the Deva, Tea and Tambre rivers, probably related with the use of C. fluminea as bait in sport fishing. Despite the wide spatial coverage of our surveys, we were unable to find live individuals in the Tea or Tambre rivers, supporting the idea of human-mediated dispersal. This concept has been corroborated by personal interviews and it is reported in grey literature (e.g., Lorenzo 2014), although there is no evidence from our analysis to support such assumption. A social-ecological system framework to assess biological invasions: Corbicula fluminea… The economic dimension (mainly given by the GDP, gross imports, and IPI variables) suggests that economic health of the region was associated with the dispersal of C. fluminea. Indeed, this economic dimension was previously identified as one of the main predictors of non-native species richness (Gallardo 2014; Keller et al. 2009). One component in the increasing globalization of the economy is the biological exchange among distant geographical regions through international transport and trade. In fact, countries with the greater degree of international trade tend to have more non-native invasive species by the development of terrestrial transport networks, migration rates, number of tourists visiting the country, and trade commodities (Dalmazzone 2000; Westphal et al. 2008). In addition, current trade agreements are fostered through relaxing restrictions that may prevent the introduction of non-native invasive species (Campbell 2001). Large development projects, such as dams, irrigation schemes, land reclamation, and road construction, have also contributed to dispersal of non-native invasive species (McNeely 2001). The commercial fishing industry is a key economic engine in Galicia. There, the River Miño supports an important traditional fishing community that carries out its commercial activity using artisanal fishing methods. Since the 1960s, barriers to fish migration (in the form of dam construction), loss of water quality (associated to urban waste, agricultural runoff, and livestock farms) and overexploitation have resulted in the extirpation of species of local cultural and economic importance (e.g., European Atlantic sturgeon Acipenser sturio Linnaeus, 1758) and a decline in species which depend on the continuity of the river to complete their life cycle [e.g., Atlantic salmon Salmo salar (Linnaeus 1758), European eel Anguilla anguilla (Linnaeus 1758)] (Dill 1993). Since the collapse of fisheries in the mid-1980s (Online Resource 3), and encouraged by its occasional consumption, exploitation of C. fluminea stocks has been highlighted as an effective management action and a new economic alternative for local fishers (e.g., Lombardero 2006; Vizoso 2008; Ilarri and Sousa 2012). However, this strategy may result in unintended environmental impacts, like deliberate introductions, if it is organized around economic goals rather than environmental goals (van Beers and van den Bergh 2001). 597 Ecological dimension The increasing body of scientific knowledge allowed the identification of new C. fluminea invaded water bodies in Galicia. This knowledge was a key factor to decipher the important role of native biodiversity to strengthen ecosystem resilience and/or resistance to environmental fluctuations and disturbances. In this regard, high biodiversity was signaled as a key component of ecosystems preventing the establishment of non-native species at high densities (e.g., freshwater mussels providing ecosystem resistance to C. fluminea invasion; Vaughn and Spooner 2006). Our results are consistent with this observation that native biodiversity may be negatively related to the increasing area occupied by C. fluminea. Hence, native species conservation should represent an effective strategy preventing the establishment of non-native species at high densities. The benthic community structure of the freshwater environments of northwestern Iberian Peninsula has been impacted by human activities for thousands of years (de Agüero et al. 2014). Nonetheless, the impact upon the benthic community has clearly become much more intense since the middle of the 20th century due to hydromorphological alterations (i.e., dam construction), fish overexploitation and pollution (Dill 1993). These changes to community structure have left the ecosystems more vulnerable to biological invasions (Manchester and Bullock 2000; Carlsson et al. 2009). One example is the decline of native freshwater mussel species such as Unio delphinus Spengler, 1793 and Margaritifera margaritifera (Linnaeus 1758) (Bauer 1986, 1988; Araujo 2011) that could have competed with C. fluminea and prevented the establishment of high density populations. In the last three decades, the improvement of environmental conditions (e.g., derived from the increasing number of wastewater treatment plants, reduction of agriculture land surface, and reduction in the livestock), the absence of potential predators (e.g., extirpation of the European Atlantic sturgeon; Ferreira-Rodrı́guez et al. 2016) and the presence of empty niches (e.g., by decline of native freshwater mussels) might have favored the successful establishment of C. fluminea. 123 598 N. Ferreira-Rodrı́guez et al. Governance dimension Management implications Our results highlight that increasing surveillance was positively related with the detection of new C. fluminea invaded water bodies. Such effectiveness of environmental guard corps represents a management opportunity in the implementation of early detection and eradication programs. Legislative instruments detailed well-defined measures for non-native invasive species governance and governability. As a result, rules-in-use (i.e., Law 42/2007 and Royal Legislative Decree 630/2013) should stop repeated introductions of enlisted species. Nevertheless, despite the implementation of more restrictive legislative instruments, new water bodies were invaded in the region year to year. In recent years, the Galician government has demonstrated high dynamic resilience, implementing specific management actions as a rapid response to non-native invasive species. This is the case of species able to cause ecological, economic and social impacts (e.g., the red palm weevil Rhynchophorus ferrugineus Olivier, 1870, the pine wood nematode Bursaphelenchus xylophilus (Steiner and Buhrer 1934) Nickle 1970 and the eucalyptus weevil Gonipterus scutellatus Gyllenhal, 1833; for more details on these preventive and control plans see http://cmaot.xunta.gal/). The governability dynamics concerning these non-native invasive species include collaborative efforts among different stakeholders (authorities, scientists, society and producers) to achieve ‘primary management’ goals (control and eradication measures). In addition to government surveillance, our analysis indicated that the number of environmental NGOs was positively related with the record of new invaded waterbodies. It is therefore reasonable to think that the pro-environmental stance and educational mission of NGOs may strengthen the implementation of management strategies (Gemmill and Bamidele-Izu 2002). Although legislative instruments for managing nonnative invasive species have been approved, they are yet to be successfully implemented. Additionally, two recent legislative instruments (Royal Legislative Decree 630/2013, National legislation regulating the Spanish list of invasive non-native species; Regulation (EU) 1143/2014, European legislation on invasive non-native species) have been approved, but insufficient time has elapsed to assess their effectiveness. Our results suggest that diagnostic application of the SES framework is useful for understanding the complexity of biological invasions. Despite increasing regulatory pressure on non-native invasive species, governments still often fail to take notice and face the real dimension of the problem. An innovative aspect of the SES framework here presented is its utility for managers to select which of the framework’s variables will be particularly relevant for avoiding non-native species introduction and dispersal. We have shown that legislative instruments alone cannot adequately avoid such introductions because their implementation is conditioned by the context in which they are applied; i.e., the socioeconomic development and environmental characteristics of the system addressed should be accounted for. In this regard, the SES framework organizes key variables for the governance of the SES at multiple hierarchical levels, which is also useful for identifying key action items for legislative instruments addressing non-native species introduction. In the case of Galicia, the resource system is characterized by a vast river network that makes the region particularly susceptible to secondary dispersal processes of freshwater non-native invasive species once introduced. Additionally, biological features of non-native invasive species in conjunction with human activities may facilitate their introduction into new water bodies. Our field results suggest that leisure activities such as sport fishing may be related with C. fluminea secondary dispersal and should be the focus of policy prescriptions and environmental education (preventive) programs. Since a single individual of C. fluminea can establish a new population through self-fertilization, its eradication is impossible in most cases (Vander Zanden and Olden 2008). Hence, prevention of further introductions is of primary importance. In this regard, our study and other previous works (e.g., Szekeres et al. 2013) highlight the importance of collaborating efforts between stakeholders (fishers, recreational users, local populations, environmental guard corps and NGOs) and academic institutions for early detection of new invasions and to develop management strategies jointly with decision-makers. As an example, local fishermen in the River Tagus recognized the presence of C. fluminea and its use as fishing bait in the 123 A social-ecological system framework to assess biological invasions: Corbicula fluminea… early 1950s (Sousa et al. 2007a), thirty years before the first scientific record in Europe (Mouthon 1981). This early detection could have provided an opportunity to address the invasion before the clam’s dispersal throughout the Iberian Peninsula. On this subject, Prince (2003, 2010) highlighted the importance of onsite fishers as civilian aquatic census takers (‘‘barefoot ecologists’’) that should play a key role in management action plans. This concept has been applied in fisheries management worldwide (Macho et al. 2014), mainly in marine fisheries like the smallscale Galician shellfisheries (Macho et al. 2013), but also in freshwater basins like in the Amazon (Castello et al. 2013). The involvement of onsite fishers will provide a rapid and cost-effective detection of nonnative species using a bottom-up strategy (DehnenSchmutz et al. 2010). The involvement of these and other stakeholders will enhance the development and implementation of management plans specifically designed to address the distinctive characteristics of local, small-scale environments, especially when scientific knowledge is limited (Cochrane et al. 2011). Conclusion C. fluminea started its worldwide expansion at the beginning of the 20th century associated to human activities. Due to the increasing number of invaded water bodies, identifying the introduction pathways and dispersal vectors of C. fluminea is a key factor in guiding preventive management actions. Economic variables, loss of native biodiversity, habitat alteration, improvement of water quality, scientific and public knowledge, legislative instruments or governmental and non-governmental surveillance are all predictive of a water body’s susceptibility to invasion in the short term. Therefore, governments have approved legislative instruments aimed at addressing this and other biological invasions. However, they are yet to be successfully implemented. In this regard, the SES framework applied here is a useful tool for future preventive and control plans regarding non-native invasive species management. Effective management actions could include freshwater mussel conservation programs and increasing surveillance, providing ecosystem resistance to invasions and mitigating the introduction of non-native species, respectively. In addition, the ability of decision-makers involving 599 different stakeholders in the real implementation of legislative instruments should influence how much we invest in non-native invasive species management and its outcome. Acknowledgements We are particularly thankful to J. Iglesias for his enthusiastic support during field work. Authors are grateful to T. B. Parr from the Oklahoma Biological Survey (University of Oklahoma) for his valuable comments that improved the manuscript. We thank the reviewers for their careful reading of our manuscript and their many insightful comments and suggestions. NF-R was supported by a postdoctoral fellowship from the Government of the Autonomous Community of Galicia (Xunta de Galicia; Plan I2C 2016-2020, 09.40.561B.444.0). References Angueira M (2013) La almeja asiática ‘‘invade’’ el Ulla y la Estación de O Con alerta de la amenaza para especies autóctonas. Diario de Arousa. http://www.diariodearousa. com/. 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