Scientific and grey literature on invasive alien species (IAS) is conditioned by social, economic and political priorities, editorial preferences and species and ecosystem characteristics. This leads to knowledge gaps and mismatches between scientific research interests and management needs. We reviewed the literature on IAS management in Spain found in Scopus, Web of Science, Google Scholar and Dialnet to identify key deficiencies and priority research areas. The collected literature was classified, employing features describing formal aspects and content. We used bibliometric and keyword co-occurrence network analyses to assess the relationship between features and reveal the existence of additional topics. Most of the compiled documents (n = 388) were focused on terrestrial ecosystems and inland waters, whereas marine and urban ecosystems were under-represented. The literature was largely generic and not species-specific, focusing on raising awareness and proposing changes on current regulation as prominent approaches to prevent further introductions. The compiled authors exhibited many clear publishing preferences (e.g. language or document type), but less regarding target taxa. In addition, there was a strong association between species and the different features considered, especially between the methodological approach (e.g. review, field experiment) and the primary emphasis of study (i.e. basic/theoretical, applied or interdisciplinary). This indicates that research on IAS has had a strong species-specific focus. References about terrestrial species focused mainly on vascular plants, whereas references about inland waters were mostly on fishes and the giant reed (Arundo donax), which has been managed with partial success. Animal culling and plant removal were the most frequent eradication and small-scale control treatments, whereas the documents addressing wider spatial scales were largely theoretical. Consequently, the success of described treatments was largely uncertain. Spanish invasion science research has been occasionally innovative, incorporating novel technologies (e.g. species distribution modelling) and engaging society with citizen-science approaches. However, the ratio between basic/theoretical and applied studies indicates that more applied research/management is needed, especially in inland waters and marine ecosystems. We call for increasing effort in the effective dissemination of experience in IAS management to enhance current practical knowledge, including that of schemes undertaken by public agencies.

Biological invasions, eradication, control, Iberian Peninsula, Mediterranean, Spanish archipelagos, keyword co-occurrence analysis, prevention

Biological invasions are human-assisted global phenomena with ravaging effects, not only on biodiversity and ecosystem services, but also on human well-being (McGeoch et al. 2010; Vilà and Hulme 2017). Although transport and introduction of alien species into novel ecosystems is inherent to humankind’s expansion (Crees and Turvey 2015), the number of new introductions has increased exponentially since the mid-twentieth century (Seebens et al. 2018, 2019). Invasive alien species (IAS) can reshape ecosystem processes, decrease native species richness and abundance (e.g. McGeoch et al. 2010 and references therein) and cause impact on the economy and human health (Zenni et al. 2021). Thus, preventative, eradication and control actions are required to impede their entry and establishment or minimise their long-term impacts (Robertson et al. 2020).

The incidence of biological invasions and their related costs have led to substantial management efforts worldwide to prevent new introductions and control those already established, by means of eradication or containment (García-de-Lomas and Vilà 2015; Diagne et al. 2021). However, to offer efficient responses is challenging and, regrettably, the specific literature on IAS management is often neglected during the decision-making process (Walsh et al. 2015). In Spain, managers, consultants and assessors face obstacles to find and access suitable references, as a large proportion of papers are written in English, are too scientifically orientated or narrowly focused to be directly applied or are not open-access (Andreu and Vilà 2007; Mungi et al. 2019; Copp et al. 2021). In addition, many applied studies are scattered amongst the grey literature (often in many different languages), tending to be largely inaccessible to international readers, which further limits the transfer of knowledge on both local and international levels (Haddaway and Bayliss 2015, Jeschke et al. 2019).

In addition to accessibility barriers, the content of scientific literature is also taxonomically and geographically biased (Hulme et al. 2013). This is accentuated by the fact that reviews and positive rather than negative results are more likely to be published (Fanelli 2012). Thus, several characteristics inherent to IAS and recipient ecosystems utterly favour the availability of literature on specific taxa, regions or ecosystems that are easier to study and/or manage (Thomsen et al. 2014; Nghiem et al. 2016; Shackleton et al. 2019). Moreover, trends in invasion science are also affected by social and political priorities, which are, in turn, influenced by communication media and the outcomes of previous research (Gläser and Laudel 2016; Geraldi et al. 2019; Shackleton et al. 2019). This can lead to changes in the importance of research topics over time, for instance through fostering a given species over others or by abandoning certain research areas.

Bibliometric and keyword co-occurrence network analyses are useful to reveal the main knowledge components of any discipline, such as areas with deep insights, outstanding gaps and peripheral research areas (Aria and Cuccurullo 2017, Radhakrishnan et al. 2017). Despite some recent contributions (Enders et al. 2019, 2020), such analyses are scarce in previous reviews of IAS literature, particularly related to management. Consequently, we chose to use them to identify the main patterns in the scientific literature shown by IAS management in Spain. Our specific aims were to: (i) characterise the species, topics and approaches from a management viewpoint, (ii) detect relationships between them and (iii) identify research areas deserving further attention. We focused on Spain due to its diversity of climates and ecosystems, along with its insular and continental territories, which have favoured the establishment of a large and diverse number of IAS (Muñoz-Mas and García-Berthou 2020). Mainland Spain is part of the Mediterranean biodiversity hotspot (Williams et al. 2011) and encompasses an enormous range of ecosystem types, from arid coastal regions to mountain ranges and woodlands. Spain includes two markedly-different populated archipelagos: the Mediterranean Balearic Islands and the Macaronesian Canary Islands off the north-western African coast (Andreu et al. 2009; Benito-Calvo et al. 2009). Islands are in themselves biodiversity hotspots, but they have especially suffered from the establishment of IAS, which have caused numerous extinctions (Lenzner et al. 2020). In addition, Spain has two autonomous cities located on the Mediterranean coast of the African continent, which may require different IAS management approaches. Former reviews on invasion management in the country were sectorial and focused on stakeholder perceptions and management of alien plants (Andreu et al. 2009) or were based on the most common deficiencies in IAS management (Dana et al. 2019). Thus, reviewing past and present experience may help to improve current IAS management actions and identify future research lines in Spain and similar territories. Our results may help funding agencies to target as yet unidentified research needs.

The number of documents published on IAS management has grown steadily since 1995 (Figure 1A). However, those published in Spain are under-represented compared to the scientific production trends both globally and in Spain, although during the last decade, the scientific production accelerated significantly. Most documents were scientific articles (72.4%) (Figure 1B). Review/meta-analysis was the most common methodological approach (54.9% out of the 388 documents), followed by field experiment (19.0%) and observational field study (18.9%) (Figure C). The total number of authors was 1,280. The most prolific author was Montserrat Vilà (13 documents/3.4%), followed by Pilar Castro-Díez, Elías D Dana and Juan García de Lomas (7 documents each/1.8%), whereas 1,067 authors appeared in one single document (Figure 1D). Most documents were written in English (59.3%) and the remainder in Spanish.

Figure 1.

Cumulative number of documents on IAS management in Spain and cumulative number of scientific documents included in the Journal Citation Reports (JCRs), with no distinction by research area (A). Global series encompasses all the documents included in the JCRs, whereas the Spain series includes exclusively those produced in Spain (data retrieved from www.scimagojr.com/countryrank.php). Numbers of documents by types are represented in (B), methodological approaches in (C) and number of documents by the most prolific authors in (D).

Management (i.e. eradication and control) was the most frequent topic (31.8% out of 388 documents), followed by risk assessment (9.6%), survey/monitoring (9.3%), biology/ecology (7.6%), and impacts (7.4%) (Figure 2A). Studies focusing on populations and the autoecology of a single species were the most frequent (45.2%), followed by studies at the community/assemblage level (24.0%) (Figure 2B). The dominant management topic was generic/not applicable (48.0%) — i.e. documents that did not focus on prevention, eradication or control of IAS and did not involve direct manipulation of target IAS — followed by control (22.7%), whereas studies focusing on prevention and eradication were less frequent (17.5% and 11.7%, respectively) (Figure 2C). Studies on a local or national scale were more frequent (28.9% and 24.2%) than those on the intermediate (regional) or largest (supranational/global) scales (20.6% and 18.6%, respectively) (Figure 2D). The most frequent primary emphasis was basic (i.e. without direct manipulation of target taxa) (45.6%), followed by applied or interdisciplinary scopes (i.e. encompassing social and policy issues), both with similar prevalence (28% and 25.5%, respectively) (Figure 2E). The most frequent management treatments concerned impeding the establishment of further IAS through awareness-raising, regulation and survey monitoring (18.0%, 15.3% and 11.8%, respectively) (Figure 2F). Animal culling (9.1%) and plant mechanical and manual removal (11.5%) were the most common eradication and control treatments, whereas 8.2% of documents did not report specific treatments of target IAS (i.e. not applicable). The success of the proposed treatments as eradication or control methods was largely uncertain (not applicable/42.8% or unknown/23.4%) or partial (15%), which implies sustained management actions to control the target IAS (Figure 2G). About 13% of the studies focused exclusively on insular territories and 17% on protected areas.

Figure 2.

Numbers of documents by main topics are represented in (A), ecological organisation levels in (B), management topics in (C), spatial scales in (D), primary emphasis in (E), treatment in (F) and success in (G).

The compiled studies involved 159 species or higher taxa (e.g. vascular plants). The largest number dealt with terrestrial ecosystems (50.4%), followed by inland waters (36.2%), whereas marine ecosystems and urban environments were the least common (8.3% and 5.0%, respectively) (Figure 3). Terrestrial vascular plants (10.4%) were the most frequent, especially eucalypts (Eucalyptus spp.) (1.6%), prickly pear species (Opuntia spp.) (1.3%), Hottentot figs (Carpobrotus spp.) (1.3%), Monterey pine (Pinus radiata) (1.3%) and cordgrasses (Spartina spp.) (1.0%). Amongst terrestrial animals, the most frequent species were the yellow-legged hornet (Vespa velutina) (1.6%), the pinewood nematode Bursaphelenchus xylophilus (1.3%), American mink Neovison vison (1.0%) and generic studies on vertebrates (3.3%), mainly birds (1.3%). In inland waters, generic studies were also the most common (7.5%), followed by those on fish management (7.0%). The most studied species in inland waters were the giant reed (Arundo donax) (3.1%) and the red swamp crayfish (Procambarus clarki) (1.8%), followed by generic studies on riparian vegetation (1.6%). Most studies on marine environments were generic (4.4%) or focused on algae (1.8%) or polychaetes (1.3%). In urban environments, generic (2.1%) and bird studies (0.8%) prevailed, in addition to those on tiger mosquito (Aedes albopictus) (0.5%).

Figure 3.

Word cloud depicting the proportion of documents per ecosystem type and taxon obtained using the function comparison.cloud of the R package wordcloud (Fellows 2018). The species or taxa occurring in less than two documents have been grouped within the category Other. Frequencies have been square root-transformed to facilitate reading the species.

The association network displaying Cramér’s V Index between pairs of features indicated great specificity regarding authors’ preferences about publishing language and document type and on the management scales and treatment success of the conveyed experiences and approaches (Cramér’s V > 0.91), but inferior regarding the target species (Cramér’s V = 0.76). Subsequently, the association network highlighted the strong association between species and all other features (Cramér’s V > 0.50), except with document type and protected area (Figure 4). The highest association of this group occurred between species and ecosystem type (Cramér’s V = 0.66) and between species and insular territory (Cramér’s V = 0.65). The connections between species, methodological approach and primary emphasis, as well as those amongst the latter two, were noteworthy. There was no strong association between the remaining features, except between the main topic and primary emphasis and the management topic (Cramér’s V > 0.50). Insular territory and especially document type depicted the lowest association with the remaining features.

Figure 4.

Association network displaying Cramér’s V between features characterising the documents on IAS management in Spain. Connections are only depicted when the χ² statistic is significant (P < 0.05). Connection width and colour are based on Cramér’s V and range from 0 (no association) to 1 (perfect association).

The most frequent studies were reviews and meta-analyses focusing on terrestrial ecosystems and providing general guidelines to manage invasive species at the national (6.5% out of the 388 documents), supranational/global (5.3%) and regional scales (4.3%) (Figure 5). They were followed by field experiments (manipulative) on terrestrial ecosystems addressed to locally eradicate specific taxa (3.3%). Reviews and meta-analyses on inland waters and those providing general guidelines to manage IAS at higher scales (i.e. regional to supranational/global) were also frequent (3.2% and 2.8%, respectively). Field experiments (manipulative) in inland waters addressed to eradicate (3.0%) or control (2.8%) specific taxa were in all cases local. The most frequent observational field studies on both terrestrial ecosystems (2.3%) and inland waters (2.0%) addressed generic aspects and were local. In contrast, documents focusing on marine ecosystems were scarce and studied generic aspects of marine invasion science with supranational/global (1.0%) or local (0.9%) perspectives. Manipulative field experiments to locally eradicate marine species were markedly rare (0.3%). Studies in urban environments were mostly reviews and meta-analyses addressing generic aspects at both supranational/global and local scales were the most abundant (0.5% and 0.5%, respectively). The proportion of field manipulative experiments to eradicate or control species was negligible.

Figure 5.

Alluvial diagram relating the methodological approach, ecosystem type, management topic and spatial scale of the compiled documents on invasive alien species (IAS) management in Spain. Connection width is proportional to the number of documents (An interactive version of this figure can be downloaded from https://doi.org/10.6084/m9.figshare.16547790.v1).

The most frequent approaches, described within terrestrial studies, focused on awareness-raising, survey/monitoring and regulation (7.3%, 6.8% and 3.1%, respectively out of the 388 documents). Most of them had no quantifiable (not applicable/17.2%) or unknown success (2.1%). This was followed by the use of biocontrol agents whose efficacy has not yet been tested (i.e. unknown, 1.6%) and unsuccessful culling of IAS (none and partial, 1.7%). The successful approaches most frequently reported combined herbicides and mechanical and manual plant removal (2.9% in total). In inland waters, awareness-raising and regulation were amongst the most frequently indicated approaches (3.9% and 3.0%, respectively), although with no quantifiable success (not applicable, 6.9%). Water level and flow regime manipulation was the most frequent management approach, but it was not tested (unknown, 2.3%) and occasionally turned out useless (0.7%). Culling and poisoning seldom worked (0.8% and 0.5%, respectively) and the success of most reported experiences was unknown or partial (1.6% and 0.9%, respectively). Hydrogeomorphological restoration, re-afforestation, plant removal and herbicide use were the most common approaches to control invasive riparian vegetation (1.2%, 1.1%, 1.6% and 0.7%, respectively). However, success of these treatments was partial or uncertain and only 0.6% reported successful experiences. The treatments for marine ecosystems followed a similar pattern and focused on preventative approaches: awareness-raising, survey/monitoring and regulation (1.9%, 1.0% and 0.9%, respectively), most of them with no quantifiable (not applicable/2.0%) or unknown success (1.6%). The only successful study involved raising awareness and DNA metabarcoding to confirm the elimination of the pygmy mussel (Xenostrobus securis) (Miralles et al. 2016). Awareness-raising, regulation and survey/monitoring were the most common approaches for urban environments (3.7% in total), but the few applied experiences indicated partial success through culling and plant removal and subsequent herbicide spraying (0.3%).

The collected keywords encompassed 1,145 different terms. The aggregation algorithm revealed 39 different research areas or clusters (Figure 7A), with 67 keywords occurring on ≥ 4 occasions (Figure 7B). The largest research area highlighted the importance of invasive plants amongst the Spanish literature on IAS management, the Mediterranean nature of much of the territory and the numerous studies carried out on this taxon in the Balearic Islands. The simplified network reflected the main topics of the study: invasive species and management, eradication and control. It also reflected research carried out on specific taxa, such as on the elimination of American mink (Neovison vison), which clustered with invasive species. Exclusion experiments on the European rabbit (Oryctolagus cuniculus) and feral cats (Felis silvestris catus) appeared in different research areas. The studies on feral cats were undertaken mainly in the Canary Islands as their research areas were connected. In addition, the resulting network highlighted the impacts on freshwater biodiversity caused by giant reed (A. donax) and aquaculture activities. The simplified network reflected the importance of the yellow-legged hornet (V. velutina) and mosquitoes (mainly the tiger mosquito A. albopictus) and the extensive use of species distribution models (SDMs), such as MaxEnt (Phillips et al. 2004), to foresee expansion trends and suitable regions. Specific topics (propagule pressure and ecosystem services) and introduction vectors (ballast waters, aquaculture and hunting) appeared in separate research areas. The simplified network reflected specific management approaches, such as general forest management, use of herbicides or emergence of citizen science. It also reflected studies addressing the interaction between invasions and climate change. In addition to the main toponymy used during the bibliography search, the network reflected specific regions and environments, such as the Strait of Gibraltar and Galicia (NW Spain) and the importance of wetlands and salt marshes. Moreover, the simplified network highlighted the importance of the archipelagos and islands within Spanish invasion science research, given that island and Canary Islands appear in differentiated research areas. It also revealed the multiplicity of terms used to name similar concepts, such as the terms alien species and exotic species that appear scattered throughout different research areas.

Figure 6.

Alluvial diagram relating the ecosystem type, treatment and success of the compiled documents on invasive alien species (IAS) management in Spain. Connection width is proportional to the number of documents. The category Other includes mulching, prevention, sterilisation, heating, incineration, containment, baiting, restocking, DNA metabarcoding, insecticide and genetic selection (Alternative static and interactive versions of this figure relating species/taxa, treatment and success can be downloaded from https://doi.org/10.6084/m9.figshare.16547790.v1).

Figure 7.

(A) Complete keyword co-occurrence network developed to visualise the importance of the research areas. Vertex colours are based on research areas or clusters and vertex size is proportional to the frequency of keyword occurrence. (B) Simplified network based on cluster centres and most frequent keywords (> Q₉₅ or number of occurrences ≥ 4). Overall vertex sizes are proportional to the log-transformed number of occurrences and inner circles to the proportion within each research area. Label sizes have been rescaled to avoid overlapping.

Spanish literature on IAS management has been mainly theoretical (55% review/meta-analysis), with a balance between theoretical and applied studies similar to that reported in other studies on biological and ecological aspects of biological invasions (Andreu and Vilà 2007; Bayliss et al. 2013; Matzek et al. 2014, 2015). Nonetheless, IAS management literature has been under-represented compared to the Spanish and global trends on scientific production, although recently its share has grown. Altogether, this suggests that Spanish invasion science may also suffer from a knowing-doing gap caused by a preponderance of theoretical studies (Matzek et al. 2014, 2015). Moreover, scientific papers written in English constituted a large proportion of the compiled literature, which suggests that most of it may be too scientifically orientated to be directly applied (Andreu and Vilà 2007; Mungi et al. 2019; Copp et al. 2021). Nevertheless, scientific activity in Spain has also incorporated new forecasting technologies (e.g. SDMs/MaxEnt, de Medeiros et al. 2018) along with new approaches to engage society in IAS control (e.g. citizen science, Clusa et al. 2018) and for biomonitoring (e.g. DNA metabarcoding, Borrell et al. 2017). In addition, risk assessments and horizon scanning studies for decision-making formed a notable part of the literature, with direct implications for IAS regulation (Bayón and Vilà 2019). There was also discussion regarding the inefficiency of current codes of conduct and laws (Maceda-Veiga et al. 2013), aimed at enforcing policies to overcome the highlighted deficiencies.

The proportion of applied studies and field experiments was markedly low, which impeded rating the efficacy of most of the described approaches (66.2%). Moreover, studies on novel biocontrol agents, such as the use of pathogens (McColl and Sunarto 2020) were infrequent. In part, this is because these agents require rigorous risk assessment studies and are, hence, subject to strict regulations (Loomans 2021). Thus, species eradication and control experiences often relied on the use of herbicides, mechanical elimination or culling (e.g. Melero et al. 2010; Mateos-Naranjo et al. 2012) and successful approaches were restricted to small-scale areas, such as islets and ponds (e.g. Ferreras-Romero et al. 2016; Maceda-Veiga et al. 2017). Consequently, concerns of Spanish managers about the problem that too much research focuses generally on the ecological aspects of alien plants, rather than on specific cost-efficient management strategies (Andreu et al. 2009), can be considered, to some extent, applicable to most invasive alien taxa.

The proportion of documents per species and ecosystem type in Spain was similar to that estimated in other countries (Thomsen et al. 2014). Terrestrial species, mainly plants, attracted the bulk of the literature, followed by studies involving species of inland waters. By contrast, marine and urban environments were infrequent in the collected literature. The preponderance of terrestrial ecosystems and the associated species can be justified by the primary introduction pathways of terrestrial species (i.e. release and/or escape), which are largely related to forestry, livestock, agriculture and wildlife trade (Essl et al. 2015). Vascular plants are the most frequently introduced taxon worldwide; consequently, they were expected to receive the largest proportion of studies. However, the feasibility of managing terrestrial invaders or aliens, especially sessile species and stages of their life history (e.g. nests, Enríquez et al. 2013), is greater compared to aquatic species, due to the lower accessibility of these environments. Therefore, a publication bias towards successful studies with positive results on tractable terrestrial species is not discernible (Fanelli 2012; Booy et al. 2017).

Studies on terrestrial ecosystems involved all spatial scales (i.e. local, regional, national and supranational). However, applied experiments and experience were mostly local interventions, such as management of prickly pear species (Opuntia spp.) and the sentry plant (Agave americana) (Arévalo et al. 2015). By contrast, eradication and control of vagile terrestrial organisms proved to be economically unaffordable (e.g. American mink N. vison, Melero et al. 2010; Mañas et al. 2016), especially in a context of multilevel overlapping or competing public administrations with ill-defined jurisdictions and pervasive budget shortages (Tollington et al. 2017; Dana et al. 2019). In this regard, island territories were well covered by the collected literature, with several successful management experiences in these territories, such as the eradication of American mink (N. vison) in the Atlantic Islands National Park (Velando et al. 2017) or of the red palm weevil (R. ferrugineus) from the Canary Islands (Fajardo et al. 2019). Impacts of IAS on islands are likely to increase in the future, especially on oceanic islands, such as the Canaries and, to a lesser extent, on the continental Balearic Archipelago. Indeed, insular terrestrial ecosystems are generally the most threatened (Lenzner et al. 2020). Therefore, these two successful examples shed hope on the future management of IAS in Spanish insular territories.

The published research on terrestrial invertebrates and microorganisms appeared to be concentrated on a few species with direct impacts on economics and human health (e.g. yellow-legged hornet V. velutina, pinewood nematode B. xylophilus or tiger mosquito A. albopictus). However, further applied research and knowledge transfer is particularly needed to control invertebrates, due to the rising number of introductions worldwide (Saul et al. 2017; Seebens 2019). Such a task often requires strategies based on prevention and prompt eradication (e.g. ballast water sanitation) (Booy et al. 2017, 2020), but these were the least frequent of the categories amongst those implying direct manipulation of the target taxa. From a theoretical viewpoint, the compiled literature raises awareness and proposes changes to current regulations as its main approach, directed at preventing further introductions at a higher level of organisation. Legislation regarding IAS has become more restrictive over the years worldwide and Spain is also immersed in this useful trend (Turbelin et al. 2017; Maceda-Veiga et al. 2019). However, in light of the number of recent introductions (Muñoz-Mas and García-Berthou 2020), it can be concluded that the real effective capability of Spain to impede the establishment of further species is limited. This general pattern is shared with other European countries and is unlikely to change substantially in the near future (Seebens et al. 2021). Tackling the establishment of further terrestrial invertebrates and microorganisms will require further and stronger innovative and well-funded preventative approaches.

The number of studies conducted in inland waters was notably high due to the enormous number of established species and their associated economic costs (e.g. Durán et al. 2012; Muñoz-Mas and García-Berthou 2020). Some of this research was promoted by the Water Framework Directive (European Parliament & Council 2000), even if IAS are not explicitly mentioned therein (Boon et al. 2020). Our keyword co-occurrence network reflected the numerous studies recommending flow management as a way to control IAS (Sabater et al. 2008; Fornaroli et al. 2020). However, despite the schemes conducted in other countries (Kiernan et al. 2012), no applied examples in large and intermediate regulated river systems were found in the literature. River basin management plans increasingly account for the presence of IAS, but more emphasis on applied management of medium-to-large river systems is necessary (Boon et al. 2020). Nonetheless, applied studies of inland water ecosystems dealt with control of the giant reed A. donax (the most managed species appearing in the compiled literature) in relatively small areas (Bruno et al. 2019) or described experience in relatively small lentic environments (i.e. common carp Cyprinus carpio in ponds and lakes; Ferreras-Romero et al. 2016) and small streams (signal crayfish Pacifastacus leniusculus; Dana et al. 2010). Unfortunately, more research is needed to optimise water allocation schemes because climate change is facilitating the establishment of further IAS. Meanwhile, the increased demands of agriculture will reduce the availability of water resources to undertake the aforementioned actions (Rahel and Olden 2008; Escribano Francés et al. 2017).

In Spain, stowaway introductions in brackish and marine environments have also gained prominence (García-Gómez et al. 2020; Painting et al. 2020), causing a shift in the type of introduced species that can be framed within the current increase in global maritime traffic (Saul et al. 2017; Seebens 2019). In this regard, Spain enacted in 2004 the International Convention for the Control and Management of Ships' Ballast Water and Sediments (BWM Convention), which has been addressed in local studies (e.g. Moreno-Andrés et al. 2017), dissertations (e.g. Bartolomé Lamarca 2014) and is highlighted in the keyword co-occurrence network. However, new IAS records are being reported frequently. For example, the gastropod Mitrella psilla was recently found thriving on western coasts of Spain (Martínez-Ortí et al. 2020). The effectiveness of measures to fulfil the BWM Convention remains limited, which underscores the difficulties faced in managing marine IAS (Thomsen et al. 2014; Cuesta et al. 2016). Indeed, the number of first records whose most probable introduction pathways are ballast waters or biofouling (Davidson et al. 2018) and the increasing importance of aquaculture and related introductions (Nunes et al. 2015; Garlock et al. 2020) suggest these introduction pathways must be taken into account. It can, therefore, be concluded that marine invasion science should move towards our central focus in the future.

Worldwide, urban environments are becoming active introduction hubs (Gaertner et al. 2017). Indeed, there are several examples of initial IAS establishment in urban zones that have spread outwards over natural environments in Spain, such as Lippia filiformis (Casasayas i Fornell 1989) or the black-headed weaver Ploceus melanocephalus (Grundy et al. 2014). Ornamental plants and alien exotic birds kept as pets are perhaps the most striking and troublesome introductions in city surroundings (Riera et al. 2021), but other less-known taxa have been found on numerous occasions (e.g. Pseudosuccinea columella (Mollusca) Martínez-Ortí 2013). Besides the introduction of ornamental plants and tortoises and terrapins (Trachemis spp.), which have already spread over natural environments and prompted specific studies (Muñoz-Mas and García-Berthou 2020), the majority of Spanish management literature on urban environments has focused on birds (e.g. the monk parakeet Myiopsitta monachus and the rose-ringed parakeet Psittacula krameri (Álvarez-Pola and Muntaner 2009; Maceda-Veiga et al. 2019; Hernández-Brito et al. 2020; Saavedra and Medina 2020). Therefore, in the light of increasing urbanisation of the Spanish population and the relatively low number of specific studies, we conclude that prevention protocols and further studies, specifically addressed to urban environments, should be strengthened to encompass the full spectrum of potential introductions.

The compiled literature on IAS management in Spain does not particularly reflect budget reductions related to the Great Recession of 2008 (Catanzaro 2018). However, IAS management literature has been under-represented compared to overall scientific production trends and the current rise in publication rates has not compensated for this historical delay. Nonetheless, our compiled references only represent a small fraction of the total number of reviewed documents (388/3796 = 10.22%). Moreover, a significant number of documents were written in Spanish (n = 158; 40.7%). This finding deserves special attention as it handicaps knowledge transfer (Di Bitetti and Ferreras 2017) and biases conclusions inferred by international agents (Konno et al. 2020). However, it is not problematic from a national point of view because texts, documents and software packages assisting the process of decision-making by administrators and functionaries have proven to be most efficient when presented in local languages (Copp et al. 2021). Nevertheless, despite the proliferation of public repositories and open access publications, a wealth of information is still hidden away, not easily accessible to risk assessors, managers and researchers through standard search engines. For example, it is known that more than a hundred plant species have been managed by Spanish regional administrations (Andreu and Vilà 2007), but only the most frequent species generated accessible documents (e.g. reports). Altogether, it indicates that public agencies produce insufficient literature (sensu lato) as they focus on other tasks. Neither communication between managers and scientists beyond undertaking management action schemes nor protocols to evaluate their success are common practices in Spain. It would be beneficial for public agencies to encourage and facilitate such interguild contact, perhaps using legislative and labour changes, to disseminate applied experience in accessible ways.

Despite these recommendations and the highlighted deficiencies, Spanish literature on IAS management should not be considered completely defective. Recent studies on alien animal species, currently thriving in Spanish inland waters, indicate that no single management protocol can be applied to every taxonomic group, due to marked differences amongst species, introduction pathways and invaded habitats (Muñoz-Mas and García-Berthou 2020). Likewise, our study shows a strong association between species, taxon or group of taxa and the features/categories used to describe the compiled literature. This indicates that species-specific studies are often needed, which highlights how difficult and complex the task of IAS management is (Woodford et al. 2016; Portela et al. 2020; Yelenik et al. 2020). Our results should help to properly drive future research efforts towards IAS management in Spain. We recommend more research into applied techniques to shift the balance between theoretical and empirical studies, especially in inland waters and marine ecosystems due to their lower accessibility. The same need for more studies applies to urban environments, as they are often the bridgehead of IAS introductions. Renewed effort in prevention and prompt eradication should be made to fulfil, for example, the BWM Convention and impede further introductions into marine ecosystems. Finally, we encourage public agencies to support and strengthen the dissemination of applied experience and thus enhance know-how and knowledge transfer in the field.

We thank Dr. Núria Roura-Pascual, an anonymous reviewer and the Editor Dr. Moritz von der Lippe for their helpful comments about the manuscript.

This research was funded by the Spanish Ministry of Science and Innovation (InvaNET network, RED2018‐102571‐T). Additional financial support was provided by the Spanish Ministry of Science and Innovation, Spanish State Research Agency (AEI) and European Regional Development Fund (FEDER, UE) (grants PID2020-118550RB, PID2019-103936GB-C21, RTI2018-093504-B-I00) and the Government of Catalonia (ref. 2017 SGR 548). RMM benefitted from a postdoctoral Juan de la Cierva Fellowship from the Spanish Ministry of Science and Innovation (FJCI-2016-30829).