Introduction

Invasive species are considered as one of the main causes of biodiversity loss (IPBES 2019; Leadley 2010; Sala et al. 2000). These are species that are introduced by humans into an area where they are not naturally present and cause damage, such as harm to local biodiversity, human development or health (Mainka and Howard 2010; Pyšek et al. 2020). In addition to the decline of native species (Bradley et al. 2019), invasive species can have very different effects, for example the reduction of the abundance of aquatic communities (Gallardo et al. 2016), alteration of food webs (David et al. 2017) or reduction of ecosystem services (Pejchar and Mooney 2009). Human life satisfaction can also be reduced by invasive species (Jones 2017).

Further, the migration and spread of invasive species is exacerbated by other environmental problems: For example, climate change (Rahel and Olden 2008; Walther et al. 2009), pollution (Crooks et al. 2011; Varó et al. 2015) and land use change (Bellard et al. 2013; Wang et al. 2016) enhance biological invasion. Estimating the annual damage caused by invasive species is a challenge due to the complexity of the issue. While it is impossible to determine the exact figure, various studies have attempted to provide quantitative estimates for different regions of the world (Pyšek et al. 2020). It is estimated that there are now more than 3500 invasive species worldwide (IPBES 2023) and the number of invasive species does not yet appear to have reached saturation point (Seebens et al. 2017). Projections suggest a potential increase of up to 36% in invasive species on different continents by 2050 (Seebens et al. 2020). In Europe, several thousand invasive species have been confirmed so far (DAISIE 2009; Keller et al. 2011), with calculations estimating annual damages at around 100 billion dollars (Haubrock et al. 2021a). Globally, the economic impact of invasive species is predicated to be between US$46.8 billion and US$162.7 billion in 2017 (Diagne et al. 2021), while other calculations estimate a $137 billion in damages in the United States alone (Pimentel et al. 2000). For Australia alone, the damage caused by invasive species is projected to be 13.6 billion dollars (Hoffmann and Broadhurst 2016). Especially in agriculture, the problem of invasive species will increase significantly in the future and lead to a productivity loss in the billions of US-Dollars (Ziska et al. 2011). Biodiversity hotspots in developing countries will be particularly affected by invasive species in the future (Early et al. 2016; Paini et al. 2016). Since not all invasive species cause the same amount of damage, they can be divided into different impact categories, also in the context of whether they cause ecological or socio-economic damage (Bacher et al. 2018; Blackburn et al. 2014).

As invasive species pose a major problem for biodiversity (IPBES 2019) and can therefore also have negative consequences for humans and ecosystem services (Jones 2017; Pejchar and Mooney 2009), their management is becoming increasingly important. Depending on the habitat and species affected, different methods can be used to control invasive species. These methods are often based on the relocation or lethal control (Giakoumi et al. 2019). The prevention of the introduction or the spread of new invasive species are also part of management (Tobin 2018). However, especially when it comes to lethal management methods, there is always a risk of social conflict, which can disrupt or prevent management measures (Crowley et al. 2017). In order to coordinate and support management measures, government agencies are developing management plans (e.g. U.S. Department of the Interior 2021) and more and more scientific studies are establishing frameworks for management (Gaertner et al. 2016; Green and Grosholz 2021).

Public awareness and perception are essential for the implementation of management efforts for invasive species: Without public support, management processes for invasive species are almost impossible to implement (Veitch and Clout 2001). Especially when charismatic species are invasive or lethal methods are to be used as a management method, public support is vital (Höbart et al. 2020). The perception of invasive species depends on a number of factors. These include the attractiveness of invasive species (Jarić et al. 2020), as well as individual factors such as the perceived distance of species and psychometric factors encompassing values, emotions, and anthropomorphic views (Straka et al. 2022). People often also see the positive sides of invasive species, for example when they provide ecosystem services. It is especially in such situations that management becomes difficult (Ngorima and Shackleton 2019) and social conflicts can arise (Crowley et al. 2019). Age and level of education also play an important role in the perception of invasive species: it has been found that especially older and more educated people perceive invasive species as a problem (Potgieter et al. 2019). In this context, it was found that even simple and brief educational measures can raise social awareness of the problem (Bravo-Vargas et al. 2019) and community awareness can make an important contribution to management (Shrestha et al. 2019).

Various studies have shown that the public has little knowledge about invasive species (Fraser 2001; Radek; Remmele and Lindemann-Matthies 2020). People are often unaware of invasive species and their impacts or even see them as beneficial (Jubase et al. 2021). Compared to other factors threatening biodiversity, invasive species tend to be underestimated (Kleespies and Dierkes 2020). The extent of the threat posed by invasive species is assessed very differently by the population (Bravo-Vargas et al. 2019). When it comes to dealing with invasive species, particular important groups are stakeholders, such as interest groups or decision makers in society, who are dealing with invasive species and their management in some form. Their perceptions are especially important because they help to shape public opinions about invasive species and are involved in management processes (Vanderhoeven et al. 2011; Vaz et al. 2020; Veitch and Clout 2001). Involving such stakeholders in the management process can make an important contribution (Shackleton et al. 2019b). However, knowledge deficits are often found in these groups as well. For example, one study revealed that even horticulture professionals have little knowledge regarding invasive species (Vanderhoeven et al. 2011). Additionally, most stakeholders and decision-makers also have problems identifying non-native species or appropriate management methods (Vaz et al. 2020). Studies on Mediterranean islands even found that stakeholders have a high tolerance for environmental changes triggered by invasive species (Bardsley and Edwards-Jones 2007).

While in recent years, the general population’s awareness and perception of invasive species has been frequently studied in different disciplines using a great methodological diversity (Kapitza et al. 2019), there are currently only a few studies in Europe investigating the perceptions and views of stakeholders on invasive species (Andreu et al. 2009; Rodríguez-Rey et al. 2022). Therefore, the aim of this study is to investigate perception and knowledge of nine different groups of stakeholders on invasive species in Germany. To this end, the following three questions need to be answered:

  1. (1)

    Can the number of invasive animal and plant species found in Germany and the economic damage caused by them be correctly estimated by the stakeholder groups?

  2. (2)

    When invasive species are to be named, which species are primarily mentioned?

  3. (3)

    Do stakeholders have an interest in invasive species and perceive invasive species as a problem?

Especially because there are sometimes significant differences in perceptions of invasive species among different stakeholder groups (García-Llorente et al. 2008), this study should provide important information about different stakeholders perceptions.

Methods

For the study, stakeholders and expert groups had to be selected in contact with the topic of invasive species. For this purpose, nine groups were selected by the team of authors (Table 1). On the one hand, groups were chosen that play an important role in the management of invasive species and, on the other hand, groups that have contact with the topic of invasive species. In addition, we selected groups organized in regional associations that were interested in helping us distribute the questionnaire. For the survey of hunters and speleologists, personal contacts with members were also used to distribute the questionnaire. Explanations of why the groups were selected can be found in Table 1. Appropriate survey instruments also had to be selected to capture the perceptions and interest of the expert groups.

Table 1 List of stakeholder groups surveyed in the study
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Survey groups and procedure

Overall, 2272 participants (72.94% male, 26.12% female, 0.13% diverse, 0.8% no answer) have responded to the survey. As the gender distribution is also a characteristic of the stakeholder groups, this distribution does not distort the results of the study. In order to obtain meaningful results, the minimum sample size per expert group was set at 50. The sample sizes and an explanation of why each group is relevant in the context of the study can be found in Table 1. Additional information on the demographic data of the individual stakeholder groups can be found in Table S1. During data collection, there was an imbalance in the sample size between the individual stakeholder groups. Due to the support of the German Hunting Association, the number of participants in the group of hunters and members in hunting associations was much higher than in the other groups. As mean values were calculated for all evaluations and percentage distributions were considered, this uneven distribution of the sample size should have no influence on the results of the study.

In order to contact the mentioned groups, the various organizations, state associations, district associations, regional offices, etc. were contacted by an informal e-mail. In a correspondence letter, the research project and the objectives were presented and a request was made to distribute the digital questionnaire among members. At the beginning of the questionnaire, reference was made to the voluntariness of participation, anonymity and data protection. The survey was carried out from February 2022 to June 2022 and was conducted in German.

Measurements

Number of invasive species and damage caused by invasive species in Germany

The stakeholder groups were asked to estimate how many invasive animal species and how many invasive plant species currently occur in Germany. In addition, the respondents were asked to estimate the economic damage caused by invasive species in Germany per year. The aim of this question was to examine whether the extent of the invasive species distribution and the damage is correctly determined.

Naming of invasive species

This question asked study participants to name three invasive species which are found in Germany. The purpose of the question was to determine which species were most present in the groups’ awareness and how many animals or plants were mentioned. If more than three invasive species were named, only the first three mentions were considered.

Perception of invasive species

To investigate whether there is an awareness of invasive species as an environmental problem in the stakeholder groups, six statements about invasive species, hereafter referred to as items, were created. As there was no existing validated instrument available for the specific research context, the items were created based on the researchers’ expertise and judgment. The items focus on whether invasive species are seen as an environmental problem and therefore should be managed or not. The questions were to be rated on a five-point Likert scale from “strongly disagree” to “strongly agree”. Half of the questions were coded negatively so that the respondents could not mark the questionnaire without reading the questions completely. The selected items can be found in Table 2.

Table 2 Factor scores of the six items to examine the perception of invasive species
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Interest in the topic of invasive species

To assess the interest of stakeholder groups in the topic of invasive species, an adapted version of the Nature Interest Scale (NIS; Kleespies et al. 2021) was used, in which the word "nature" was replaced by "invasive species" (Table 3). The scale is a validated instrument measuring interest based on the psychological construct of interest, that incorporates value-based, emotional, and cognitive interest in an subject (Krapp 1993). The items were to be rated on a five-point Likert scale from “strongly disagree” to “strongly agree”.

Table 3 Factor scores of the nine items to examine interest in invasive species adapted from the NIS by Kleespies et al. (2021)
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Analysis

For the estimation of the number of invasive animal species, plant species and the annual damage caused in Germany, the median was determined for each of the nine groups. The negatively coded items were recoded positively before calculating the mean. For the three invasive species that were to be named, each entry was assessed to determine if it was in fact an invasive species. To confirm or reject the status of an invasive species, the EU list of invasive species (No. 2022/1203) as well as the EASIN (European Commission 2023) database, Global invasive species database (GISD 2022) and published assessments of species not included in these databases were used.

All statistical analysis was executed using IBM SPSS 28. To obtain an overall interest score for each person, the mean value was calculated from the nine interest items for each respondent. The same procedure was used for the perception in invasive species items, but the negatively formulated items were reversed first. In order to verify the appropriateness of this procedure, the factor structure of the items was verified using principle component analysis (PCA) after the Bartlett test and the Kaiser–Meyer–Olkin test had confirmed the applicability of this analysis method. PCAs are often used in social sciences to reduce the dimensionality of data sets while minimizing the loss of information. (Further information on the methods: Field 2018 and Jolliffe and Cadima 2016). Cronbach’s alpha was calculated to assess the reliability of the instruments. Based on the theoretical considerations, the number of factors was set to one. The results of the factor analyses show that all items have a high factor loading ( >|.600|; Tables 2 and 3). Thus, the usability of the items as a single factor construct can be assumed. The negative factor loadings can be explained by the negative wording of the items. Even though the hunters are overrepresented by their numbers, there is no bias in the analysis because mean and median values were used.

Results

The results show both similarities and differences between the different stakeholder groups. The median of the estimated number of invasive plant species ranged from 100 to 200, and that of invasive animal species between 60 and 100. Overall, it was estimated that there are more invasive plant species than animal species (Fig. 1A and Table S2). The median of the estimated annual damage caused by invasive species in Germany was between 4 and 20 million among the groups (Fig. 1B and Table S2).

Fig. 1
figure 1

A Estimation of the number of invasive species by the expert groups (median). Green represents the number of estimated invasive plant species, red the number of estimated invasive animal species. B Estimated annual damage caused by invasive species in dollar, estimated by each expert group on a logarithmic scale (median). The yellow line represents the annual damage for Germany (160 million US-Dollar) calculated by Haubrock et al. (2021b). The surveyed stakeholder groups from top center clockwise are: animal welfare supporters, divers, speleologists, members of environmental and conservation organizations, farmers, hunters and members in hunting associations, members in allotment garden clubs, administrative employees in the regulatory authority, employees in zoological gardens

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When naming three invasive species occurring in Germany, only a few invalid answers were given in all groups (< 15%). Most invalid responses were those that referred to an entire group of species rather than a single species (e.g. frogs, wasps, beans). Other invalid responses included animals or plants that do not exist, native species, or species that do not exist here outside of zoos (e.g. lions). In most stakeholder groups, more animal than plant species were mentioned. An exception were members in allotment garden clubs and members of environmental and conservation organizations, where the number of animals and plants was almost equally distributed. In the case of hunters, employees in zoological gardens, divers and animal welfare supporters, animals were named predominantly (Fig. 2). Regardless of stakeholder group, raccoon (Procyon lotor), common raccoon dog (Nyctereutes procyonoides), nutria (Myocastor coypus), Himalayan balsam (Impatiens glandulifera), giant hogweed (Heracleum mantegazzianum), and Japanese knotweed (Polygonum cuspidatum) were frequently mentioned as invasive species (Table 4).

Fig. 2
figure 2

Balance of responses between invasive plant species (green), animal species (orange) and invalid answers (gray) mentioned by each stakeholder group in percent. The striped areas represent incorrect answers (animal or plant species that were mentioned but are not invasive). The surveyed stakeholder groups left to right are: members in allotment garden clubs, hunters and members in hunting associations, farmers, members of environmental and conservation organizations, administrative employees in the regulatory authority, speleologists, divers, animal welfare supporters, employees in zoological gardens

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Table 4 The three most commonly named invasive animal and plant species for each of the stakeholder groups
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Both the awareness that invasive species are a problem and the interest in invasive species were high in all groups. Administrative employees in the regulatory authority showed a slightly lower interest than the other groups, while hunters tend to see invasive species as more of a problem than the other groups (Fig. 3 and Table S2).

Fig. 3
figure 3

Perception of invasive species as a problem plotted against interest in invasive species. A high value represents high interest or strong perception as a problem. Each circle stands for a stakeholder group

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Discussion

The collected data and results in the nine stakeholder groups in Germany provide important insights into how these groups view the issue of invasive species.

Number of invasive species and economic damage

Having stakeholders estimate the number of invasive species indicates whether the extent of the problem is properly understood and assessed. Awareness of invasive species is considered an important factor because it directly affects whether management is considered necessary (Burt et al. 2007). The exact number of invasive species in Germany is difficult to determine. Different lists and databases provide different results. The EU list for invasive species, which is also valid in Germany, names 88 invasive species occurring in Europe according to the 2022 update (European Commission, 2022). The Global invasive species database lists 196 invasive species (101 animals, 86 plants, 9 other kingdoms) occurring in Germany (GISD 2022). Despite differences in the numbers, there is an approximate order of magnitude of the number of invasive species in Germany.

Regardless of the stakeholder group, the results of this study indicate that the estimated number of invasive species was at least in the correct order of magnitude (in the upper two or lower three-digit range). Thus, this result is consistent with a past study from New Zealand where respondents did not know the exact number of invasive species, but were at least in the correct range (Fraser 2001). The good estimates of the stakeholder groups surveyed could be due to a variety of reasons. Membership in one of the surveyed stakeholder groups puts the respondents in direct contact with the topic of invasive species. Past studies have shown that proximity to the topic or being personally affected by invasive species leads to individuals being aware and motivated for the topic (Schüttler et al. 2011; Verbrugge et al. 2013). However, it’s important to acknowledge the limitations associated with these estimates. For instance, knowing the number of invasive species alone does not provide information about their distribution within Germany, which may differ regionally. This limitation could be addressed in future studies by conducting more localized surveys or incorporating geospatial data.

Similar to the number of invasive species, the economic damage caused by invasive species is also a significant factor influencing the management processes (Fraser et al. 2014; Larson et al. 2011). Similar to their numbers, the annual damage caused by invasive species is difficult to determine. An estimate conducted for Germany suggests approximately 160 million US-Dollar per year (Haubrock et al. 2021b). Here, the annual damage due to invasive species was significantly underestimated by all stakeholder groups. This result is also consistent with a previous study, which also found the damage done by invasive species is often underestimated (Waliczek et al. 2017). One possible explanation for this could be that invasive species are not only associated with economic or environmental problems, but also perceived positively in the population, especially if they are attractive or bring value in the form of ecosystem services (Dickie et al. 2014; Jubase et al. 2021). In addition, the economic damage caused by invasive species is not always directly visible. Therefore, this is why the extent of damage is not perceived and thus underestimated. It is also possible that the study participants only took into account the direct economic damage and not the management costs, which also make up part of the total economic damage.

Another reason for the significantly undervalued damage could be that large numbers and their relations to each other are often misjudged: First, a million is often perceived as a half way point between a thousand and a billion, or scale words such as thousand or million often serve as categories for large values rather than denoting exact sizes (Landy et al. 2013, 2017). Second, particularly large values are often underestimated (Izard and Dehaene 2008).

Regularly mentioned invasive species

When invasive species are named, there are only a few invalid or incorrect responses in each group (< 12%). This indicates that at least some invasive species are present in the stakeholders’ knowledge and that the study participants were generally able to name at least three invasive species correctly. In almost all groups, significantly more animals were named than plants. This result could be due to the concept of "plant blindness" (Wandersee and Schussler 1999). This term describes the phenomenon that individuals are less interested in plants than animals, and plants are more likely to be overlooked (Jose et al. 2019). For example, although there is a greater amount of invasive plant species estimated, animal species are listed for the most part. Interestingly, the number of studies dealing with invasive plants also predominates (Kapitza et al. 2019). An exception are the members in allotment garden clubs and members of environmental and conservation organizations, where the number of plants mentioned was higher or animals and plants almost equally distributed. The reason for this could be the focus of their work: Garden clubs focus on plants, so invasive plants play an important role in their work. Environmental and conservation organizations focus on the protection of ecosystems, which are also threatened by invasive plants.

On the contrary, a particularly high proportion of animals were mentioned by hunters, farmers, divers, and employees in zoological gardens, who prioritize animals: Animal rights activists advocate for the protection of animals, diving is primarily focused on observing animals in their habitat, zoo employees mostly manage animals, and hunters are usually confronted with the consequences and management of invasive animal species.

For the invasive species mentioned by the participants, there is a clear overlap between the expert groups for both plants and animals. Large mammals such as the raccoon (Procyon lotor), the nutria (Myocastor coypus) or the raccoon dog (Nyctereutes procyonoides) are particularly often mentioned as invasive animal species. Other vertebrate classes or other taxa are named much less often. It is proven that animals that are larger and whose appearance is more similar to humans are preferred (Gunnthorsdottir 2001). Particularly charismatic species that are perceived as beautiful or attractive and are portrayed in a positive light in the media are more likely to be accepted by society (Jarić et al. 2020). Especially for mammals more empathy and compassion is felt (Miralles et al. 2019) and they are perceived as particularly attractive and interesting (Landová et al. 2018; Moss and Esson 2010). This may result in larger mammals among the invasive species being noticed and remembered. Invasive carnivores in particular are widespread in the survey area and are especially conspicuous in their appearance (Peter et al. 2023). It is therefore understandable that the stakeholder groups are aware of these species in particular.

In almost all stakeholder groups, the most frequently mentioned plants were Himalayan balsam (Impatiens glandulifera), Japanese knotweed (Polygonum cuspidatum) and the giant hogweed (Heracleum mantegazzianum). All three species have distinctive characteristics that may be one reason these invasive plants are well known and therefore often named regardless of stakeholder group. Heracleum mantegazzianum has a whole range of defense mechanisms. In particular, the UV-dependent toxins that can cause photo dermatitis present a risk to the public (Hattendorf et al. 2007). Because of this negative impact, Heracleum mantegazzianum is well known and present in people’s knowledge. Impatiens glandulifera on the other hand originated in the Himalayas and was brought to Europe in the seventeenth century. Because of its appearance, it has long been a desirable addition to gardens (Cockel and Tanner 2012) and because of its conspicuous appearance and its rapid growth rate it is unlikely to be overlooked (Pyšek and Prach 1995). Therefore, it is likely that this species is known to many people because of its aesthetics and spread. Polygonum cuspidatum is a perennial plant up to three meters tall (Barney et al. 2006). Daily growth can be up to 30 cm a day during the main growing season. This plant was also introduced in the seventeenth century as an ornamental and forage plant (Böhmer et al. 2006). Due to its size, distribution, and rapid growth, it is understandable that Polygonum cuspidatum is often recognized as an invasive species, regardless of stakeholder group.

Additionally, the attention that various species receive in the media could also be a reason for the increased mentions. A search in google news shows 26,300 hits for "Waschbär" [Procyon lotor] in Germany, while for example there were only 559 hits for "Drüsiges Springkraut" [Impatiens glandulifera] (Google 2023).

Interest and perception of invasive species

A positive picture emerges for interest in invasive species and perception of invasive species. Interest in a subject is an important factor that is closely related to learning and understanding of a topic (Schiefele and Schreyer 1994). For instance, interest leads to a more focused performance of tasks (Renninger and Hidi 2002) and more successful learning about a topic (Ainley et al. 2002). Therefore, interest in invasive species can help acquire this necessary knowledge. Knowledge and awareness are important influencing factors when dealing with invasive species and their management process (Bravo-Vargas et al. 2019; Cole et al. 2016; Cordeiro et al. 2020; Höbart et al. 2020; Sosa et al. 2021). In our study, all stakeholder groups demonstrated a high level of interest in the topic of invasive species. This clearly distinguishes the stakeholder groups from the general population, where a previous study found that a good third were not interested in invasive species at all (Jubase et al. 2021). Only administrative employees in the regulatory authority showed slightly less interest, but this can be explained by the regulatory authority’s focus. The authority’s tasks cover a very broad spectrum to ensure public order and the reporting of invasive species is therefore only a small part of these tasks. As a result, not every employee actually deals with invasive species on a regular basis.

Invasive species are also considered an environmental problem in all groups. These results are consistent with previous studies. Although individuals in the general population tend to care less or only partially about invasive species and are less likely to perceive them as an issue (Verbrugge et al. 2013), stakeholder groups, on the other hand, are generally aware of their responsibilities and also perceive them as a problem (Bardsley and Edwards-Jones 2007; Burt et al. 2007; Colton and Alpert 1998; Kapler et al. 2012).

Implications

Stakeholder groups play a crucial role in invasive species management and the results of the study provide important evidence how these critical groups perceive invasive species. While the estimate of the number of invasive species had decent accuracy, the economic damage caused by these species was significantly underestimated in all groups. Because the impacts caused by invasive species are an important factor in how the issue is perceived (Shackleton et al. 2019a; Veitch and Clout 2001), more should be done to educate key stakeholder groups about the potential consequences of invasive species. For example, educational programs addressing invasive species and their consequences could be provided to stakeholder groups. These programs need to be designed to meet the needs of each stakeholder group. For instance, invasive terrestrial plants could be a particular focus for members in allotment garden clubs, while for divers the focus should be on aquatic invasive species. Other educational initiatives such as citizen science projects could also make a positive contribution. Stakeholders themselves could participate in monitoring or data collection on invasive species. This would help them to learn more about the problems and the damage caused by invasive species. For this purpose, such programs must be specifically developed and also made accessible to stakeholders.

In the case of the mentioned invasive species, especially conspicuous animal species were mostly named, while smaller inconspicuous species and plants were mentioned less often. To overcome this focus on charismatic invasive plants and animals, awareness campaigns and education programs should highlight the ecological and economic impacts of inconspicuous and smaller species. Information material could be provided to the stakeholder groups that also educates them about less conspicuous or small invasive species.

Interest in and awareness of invasive species as a problem was high among all groups. This result shows that the stakeholder groups recognize the problem, and through their interest are probably also open to receive more knowledge on the topic. Therefore, education in all different forms like seminars, courses or in online formats can be seen as a potential opportunity to improve stakeholder perceptions. However, it is important that educational programs are tailored to the target group. It is possible, for example, that arguments that work for hunters may have less of an impact on other groups such as animal rights activists. Social conflicts that can arise as a result of the management of invasive species in this context must also be taken into account (Crowley et al. 2017).

Conclusion

Stakeholder groups play an important role in the management of invasive species. The views and perceptions of invasive species by stakeholder groups are therefore an important topic that is still in urgent need of research. This study investigated how different stakeholder groups in Germany assess the spread and damage of invasive species and which invasive species are particularly present in their perception. It also investigated whether there is interest in the topic and how invasive species are perceived.

The results reveal both common views and nuanced differences between the groups. The number of invasive animal and plant species occurring in Germany was estimated to be in the right order of magnitude by all stakeholder groups. However, the underestimation of economic damage indicates a potential awareness gap, highlighting the need for education programs that should be customized to the needs of stakeholders. When invasive species were named, particularly conspicuous and charismatic species were mentioned. Species with special characteristics were also mentioned particularly often, while smaller and inconspicuous species were rarely referred to. The high level of interest and perception of invasive species as an environmental problem across all stakeholder groups shows that the topic is important to them and that invasive species are perceived as a problem that requires action.

The results make an important contribution to the current scientific discourse. Despite the contribution, however, further research is needed, especially on the perception of stakeholders at an international and global level.