Although only a small proportion of introduced species become naturalized or invasive and have a measurable impact (Lockwood et al. 2013; but see Ricciardi et al. 2013), biological invasions by alien species (introduced to regions outside their native distribution range due to human activities; Richardson et al. 2000, Blackburn et al. 2011) affect the majority of habitats, including semi-natural ones. Invasive alien species (IAS), with their widely documented impacts on biodiversity, ecosystem functioning and economy (Pyšek and Richardson 2010, Vilà et al. 2010, 2011, Pyšek et al. 2012c, Scalera et al. 2012, Follak et al. 2013, Blackburn et al. 2014, Jeschke et al. 2014) are recognized as one of the key components of global environmental change (MEA 2005). Costs due to IAS were estimated to reach up to 5% of global GDP (Pimentel et al. 2001, 2002). In Europe, recent estimates of direct costs due to IAS reach at least 12.7 billion € per year (Kettunen et al. 2009). It is also important to note that direct environmental and eradication costs associated with environmental weeds or pests are only a small fraction of costs caused to agriculture or forestry. Nevertheless, even these figures on overall costs for environmental weeds and pests illustrate the need for an urgent policy response at all scales, from national to international and global, supported by a corresponding scientific knowledge base; the fact that the majority of alien species are introduced intentionally or in association with imported/transported commodities (Hulme et al. 2008, 2009) provides an opportunity for interventions (Roques and Auger-Rozenberg 2006, Dehnen-Schmutz et al. 2007, Kenis et al. 2007).

In Europe, more than 12,000 alien plant and animal species are recorded (DAISIE 2009, www.europe-aliens.org) and the numbers of successfully establishing species continue to grow (Hulme et al. 2009, van Kleunen et al. 2015). Unfortunately, research, legislation, and management of IAS are not fully coordinated, neither within individual countries, nor continentally (Hulme et al. 2009), which leads individual countries to cope with alien species in different ways. The most common approach is based on producing lists of prominent IAS that receive much attention and are prioritized in terms of prevention, monitoring and management. These so-called Black, Grey and Watch (alert) Lists represent a convenient starting point for setting such priorities (European Commission 2014). The necessary condition for making such lists trustworthy is, however, a robust and transparent risk assessment, based on the impacts of individual species, allowing their scientifically defensible selection (Wittenberg and Cock 2001, Verbrugge et al. 2012, Lewis and Porter 2014). The transparency is important so as to accommodate interests and perception of impacts by groups of concerned authorities and stakeholders representing sectors as diverse as, e.g. forestry, horticulture, aquaculture, hunting and nature conservation, and to justify possible trade restrictions (Bayliss et al. 2013, Kelly et al. 2013, Ööpik et al. 2013). Therefore, when developing regional Black Lists, interests that differ among the above-mentioned sectors need to be taken into account. Many intentionally imported alien species are of a high economic value (DiTomaso et al. 2010, Richardson and Rejmánek 2011, Woziwoda et al. 2014), but can have negative impacts on native populations, species and communities due to a wide range of mechanisms and processes that have been described in the literature in the last decade (e.g. Levine et al. 2003, Gaertner et al. 2009, 2011, Mitchell et al. 2010, Pyšek and Richardson 2010, Vilà et al. 2010, 2011, Dodet and Collet 2012, Pyšek et al. 2012c, Scalera et al. 2012, Blackburn et al. 2014, Jeschke et al. 2014). However, although these processes are becoming reasonably well understood, there is still much uncertainty about which particular species will have an impact in specific environmental settings and how the invaded habitats and ecosystems will be impacted (Leung et al. 2012, Blackburn et al. 2014). Ideally, each intentional introduction of a new alien species should be thus preceded by a cost-benefit analysis of negative vs. positive effects on both the environment and socioeconomy (Keller and Drake 2009). The decision should then reflect the climatic and habitat match between the current range of the species and the region to which it is proposed for import, as well as information about previous invasion history and life history traits of the species itself, or its close relatives (Kolar and Lodge 2001, Keller and Springborn 2014).

The urgent need to tackle biological invasions, develop a common policy and establish an early warning system in Europe, has been recognized by the European Commission (see the Communication ‘Towards an EU Strategy on Invasive Species‘, (COM (2008) 789 final) and EU Biodiversity Strategy to 2020 (http://ec.europa.eu/environment/nature/biodiversity/comm2006/2020.htm). Part of this activity is aimed at the new EU Regulation on IAS COM (2013) 620 (European Commission 2014), which is an important legislation on invasive species threatening biodiversity and human well-being (Genovesi et al. 2015). Besides setting a framework for roles and responsibilities among the different bodies dealing with IAS it will include a list of species that pose the most significant threats (list of alien species of the Union concern) and thus should be prohibited from the import, sale, and use in Europe. This list will be prepared by the European Commission on the basis of the criteria set out in the Regulation; the EU member states participate in the process of the preparation of the list (by providing comments and proposals for individual IAS inclusion). Although national Black Lists may play an important role in the process of preparation of the EU list, so far only a few countries have developed their own Black Lists with some legislative support (Essl et al. 2011).

The development of national and regional Black Lists and identification of important species, based on using standard and transparent criteria, is a key aspect of the early warning and information systems. Some European countries or trade sectors (agriculture, aquaculture) already regulate the introduction and transport of selected species, based on risk assessments provided by the European Plant Protection Organisation (EPPO), European Food Safety Authority (EFSA) and UK Department for Environment, Food and Rural Affairs (DEFRA). An example of a working system is international cooperation in the field of agriculture pests (EPPO, DEFRA) which can serve as a template to be followed for the management of IAS in Europe in general (Brunel et al. 2013). Not only legislative tools are affecting the policy on IAS. To prevent the spread of alien species and restrict their trading, a significant component of policy and public involvement are voluntary codes of conduct developed for example for horticulture or sheltered under the Bern Convention (Heywood and Brunel 2011, Caffrey et al. 2014, Halford et al. 2014, Heywood 2014).

In the Czech Republic (78,866 km², 10.5 millions of inhabitants), as in many other European countries, there is an elaborate and legislatively well-anchored system of the approach to harmful organisms in agriculture. In the field of nature conservation, legislation is not sufficient and does not adequately respond to the current threats from biological invasions, but the issue of IAS has become in the last years one of the priorities in the Czech national strategic environmental documents (State Environmental Policy 2012–20, State Programme of Nature and Landscape Conservation 2009, Biodiversity Strategy 2005). These documents emphasize the need to focus on IAS, including development of priority lists of species for management, creating financial tools and preparation of new legislation, which will be encouraged by the adoption of the new IAS EU legislation.

Despite significant progress in producing lists of important alien species for individual countries (see review in Essl et al. 2011), a standard methodology for the complex assessment of their impacts only started to appear recently (e.g. Blackburn et al. 2014). Such a framework needs to be accompanied by a close cooperation between policy makers, researchers and practitioners in nature/biodiversity conservation and IAS management, to allow for harmonization of the information flow on IAS (Ricciardi et al. 2000, Kettunen et al. 2009, Shine et al. 2009, Caffrey et al. 2014).

Species with documented strong negative impacts, that threaten ecosystems, habitats or native biota, should be eradicated from the newly invaded sites as fast as possible, and further introductions of such species avoided (Convention on Biological Diversity 1992, Genovesi 2005). However, if resources are limited, the question remains which species, which locations and how (considering feasibility and control methods) should be targeted first, and this prioritization can be addressed by different methods (Humair et al. 2014).

The criteria for placing individual species into particular Black List categories need to be general enough to accommodate differences among various taxonomic groups (plants, invertebrates, vertebrates) and invaded environments (e.g. terrestrial, aquatic; urban, suburban, seminatural), take into account invasion dynamics, the environmental and socio-economic impact they pose and management strategy suitable for each particular invader. The existing Black Lists do not take differences between invaded habitats and management feasibility into account in their assessment, do not cover socio-economic impacts and are restricted to selected taxonomic groups (Essl et al. 2011). Some of the existing impact assessments, serving as a basis for Black Lists, multiply the impact scores by a given species’ population status (Gederaas et al. 2012, http://ias.biodiversity.be) but as far as we know, there is no system that incorporates information on the type of invaded habitat and management feasibility into the Black List classification.

In the Czech Republic, there is a thorough knowledge of biological invasions that has resulted in publications of comprehensive and updated lists of alien plants and animals (Pyšek et al. 2002, 2012b, Šefrová and Laštůvka 2005) with an indication of their invasion status using commonly accepted classification (Richardson et al. 2000, Pyšek et al. 2004, Blackburn et al. 2011). However, the classification of alien species based on management criteria has not been available up to now. Still, for any management planning, setting the priorities among species and habitats is crucial. In this paper we thus combine information on the potential environmental impact of alien species in the Czech Republic, their current or predicted population status, the feasibility of management, and type of invaded habitats. As a synthesis, we present Black, Grey and Watch Lists of alien species for the country, with recommended categorized management measures for land managers, policy makers and other stakeholders.

The proposed Black and Grey Lists of alien species in the Czech Republic are based primarily on the existing inventories of plant (Pyšek et al. 2012b) and animal (Šefrová and Laštůvka 2005) alien species. The data from these lists were amended by recent updates of the alien biota in the Czech Republic for particular groups such as fishes (Musil et al. 2010), national museum collections or unpublished records (personal communications and databases). The Watch List of alien species includes those currently not present in the wild in the Czech Republic and occurring there only in captivity or cultivation, but reported from the wild in other European countries with similar climate and habitats. Existing lists of aliens in these comparable countries, as summarized in e.g DAISIE or Nobanis, were thus screened to generate the Watch List for the Czech Republic.

To minimize the possible subjective bias of experts assessing species on original lists, each species was reassessed according to the current state of its population status, invaded habitats, cultivation and farming history, impact on environment (ecology) and socio-economy and with respect to the knowledge of its effective management. The species sharing similar patterns of classification were then grouped into subgroups of Black and Grey Lists (see details below). Species included in Black Lists were those posing significant strong negative effects on the environment and where some management, if available and feasible, should be applied. Grey List was used for species with limited negative environmental impact, where monitoring and local management is also relevant. Species for Watch List were selected from those that may in the near future colonize the territory Czech Republic and whose monitoring and management, due to possible substantial negative environmental impact, is recommended.

The evaluation of alien species occurring in the Czech Republic was done for vascular plants, vertebrates and most invertebrate groups. As the classification of alien plant species in the Czech Republic is more elaborated than that of animals, in terms of their regional population dynamics or abundances (Pyšek et al. 2012a, b), the criteria for the Black List species’ assessment were first developed for plants and then adapted for other taxonomic groups.

For each species included in the Black, Grey and Watch List based on the above criteria, the following information on their populations was assessed, if available, and used to classify species.

A. Mode of current spread:

1. Plants and animals that are intentionally released into the environment for landscaping, restoration or hunting (the ‘release’ pathway according to Hulme et al. 2008) and distribution of the species is highly dependent on human activities. Without presence of humans activities the species will disappear in relatively short time.

2. Current spread is mostly spontaneous without direct contribution of humans. For this category it is not crucial if the initial occurrences resulted from past human activities (abandoned plantations, populations of animals escaped from cultures, contaminants) or results of spontaneous spread from other areas where they are alien. Without presence of human activities the species will remain in the landscape for relatively long time.

3. Combination of release and spontaneous spread.

B. Distribution:

Current distribution regardless of whether the species occurs as a result of release or spontaneous introduction. This categorization does not take into account abundance of the species. Both groups can be represented by dense or sparse populations. Especially in case of regionally widespread species, which are present in numerous, well established and continuously replenished populations, their local management cannot be usually efficient. However, in some cases local management may still be performed to reduce specific impacts, e.g. local and time-restricted trapping of Neovison vison (American mink) before the bird breeding season.

1. Regional: Present distribution of the species at a large scale or future expasion not strongly restricted by environmental constraints is expected. Clusters of local populations dispersed across country exchanging individuals due to the transport of propagules or active migration.

2. Local (isolated populations): current and also future distribution in localized area(s) within the Czech Republic. The distribution can be limited by e.g. climate or habitat specificity. The localized distribution makes management efficient if there are effective methods available.

C. Evaluation of environmental impact

Standardized assessment of environmental and socio-economic impact is not available for all alien species in the Czech Republic. Therefore it was assessed using the simplified rationale of GISS (Nentwig et al. 2010, Kumschick et al. 2012, Vaes-Petignat and Nentwig 2014) and the recently suggested unified classification of alien species based on the magnitude of their impacts (Blackburn et al. 2014). The black listing in this study is based primarily on the environmental impact of populations occurring in the outdoor environment, and excludes e.g. alien species only having significant economic impact as storage pests. Due to the lack of direct knowledge on impacts of many species in the Czech Republic, their impact was classified as “potential impact”, taking into account any impact of the given species reported from climatically similar regions, and also considering interactions with, or impact of, ecologically similar species. The impact was classified based on expert judgement into three levels ranging from limited (minimal) to moderate and massive, with respect to whether it results in irreversible negative changes to native populations, species or ecosystems (e.g. due to predation, competition, hybridization, ecosystem functioning). For impact assessment we used data from Kumschick et al. (2015), and Rumlerová et al. (unpublished).

D. Evaluation of socio-economic impact

Socio-economic impact and impact on humans was additionally assessed for taxa with considerable environmental impacts to support final reasoning of recommended management. The weight of socio-economic impact was used and ranked high in case of species like Ambrosia artemisiifolia (common ragweed), Heracleum mantegazzianum (giant hogweed), where strong negative impact on human health is significant or Arion vulgaris (Lusitanian slug), and Varroa destructor (varroa mite), which have direct effect on agriculture. The impact was classified based on expert judgement into three levels ranging from minimal to moderate (most weeds and pests) and massive.

E. Management options

Management options were assessed along axes representing the management itself, the context of invaded habitats, and population status. The species were classified according to the applicable management strategy (see details below and in Table 1).

Table 1.Download as CSV 

List of selected management options (detailed classification) applied to alien species.

Management option	Description	Recommendation
Tolerance/resignation	This approach is relevant in many ecosystems/sectors (forestry, fishery) for several reasons. Many alien species occurring now in the landscape are of a high economic importance. This approach is also relevant for large populations of widespread alien species especially in urban and suburban environments. Direct eradication of such species is almost impossible or associated with enormous costs and likely to bring doubtful results.	Tolerance is applicable in several cases. In some urban and suburban areas we recommend to tolerate the species of a high economic value as well as species eradication of which is almost impossible because of their wide distribution. This tolerance should exclude areas of high conservation value where approaches including local eradication with subsequent change of local management can be applied. Tolerance cannot be used in rural landscape where primary aim is to prevent new alien populations from establishing. We recommend to tolerate e.g. large populations formed as a result of old abandoned plantations (e.g. Robinia pseudoacacia) or release (crayfish, white-tailed deer).
Eradication	Complete eradication of alien species at national scale. It is usually demanding in terms of financial, time and human labour resources, and would require transboundary coordination in case of species present also in neighbouring countries.	Complete eradication should be used primarily for small and pioneer populations where rapid response is likely to result in successful action. It is also applicable to small populations of relatively large animals where hunting or other effective control is feasible. Eradication is not recommended in urban and suburban environment where it usually fails for several reasons (public opinion, high propagule pressure). The complete eradication of several species currently posing strong negative socio-economic impact can be reasoned.
Containment	Local eradication or suppression of alien species’ populations. Depending on infested area and habitat type, the costs can vary. Repeated and continuous management is necessary to meet the goals.	Containment is recommended only for sites with high conservation priorities or to lower the negative impact of selected alien species. Due to high costs and need to repeat the actions regularly it is not recommended in large areas, or urban and suburban environment. Containment can be used to reduce e.g. the propagule pressure.
Removal of populations from abandoned plantations and farming facilities	Removal of populations after cessation of their planting or farming, especially related to biofuel plants and animals bred in cages, fishponds or forest enclosures.	Complete eradication of the populations at local scale is recommended, as there is a high risk of escape into natural environment following the abandonment.
Prevention of spread to (semi-)natural environment	This management option refers mainly to revegetation activities in suburban zones (along road and railway corridors) and to species released for forestry, game hunting or fishery.	This option should be used in most cases to avoid conflicts of nature conservation with forestry, landscaping, agriculture and hunting. If a release of a species into the wild is considered, preference should be given to native or locally native taxa. Examples are e.g. brown vs rainbow trout, or red vs sika deer.
Change of management	Change of management is a widely used method applicable to a wide range of habitats. In rural landscapes such a recommended management (preferred by nature conservation) is similar to the traditional management (regular mowing, removal of shrubs, grazing). This management option includes also hunting and fishery practices.	In case of plants, change of the current management should be used to reduce the cover and therefore impact of local dominants. Important condition is that the management has to be permanent and resulting ecosystem must be of higher natural quality than the previous one. Change of management is relevant for a wide range of stakeholders including forestry, game hunting and fishery.

Complete eradication is hardly feasible in the Czech Republic, an inland state surrounded by other countries, and can be only achieved, if at all, by intensive international cooperation followed by continuous sanitary measurements. Although complete eradication is usually feasible only on islands (e.g. Chapuis et al. 2004, Genovesi 2005, Simberloff et al. 2011), in some cases it is an ideal target to which efforts should be directed. In practice, complete eradication is possible only for populations of alien species that do not yet spread. For large infestations consisting of many metapopulations, complete eradication above some threshold is almost impossible due to enormous costs (Rejmánek and Pitcairn 2002, Pluess et al. 2012a, b). High cost of management can be justified only for newly detected occurrences of highly important alien species. Unfortunately, intentions behind eradication attempts are often led by wrong ideas to restore ecosystems to their “historical” state, which is often idealized. Eradication is sometimes initiated by the local public or little-informed conservation activists, and often is accompanied by damages to native communities.

Tolerance (resignation) means to refrain from any systematic attempts to manage the given alien species; although both lead to the same result, reasons for them are fundamentally different: tolerance is result of a decision based on the fact that the given IAS has a low impact, while resignation is an enforced attitude if there are no existing management options. The latter currently happens in e.g. mine disposal sites in northern Bohemia, where management is passive approach, and eradication efforts focused on a few selected plant species and habitats. Many newly introduced plants continuously spread as a result of restoration of brownfields and landscaping (Kabrna et al. 2014). Similarly, for some insects, e.g. Harmonia axyridis (harlequin ladybird), any management action is almost impossible. At present we are unable to stop the invasion of such species, let alone eradicate them completely.

Stratified approach reflects the local/regional context of the invasions and therefore represents, in the vast majority of cases, the optimal strategy. An example is the management of Robinia pseudoacacia (black locust) in the Czech Republic, whose planting can be allowed in areas where the stands do not represent an imminent threat to the landscape, but should be prohibited, and extant stands eradicated, from sites with nature conservation needs, such as in and around steppe habitats. Similarly, some economically important alien fish species are tolerated in aquaculture ponds (many of which are localities of high conservation value, and even listed among protected nature reserves and Natura 2000 sites), but in other localities might be subject to management. For example, the native Salmo trutta (brown trout) should be preferred over alien salmonid fish, such as Oncorhynchus mykiss (rainbow trout), in stream habitats, but alien fish species are less likely to pose a conservation problem in ponds used for recreational fishing. The stratified approach thus discriminates where and when the management of alien species is needed and efficient, and where the eradication is neither effective, nor necessary (e.g. in urban and suburban areas). The stratified management limits counterproductive and useless actions against alien species and places them into the framework of nature protection and traditional land use management.

There are in total 1454 alien vascular plant species recorded in the Czech Republic (36.6% of the total flora; Pyšek et al. 2012a, b), however, the vast majority of them do not have a measurable impact. This group of “low impact species” consist of species that (i) are unable to reproduce or develop viable populations outside cultivation (casuals); (ii) are naturalized but have not expanded their range for a long time, or even failed to persist and became rare (e.g. Agrostemma githago, common corn-cockle) and (iii) are locally naturalized, having potentially negative impact (e.g. Celastrus orbiculatus, oriental bittersweet), but their sparse distribution still makes management feasible. Within the last group belong species which are candidates for priority monitoring (e.g. biofuel plants like Paulownia tomentosa, princess tree). Alien plant species with potentially high risk of environmental and potential negative socio-economic impact thus recruit from naturalized species starting to spread (85 species), or species with continuing spread (61 species).

The assessment of fauna was based on several sources providing an overview of alien animal species occurring in the Czech Republic: 662 species from the DAISIE database (Pergl et al. 2012), 595 species from the catalogue of alien animal species (Šefrová and Laštůvka 2005), and 490 species from the list of alien terrestrial insects occurring in indoor and outdoor environments (Šefrová 2005 and unpublished database of Šefrová et al.). This screening resulted in a total of 680 alien animal species, the majority of which are terrestrial insects (490), followed by other terrestrial and aquatic invertebrates (110) and vertebrates (80). Of the alien terrestrial insects, 249 are known to be restricted to indoor spaces where stable temperature allows them to shelter from harsh winter conditions outside, and the same holds for the majority of arachnids and gastropods. These species, unable to escape into the outdoor environment, were thus not included in the assessment for the Black List. As a result, we identified 184 animal species that occur outdoors and have (or potentially may have) an environmental impact.

There are 102 established (naturalized) but not invasive insect species that have not spread significantly or had already spread in the past and now are considered as a part of resident communities. Among the invasive insects, seven species have an impact on native insects and 41 can be classified also as pests in agriculture, forestry or horticulture. Of these, 28 species cause significant losses to the economy and are therefore permanently monitored and managed; monetary value of the damage to the environment, if at all possible to estimate based on current knowledge, is by an order of magnitude lower than that to economy.

In the list, we retained two invertebrate species known to have more devastating effect on agriculture than on biodiversity, Arion vulgaris (Lusitanian slug) and Varroa destructor (varroa mite), which potentially can also have a strong environmental impact. Arion vulgaris is generally widespread and may influence also natural communities by herbivory and competition with native gastropods; the environmental impacts of V. destructor are indirect, through its potential effect on the pollination by honeybees. In aquatic environments, the proportion of invertebrates with possible impact on native species or ecosystems is relatively high, with representatives from macrozoobenthic molluscs, such as Dreissena polymorpha (zebra mussel), or crustaceans, such as the amphipod Dikerogammarus villosus (killer shrimp), or invasive crayfish (Orconectes limosus, spiny-cheek crayfish; Pacifastacus leniusculus, signal crayfish).

Alien vertebrates are the smallest group in terms of species number, but host the highest proportion of species causing ecological impacts. There are marked differences among vertebrate groups. There is no alien bird with negative ecological impact in the Czech Republic, and only one reptile (Trachemys scripta, pond slider), which so far does not seem to be able to reproduce in the wild under the local climatic conditions. In contrast, fish and mammals with well documented or potential impact are quite common. Several of these fish (~10 spp.) and mammals (~15 spp.) are already widely distributed in the Czech Republic, and their complete eradication is not feasible. However, local/regional eradication or suppression by management action may be possible. It is therefore important to reduce new introductions and releases and strictly control the vicinity of farming and breeding facilities (e.g. deer parks, fishponds) to prevent or at least diminish escapes into nature.

The groups of alien species classified within the Black (BL1–3) and Grey Lists are characterized mainly by level of impact, type of spread (affecting the management and regulation). Species with high environmental and high socio-economic impact are in BL1. Species with high or medium environmental impact and almost negligible socio-economic impact are then classified according prevailing mode of their spread (BL2, BL3). Species, the environmental impact of which is limited at present, are included in the Grey List (GL). The detailed description of the groups is following:

Species group BL1: Species with the greatest impact and with the strongest regulations recommended/needed; their populations should be managed whenever possible although they are already present in large numbers in the Czech Republic and their complete eradication is not feasible. Whenever feasible, it is important to limit further spread of these species; for species where efficient management strategy is not available at present, research that may provide management options is warranted. The group includes two plant and three animal taxa. Plants listed in these category are rapidly spreading neophytes, an annual Ambrosia artemisiifolia (common ragweed) and monocarpic perennial Heracleum mantegazzianum (giant hogweed), having strong impacts on native biodiversity and/or posing direct threats to human health (allergy and photodermatitis) (Nielsen et al. 2005, Hejda et al. 2009, Pyšek et al. 2012a). Animal taxa comprise heterogeneous group of species which include Varroa destructor, a mite affecting bees, and two mammal species (Neovison vison, American mink; Procyon lotor, racoon). As Varroa has also significant socio-economic impact and is restricted to honey bee colonies, its distribution is monitored and management is already driven by state authorities.

Species group BL2: Species depending highly on human actions that promote their spread (mostly combination of release and spontaneous spread), both types of distribution, and mostly with moderate to massive environmental impact, but minimal socio-economic impact; 49 plant and 8 animal taxa. These species are often found as remnants of planting in gardens and plantations or in case of animals introduced for hunting and fishing, which facilitates their further spread. Instead of economically important species, alternative native species should be promoted. If necessary for economic activities in areas with low conservation value, keeping in capture could be permitted, with prerequisite of good prevention of escape, and removal of the captive population once the economic activity has ceased. Spontaneous populations outside urban areas or areas of captivity should be reduced by change of local management, or by local eradication campaigns when feasible. Specific focus should be on areas with high conservation value.

Species group BL3: Species whose current distribution results from spontaneous spread and unintentional introductions. They cover species with both types of distribution and impact ranging from limited to massive (Appendix). The recommended strategy for these species is stratified approach balancing between the local needs and the available resources for eradication. As none of the species is planted or released intentionally, the management and trade regulations can be more straightforward than in BL2. If locally necessary and there are known efficient eradication methods for the given species, eradication should be attempted. In urban and suburban environments species can be tolerated, but eradication or suppression by change of local management (land use) is recommended.

Species group GL: Species with limited environmental impact at present, distributed both regionally and locally, and with current distribution as a results of spontaneous or combined spread. For the listed species outside areas of a high conservation value there is no need to take actions against them, or restrict them. Change in management may be actively taken into account to reduce their distribution. This group consists of 47 plants and 16 animals, and is substantially formed of several weedy plant species and parasites.

The Watch List (Appendix) contains selected high-impact species that (1) have not yet been recorded from the Czech Republic but occur in other European countries with similar climatic conditions and habitats (and thus may be successfully introduced to or invade the Czech territory), (2) species that are at present kept in culture or enclosures only (such as Capra aegagrus, wild goat, or Bison bison, American bison), or (3) species introduced in the past but without presently known wild populations, which may be considered potential competitors for native species (several fish species). In case of plants this is analogous to species already present in e.g. gardens, parks or aquaculture (e.g. Azolla filiculoides, Pacific mosquitofern; Paulownia tomentosa, princess tree) which may in the future establish in the wild and became problematic. There are 25 plant and 27 animal taxa on the Watch List. For these species, as well as for some sparsely distributed species from the Black or Grey Lists, preventive actions against their introduction to and subsequent spread in the country, or uninvaded regions, are justified.