To counteract misleading signal-effects for management decisions, we established a mechanism in the assessment procedure to ensure that all significant criteria have an influence on the resulting RLG. Thus, RLG is determined by a step-by-step evaluation procedure (Fig. 2).

Figure 2.

Stepwise Red List assessment for habitat types in Germany. The ‘National Long-term Threat’ (nTH) is derived from the ‘Regional Long-term Threats’ (rTH) of eight major landscape regions (Step 1, 2) (see Fig. 3). After that, the degree of endangerment is upgraded or devalued consecutively, first by applying criterion T (Step 3) and then criterion R (Step 4). RLG represents the overall ‘Risk of loss’ (Step 5) (cf. BfN 2017).

For the long-term risk assessment mainly anthropogenic spatial (sub criterion AL) and qualitative (sub criterion QUL) changes over the last 50–150 years (sliding time frame) are assessed for the major landscape regions (see Fig. 3). For this purpose, the time period between 1850 and 1950 is set as the reference. In most cases, an earlier reference stage cannot be used due to insufficient data. Hence, the considered reference period does not represent the pristine stage of nature as still existed in the Middle Ages in greater parts of Europe. Specific to the habitat, the initial phase of industrialisation (~1850) or rather the situation before the massive intensification in agriculture after the Second World War started (~1950) was chosen.

Figure 3.

Map of the regions delimited for the regional threat classification of habitats (major landscape regions; red outlines). For ecological characterisation, Germany can be subdivided into natural units. The figure is based on the system of Meynen and Schmithüsen et al. (1953–1962). For the application in the habitats directive (Natura 2000) and the risk assessment of habitats Ssymank (1994) has restructured and generalised the system. The classification of major landscape units is based on physiographic units (black outlines; for a reference list see Annex V.6, Finck et al. [2017]) according to Ssymank et al. (1998) and Petersen et al. (2003).

A similar reference period for the assessment of the long-term threat in Germany is used by Ludwig et al. (2009) for species, and also in Red Lists of habitats from several German federal states which were used as data sources (e.g. Buder and Uhlemann 2010; Von Hengel and Westhus 2011; Zimmermann et al. 2011; Von Drachenfels 2012). The IUCN uses an earlier reference period for the long-term trend; here the relative changes since 1750 are considered (Bland et al. 2017).

For each of the defined eight major landscape regions (see Fig. 3) a risk assessment is performed with regard to the two sub-criteria AL and QUL. Subsequently, consolidation of these sub-criteria into the ‘Regional Long-term Threat’ (rTH) is carried out (see Fig. 2, step 1). Following the ‘precautionary principle’, the highest risk category obtained by any of the two sub-criteria is used as the overall rTH. The verbal-descriptive definitions for the categories of the sub criteria AL and QUL, and the overall categories for rTH, are presented in Table 1 as they also correspond to criterion I (nTH), which only differs in spatial scale of assessment.

The sub-criterion AL represents the estimated long-term loss in area of occupancy and the decline in number of sites of habitats (by demolition, building activities, changes in land use, etc.). AL has been described in detail by Blab et al. (1995) (here criterion I). Hereby, the historical ideal condition that belongs to a habitat concerning total area and site density is used as a hypothetical reference to assess threats. In fact, this ideal situation currently rarely exists for any habitat type and can only be described in approximation. In some well-documented cases (e.g. bogs, heathland, ponds, hedges, and unmodified running waters) precise data for the net loss of area are available over a longer period. However, in most cases additional expert judgement is needed to assess this sub-criterion.

Apart from direct loss of total area and decrease in number of sites, habitats can be threatened in particular by qualitative changes and deterioration represented by sub-criterion QUL. Typically, this has adverse effects on the abiotic conditions as well as on the structural appearance, the typical set of characteristic species, and on ecological interactions (see Blab et al. 1995). As the discrepancy from a habitat’s ideal or (semi)-natural state increases, it becomes more endangered. An ‘ideal state’ in quality for each habitat type must be elaborated to serve as a reference with regard to all relevant parameters (essential for the value and possible colonisation of the habitat type by typical species). This reference has to consider, among other parameters, the historic conditions, known abiotic requirements, and ecological requirements of typical animal or plant species or plant communities. However, this is linked to methodical problems. In a number of cases the ‘ideal’ or ‘historic’ state is not sufficiently known or can only be described in general terms. Therefore, expert judgement is additionally needed to assess QUL. Given that it is often difficult or impossible to separate biotic and abiotic degradation processes as proposed by Rodríguez et al. (2011), theses aspects are combined to QUL in the German Red List. This corresponds to a similar approach e.g. in the European Red List of Habitats (Janssen et al. 2016) and in the Red List of Ecosystems of Switzerland (Delarze et al. 2016).

The assessment of nTH in the current edition corresponds to the overall Red List category of the second edition because in 2006 only nTH was considered to determine RLG (Riecken et al. 2006). The assessment is based on an upscaling from rTH to nTH, i.e. from the regional to the national scale (Fig. 2, step 2). Median values of all rTH values for every habitat type are calculated (of a maximum of seven terrestrial regions, i.e. all regions where the habitat type is present).

If regions differ extremely in rTH, the most representative region(s) for each habitat turned the balance. The reference period corresponds to that of rTH. For nTH the categories and definitions remain largely unchanged compared to earlier editions of the Red List Germany (Table 1). However, in contrast to previous editions, intermediate values (1–2, 2–3) are no longer used in this context. The evaluation of nTH is the starting value underlying the Red List assessment scheme (Table 3), whereas the following criteria T and R ‘only’ cause an upward or downward revaluation of the category.

The ‘Current Trend’ (T) in total area (and number of sites) is assigned at the national level. The estimation of T is based on development over the last ten years and a forecast for the near future (maximum ten years). This period corresponds to the updating cycle of the Red List Germany. A comparable criterion is used in Germany for the Red List assessment of species, but without the future assessment (short-term population trend, cf. Ludwig et al. 2009). The reporting format for the main results of the monitoring referred to in Article 11 of the European Union (EU) Habitats Directive for habitat types in Annex I also considers short-term trends over a similar time horizon (sliding window over 12 years, cf. DG Environment 2017.). A criterion with a similar idea was also integrated into the Finnish assessment (Kontula and Raunio 2009). They estimate the ‘projected quantitative and qualitative change in the near future (criterion A2/B2)’ in a time frame of 20–30 years. Bland et al. (2017) use a 50-year period for short-time assessments. T is included in the overall assessment of RLG in this new edition for the first time. Considering the availability of data sources, five categories are used (Table 2). For all endangered and near threatened types of habitats, which show a negative short-term trend, the threat category increases by half a value based on the assessment of nTH. For endangered/near threatened types of habitats which have a stable trend the threat category decreases (i.e. improves) by half a value because we interpret stabilisation as success of nature conservation activities. For endangered habitat types with a currently positive short-term trend, the threat category improves by one value (Fig. 2, step 3).

In the revised assessment scheme, a higher risk of loss is basically assumed for habitat types which are extremely rare. They are characterised through very few or very small occurrences and are therefore usually very sensitive to the loss of individual sites since one single event or a critical hazard could destroy the whole inventory (cf. Williams et al. 2015). In the German procedure criterion R functions as a regulating upgrading factor. The main objective of this assessment is to emphasise the higher risk of loss of extremely rare habitats. A similar approach was introduced by Kontula and Raunio (2009), even though thresholds and the degree of differentiation differ due to the specificity of national data sources and natural conditions. The IUCN sets graded thresholds of ‘restricted geographic distribution’ which are only decisive if defined threat conditions are given (e.g. continuing decline, inferred threatening processes, low number of locations) (IUCN 2016).

Criterion R is not classified in a full system from widespread to extremely rare. All types of habitats are examined and classified as either ‘Extremely Rare’ or ‘Not Extremely Rare’. All types which had been assessed as category ‘R’ (extremely rare) for the Red List status in the second edition (Riecken et al. 2006) were transferred to the category ‘Extremely Rare’ of criterion R. Furthermore, extremely rare types of habitats were derived from the area sums of the related Natura 2000 habitat types from the national report of 2013 (Ellwanger et al. 2015) (reporting obligation under Article 17 of the EU Habitats Directive). A maximum threshold for ‘Extremely Rare’ in terms of area size was set at a total area of 500 hectares in Germany. The corresponding assessment tightens RLG by half a value (Fig. 2, step 4).

RLG describes the overall ‘Risk of Loss’ under current national threat conditions. Based on nTH, criteria T and R have a downgrading or an upgrading effect (Fig. 2, step 5). All possible evaluation constellations are defined in the assessment scheme (Table 3). In principle, only long-term endangered habitat types and types classified in the early warning stage are taken into account in the assessment scheme. For non-endangered types, types with unknown threat-status, and types not relevant for nature conservation purposes, nTH corresponds to RLG. Due to the algorithm used, intermediate values can also occur. The stepwise assessment results in a wider spread of Red List categories (Table 4). The (verbal-descriptive) definitions of the Red List categories are derived from the possible combinations of the individual criteria according to the evaluation scheme (see Tables 1, 2).

The categories ‘Imminently Threatened By Complete Destruction’ (1!) and ‘Imminently Threatened’ (V–3) are newly introduced. These new categories represent both extremes of ‘collapse risk’ in the German approach.

The revised assessment system has been tested and applied in the current edition of the ‘German Red List of threatened habitats’ (Finck et al. 2017) (Table 5). The assessment covers a total of 863 marine, coastal, inland water, open terrestrial, shrubs/trees/forests, and alpine types of habitats in Germany (not considering so called ‘technical habitats’). While two-thirds (65.1%, n = 562) of the assessed habitat types were assigned with different degrees of ‘risk of loss’ (Red List categories ‘0’ to ‘3–V’), 24.7% (213) are currently of ‘Least Concern’. Thirteen marine types of habitats (1.5%), mainly characterised by the European oyster (Ostrea edulis) or Honeycomb worm reefs (Sabellaria sp.), had to be classified as ‘Collapsed’ (category 0). Comparing the main habitat groups in Germany, the proportion of threatened coastal habitats (RLG categories 0 to 3–V) is the highest (82.8%). Alpine (58.8%) and marine (52.5%) habitat types represent the least threatened habitat groups. Inland waters (76.4%), open terrestrial habitats (68.8%), and shrubs/trees/forests (69.5%) show proportions of threatened habitat types above the average (65.1%). Open terrestrial habitats represent a significant proportion of habitat types classified in the highest threat category ‘1!’ (16.3%). Intensive land use still represents the main threat factor especially for open terrestrial habitats and (to a lesser extent) forest habitats. A detailed analysis of major threat factors for habitat types in Germany was published in 2019 (Heinze et al. 2019).

The application of the assessment scheme results in a clear spread of the realised categories for the Red List-status. Only a total of 101 (17.1%) of the long-term endangered habitat types (nTH = 0, 1, 2, 3, V) were classified in the same category for the overall RLG-status (Fig. 4). The newly introduced categories ‘1!’ and ‘3–V’ are frequently used: 16.2% of all assessed habitat types were classified in category ‘3–V’. 6% had to be assessed in category ‘1!’ (Table 5). Two thirds of the habitat types, which are valued as ‘Critically Endangered’ (1) for nTH had to be upgraded to the category ‘1!’ for RLG (Fig. 4, second bar). In contrast, almost 60% of habitat types that were assessed as ‘Vulnerable’ (3) according to nTH could be downgraded to category ‘3–V’ or V, respectively, as they had a stable or positive short-term trend (Fig. 4, fourth bar).

Figure 4.

Spread of RLG-values (y-axis) by applying the matrix algorithm (Table 3). The analysis is based on nTH (criterion I, x-axis) for habitat types in Germany. Full dark pigmented bar = no change in category; full light-coloured bar = downgrading of threat category value; brindled bar = upgrading of threat category value; Labels: number of attributive habitat-types in the [resulting RLG-category].

The IUCN criteria catalogue (Keith et al. 2013) was applied to the national situation in Germany for raised bogs by Riecken et al. (2013). The overall status was assessed to be ‘Critically endangered’. This result corresponded exactly to the national assessment at that time (Riecken et al. 2006). The condition of bogs is even better represented by the new methodology. The degradation started at the beginning of industrialisation during the 18^th century, especially in the North-western lowlands of Germany but also in other parts of Europe. Bogs were drained and the peat was cut, dried, and transported so that only about 1–2% of the original area is preserved today (LLUR 2012; Ellwanger et al. 2015). The remaining sites are of relatively small size and isolated from other stocks. In the assessment period of the current edition of the German Red List, the long-term threat situation (rTH) of ‘raised bogs’ did not change significantly in most landscape regions (Table 6). Nevertheless, agricultural utilisation of former bogs continues and has increased in intensity during recent decades (Rath and Buchwald 2010). Additionally, climate-induced changes in abiotic conditions are having an increasingly negative impact (Essl and Rabitsch 2013). Therefore, T is still classified as negative. Applying the new assessment scheme, RLG had to be upgraded to the highest threat category (‘1!’). Compared to the European assessment (Janssen et al. 2016), the situation in Germany is much more critical (European assessment [EU 28] for the decisive criterion A3/historical decline: EN). For the evaluation of the revised assessment of ‘raised bogs’, it must be considered that ‘raised bogs’ are characterised by very slow regeneration ability (‘RE’, Table 6, symbol ‘N’). The regeneration ability was estimated for each habitat type. The result is ‘additional information’ and does not influence the assessment of the degree of threat (Blab et al. 1995). For this reason, no major improvements in the ‘Long-term threat’ can be expected in the near future. Only limited areas are available for bog restoration in Germany. With these preconditions, ‘raised bogs’ may always remain in a high long-term risk category in Germany. Criterion T was integrated into the evaluation process in order to be able to display current trends. The method-inherent increased Red List status in 2017 reveals an acute need for action to counteract specific causes of threat. Differences in the rTH show that the situation for characteristic habitat-subtypes in the North-western and North-eastern lowlands and the highland regions is even worse than for (subtypes of) the alpine region. Especially in the Alps, impacts of climate change and anthropogenic use can be observed, but so far have not changed the threat situation of ‘raised bogs’. However, there may be a threshold for observable detriments, which has not yet been reached.

In the German habitat classification used for the Red List, pristine woods are not separated but are assessed together with their utilized variants. There is no database available which describes different pristine Central European forest types in detail. The risk assessment of forest habitat types therefore represents a weighted median of the existing stands (structure-rich old-forest, young age-class forest, etc.). The degree of naturalness (richness of structure, mixed forest, old wood, deadwood, stratification of different age classes) is weighted by the risk assessment through the quality criterion (QUL). In many cases, the specific ground layer is also well-developed in woodland areas which are used by forestry, so that a classification of the forest habitat type is possible. The Long-term Threat to ‘beech (mixed) forests on moist, base-deficient sites (Fagus sylvatica)’ has not changed since the last assessment period and is still classified as being ‘Endangered (EN/2)’. Thus, the continued positive short-term trend has not yet affected the long-term threat assessment. However, this type is experiencing an improvement of a full threat category from EN (2) to VU (3) for RLG (Table 6) as the area of beech forests in Germany has been continuously increasing in recent decades. A general reorientation in forest management in recent decades has contributed to a significant increase in native broadleaved forests in Germany (BMEL 2016). Former main threats such as ‘reforestation with non-autochthonous trees’ have decreased, at least in protected areas. The current downgrading of the Red List status reflects these efforts. Nevertheless, the legal protection of beech forests within Natura 2000 sites may not be sufficient to reach biodiversity goals if intensive forestry continues in large parts of protected areas (Panek 2016). Therefore, programmes were initiated to increase non-intervention management areas (e.g. EU Biodiversity Strategy, National Strategy on Biological Diversity). To continue the positive development, additional focus must be set on the habitat quality as well.

In contrast to the Red Lists of species, the underlying data for habitat threat are not collected by volunteer scientists but exclusively in the context of monitoring obligations (e.g. EU Habitats Directive) or in the course of habitat mapping by the federal states. Thus, the national Red List assessment in Germany mostly relies on regional data sources collected by federal state administrations. Marine habitats are an exception, because here the German Federal Agency for Nature Conservation is the directly responsible nature conservation authority. Data collection in the ‘Exclusive Economic Zone’ is therefore carried out by the Federal Government and in coastal areas by the Federal States. Even though data from current habitat mappings were not available for all federal states in the current Red List, the existing baselines provides a good overview of all major landscape regions (see Fig. 3). However, there are still considerable differences in the actuality, evaluation and mapping methodology (Kaiser et al. 2013). In addition, classification systems of habitat types of the sixteen federal states are not completely comparable. For this reason, data originating from federal states often cannot be transferred directly. Thus, a supplementary, case-by-case expert assessment was often necessary up to now. However, there are approaches to minimise problems with data availability and transferability. On the one hand, there are efforts on the part of the federal states to standardise mapping (e.g. in a benchmark paper; Beck et al. 2013). On the other hand, a universally applicable standard list of habitat types is being developed in a current research project which aims to establish a nationwide, uniform random sample of habitat mapping (BfN 2018). This will make it easier to match the data originating from the federal states in the future. In addition, the planned nationwide random samples would represent a kind of calibration. The establishment of frequent, standardised ecosystem monitoring could function as a solid regular database for the assessment of changes in the actual threat situation of many habitat types. Great efforts have also been made in recent years in the classification and monitoring of marine habitat types (Finck et al. 2017; BfN 2019). There is still a lack of national standardised monitoring data for a wide range of habitat types occurring in Germany and Europe. However, through the monitoring obligations of the EU Habitats Directive, a standardised tool is available at least for habitats which correspond to types in Annex I of the Directive. Nevertheless, it is not always possible to assign the types defined in the Habitats Directive directly to the German standard list of habitats due to different definitions or development goals. A crucial question remains: to what extent the habitat data, which were collected under the Habitats Directive, can be used to draw conclusions about the current frequency, distribution, and quality of habitat types within the framework of the national Red List. At least we tried to keep the European types as distinct as possible in order to guarantee a transferability of the data into the German standard list. Nonetheless, summarising the given data into a nationwide Red list is usually a standardisation step for which expert assessment remains necessary.

Comparisons with the previous edition of the Red List (Riecken et al. 2006) are only possible to a limited extent for the individual criteria due to the changed evaluation methodology. Unfortunately, changes in methodology are always at the expense of comparability; at least the determination of the individual criteria nTH and T was kept unchanged.

As a first step in our assessment procedure the long-term threat situation in area and quality is always assessed – if detailed databases are lacking –, based on expert estimation. In today’s intensively cultivated landscape, we can assume that the historical conditions of many habitat types with significance for biodiversity were more favourable. Therefore, a comparison with the ‘historical more ideal condition’ of habitat types is the starting point of our Red List assessment. This rationale can be confirmed by the application of IUCN criteria for the ‘European Red List of Habitats’ (Janssen et al. 2016). In some European countries, only limited data for long-term trends were available, so that criterion A3 (reduction in geographic distribution since 1750) was assessed as being ‘data deficient’ (cf. Janssen et al. 2016; Biró et al. 2017). Following the ‘precautionary principle’, the resulting Red List category is therefore based on one or two criteria which often only reflect short-term threat situations and can therefore be misleading to an overrated positive or negative evaluation compared to the ‘historical condition’ of the habitat. For some habitat types the reference period already reveals a depleted situation. In a study from Hungary, Biró et al. (2017) have shown that the number of highly endangered habitat types increase dramatically if the long-term trend is taken into account. To deal with the possibility of ‘earlier decline’, Kontula and Raunio (2009) proposed tightening the assessment in a sub-step of their stepwise procedure (here criterion A3, B3). Since ‘early decline’ represents a temporal shift in the historical ‘ideal state’ for particular habitat types, this factor is taken into account in the long-term assessment of the German procedure by setting a sliding time frame. Thus, the selection of an adequate historical reference period for Red List assessments is also a question of the specific history of landscape development, as well as national nature conservation objectives. In Germany, for example, the preservation of extensively used semi-natural habitat types is a legal objective regulated in the Federal Nature Conservation Act. This is one of the reasons why we chose a later and dynamic long-term reference period than the IUCN. Since habitats are dynamic systems, which typically do not disappear but rather replace each other, vulnerability must be assessed individually for each type. For this reason, we state that reference periods (particularly long-term evaluations) can also differ for habitat groups. Overrated positive evaluations through the IUCN method may particularly apply to European forest habitats, which have experienced severe historical losses but are currently increasing or stable (see case study beech forests).

By extending the criteria system, RLG is particularly intended to reveal successes in nature conservation and the need for action. In this context, we agree with the argumentation of Delarze et al. (2016), that the objective of national Red Lists is to demonstrate current trends and to indicate needs for action. This effect becomes clear in various ways by analysing the results of the current German Red List. For example, in an alarming way, many of the open terrestrial habitats were assigned to the highest threat category ‘1!’ (16.3%, Table 5). There is still an ongoing negative trend, especially for many terrestrial open landscape habitat types, mainly caused by the intensification of agriculture accompanied by grassland loss and levelling of site conditions, which results in a severe loss of extensively used rural habitats (Heinze et al. 2019). The loss of biodiversity in the cultural landscape is also a topical issue in European politics. Here the result of the Red List fits into the general picture. The Common Agricultural Policy severely impacts biodiversity and ecosystem services (Simoncini et al. 2019). The “Red List tool” must therefore also be able to reveal short-term changes in intensively used landscapes, which can change very quickly due to initial agricultural policy conditions.

The urgent need for an accentuation of “critically endangered” as well as currently declining habitat types is clearly illustrated by case study of raised bog ecosystems. On the other hand, widespread beech forest habitat types are experiencing an improvement of a full threat category from EN (2) to VU (3) for RLD, as the area of beech forests is continuously increasing (see case study) in recent decades. To show actual tendencies by means of the Red List category, short-term trends function to illustrate modification in the threat situation. Generally, habitats which are characterised by very slow regeneration ability, which have been severely destroyed or deteriorated in historical times (e.g. forest types, raised bogs cf. case studies), can only achieve minor improvements in their ‘National Long-term Threat’ (nTH) status. Once severely degraded, the period of time required for re-establishing defining features may exceed the reference period of Red List assessments. By using a consecutive assessment scheme, a change in the Red List status is possible even if the long-term threat remains the same. Applying the assessment procedure of previous German Red Lists, a change in the Red List status of a habitat type was only possible if a significant change in the threat situation was achieved with the historical optimum state as a reference.

Overall, the approach of assessing stable trends as (first) successes in nature conservation and therefore with a reduction of the overall threat has proven successful. Otherwise, the current extinction risk for extremely rare but currently stable habitat types that are endangered in the long term would be overestimated when applying the assessment scheme. In nature conservation, the short-term focus should be on the many habitat types that are currently in decline. In the intensively used European cultural landscape, maintaining the same conditions of conservation is also a (small) success. This approach is also laid down in the EU Habitats Directive, whereby a ban on deterioration of the conservation status of habitat types is taken as the minimum objective (Council of the European Communities 1992). However, a long-term goal must also be the improvement of conservation status or rather a “Least Concern” condition. In general, since continuous short-term trends have a long-term effect only after several decades, criterion T functions as a short-time control value. However, the preceding interpretations should be seen with the limitation that the actual impact of the signal effect of RLG through the integration of short-term changes for the necessity of nature conservation action can only be evaluated when regular assessments of the endangerment of Germany’s habitat types are available. By comparing Red List versions, it will be possible in future to map actual developments and thus initiate direct nature conservation measures towards habitats with negative developments. In order to exactly reflect the development in the update cycle of ten years, introduction of two sub criteria of T should be considered, looking ten years into the past on the one hand and ten years into the future on the other (corresponding to the parameter ‘future prospects’ in Habitat Directive assessments of conservation status). As a result, if repeated assessments are available (as in Germany), short time tendencies become particularly relevant for management decisions.

Extremely rare habitat types are naturally exposed to a higher risk of severe impairment by individual events (Finck et al. 2017). In the German Red List, the assessment scheme only includes threatened (and near threatened) habitats, so that corresponding conditions were examined before criterion R could have any effect on RLG. Thus, our approach prevents misleading evaluations of naturally extremely rare habitats which are actually not decreasing or degrading (c.f. Gigante et al. 2016). Because rarity has a further aggravating effect in our assessment, extremely rare but currently stable types do not easily lose the focus of nature conservation.

Bland et al. (2017) have the reverse approach, so that thresholds of ‘restricted geographic distribution’ are only decisive if special threat conditions are given (e.g. continuing decline, inferred threatening processes, low number of locations). The criterion of rarity in our approach is so far a yes/no criterion. On the basis of the available data, it should be analysed in more detail as to whether further differentiation of this criterion makes sense in future. A differentiation among various types of habitats would be more appropriate. For example, 500 hectares of Alpine rivers cannot be compared to 500 hectares of beech forests, merely from the surface area point of view.

The assessment system applied to the third edition of the German Red List relies on a full assessment of all criteria and a use of all individual values to determine RLG. In contrast, in the IUCN procedure the highest risk category obtained by any of the assessed criteria represents the overall risk status. Nevertheless, all three criteria of the German methodology indicate spatial changes as symptoms of ‘ecosystem collapse’ (c.f. Bland et al. 2017; Rowland et al. 2018). The change in quality is also assessed in the case of criterion nTH. However, the effect of individual criteria can deviate greatly from each other (Finck et al. 2017). Therefore, in our opinion, the overall ‘risk of loss’ can only be assessed by taking all criteria into consideration.

Some recently published European Red List assessments (e.g. Härtel et al. 2009 [CZ]; Essl and Egger 2010 [AT]; Biserkov et al. 2015 [BU]; Finck et al. 2017 [DE]) do not strictly follow the IUCN approach, which has been significantly developed since 2009 (Keith et al. 2009). However, in most of these lists the early draft of the IUCN approach was considered. At least the basic concept of ‘ecosystem collapse’ has also been applied in the German method. The assessment procedure presented here allows for a clearly defined differentiated assessment of the overall ‘risk of loss’ (Keith et al. 2009) for individual habitat types under current threat conditions. In principle it has been shown that IUCN criteria for Red List assessment of habitats are applicable within small countries or regions (Bland et al. 2019). Some countries use already an assessment procedure very close to the methodology proposed by the IUCN (e.g. Lindgaard and Henriksen 2011; Delarze et al. 2015, 2016; Gubbay et al. 2016; Janssen et al. 2016; Kontula and Raunio 2019; Chytrý et al. 2019). However, they also had to allow for national or European specifications of the IUCN protocol (e.g. workshop documentation; Finnish Environment Institute 2019). In addition, it is often the case that only some of the IUCN criteria could be evaluated, which can lead to incomplete and sometimes inconclusive risk assessments (see case studies). A detailed comparison of the assessments of these lists is not the subject of this paper. However, in this context the objective of habitat red-listing must be discussed, considering the background of nature conservation goals (see also Delarze et al. 2016; Gigante et al. 2018; Bland et al. 2019; Rowland et al. 2019).

Ultimately, a ‘standard criteria system’ should offer sufficient flexibility to adapt to national and regional requirements. In this regard, we may need to discuss different thresholds and reference time frames for different habitat groups depending on specific spatial pattern and distribution history. For example, Delarze et al. (2016) have lowered the thresholds for IUCN criteria B1 and B2 in view of the relatively small size of the country. A future prospect will be to integrate useful national approaches to international standards. Since the distribution of ecosystems may extend over different countries (evaluation units), threats to specific ecosystems or habitat types should be determined in a broader spatial scale with the precondition that evaluation systems are comparable. However, this proposal is limited by the actual availability of significant data for the assessed area, which determines the applicability of criteria. Analogous to the improvement of data sources the catalogue of criteria and categories should be adapted and improved. Modifications to apply the IUCN criteria for Red List assessment are a realistic response to the amount of available data for a landscape that is highly diverse, fine-grained and dynamic, as well as strongly affected by cultural influences (cf. Janssen et al. 2016).