The new IUCN threat categories include many numerical thresholds that require quantitative data. Since such data are rare for bryophytes, the evaluation against the threat categories must often be done by inference from the available data. The most relevant data that can be used for bryophytes are population decline, present distribution and total population size, number of sites, and estimated loss of relevant habitats over the last 10 years or three generations. For assessing species at a regional level, evidence of an inflow of propagules from outside the Red List region is discussed. The use of the terms ‘individual’, ‘fragmentation’, ‘location’ and ‘generation time’ is also discussed.

The recent Red Data Book of European Bryophytes (European Committee for the Conservation of Bryophytes 1995) included an extensive listing of bryophytes considered threatened in Europe. It used the original IUCN threat categories of Extinct, Endangered, Vulnerable and Rare, but with additional definitions. The revised IUCN threat categories (IUCN 1994) had been produced by the time the RDB was published, but it was too late to assign species to the new categories, although a small number of 'endangered' species were provisionally allocated to the new Critically Endangered category. However, now that the revised IUCN guidelines and threat categories are becoming more widely accepted, and used, as a substantial improvement on the previous methods, it is necessary to re-assess the threat status of the species in the European bryophyte flora. Hallingbäck et al. (1995) provided a background to the adoption of the new threat categories for bryophytes and some recommendations for their application. These were tested during a workshop meeting on bryophyte conservation in Europe in Reading in August 1996. The purpose of this paper is to present the guidelines to a wider audience of bryologists, with more specific examples on their use, in the light of the Reading workshop. It should be possible to use the same guidelines at all levels, from world-wide to national or regional.

A summary is given below of the revised IUCN threat categories and qualifying criteria appropriate for bryophytes (see also Figure 1).

This system is complex and offers a range of alternatives for identifying the status of threatened species. A species is required to fulfil a minimum of one criterion (of criteria A to E) to qualify for any of the threatened species categories on the Red List. Species are tested against all criteria, working 'downwards' through the threat categories (starting with Extinct) until the appropriate category for that species is found. The species is allocated to the 'highest' category that it fits. In other words, if a species is determined as Critically Endangered using criterion B but only Endangered using criterion C, its status is Critically Endangered. Decline may be measured as a reduction in the number of 'individuals' observed or, in the absence of this information, inferred from distribution data.

The guidelines are intended for world Red Lists as well as Red Lists for smaller regions. Numbers of individuals, locations or squares are not related to the size of the area for which a Red List is made. This, however, lead to some confusion. Gärdenfors (1996) recently proposed guidelines on how to apply the IUCN system on a regional scale. He clearly showed that size of the Red List region as such does not make any difference in the system. The IUCN category system judges the risk of extinction of a population within a given geographical region. This region can be the world or a small country. The risk that a species which for example is considered to have declined by 50% or to consist of less than 10 individuals or to be restricted to fewer than 5 sites will go extinct is independent of the total size of the region. A higher percentage of the species stock will probably be Red Listed in smaller countries. This is because smaller countries generally house smaller populations and smaller populations are more prone to extinction, but not because the thresholds of the criteria are wrong or should be different on different geographical scales. One exception is if an inflow of diaspores from outside the Red List region is counteracting the extinction risk of the population (rescue effects). The smaller the country, the higher the probability of such a flow.

If there is no reason to believe that an inflow of propagules occurs or if the species is endemic, the criteria are applicable with those numbers given in the IUCN manual. However, if new individuals disperse into the region from surrounding regions, the species is probably less prone to extinction and must therefore be ‘downgraded’ to the next appropriate threat category in the IUCN system. Gärdenfors (1996) stressed that the scale problem is ‘more pronounced in countries surrounded by other countries than those surrounded by sea or other barriers preventing dispersal.’ If a downgrading takes place, this movement should be indicated by giving the category first met by the IUCN criteria within parentheses. This process was approved by IUCN at a meeting in England 1997. Thus, the same thresholds should be used in Luxembourg, Great Britain and Europe as well as at world level, although a downgrading step must take place in countries where the species is healthy and not threatened just outside the country border.

According to Gärdenfors (loc. cit.) those species which are considered extinct in the red-listed region but not in the world, should preferably be called Regionally Extinct (RE).

What constitutes an individual bryophyte is not always clear and, without genetic studies of each unit, a reliable estimation of the number of genetical individuals is impossible. It is recommended that authors of all investigations and Red Lists that use the term 'individual' define the way they have used it. The sort of individual within which all the shoots are connected to one another is also often difficult to determine without extensive destruction of populations. However, it is possible to use a pragmatic definition of an individual, in the case of those species that have a growth form that makes it easy to separate colonies or stands. For example, a single tuft of Ulota or a single discrete patch of Brachythecium can be regarded as an individual.

In bryophytes, information on the effects of isolation of subpopulations is more or less lacking. However, in general, those taxa with a large production of small diaspores are considered probably more easily spread (cf. Söderström & Herben 1997) and hence not so vulnerable to isolation through fragmentation of their habitats. Species that produce only small numbers of diaspores (or none at all), or only large ones, are less efficient at long distance dispersal and the subpopulations may be considered more easily isolated if the population has become fragmented.

If the natural habitats have been fragmented (for example old growth forests and rich fens) this can be used as indirect evidence for fragmentation of populations in species with poor dispersal ability. The IUCN system uses 'number of individuals of the largest subpopulation' as an estimation of fragmentation. This is not applicable to bryophytes since it is not normally possible to count individuals.

For practical reasons we recommend that, in most circumstances, a minimum distance greater than 50 km between populations of species without spore dispersal can indicate severe fragmentation, 100-1000 km for species with spores (shorter for species with low production and large spores and longer for those with high production and small spores).

Another difficulty is the definition of a location. The IUCN definition of a location is: "Location defines a geographically or ecologically distinct area in which a single event (e.g. pollution) will soon affect all individuals of the taxon present. A location usually, but not always, contains all or part of a subpopulation of the taxon, and is typically a small proportion of the taxon´s total distribution." The IUCN term 'Location' in this study was regarded as what botanists usually call a 'site' or 'locality' (Georgina Mace, pers. comm.). The only guideline that it is possible to offer on this matter is to choose 'locations' that are sensible and appropriate for the data available. Areas with good data are likely to have smaller 'locations' than those with poor data. We recommend that all authors that use the term ‘location’ define the way they use it.

For the estimation of rates of decline either a time span of 10 years or a span of three generations (whichever is longest) should be used. The IUCN system defines ‘generation’ as the ‘average age of parents in the population’. The ‘average age’ is impossible to estimate for most bryophytes since age can vary between a few years and several thousands for most perennial species, depending on environmental conditions and disturbance dynamics increasing mortality.

For practical reasons, ‘parents’ should be individuals which have started to produce spores. Individuals with only asexual reproduction cannot, we think, be classified as true ‘parents’. We believe that 'generation time' can be useful for some perennial ‘slow’ species which take a very long time to produce spores for the first time and where we can see a retreat/recover only after many years. Note that the IUCN Red List of Threatened Animals recommends a maximum length of 25 years for one generation (Baillie & Groombridge 1996). For species never found with sporophytes in the region we recommend using 25 years as a generation length. For those which produce sporophytes only now and then, the generation length has to be linked to life strategy type (as defined by During 1992): ‘short’ for typical colonisers and fugitives, ‘medium’ for short-lived shuttles and ‘long’ for perennial stayers.

Therefore we strongly recommend that ‘length of generation’ should be considered, (but not in all cases) and always used with common sense. We suggest the following definitions based on the differences in potential life span of the gametophyte:

Be aware that the synopsis given below includes only part of the original text about the new categories. We therefore strongly recommend all those who want to adopt the revised IUCN categories to study also the original IUCN publication (IUCN 1994) very carefully.

IUCN definition: "A taxon is Extinct when there is no reasonable doubt that the last individual has died." For bryophytes, this means, in theory, that no living material of the taxon exists in the world. See the comment under the next category (EW).

IUCN definition: "A taxon is Extinct in the wild when it is known only to survive in cultivation, in captivity or as a naturalised population (or populations) well outside the past range." Since bryophytes have a very high regenerative potential, this implies that no living material should exist in the wild.

To interpret them in a reasonable way for bryophytes, EX and EW need additional definition. For the purposes of the European bryophyte Red List (ECCB 1995), extinct (including EW) is defined as: "Taxa for which all known localities have been checked repeatedly in the last 30 years without success, or taxa listed as extinct or vanished in all available Red Lists, if the total area of distribution is covered by Red Lists." At the Reading workshop, it was agreed to extend the threshold to 50 years. It must, however, be stressed that all known localities should have been carefully checked. For short-lived species with ephemeral gametophytes, the localities must be searched at the appropriate time of year and preferably over several years, in order to take account of possible population fluctuations and survival of the species by a diaspore bank only.

When applied on a regional level those species which are considered extinct in the red-listed region but not in the world, should preferably be called Regionally Extinct (RE) (Gärdenfors 1996).

IUCN definitions: "A taxon is Critically Endangered when it is facing an extremely high risk of extinction in the wild in the immediate future, as defined by any of the criteria (A to E)".

"A taxon is Endangered when it is not Critically Endangered but is facing a very high risk of extinction in the wild in the near future, as defined by any of the criteria (A to E)".

"A taxon is Vulnerable when it is not Critically Endangered or Endangered but is facing a high risk of extinction in the wild in the medium-term future, as defined by any of the criteria (A to E)".

Only IUCN criteria A to D are indicated below, for the purposes of evaluating bryophytes. Criterion E - ‘Quantitative analysis showing the probability of extinction in the wild is at least 50% within 10 years or 3 generations, whichever is the longer (CR); at least 20% within 20 years or 5 generations, whichever is the longer (EN); at least 10% within 100 years (VU)’ - is, as long as no PVA or equivalent proper data not yet are available, not possible to use. However, it may be possible to use it in the future, following more research on bryophyte extinction processes and threats.

A.

Large decline

Major population decline ‘observed, estimated, inferred or suspected’ in the last ‘10 years or 3 generations, whichever is the longer, based on’:

B. Restricted area of occupancy, few localities, decline

To qualify under this criterion, a species must occupy a restricted area and have few localities and have a ‘continuing decline observed, inferred or projected’. For bryophytes, the area of occupancy has been interpreted as the number of grid squares in which a species has been recorded. The IUCN option ‘Severely fragmented’ for species with small subpopulations which are all more or less isolated can also be used for bryophytes. Extreme fluctuations, used in the IUCN criteria, have not been used here, as there is seldom any information on the dynamics of bryophyte populations. However, when they are known, fluctuations can be included in the evaluation. Investigation into the fluctuations of listed species should be a principal objective before a final estimate can be made of the chances of their survival.

In the event that a species occurs in, for example, three 10 km x 10 km squares and six localities, thus apparently falling between two threat categories, the species falls into the 'lower' category (i.e. Vulnerable).

C. Small population and decline

Small population:

together with either:

C1. Large decline:

D.

Very small or restricted populations

Species with insufficient data to categorise them but which are thought likely to qualify as Extinct, Critically Endangered, Endangered or Vulnerable when they are better known. Data Deficient species should be listed on an appended list. Listing taxa in this category indicates that more information is required.

A taxon belongs to the Lower Risk category when it has been evaluated but does not satisfy the criteria for any of the categories EX, EW, RE, CR, EN, VU or DD. This category does not include species thought to be significantly under-recorded. The LR category is divided into three: least concern (lc), conservation dependent (cd) and near threatened (nt). Taxa in the last subcategory are close to qualifying for VU. Be aware of the importance of the LR (nt) species as any of them could rapidly become threatened and they should therefore be re-evaluated at appropriate intervals. A list of LR (nt) species includes many that are close to qualifying for VU, and should therefore always be included in an appendix to a Red List. The LR (cd) category is probably not applicable to bryophytes since conservation programmes are rarely directed specifically at bryophyte species. The LR (lc) category includes the majority of common, non-threatened species.

This final category includes species that have not yet been evaluated against the IUCN criteria. It should not be confused with Data Deficient.

All taxa in the categories CR, EN, VU are classified as 'threatened'. Taxa in LR are 'not threatened' and those in DD are neither 'threatened' nor 'not threatened'. The following remark from IUCN 1994 (preamble, 5) is most important: ‘Listing in the categories of Not Evaluated and Data Deficient indicates that no assessment of extinction risk has been made, though for different reasons. Until such time as an assessment is made, species listed in these categories should not be treated as if they were non-threatened, and it may be appropriate (especially for Data Deficient forms) to give them the same degree of protection as threatened taxa, at least until their status can be evaluated.’

The examples below give the reasons why species do or do not qualify for their status in specified Red Lists under each of the criteria A to E, for different regions or all the world.

Jamesoniella undulifolia. Status: EN (Great Britain).

This endemic species has not been seen since 1924, in spite of repeated searching. Evaluation against criteria A-E is therefore unnecessary (herbarium specimens are assumed to be dead!)

Orthotrichum gymnostomum. Status: LR (nt) (Sweden).

This species, which is an epiphyte on aspen Populus tremula, does not qualify for the Swedish Red List. However, it may become Vulnerable in the near future unless appropriate conservation action is taken to improve the air quality and the proportion of aspen trees in Swedish woodlands, and is thus considered ‘near threatened’ LR (nt).

Pterogonium gracile. Status: CR (Sweden).

At a regional level the new criteria have been tested in Britain (Palmer et al. 1997) and in Sweden. In Sweden, 40 species red-listed according to the old system have been compared with the new (Hallingbäck, in prep.).

Compared to the previous system, the new one has many numerical thresholds which require quantitative data. For example, population size, decline over the last 10 years, generation time and number of mature individuals. Since bryologists usually completely lack data for conducting PVAs (Population Viability Analyses), seldom are able to count individuals or measure 'generation length' (sensu IUCN), and seldom have information about bryophyte total population sizes, some of the criteria are often not applicable or are inappropriate when dealing with bryophytes.

The most relevant data that can be used for bryophytes are:

* Population decline over the last 10 years and 3 generations (criteria A, C, D)

* Decline in habitat quality (criteria A, B)

* Present distribution i. e. area of occupancy, counted as number of grid squares (B, D)

* Number of sites (B, D)

* Number of individuals in total population and in all subpopulations (to estimate the degree of fragmentation), (C).

* Evidence of inflow of propagules from outside the Red List region in regional listing (Downgrading)

We believe that most of the four criteria A-D can be applied to bryophyte species that have been relatively well monographed or studied on a world-wide basis. However, our global overview of bryophyte distribution is very poor, mainly because there are too few bryologists.

The preamble to the revised categories (IUCN 1994) emphasises the importance of attempting to allocate threat statuses to even relatively poorly-known species by estimating and extrapolating the current or potential threats into the future, using the precautionary principle. This is important for the assessment of bryophytes since it enables us to use indirect factors such as habitat destruction and air pollution. Without this consideration, most bryophyte species would end up in the category Data Deficient, because bryophytes in general are less known compared to vascular plants and vertebrates. However, taxa which are likely to be overlooked often should be placed in the Data Deficient category.

Decline in habitat quality can be very useful if up-to-date information about the population status of the species is lacking. The problem is to collate the up-to-date information about destruction of the habitat for all known sites for a taxon. While bryologists usually know the taxa well, they seldom have access to appropriate data on recent habitat destruction, creeping degradation or air pollution throughout the range of a taxon.

As stressed in the preamble to the new categories (IUCN 1994), Red List threat categories alone are not sufficient to determine priorities for conservation. A system of assessing priorities for action should also include other factors, such as international responsibilities, taxonomic uniqueness, logistics, chances of success and perhaps costs.