1.5.4 UK interpretation of site selection criteria and principles

 

1.5.4.1 Selection criteria and principles for Annex I habitat types

1.5.4.1.1 Representativity
Representativity is the degree to which a given habitat corresponds to a described type, including not only the most typical form of the habitat, but also its main lines of variation. The Interpretation manual of European habitats (European Commission DG Environment 2003) is the reference source for defining the habitat types. A first requirement in the evaluation of any habitat example for inclusion on the national list has therefore been to ensure that it conforms to the general habitat type description in the Manual.

It is recognised in the introduction to the Manual that judgement plays a part in determining the degree to which a given habitat example fits the general description. This is because that description reflects the full variation of the habitat type in Europe and at a given site it is unlikely that the full range of variation will be encountered. The Manual is necessarily general in character and cannot fully accommodate every local facet of variation of each habitat in all the countries in which it occurs. Habitat selection in the UK has sought to cover the range of variation a habitat type encompasses, including its most typical form and the main variations.

In most cases, the decision concerning whether a given habitat example conforms to a described Annex I type has been straightforward. However, unlike most species, habitat types show a continuum of variation, and in the field there are frequently uninterrupted zonations between types. For example, transitions between 4010 Northern Atlantic wet heaths with Erica tetralix and 4030 European dry heaths frequently occur on lowland heaths and upland moorlands, with the former habitat gradually changing to the latter as soils become drier. Such transitions may be important in their own right and contribute towards the diversity of some sites. Some habitat examples may therefore be intermediate in character between two or more Annex I types. An extreme example of this problem for site selection is illustrated by the selection of rias. These are flooded river valleys typical of south Wales and south-west England and which also occur in France, Spain and Portugal. For the most part, such flooded valleys have limited freshwater influence and are classified as 1160 Large shallow inlets and bays. However, in a few cases substantial rivers enter into rias and give, in the upper part of these inlets, brackish conditions more typical of 1130 Estuaries. For this reason, parts of the ria systems at, for example, Milford Haven and Plymouth Sound have been listed as1130 Estuaries.

The range of variation exhibited by terrestrial habitats has been described in terms of the National Vegetation Classification (Rodwell 1991a,b, 1992, 1995, 2000), where possible. In the case of a small number of habitats, there is a simple correspondence between a single NVC type and an Annex I type, for example 6520 Mountain hay meadows, which shows a limited range of variation and corresponds to NVC type MG3 Anthoxanthum odoratum - Geranium sylvaticum grassland.

In other cases, an Annex I habitat type corresponds to a series of NVC types, e.g. 4030 European dry heaths corresponds to twelve different NVC types. For habitat types such as these, which show a wide range of variation in relation to climatic, edaphic and other factors, the NVC classification of the type has influenced site selection by providing the framework within which the diversity of the habitat type can be categorised and sites selected to reflect this diversity.

In other instances, the NVC is less valuable as a source of reference about the character or variability of a given habitat. For example, the NVC classification did not cover Northern Ireland, and, whilst there is general correspondence of vegetation types between Northern Ireland and England, Scotland and Wales, some vegetation types are probably restricted to Northern Ireland. In other cases the variability of the type is not fully described in the NVC or has been completely omitted, e.g. 7150 Depressions on peat substrates of the Rhynchosporion, various types of vegetation associated with 1220 Perennial vegetation of stony banks (see Sneddon & Randall 1993), and the complex vegetation assemblages associated with types of soft rock cliff within 1230 Vegetated sea cliffs of the Atlantic and Baltic coasts. In the case of certain coastal habitats, such as sand dunes, representative communities can be variable depending on the initial colonising species.

Marine habitats are often characterised by physical features or by sedentary animals such as molluscs and barnacles, as well as by their flora. These are not covered by the NVC, which is a terrestrial and freshwater vegetation classification. Coincident with the selection of sites under the Habitats Directive, work was in hand within the JNCC to develop a National Marine Habitat Classification (Connor et al. 2004), and this ongoing work helped inform the process of selection.

Certain habitat types, such as 1130 Estuaries and 1160 Large shallow inlets and bays, are broad physiographic units, within which other Annex I habitat types, such as 1140 Mudflats and sandflats not covered by sea water at low tide, may occur. In these cases, the larger physiographic habitat type and those included within it have been evaluated independently, as part of the national series for each type.

1.5.4.1.2 Area of habitat type
For the most part, the sites selected contain the largest examples in the UK of the habitat types for which they have been selected. Particular attention has been paid to the selection of sites that host a substantial proportion of the total habitat resource in the UK. For example, Salisbury Plain contains more than 36% of the UK resource of 6210 Semi-natural dry grasslands and scrubland facies on calcareous substrates (Festuco-Brometalia), and Dungeness supports nearly 44% of 1220 Perennial vegetation of stony banks in the UK.

In general, this selection has been reinforced by other criteria; for example, habitat structure and function are most often best conserved in sites that are extensive. There are, however, circumstances where relatively small sites are selected. The most obvious cases are situations where only small sites for a given habitat survive, e.g. the last known surviving area of natural 1340 Inland salt meadows is only about 0.1 ha in extent.

In other cases, relatively small sites are selected to encompass the range of ecological variation or to take account of the geographical range of a habitat. For example, 6210 Semi-natural dry grasslands and scrubland facies on calcareous substrates (Festuco-Brometalia) which occurs on magnesian limestone in north-east England is recognised in the NVC as a distinct type, MG8 Sesleria albicans - Scabiosa columbaria grassland. This distinct type has an ecological character intermediate between southern limestone grasslands and the limestone grasslands typical of northern England. It was always very localised and is now severely reduced. Thrislington, County Durham, is selected as representative of this type, although it covers only 23 ha, contrasting with other sites in the calcareous grassland SAC series, which includes Salisbury Plain, Hampshire/Wiltshire, which at over 21,000 ha is thought to be the largest remaining calcareous grassland in Europe.


Some Annex I habitats are very extensive in the UK, e.g. 4030 European dry heaths and 7130 Blanket bogs which are major landscape components in the north and west of the UK. Many large examples of these habitats have been selected as SACs, but because of their distribution patterns, the SAC series contains a relatively small proportion of their total national extent.

1.5.4.1.3 Conservation of structure and function
The vegetation of most terrestrial sites in the UK's proposed list is either the result of or has been appreciably affected by past and present management practices. Habitat structure and function involve a number of inter-related components. Structure can relate to a variety of biotic and abiotic features, including species composition, the physical architecture of the vegetation, the ground morphology, the successional status of the vegetation, and species assemblages of plants, animals or both. Function relates to the way in which the biotic and abiotic features interact over time. Functions may include energy flows, biogeochemical cycles, hydrology and many other processes.

The value of structure as a criterion for comparing the quality of sites is very variable. Some types have very limited variation in structure. In the example of 6510 Lowland hay meadows (Alopecurus pratensis, Sanguisorba officinalis), the vegetation is characterised by a narrow range of plant forms that provide a vegetation canopy whose structure varies little between sites. Damaging activities, such as fertiliser application (which changes the botanical composition and reduces the biological diversity of such grassland), may cause only very modest changes to physical structure.

In other cases, structural features play a critical part in discrimination between sites. In the case of 8240 Limestone pavements the selected sites are those that are of the highest quality in terms of well-developed 'clint' and 'grike' rock structure, reflected in the pattern of vegetation. Similarly, 7110 Active raised bogs in which the central peat dome and surrounding lagg fen are most complete have been seen as of high value.


Many habitat types occur not as isolated examples surrounded by intensive land use but as parts of mosaics of habitat, notably on the coast and in the uplands. In these situations the juxtaposition of communities and the transitions between them have been seen as an important element of habitat structure.

The evaluation of habitat function is in many cases more difficult than that of structure, because of the complexity of functions and the limitations of our information and understanding of these functions. In some cases certain features are known to be of overriding importance for the maintenance of function. For example, in the freshwater habitat types listed in Annex I, an increase in nutrient status of the water will cause adverse change. However, in many cases the maintenance of habitat function is dependent upon a wide range of biotic and abiotic processes. It would be difficult to define and evaluate these individually, and in general the lack of significant deterioration of the habitat, as evidenced by, for example, the presence of typical native species, has been seen as evidence that habitat function is being conserved. In some cases these influences have been operating for a very long time. Current structure and function may be related to management, and such practices must be continued if the interest of the site is to be maintained.

Annex III also refers to possibilities for restoration of habitat structure and function. Where a sufficient number of examples of habitat types in good condition can be identified, it has been considered unnecessary to select sites that are damaged or in relatively poor condition. However, many sites may require adjustments to management or a modification in human impacts over part of their area. In these cases, the likelihood of successfully restoring structure and function has been a helpful consideration. Where the habitat type is rare in all or part of its range, options for site selection are more limited, and sites needing more significant restoration management may be selected. This is true with a proportion of the listed sites in the lowlands supporting 7230 Alkaline fens, for example, where traditional management has been abandoned. Examples of 7120 Degraded raised bogs still capable of natural regeneration have been selected to complement the SAC series for 7110 Active raised bogs, and all sites require significant restoration management, for example to restore more natural hydrological processes.

1.5.4.1.4 Global assessment
The global assessment is an expert judgement of the overall value of the site for the conservation of the relevant Annex I habitat. It provides an integrated assessment of the other selection criteria, and may also take into account other relevant factors, such as ecological relationships between different habitats and species (European Commission DGXI 1995).


As an overall index of the site's conservation value, particular attention has been paid to the global assessment. Sites have been graded A, B or C, as described in European Commission DGXI (1995). In the UK these gradings have been interpreted as follows:

 

A: Sites holding outstanding examples of the habitat in a European context.


B: Sites holding excellent stands of the habitat, significantly above the threshold for SSSI/ASSI notification but of somewhat lower value than grade A sites.


C: Examples of the habitat which are of at least national interest (i.e. usually above the threshold for SSSI/ASSI notification on terrestrial sites) but not significantly above this. These habitats are not the primary reason for SACs being selected.

There is therefore a distinction between the principal features for which sites have been selected (those graded A or B) and those which are only of secondary interest (those graded C). This is a useful distinction but it is important to note that all three grades are qualifying SAC interest features.


Only examples of features graded A or B for global assessment are described in detail in the present report.

1.5.4.2 Selection principles for Annex II species

1.5.4.2.1 Proportion of UK population
For the most part the sites selected for individual species are those where the evidence indicates that the largest populations occur. In many cases these judgements have to be based not on precise counts of individuals but on estimates of abundance. For example, the number of populations has been taken into account in the case of 1065 Marsh fritillary Euphydryas aurinia. This species has naturally highly variable populations, so the numbers of individuals in a given year are not necessarily an indication of the value of the site in the longer term. In other cases, extent within the site has been relevant and is typically used as a measure of abundance for perennial plants that spread by vegetative reproduction, such as 1614 Creeping marshwort Apium repens.

For widely-distributed species, e.g. 1365 Common seal Phoca vitulina, or where a distinct form occurs, such as the populations of 1065 Marsh fritillary E. aurinia scotica in Scotland, some sites supporting a relatively small proportion of the UK population have been selected to ensure representation of geographic range. Where species occur widely at low population densities, e.g. 1355 Otter Lutra lutra, or are relatively abundant within a more restricted range, e.g. 1083 Stag beetle Lucanus cervus, the differentiation of sites on the basis of population size was rarely possible, and site selection has endeavoured to reflect the range of geographical areas and ecological conditions in which the species is found. For some widespread species the SAC series consequently contains a relatively small proportion of the total UK population. Effective conservation of such species will depend on a combination of site-based and wider environment measures.

1.5.4.2.2 Conservation of features important for species survival
The value of this factor in the comparison of sites is variable. For some Annex II species, such as 1654 Early gentian Gentianella anglica, a small number of habitat features are required to ensure survival: essentially, short open turf and calcareous soils with low nutrient status. For other species, as for example 1355 Otter Lutra lutra, a complex range of site features may be required, including all-year-round availability of food, suitable areas providing cover for young otters and good water quality. The features will vary from site to site, particularly depending on whether it is freshwater or coastal. For some species, the features required for survival are not fully known. In these cases the presence of a persistent population that is known to be stable has been seen as prima facie evidence that habitat conditions are favourable.

1.5.4.2.3 Isolation of species populations
This factor has been found to be relevant to only a small number of species populations in the UK. Isolation has been viewed positively only where populations are large or display distinctive physiological, ecological or genetic features, e.g. the distinctive form of 1065 Marsh fritillary Euphydryas aurinia scotica in Scotland.
1092 White-clawed crayfish Austropotamobius pallipes is a special case for which this factor has been of primary significance in site selection. Crayfish plague (a virulent disease caused by the fungus Aphanomyces astaci) has been introduced into Britain and is spreading through the country, wiping out native crayfish populations. Therefore, selected sites are those which support a significant population that has been recorded over a number of years, but, most importantly, they are isolated from areas of crayfish plague infection and are often cut off from other populations. The degree of isolation is variable and the possibility of crayfish plague spreading to the selected sites cannot be ruled out entirely.

1.5.4.2.4 Global assessment
The global assessment is an evaluation of the overall value of the site for the species concerned. The same grading system has been used as for habitats (see Section 1.5.4.1.4), distinguishing the primary features for which sites have been selected as SACs from those which are of secondary interest.

1.5.4.3 General principles

1.5.4.3.1 Priority/non-priority status
The Directive requires Member States to give special attention to sites containing priority habitat types and species. Although there is no requirement to select every example of priority habitat types and species, site selection has been significantly weighted in favour of priority habitat types in terms of both number of sites and area covered. Of the 23 priority habitat types in the UK, some, such as 7130 Blanket bogs (only a priority habitat if the bog is active), are very extensive, while others, such as 1340 Inland salt meadows, cover a small surface area at only one site. Site selection takes account of this wide variability in the abundance of priority habitat types.


There is only one priority species known currently as a native in the UK, the liverwort 1390 Western rustwort Marsupella profunda, and the two sites where it is a primary reason for selection support the largest known populations in the UK.

1.5.4.3.2 Rarity
The habitat types listed at Annex I of the Directive vary greatly in their abundance in the UK, ranging from 1340 Inland salt meadows, with a total extent of only 0.1 ha, to 7130 Blanket bogs, which is estimated to cover more than 2,000,000 ha. Many of the habitat types are very local and only a small proportion of them are known or estimated to cover more than 50,000 ha in the UK.

 

For the purposes of implementing the Directive, 23 Annex I habitat types are considered to be rare in the UK because their total extent is less than 1,000 ha or because there is a significant or outstanding representation of the habitat type at three or fewer sites (Table 1.5).

 

Table 1.5: Annex I habitats that are rare in the UK, covering less than 1,000 ha or with a significant or outstanding representation of the habitat type at three or fewer sites.

EU code Directive name
1180 Submarine structures made by leaking gases
1210 Annual vegetation of drift lines
1320 Spartina swards (Spartinion maritimae) 7
1340 Inland salt meadows
1420 Mediterranean and thermo-Atlantic halophilous scrubs (Sarcocornetea fruticosi)
2110 Embryonic shifting dunes
2160 Dunes with Hippophae rhamnoides
2170 Dunes with Salix repens ssp. argentea (Salicion arenariae)
2250 *Coastal dunes with Juniperus spp.
2330 Inland dunes with open Corynephorus and Agrostis grasslands
3110 Oligotrophic waters containing very few minerals of sandy plains (Littorelletalia uniflorae)
3170 *Mediterranean temporary ponds
3180 *Turloughs
4020 *Temperate Atlantic wet heaths with Erica ciliaris and Erica tetralix
4040 Dry Atlantic coastal heaths with Erica vagans
4080 Sub-Arctic Salix spp. scrub
5110 Stable xerothermophilous formations with Buxus sempervirens on rock slopes (Berberidion p.p.)
6170 Alpine and subalpine calcareous grasslands
6430 Hydrophilous tall herb fringe communities of plains and of the montane to alpine levels
6520 Mountain hay meadows
7150 Depressions on peat substrates of the Rhynchosporion
7240 *Alpine pioneer formations of the Caricion bicoloris-atrofuscae
9160 Sub-Atlantic and medio-European oak or oak-hornbeam forests of the Carpinion betuli

 

Selection has aimed to ensure that the majority of the area covered by these habitat types has been included in the SAC series (other than for 1180 Submarine structures made by leaking gases, for which site selection in offshore waters is still in progress). It should be noted that rare habitat types vary in their patterns of distribution. Some, such as 2250 Coastal dunes with Juniperus spp., are very localised and found at only a small number of sites. In these cases all or almost all of the UK resource is included within the SAC series. However, some other rare habitats, e.g. 6430 Hydrophilous tall herb fringe communities of plains and of the montane to alpine levels, are widely distributed, with a relatively small amount of the resource found at a larger number of sites. In these cases, although a large proportion of the resource is included in the SAC series, some small fragmentary examples with poor representation of the main features of the type have not been selected.

 

Twelve Annex II species are recorded from 15 or fewer 10x10 km squares of the national grid in the UK and are considered to be nationally rare (Red Data Book species) (Table 1.6). For each of these species a high proportion of the total UK population is included within the SAC series (other than for two species added to Annex II in 2003, 4056 Ram's-horn snail Anisus vorticulus and 4035 Fisher's estuarine moth Gortyna borelii lunata, for which selection rationale are still under consideration).

Table 1.6 Annex II species which are rare in the UK, recorded from 15 or fewer 10x10 km squares of the national grid.

EU code
Directive name Common name
Limoniscus violaceus Violet click beetle
4035
Gortyna borelii lunata Fisher's estuarine moth
Vertigo angustior Narrow-mouthed whorl snail
Vertigo genesii Round-mouthed whorl snail
4056
Anisus vorticulus Ram's-horn snail
Buxbaumia viridis Green shield-moss
*Marsupella profunda Western rustwort
Trichomanes speciosum Killarney fern
Rumex rupestris Shore dock
Apium repens Creeping marshwort
Cypripedium calceolus Lady's-slipper orchid
Liparis loeselii Fen orchid

 

1.5.4.3.3 Geographical range
In the case of both habitat types and species, favourable conservation status is dependent upon the maintenance of the geographical range of the habitat type or species, amongst other things. The SAC series for each habitat type and species has been selected to reflect its distribution in the UK. Habitat types and species vary considerably in their patterns of distribution. Some, such as 4030 European dry heaths, are found in all parts of the UK and the SAC series reflects this. Others, such as 4040 Dry Atlantic coastal heaths with Erica vagans, are highly localised. However, there are also habitat types and species with very disjunct distributions, in that they occur in two or more parts of the UK that are widely separated. For example, 3160 Natural dystrophic lakes and ponds are commonly associated with blanket bog in the north and west of the UK, but also occur rarely on lowland heaths in southern Britain. The sites selected reflect this disjunct distribution.


Frequently, where a very high proportion of the resource for a relatively widespread species or habitat type occurs in a given part of the UK, a high proportion of sites are selected in these centres of distribution.

1.5.4.3.4 Special UK responsibilities
The UK has special responsibility in the EU for certain habitat types and species because we hold a large proportion of the European resource. Endemic or near-endemic habitat types, such as 91C0 Caledonian forest, 91A0 Old sessile oak woods with Ilex and Blechnum in the British Isles, and 1654 Early gentian Gentianella anglica, are obvious examples. There are others, such as 4010 Northern Atlantic wet heaths with Erica tetralix, 7130 Blanket bogs and 1364 Grey seal Halichoerus grypus, where the UK resource is relatively large compared with that of other Member States. For these, and other habitat types and species where the UK hosts a disproportionately large part of the EU resource, the number, and sometimes the area, of UK sites selected is generally higher than for other habitat types and species, making a significant UK contribution to the maintenance of favourable conservation status in the EU. For some habitat types and species there is insufficient information upon which to make judgements about the proportion held by the UK. In these cases some adjustments to the list could be appropriate if EU-level assessment demonstrates the importance of the UK resource to be higher, or lower, than is currently known.

The proportion of the UK resource selected is, however, not a simple reflection of the proportion of the EU habitat type or species resource in the UK. The sites selected are themselves all of high quality when judged against other criteria. An objective of the site selection process has been to ensure that selection is consistently based upon all the relevant factors for each habitat type and species. For example, 91C0 Caledonian forest is a priority habitat, covers a small geographical area, and all the EU resource occurs in the UK, but it exhibits a relatively narrow range of variation. Taking this into account, approximately 60% of the total UK resource, and over 85% of core ancient forest, has been selected. A high proportion of the EU resource of 4030 European dry heaths also occurs in the UK, but this is a non-priority habitat, covers a large geographical area, and exhibits a wide range of variation. The proportion of the UK resource of 4030 European dry heaths proposed for inclusion on the national list is, therefore, less than for 91C0 Caledonian forest, but nevertheless ensures adequate representation of the range of variation.

1.5.4.3.5 Multiple interest
Sites with multiple interests are of high intrinsic value. The Directive recognises this in its emphasis on the maintenance of biodiversity. Special emphasis has been given to the identification and delimitation of sites containing a multiplicity of high-quality interests forming an ecologically functional unit. In general, sites supporting the largest numbers of qualifying features are amongst the largest sites. Examples include upland sites (e.g. Moor House - Upper Teesdale, the Cairngorms, and Eryri/Snowdonia), and coastal sites (e.g. Morecambe Bay, and Dornoch Firth and Morrich More). This reflects the fact that the most extensive natural and semi-natural landscapes in the UK occur in upland and coastal regions. Consequently, Scotland and Wales contain the greatest SAC land area in relative terms, and individual sites are larger, on average, than those in England or Northern Ireland. Non-coastal lowland sites with large numbers of high-quality interests are much rarer, but include some outstanding examples, such as the New Forest, Dorset Heaths (Purbeck and Wareham) and Studland Dunes, and the Broads. Few sites in Europe have an ecological character similar to these areas.

A site selection process that only included multiple interest sites would have provided an inadequate representation of some habitat types and species in the UK. A number of habitat types and species, most typically those of lowland situations, are seldom found as part of habitat mosaics, as they occur most often in intensively-managed countryside, where semi-natural habitats and associated species populations are highly fragmented. They include several rare habitat types, such as 1340 Inland salt meadows, as well as more widespread habitat types and species, such as 7110 Active raised bogs and 1166 Great crested newt Triturus cristatus. Consequently, a significant proportion of sites support only one or a few features (see Figure 1.2).

SAC selection fig. 1.2 Numbers of features on cSACs in the UK
Figure 1.2 Numbers of features on cSACs in the UK, August 2008.

 

7 In the UK stands of this Annex I type have only been selected as SACs where they are dominated by Spartina maritima, Spartina alterniflora, or the rare and local hybrid Spartina x townsendii (European Commission DGXI 1996)