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Towards a strategy for Scotland's biodiversity
THE RESOURCE AND TRENDS

2. TRENDS IN BIODIVERSITY

Alister Jones, Ed Mackey, Noranne Ellis and Joanna Drewitt

2.1 Introduction

Trends reflect change and environmental change is continuous. Scotland's environment is the result of several thousands of years of change, with ecosystems developing under particular combinations of edaphic, topographic and climatic influences, and the activities of people. Directly or indirectly, human influence has come to be a dominant cause of change in recent times.

The aim of this chapter is to identify and explain some of the most prominent and relatively recent changes in biodiversity. It is often in the nature of biological sample data that confidence limits are wide, and surveys designed in the past do not entirely satisfy present day needs. Best use has, however, been made of available data, with an emphasis on those results that are statistically significant.

This chapter is divided into three parts, examining biodiversity trends, looking more closely at reported trends in BAP priority habitats and species, and considering the key influences that have an impact on biodiversity in Scotland.

2.2 Biodiversity trends

A description of biodiversity trends characterised by habitat settings is not ideal. It may appear to be compartmentalised, with insufficient regard being given to the natural processes upon which biodiversity depends, or to ecosystem functions and interactions within and between environmental media (for example between soil and water, the edge effect of ecotones, or the importance of habitat structure and connectivity). Nevertheless, trend data have for the most part been gathered in that way, and the convention adopted is to refer to the same settings that were used in Action for Scotland's Biodiversity (Usher, 2000), with an additional brief note on soils. Each section commences with an analysis of recent and current trends and concludes with a brief statement of predicted effects of climate change.

2.2.1 The seas, coasts and firths

Much of the Scottish coast is a naturally dynamic environment of erosion and accretion in which continuous processes of habitat creation and ecological succession are vital to maintaining species-richness (Usher, 1999). Yet it is on parts of the coast, and around the major Firths, that a host of social, economic and environmental pressures come together (Sankey, 1999). About 10 per cent of the Scottish coastline has been affected by intensive urban or industrial use (Ritchie, 1999); in the upper Forth estuary, for example, it has been estimated that 50 per cent of the intertidal mudflats have been claimed by various forms of land use and development.

The recent trend has been one of improving water quality in coastal waters, and severe pollution has, in general, become localised around sea outfalls, inappropriately sited fish farms or sludge and spoil dumping sites. A build-up of nitrates from diffuse pollution within the River Ythan catchment in north-east Scotland has enriched estuarine mudflats such that dense algal mats now affect invertebrate communities living in the mud. Land-based pollution from waste water and sewage is being brought under stricter control, and the input of nutrients and contaminants to the North Sea has been reduced since the dumping of sewage sludge stopped in 1998. Although much remains to be done to bring pollution under control, coastal water quality was assessed as predominantly excellent or good between 1996 and 1999, with little more than 3 per cent of the coastline classified as unsatisfactory or seriously polluted at any time.

Seriously polluted areas within Scottish estuaries were reduced by 40 per cent between 1996 and 1999. In the 1970s, the faunal composition of the upper Clyde estuary sediments was dominated by a few pollution-tolerant species. By the late 1990s it had begun to change in favour of a more diverse invertebrate community, and fish species richness increased from 25 to 40. Similarly in the Forth, returning fish populations, including smelt ( Osmerus eperlanus) which is particularly sensitive to low oxygen levels, have been linked to increasing levels of dissolved oxygen and declining organic and metallic waste levels.

The population sizes of harbour or common seal ( Phoca vitulina) and grey seal ( Halichoerus grypus) have remained stable or increased (respectively) over the past decade. However, phocine distemper virus is now present again in British waters and is likely to impact especially upon harbour seals which are particularly susceptible. Between 1969-70 and 1985-87 four out of 18 seabird species, such as cormorant ( Phalacrocorax carbo) and roseate tern ( Sterna dougalli), showed marked declines. Conversely, 11 seabird species, including the Arctic skua ( Stercorarius parasiticus), great skua ( Stercorarius skua) and gannet ( Morus bassanus), showed marked increases in breeding populations. In some cases, however, seabird populations have become sustained at artificially high numbers due to abundant feeding on fisheries discards.

Exploitation of fish in Scottish waters has a long history, with a wide range of species being taken from intertidal, coastal and offshore habitats. Intensive trawling disturbance may have affected the species composition of the seabed community in parts of the North Sea. Compared with conditions at the start of the 20th century, species composition in parts of the North Sea had changed by 1986. Scavenging crustaceans and seastars had displaced bivalve molluscs, long-lived species had declined and opportunistic and scavenging species had increased in abundance. Of the 12 commercially exploited fin-fish species for which Scottish data are available, nine are currently considered to be fished outside safe biological limits (based on spawning stock biomass, fishing mortality and recruitment), and some individual stocks, such as cod ( Gadus morhua), have been brought to the point of collapse. The depletion of traditionally fished stocks has, in turn, stimulated the targeting of previously unexploited species and fish of deeper waters off the Continental Shelf. Current levels of exploitation of deep-water fish, such as orange roughy ( Hoplostethus atlanticus), which grows and matures slowly, are thought to be unsustainable. Furthermore, the passage of trawl gear is especially destructive to species and habitats in deep-water environments that rarely experience changes, and damage to the deep-water reef structures constructed by the coral ( Lophelia pertusa) is of international concern.

Sea temperatures increased by 0.5°C, on average, between 1871and 2000. By 2100 they may be as much as 2 to 3°C higher than now (Hulme et al., 2002), with the east coast warming at a greater rate. Increasing air and seawater temperatures are likely to result in an increased diversity of marine life, with species such as pink sea-fan ( Eunicella verrucosa), a red alga ( Anotrichium barbatum) and the honeycomb worm ( Sabellaria alveolata), probably extending their ranges northwards into Scottish waters. On the other hand, species such as northern sea-pen ( Swiftia pallida) and green sea urchin ( Strongylocentrotis droebachiensis) may retreat and ultimately disappear from our coasts (Hiscock et al., 2001).

Sea level is predicted to rise by varying amounts relative to the land, depending on the assumptions and the models used, but it is not expected to exceed 69 cm by 2080; contrast Hulme et al. (2002) and Dawson et al. (2001). The Northern and Western Isles are expected to experience the greatest rise, indicating a need for a 'managed retreat' along some areas of the coast and within the firths. Climate models are poor at simulating high intensity wind speeds and predictions for occurrences of storm events are uncertain, but when they occur they are expected have a major adverse effect on the coast. However, westerlies are expected to be the prevailing wind direction (Hulme et al., 2002).

2.2.2 Lochs, rivers and marshes

Scotland has at least 30,000 lochs and lochans, and over 10,000 burns and rivers. In 1997, sites surveyed for river habitat included some that were categorised as obviously modified (15 per cent), extensively modified (12 per cent) or heavily modified (1 per cent), reflecting the impacts of agriculture and urban development. By 1999, some 4,000 km of classified river length (about 10 per cent of the total) and 30 out of 150 classified lochs were polluted (SEPA, 1999). Nevertheless, the general trend in recent decades has been of improving water quality. Levels of contamination implicated in the former decline of the otter ( Lutra lutra) have gradually been reduced. Water quality in central Scotland has improved since the decline of heavy industry and the introduction of better sewage treatment facilities, such that the restoration of fish stocks has allowed the otter to re-occupy much of its former range in lowland Scotland (Green & Green, 1997).

Eutrophication, particularly from phosphorus-rich discharges, has degraded the ecological status of Loch Leven, with knock-on effects on its value as a trout fishery. Pollution of the only Scottish lochs in which the vendace ( Coregonus albula) was known led to its extinction, although it was subsequently re-introduced from English populations into two other lochs. By 2000, some 50 per cent of the native freshwater fish species were thought to have declined throughout Scotland, while non-native fish species had generally expanded. About half of the world's functional populations of freshwater pearl mussel ( Margaritifera margaritifera) are found in Scotland, but by the late 1990s, only 7 per cent of mussel beds were classified as being in a near-natural state and 65 per cent had no juveniles (Cosgrove et al., 2000). An increasing number of heavy rainfall events have also been associated with uprooting pearl mussel beds and transporting them downstream (Hastie et al., in press).

The acidification of Scotland's fresh waters, notably in upland parts of south-western Scotland, has been linked to declines in the distribution of the dipper ( Cinclus cinclus). Acidification has also been related to reductions in otter distribution and abundance, with otters absent from 61 per cent of km squares where acid deposition exceeded critical loads in 1997 (Fowler et al., 2002). However, many previously acidified freshwater systems in upland and south-west Scotland have recovered to the point that a number of acid-sensitive diatom species have returned (Harriman et al., 1995; Soulsby et al., 1997). Given further reduction in acidifying pollutants such improvements should continue, but it may take decades to realise the full ecological effects (Battarbee et al., 1988).

Ponds are an important habitat for wildlife, especially aquatic plants, invertebrates and amphibians. The National Amphibian Survey in Scotland (1983-1992) indicated a median percentage loss per survey of 7 per cent since the 1950s, mainly due to agricultural practices, mismanagement and in-filling. The largest decreases were in Fife (57 per cent) and the former Lothian Region area (42 per cent). Estimates from the Countryside Survey reported no significant change in Scotland for the period 1990 to 1998. By 1996, the number of sites where the great crested newt ( Triturus cristatus) had been recorded since 1876 had fallen by 55 per cent.

The Atlantic salmon ( Salmo salar) is locally extinct in some rivers in the West of Scotland but is becoming re-established in formerly polluted rivers of the Clyde and Forth catchments. Its status is a matter of current conservation concern. On the basis of catch records, including those from net fisheries that have now been mostly closed, salmon in Scotland declined by 82 per cent between the 1960s and the late 1990s. The genetic diversity of distinct populations within Scottish rivers is threatened by declines in the number of salmon returning to their natal rivers and by hybridisation with escaped farmed fish.

Of the breeding bird species dependent on fresh waters, five species expanded their range in Scotland between 1970 and 1990. These included the kingfisher ( Alcedo atthis) and the grey heron ( Ardea cinerea), both of which may have benefited from milder winters as well as improved water quality. At the same time the range of nine species reduced. In most cases the reasons are unclear, though the decline of the moorhen ( Gallinula chloropus) is possibly linked to land drainage.

The non-native American mink ( Mustela vison) was introduced for fur farming in 1938 and subsequently escaped. Mink can now be found throughout much of Scotland, except in some Hebridean Isles and the Northern Isles. The impact of mink on ground nesting birds, waterfowl and water voles has been serious (Craik, 1997).

Winters are expected to continue to become wetter and summers to become drier over Scotland as a whole (as has happened over the last century), although retaining the west-to-east gradient in the annual total rainfall. However, the models indicate that the far north of Scotland may experience an increase in summer precipitation variability (Hulme et al., 2002). Lower flow rates in small water bodies and upland streams during the summer may exacerbate the degree of warming, with potentially greater adverse effects on freshwater species. However, heavy rainfall events will lead to a greater frequency of flooding and possible damage to spawning gravels.

2.2.3 Farmland and lowland grassland

Intensification and specialisation of Scottish farming have transformed the appearance and structure of lowland farmland. Over recent decades, the west has become more dominantly pastoral and the east more dominantly arable. According to the agricultural census, hay production declined by 34 per cent and silage production increased by 8 per cent between 1991 and 2000 (FPD Savills, 2001). In north-west Scotland there was a reduction in the number of crofters between 1960 and 1985, a reduction in tillage and the replacement of cattle grazing by sheep (RSPB & Scottish Crofters Union, 1992).

Mixed farming declined by 23 per cent between 1987 and 1999. Many hedgerows, despite their importance for biodiversity, became redundant as stock barriers, and were removed - the overall length reducing by half between 1947 and 1988 (Mackey et al., 1998). The effect was noticeable in western pastoral areas, and most evident in the eastern grain-growing areas, as it coincided with the ongoing trend towards larger field units to facilitate the use of modern farming equipment. No further reduction was detected between 1990 and 1998 (Haines-Young et al., 2000). Farm woodland recorded in the June Agricultural Census doubled between 1991 and 2000.

These are all symptomatic of significant changes in the mosaic of habitats that have been sustained by mixed farming systems. Between around 1970 and 1990, 12 species of farmland bird, such as the barn owl ( Tyto alba), contracted in range by more than 10 per cent. Similarly between 1994 and 1999, the abundance of three out of 13 widespread farmland species, the skylark ( Alauda arvensis), lapwing ( Vanellus vanellus) and kestrel ( Falco tinninculus), showed a statistically significant decrease.

Environmentally friendly farming practices have been shown to enable wildlife such as the corncrake ( Crex crex) to recover or re-establish in its core areas (Green & Riley, 1999). By contrast seed banks of native flora on agricultural land are now depleted due to intensive agricultural practices and the return of arable flora is unlikely to occur naturally (Ford, 1997). The creation or re-establishment of habitats such as wildflower meadows from seed carries a potential risk of the genetic structure of native species being affected by the use of non-native provenances, either by displacing plants of local origin or through hybridisation (Welch et al., 2001). An example is the non-native variety of bird's-foot trefoil ( Lotus corniculatus var . sativus) which is more vigorous in growth and out-competes native varieties.

Cereal farming since the 1950s has seen a steady decline in the use of oats as a crop (down to less than 10 per cent of the total cereal area), whilst the area of barley has expanded. Recent studies on the genetics of the current varieties of barley show that they can be traced back to only 17 foundation cultivars, indicating a declining extent of genetic diversity in modern spring barley.

Climatic changes are not expected to increase biodiversity on farmland because, although warmer average temperatures may encourage some species of butterflies and birds to extend their ranges northwards, not all species will have the ability to shift northwards at the same rate. For those that do there may be a lack of suitable habitat(s). New crops and cropping patterns will undoubtedly need to be considered to take advantage of warmer, drier summers and the constraints of wetter autumns. Altitudinal limits of managed land could move further up the hill, reducing the extent of upland semi-natural habitats.

2.2.4 Forests and woodland

Woodlands are the climatic climax vegetation over most of Scotland, but had been reduced to less than 5 per cent of Scotland's land area by the early 1900s. Forest and woodland cover in Scotland expanded to around 17 per cent by 2001 (FC, 2002), with the steepest rise being in Dumfriesshire, Argyll and parts of the Scottish Borders. This afforestation (which was strongly driven by government policy) occurred largely between the 1940s and the late 1980s. Most of it consisted of plantations of non-native conifers, and was achieved by displacing rough grassland, blanket mire and heather moorland, as well as by conversion of former broadleaved woodland - the extent of the latter declining by 26 per cent during this period.

The area of woodland with native species increased by one-third between 1984 and 1999. This included a 9 per cent expansion of broadleaved woodland between 1990 and 1998, mainly on grasslands, and the restoration of native woodlands tree cover in plantations on ancient woodland sites. There was also a significant expansion in new woodlands of Scots pine ( Pinus sylvestris), formed by the incremental enlargement of existing native pinewoods by natural regeneration, or by planting of other suitable sites using plants raised from seed of the appropriate biochemical origin for the locality (FC, 1999).

Native woodland species, and those of previously open habitats will have been affected by the loss and fragmentation of their habitat, the change to conifer plantations or changes in traditional management (e.g. the demise of coppicing). However, given the intensification of management of adjacent (non-woodland) areas, remnant woodlands have become relatively more important for species like the pearl bordered fritillary ( Boloria euphrosyne). Perversely, for a short time, threatened butterflies were doing better in native woodlands that had been partially cleared and planted with conifers than they were in unmanaged woods (Warren & Key, 1991).

Between around 1970 and 1990, ten woodland species, including the woodcock ( Scolopax rusticola), contracted in range by more than 10 per cent. Eleven others, such as the wood warbler ( Phylloscopus sibilatrix), expanded. Between 1994 and 1999, five out of 14 widespread woodland species, e.g. the great tit ( Parus major), showed a statistically significant increase in abundance. The geographical range of the capercaillie ( Tetrao urogallus), the largest of the native grouse, contracted by 64 per cent between around 1970 and 1990. More targeted surveys have shown that the number of individuals declined by 51 per cent between around 1992 and 1999, from an estimated 2,200 to 1,070 birds. Several factors have been implicated - the main ones being habitat fragmentation and management, deer fencing, disturbance, and adverse weather conditions particularly during the chick-rearing period.

Fluctuations in species abundance and composition can be a feature of any woodland's development. This is very evident in plantations where, as they mature, they attract a higher proportion of species that are the same as, or similar to, the characteristic species of semi-natural woodlands, particularly fungi and invertebrates (Humphrey et al., 2002). Since the early 1990s there has been more emphasis on restructuring conifer plantations. This is usually carried out as they approach the end of their optimum economic life cycle, and is coupled with more appropriate management of the non-wooded areas within forest boundaries, and the replanting (after felling) of a wider range of tree species.

With mean annual temperatures expected to rise by 1 to 3.5°C by the end of the century (Hulme et al., 2002) some southerly species are expected to extend their ranges into Scotland. For example, the nuthatch ( Sitta europa) has already been observed within woodlands in the Borders. Species composition within woodland habitats may alter, initially with the addition of species with good dispersal capability from the south (particularly birds and invertebrates), and then over a much longer period as the relative dominance of the tree species within individual woods changes. For example, birch ( Betula spp.), which is already a constituent species of many pinewoods, may become a bigger component; similarly, but probably to a lesser extent, oak ( Quercus spp.) may increase around the margins of pinewoods where soil conditions are favourable. There is likely also to be an upward shift in the treeline from the 650 m altitude today.

2.2.5 Mountains, heaths and bogs

Montane environments are among the least altered in Scotland. Their arctic-alpine communities of plants and animals echo Scotland's post-glacial past. Nevertheless, they are vulnerable to disturbance by, for example, excessive trampling, climate change and air pollution. Nitrogen enrichment, together with grazing pressure, have been implicated in declines in the extent and quality of montane heaths dominated by the moss Racomitrium lanuginosum (Baddeley et al., 1994). Increased tissue nitrogen, associated with air pollution during the twentieth century, also poses a threat to snowbed bryophyte communities (Woolgrove & Woodin, 1996a,b).

Moorlands, peatland and rough grassland form a mosaic of semi-natural habitats covering more than 50 per cent of Scotland's land area. Apart from the places where tree cover is naturally limited due to severe wetness, cold or exposure, the condition of other semi-natural habitats and woodland regeneration is governed mainly by the grazing of sheep and/or deer. Overgrazing has become evident in parts of the uplands due to the number of sheep increasing by 32 per cent between 1950 and 1990, and the red deer ( Cervus elaphus) population allegedly doubling between 1959 and 1989. As a consequence the natural regeneration of trees has been arrested and the condition of heather moorland has deteriorated.

Between the late 1940s and the late 1980s, the overall area of blanket bogs and the less extensive lowland bogs were reduced by 21 per cent and 44 per cent respectively (Mackey et al., 1998). In each case the main cause was afforestation, and to a lesser extent agricultural improvement; commercial peat extraction was an additional factor on lowland bogs. Between 1990 and 1998, no change was detected in their extent (Haines-Young et al., 2000).

Conditions are optimal for the development of heather moorland in Scotland (Thompson et al., 1995). Sporting estates flourished from the 1870s onwards, but their economic value has declined since the 1930s. Fewer gamekeepers to manage the sporting interest, rising sheep numbers, and falling standards of muirburn have contributed to a decline in the heather cover upon which grouse rely (Watson & Lance, 1984). Between the 1940s and 1980s, heather moorland was reduced by about 23 per cent through afforestation and conversion to rough grassland. Weaker evidence suggests that a further decline of 5 per cent between 1990 and 1998 was associated mainly with forest cover (or canopy closure) and conversion to grassland. Also within dwarf shrub heaths, the number of species characteristic of low-nutrient soils declined, and those typical of more fertile conditions increased (McGowan et al., 2002).

Rough grassland decreased by 10 per cent between 1947 and 1988, due mainly to afforestation and grassland improvement. An estimated reduction of 16 per cent in the extent of calcareous grassland in the marginal uplands between 1990 and 1998 was due mainly to its conversion to more intensively managed grassland. However, a 19 per cent expansion of fen, marsh and swamp is partly attributed to reduced or abandoned grassland drainage in wetter areas (McGowan et al., 2002).

The number and distribution of recent cases suggests that the illegal persecution of raptors is widespread in Scotland, severely limiting the breeding success and distribution of hen harriers ( Circus cyaneus) and suppressing golden eagle ( Aquila chrysaetos) recruitment (RSPB, 2001). Incidents of alleged raptor persecution are associated geographically with parts of the country where game shooting is practised and, compared with the rest of the UK, they are disproportionately common in Scotland (RSPB, 1999).

The area affected by potentially harmful levels of nitrogen deposition on peatland ecosystems is expected to decline by 28 per cent by 2010 (Mackey et al., 2001). Global ozone concentrations are, however, expected to continue to rise over the next decade, and will be more persistent in upland areas. High levels of ozone exposure reduce the productivity of crops, although it is not yet known which species are most sensitive it.

With a rise in mean annual temperature, vegetation zones will shift by between 250 and 1,400 km northwards, and uphill by between 200 and 960 m. The extent of the arctic-alpine habitat will therefore decrease by 93 per cent on a best case scenario and, at worst, will disappear completely. However, with the expectation that the high wind speeds already experienced across many areas of Scotland will continue, then there is some hope that pockets of suitable micro-climate may continue for some arctic-alpine species. The arctic-alpine species expected to be adversely affected include lichens, such as alpine sulphur-tresses ( Alectoria ochroleuca) and snow caloplaca ( Caloplaca nivalis), vascular plants such as the tufted saxifrage ( Saxifraga caespitosa) and drooping saxifrage ( Saxifraga cernua), snow-patch moss ( Kiareria starkei), and montane birds, such as snow bunting ( Plectrophenax nivalis), ptarmigan ( Lagopus mutus) and dotterel ( Charadrius morinellus). Heavy rain events in autumn and winter may cause physical and chemical alterations in ecosystems, such as greater erosion of upland peatlands and extreme flows in upland streams.

2.2.6 Soil

The productivity of soil, the largely unseen foundation of much of Scotland's biodiversity, is often an inheritance of reclamation, cultivation and manuring over the last 400 years (Davidson & Smout, 1996). Land use change and pollution can have large impacts on soil biota and species richness (Usher, 1996). In recent decades, more intensive farming and forestry practices, together with urban and industrial development, have in places damaged soil and water processes (Bullock & Thompson, 1996). Soils may be polluted and contaminated, eroded, compacted, truncated and lose their ecosystem functions; as a terrestrial carbon reservoir, soil properties can be affected by the drainage of wetlands, afforestation, peat extraction, cultivation and climate change (Taylor, 1995; Puri et al., 1999). It has been concluded, nevertheless, that the lack of data on trends in soil properties makes it impossible to assess whether current land use practices and pollutant inputs to soil are sustainable (SEPA, 2001).

Warmer temperatures have been related to a 65 per cent increase in the concentration of dissolved organic carbon in freshwaters across the UK between 1988 and 2000 (Freeman et al., 2001). With increasing soil temperatures in the future, concurrent with drier soils in summer, the rate of decomposition may increase. The concern is that the soil carbon store is released to the freshwater environment, and thereby relocated to the oceans.

2.3 Reported trends in BAP priority habitats and species

The most recently assessed trends in the status of all UK BAP priority species is to be found in the analysis of the BAP Lead Partner reports in 1999 (Kerr et al., 2000; Jones et al., 2001). It is recognised that the size and distribution of species populations was only one of the criteria used to determine BAP status, and that the validity of some of the assessments made in 1999 were questionable, due to insufficient information. The results are summarised in Table 2.1 and given in full in Annex 2.1. Though the BAP implementation process was then at an early stage, the figures are broadly encouraging, indicating that the habitats and species for which Scotland has a dominant share (i.e. over 50 per cent of the habitat area or over 50 per cent of the species population) were performing slightly better than the other BAPs.

Table 2.1. Comparison of biological status of BAP habitats and species in Scotland. All data are percentages. The 'lost/extinct' category includes only species that had disappeared from Scotland before the SAPs were published.

2.4 Key influences

Many of the positive and negative trends are directly or indirectly attributable to key influences affecting our environment, such as those listed in Table 2.2. The examples are drawn from the National Prospectuses of Natural Heritage Futures (SNH, 2002), but have been grouped and 'scored' for their relative impacts on biodiversity. The list may not be exhaustive, and there will not be complete agreement on the scale, severity or cross-boundary effects of some impacts. The list, however, provides a framework for examining key influences in a semi-systematic way. For example, one may quickly identify those mechanisms that could be directed towards more positive outcomes, such as the EU Common Agricultural and Fisheries Policies. It can also flag up anomalies, where what is beneficial in one sector may have an adverse effect in another sector, such as the ending of sewage dumping at sea.

The BAP Lead Partners reports (Kerr et al., 2000; Jones et al., 2001) also included an analysis of specific 'constraints' to BAP implementation, now summarised in Table 2.3. Certain aspects had been referred to in the BAPs themselves as 'threats' to specific habitat or species, and the similarity between them and the key influences identified in Table 2.2 suggests that many of the issues have been evident for a long time.

Table 2.2. Key Influences on Scottish biodiversity. The following symbols have been used: ++++ for a strong positive impact, + for a low positive impact, C to be viewed in context, - for a low negative impact, ---- for a strong negative impact, and a dot (·) for possible neutrality.

Table 2.3. Analysis of 'constraints' to the implementation of the Scottish BAPs (from Jones et al., 2001). These are data based on Lead Partner reports for 21 HAPs and 150 SAPs relevant to Scotland.

2.5 Concluding remarks

Although the best available data have been used, they were not necessarily designed or captured for the present purpose. They give a partial and approximate view of some of the more obvious trends in Scottish biodiversity. More thought needs to be given to biodiversity data needs in Scotland, and it should be done in ways that facilitate meaningful comparisons with the rest of the UK and Europe.

In the meantime, we should recognise the inherent dynamism and inevitable complexity of the environment given the cumulative effect of natural processes and human impacts upon it. The rate of change has been increasing due to the intensification of land use and other development pressures in both urban and rural areas (including water catchment and forestry). This has resulted in a reduction of 17 per cent in semi-natural land in the 40 years or so up to 1988 (Mackey et al., 1998). In the subsequent decade, some declines, such as hedgerow loss, appear to have been arrested (McGowan et al., 2002). Climate change is expected to become more evident in the future.

Restoration of coastal seabed life to some, mostly unknown, pristine condition that pertained prior to exploitation is unachievable. It is probable that these ecosystems can exist in multiple stable states. However, there is a need to develop marine policies and management measures that aim to restore sustainability to the exploitation of Scotland's seas.

As a consequence of the loss, fragmentation or degradation of semi-natural habitats, the populations of many species have declined to non-viable levels; others have been directly destroyed (e.g. by collecting or over-exploitation). Non-native species have also been introduced, with some becoming invasive and damaging to native biodiversity. This includes species such as the signal crayfish ( Pacifastacus leniusculus), which carries the crayfish plague that is inimical to native white-clawed crayfish ( Austropotamobius pallipes), and feral Sika deer ( Cervus nippon), able to hybridise with the red deer ( Cervus elaphus) and now occupying about one-third of the native red deer range. It also includes locally non-native species, such as the hedgehog ( Erinaceus europaeus), which has proved highly destructive to ground nesting birds since its release within the Western Isles.

Many favourable trends in biodiversity have been brought about, or assisted, by political or other strategic level intervention, regulation and the promotion (and funding) of better land and water management practices. Biodiversity Action Plans have identified vulnerable habitats and native species for targeted action. A Scottish Biodiversity Strategy can build upon the environmental improvements that are evident, but it must also tackle the many problems that remain, recognising that new concerns will emerge through time.

Annex 2.1. The assessment of the status of BAP species described by Jones et al. (2001). The species are grouped by habitats, as in Usher (2000). The status column indicates that the species has recovered (1) or is showing signs of recovery (2), or that there is apparently no change (3). Species that have declined further are given status (4), whilst those that have become extinct in Scotland prior to the SAP publication are given status (5) - no species has become extinct since plan publication. The status of some species remains unknown (?). More recent information about BAP species is constantly becoming available; see www.ukbap.org.uk.

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