Information and Advisory Note Number 129                                   Back to menu

NATURAL HERITAGE TRENDS Species diversity: introduction

1. Species Diversity

1.1 Scotland and its surrounding seas may support up to 90,000 native species (Usher, 1997); perhaps less than 1% of the world total (Hawksworth and Kalin-Arroyo, 1995). Scotland's species complement is, nonetheless, remarkable for its diverse mix of sub-tropical, Atlantic, arctic, arctic-alpine and boreal elements, many of which are on the extreme edge of their global range. Some have highly disjunct ranges which otherwise include only a few widely dispersed localities in Asia, North or tropical America. Thirty Scottish species occur nowhere else in the world.

1.2 A relatively mild, wet climate is particularly favourable to 'lower' plants. Consequently, more than half of the liverworts and mosses, and over a third of the lichens of Europe, occur in Scotland. Being sensitive to air and water pollutants, many lower plants have their European stronghold along the Atlantic coast and Western Isles. Also of international importance are Scotland's breeding populations of seabirds and grey seals, and over-wintering geese.

1.3 Species richness varies markedly across Scotland [Figure 1], Peatlands, for example, are inherently species-poor, whereas heaths and broadleaved woodlands, particularly in areas favourable for lower plants, tend to be species-rich.

1.4 Many species have become restricted to modified remnants of their original habitats. Between the 1940s and 1980s an estimated 17% of semi-natural habitat in Scotland was converted to more intensive use, such as for farming or forestry (Mackey et al., 1998).

1.5 The way in which land and wildlife habitats are managed affects the abundance and diversity of species. For example, changes in farming practices have been linked to declines in terrestrial bird populations throughout the United Kingdom in recent decades.

1.6 As part of the UK Government's commitment to the Convention on Biological Diversity, action plans are being developed for 41 habitats and 226 species occurring in Scotland. The latter are species that are globally threatened or rapidly declining in the UK. For each, conservation actions and targets will be specified with regard to the species' range, population or breeding performance. Of the 34 species groups represented in the Biodiversity Action Plan, adequate trend data are generally available for only one; the birds. Hence, recommendations for monitoring each species' status will also be made.

1.7 An overview of Scotland's BAP Priority Species is given in Action for Scotland's Biodiversity (Usher et al., 2000).



Figure 1. Variation in species richness in Scotland, based on the distribution of 3,649 species in 14
taxonomic groups: lichens, seaweeds, bryophytes, stoneworts, rare and scarce vascular plants,
molluscs, dragonflies and damselflies, grasshoppers and crickets, carabid beetles, butterflies,
amphibians, reptiles, breeding birds and mammals. Lichens and bryophytes, many of which have an
affinity for moist, relatively unpolluted areas, form the bulk of the sample

2. Trends

2.1 Species groups for which adequate information is available are summarised in Table 1. This shows the following.

• Within a sample of rare and endemic vascular plant species examined during 1990-96, the majority appeared to have fewer populations than were known to exist prior to 1990.

• The distribution of many butterfly species is changing. Most of the species currently in decline are already scarce, whilst those that are increasing are already widespread.

• The majority of native freshwater fish species are thought to have declined recently, having been affected by a wide range of pressures on the quality of their habitats.

• During the 1970s-90s, population sizes of the majority of wintering waders and wildfowl (notably the geese), increased markedly1 in abundance. Similarly, population sizes of the majority of seabird species increased during the 1970s-80s.

• Although the geographic ranges recorded for about half of Scotland's terrestrial and freshwater bird species showed little change between c. 1970 and c.1990, almost one-third of species, including 60% of farmland birds, showed a marked reduction in range size.

• In a survey of widespread, terrestrial and freshwater birds between 1994 and 1998, 16 out of 53 species showed a statistically significant increase in abundance, five showed a significant decrease and 32 showed no significant change

• Over one-third of native mammals are thought to be in decline.

• Over one-third of species given legal protection in Scotland, and for which trend information is available, were thought to be declining in 1997. Almost half were thought to have shown little recent change.

• The geographic ranges of most non-native plant species introduced into Scotland are thought to have changed little between the 1950s and 1987-88. However, of 58 plant species that increased their range significantly, 31 are thought to have a medium or high adverse impact on native species.
2.2 No reliable trend data exist for reptiles, amphibians, the majority of invertebrates or for lower plants.

¹ Here, a 'marked' change is one of 10% or more.

3. Sources

3.1 This profile has been developed using the following key sources, in addition to other references: Atkinson et al. (2000), Lloyd et al. (1991), Fleming (1997), Maitland (1997, 1999), Welch et al. (2001), Butterfly Conservation, Mammal Society, and the Wildfowl and Wetlands Trust. Spatial and other data were provided by the Biological Records Centre, the British Lichen Society and British Trust for Ornithology.

4. References

Atkinson, P.W., Austin, G.E., Burton, N.H.K., Musgrove, A.J., Pollitt, M. and Rehfisch, M.M. (2000). WeBS Alerts 1998/99: Changes in numbers of wintering waterbirds in the United Kingdom at national, county and Special Protection Area (SPA) scales. BTO Research Report No. 239. Norfolk: British Trust for Ornithology.

Fleming, L.V. (1997). Protecting biodiversity: mechanisms and their effectiveness. In Biodiversity in Scotland: Status, Trends and Initiatives, eds. L.V. Fleming, A.C. Newton, J.A. Vickery and M.B. Usher. Edinburgh: The Stationery Office, pp. 261-272.

Hawksworth, D.L. and Kalin-Arroyo, M.T. (1995). Magnitude and distribution of biodiversity. In Global Biodiversity Assessment, eds. V.H. Heywood, and R.T. Watson. Cambridge: UNEP.

Lloyd, C, Tasker, M.L. and Partridge, K. (1991). The Status of Seabirds in Britain and Ireland. London: T&AD Poyser.
Mackey, E.C., Shewry, M.C. and Tudor, G.J. (1998). Land Cover Change: Scotland from the 1940s to the 1980s. Edinburgh: The Stationery Office.

Maitland, P.S. (1997). Sustainable management for biodiversity: freshwater fisheries. In Biodiversity in Scotland: Status, Trends and Initiatives, eds. L.V. Fleming, A.C. Newton, J.A. Vickery and M.B. Usher. Edinburgh: The Stationery Office, pp. 167— 178.

Maitland, P.S. (1999). Priority freshwater fish in Scotland. A report to SNH. Haddington: Fish Conservation Centre.

Mammal Society (1999). The State of British Mammals. London: The Mammal Society.

Noble, D.G., Bashford, R.I:, Marchant, J.H., Baillie, S.R. and Gregory, R.D. (1999). The Breeding Bird Survey 1998. BTO Research Report 225. Thetford: British Trust for Ornithology.

Usher, M.B. (1997). Scotland's biodiversity: an overview. In Biodiversity in Scotland: Status, Trends and Initiatives, eds. L.V. Fleming, A.C. Newton, J.A. Vickery and M.B. Usher. Edinburgh: The Stationery Office, pp. 5-20.

Usher, M.B. (ed.) (2000). Action for Scotland's Biodiversity. Edinburgh: The Stationery Office.

Welch, D., Carss, D.N., Gornall, J., Manchester, S.J., Marquiss, M., Preston, CD., Telfer, M.G., Arnold, H.R. and Holbrook, J. (2001). An audit of alien species in Scotland. SNH Review No. 139. Perth: Scottish Natural Heritage.

5. Further Information

Further detailed information on Natural Heritage Trends: Species Diversity can be found in the following l&A Notes.

6. Contact

To obtain further information about any of the issues raised in this l&A Note, please contact

Dr Phil Shaw or Ed Mackey
 Environmental Audit Group Chief Scientist's Unit
 Scottish Natural Heritage
2 Anderson Place
 EDINBURGH EH6 5NP

Tel: 0131-446 2464
Fax:0131-446 2405
E-mail: phil.shaw@snh.gov.uk

Table 1. Species trends

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