Information and Advisory Note Number 130                                   Back to menu

NATURAL HERITAGE TRENDS Species diversity: plant species


1. Introduction

1.1 Scotland has 47 vascular plant species and subspecies considered nationally endangered, or vulnerable to extinction. Some of these are known from only a small number of localities, at which their presence has been monitored over recent decades. Changes in the number of Scottish populations of these species detected before and after 1990 are described below.

2. Trends

2.1 Of 79 rare or endemic plant species with few Scottish populations prior to 1990, the majority had similar numbers of populations when resurveyed in 1990-96 [Table 1]. Among those species for which at least 10 populations were revisited in 1990-96, 40% showed a net decline. Species showing changes of at least 10% are listed in Figure 1.

Table 1. Change in the numbers of populations located in 1990-96, compared with those known to exist prior to 1990

3. Source


(184 species) and grasslands (301 species). By contrast, only 14 introduced plant species have been recorded in association with upland or freshwater habitats.


4. Introduction

4.1 An 'introduced' species is one present in an area outside of its historically known natural range, as a result of intentional or accidental dispersal by human activities. Such species are often referred to as 'alien' or 'non-native' species, of which several categories are recognised (Table 2). Some non-native species, such as the pheasant and brown hare, are long-established and considered 'naturalised'. Others, such as the hedgehog, are native to much of Scotland, but have been introduced to the Western Isles, with adverse consequences for breeding waders.

4.2 The effects of introduced species on their host environment include competition with native species, habitat alteration, hybridisation, predation and the transmission of disease or parasites. Generally, such detrimental effects tend to outweigh any benefits gained, and may ultimately reduce species diversity. Thus, over the past 400 years, more global extinctions have been attributed to introduced species than to any other factor (World Conservation Monitoring Centre, 1992). Currently, nearly 20% of vertebrate species at risk of global extinction are threatened by introduced species (McNeely era/., 1995).

4.3 At least 952 non-native species have become established in the wild in Scotland, within taxonomic groups for which adequate data are available (Welch et a/., 2001). Some 43% of vascular plant species in Scotland, are non-native. There are probably no sizeable areas of Scotland now entirely free of introduced species. Non-native vascular plants are particularly well established in southern and central Scotland, reflecting their origins in parks and gardens, from which they have spread mainly into invaded woodlands

5. Trends

5.1 A recent review has suggested that at least 104 introduced plant species increased their geographical range in Scotland from the 1950s until 1988. Species showing significant increases outnumbered those showing significant decreases by a factor of 12. Of the 58 plant species that increased their range significantly, 31 are thought to have a medium or high adverse impact on native species [Figure 2].

5.2 Changes in the range size of invasive plant species during this period show north-south contrasts [Figure 3] in that:

the number of invasive species showing an increase was greatest in southern and central Scotland,
the proportion showing an increase was greatest in northern Scotland, the Outer Hebrides, and the Western Isles.

Table 2. Six categories for classifying species according to their nativeness (from Usher, 2000).

5.3 Wilson's pouchwort, a leafy liverwort found in humid, wooded ravines, is thought to be partially threatened by shading and encroachment of rhododendrons at two sites in southern Argyll.

6. Source

6.1 This profile has been developed from data extracted from Welch et a/. (2001). Additional sources were: UK Steering Group (1995), Hill era/. (1999), Usher (2000).

7. References

Hill, M.O., Davies, C.E., Harris, MP., Marquiss, M., Harding, P.T., Preston, CD., Roy, D.B., Telfer, M.G. and Welch, D. (1999). Biodiversity Assessment: The State of and Changes in Scotland's Biodiversity. Final report to Scottish Natural Heritage. Monks Wood: Institute of Terrestrial Ecology.

McNeely, J.A., Gadgil, M., Leveque, C, Padoch, C. and Redford, K. (1995). Human influences on biodiversity. In Global Biodiversity Assessment, eds. V.H. Heywood and R.T. Watson. Cambridge: UNEP, pp. 711-821.

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

UK Steering Group (1995). Biodiversity: The UK Steering Group Report. Volume 2: Action Plants. London: HMSO.

Usher, M.B. (2000). The nativeness and non-nativeness of species. Watsonia, 23, 323-326.

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. Review No. 139. Perth: Scottish Natural Heritage.

World Conservation Monitoring Centre (1992). Global Biodiversity: Status of the Earth's Living Resources. London: Chapman & Hall.

8. Further Information

Further detailed information on Natural Heritage Trends: Species Diversity can be found in Information & Advisory Note No. 129.

9. 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

Tel: 0131-446 2464
Fax:0131-446 2405

Species mentioned in the text

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