Table 1. Change in the numbers of populations located in 1990-96, compared with those known to exist prior to 1990
Information and Advisory Note Number 130 Back to menu
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.
2.1 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
SNH.
(184 species) and grasslands (301 species). By contrast, only 14 introduced
plant species have been recorded in association with upland or freshwater
habitats.
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
5.1 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.
6.1 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).
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.
Further detailed information on Natural Heritage Trends: Species Diversity can be found in Information & Advisory Note No. 129.
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
Species mentioned in the text
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