S17 Non-Native Species: Terrestrial, Freshwater and Marine Environments
North American signal crayfish
Pacifastacus leniusculus
Non-natives: progress in terms of their range
Data sourced from Welch et al. 2001; 'n' refers the number of species and subspecies in each taxa
TREND: DETERIORATION | DATA CONFIDENCE: LOW |
A 'non-native' species (also known as an 'introduced' or 'alien' species) is one present in an area outside of its historically known natural range, as a result of dispersal by human activities. The known impacts of non-native species vary from benign to destructive. Within the taxonomic groups for which adequate data are available, at least 915 non-native species and 95 non-native sub-species (a morphologically distinct subdivision of a species including sub-species, hybrids and varieties) have become established in the wild in Scotland.
Assessment
Where adequate data are available (excluding the unknown category):
- The majority of non-natives (84% across groups; 33% birds, 31% mammals, 77% fish, 43% invertebrates, 86% higher plants, 67% lower plants) showed no change.
- In total 23% of non-native mammals and 2% of non-native higher plants decreased.
- Non-natives that increased (14% across groups; 67% birds, 46% mammals, 23% fish, 57% invertebrates, 13% higher plants, 33% lower plants) outnumbered those that decreased by a factor of 6.
Evidence
Trends (measured as geographical distributions) among 867 of the 1010 non-natives could be assessed by comparing records, e.g. from 1987/08 (vascular plants) or 1990s (birds) with those from the 1950s.