Non-native plants add to the British flora without negative consequences for native diversity

Area of Science:

• Ecology
• Botany
• Environmental Science

Background:

• Non-native plants are frequently classified as invasive species, presumed to cause ecological harm.
• In Britain, approximately 40% of listed invasive species are plants, yet they are seldom linked to native plant extinctions.

Purpose of the Study:

• To investigate the impact of non-native plant species on native floral diversity in Britain.
• To test the "time-to-exclusion hypothesis" which suggests invasive plants will eventually eliminate native species.

Main Methods:

• Analysis of British Countryside Survey data from 479 sites between 1990 and 2007.
• Comparison of changes in plant occurrence and cover between native and non-native species.
• Examination of the relationship between native and non-native species diversity.

Main Results:

• Changes in plant occurrence and cover did not differ significantly between native and non-native species.
• Native species dominated vegetation changes, with cover increases over nine times greater than non-native species.
• Native and non-native species diversity were found to be compatible, with increasing non-native diversity correlating with increasing native diversity.

Conclusions:

• The "time-to-exclusion hypothesis" is rejected as extinctions have not been observed.
• Non-native plant species do not pose a significant threat to floral diversity in Britain.
• Factors other than plant invasions appear to be the primary drivers of vegetation change.