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

Abstract

Plants are commonly listed as invasive species, presuming that they cause harm at both global and regional scales. Approximately 40% of all species listed as invasive within Britain are plants. However, invasive plants are rarely linked to the national or global extinction of native plant species. The possible explanation is that competitive exclusion takes place slowly and that invasive plants will eventually eliminate native species (the "time-to-exclusion hypothesis"). Using the extensive British Countryside Survey Data, we find that changes to plant occurrence and cover between 1990 and 2007 at 479 British sites do not differ between native and non-native plant species. More than 80% of the plant species that are widespread enough to be sampled are native species; hence, total cover changes have been dominated by native species (total cover increases by native species are more than nine times greater than those by non-native species). This implies that factors other than plant "invasions" are the key drivers of vegetation change. We also find that the diversity of native species is increasing in locations where the diversity of non-native species is increasing, suggesting that high diversities of native and non-native plant species are compatible with one another. We reject the time-toexclusion hypothesis as the reason why extinctions have not been observed and suggest that non-native plant species are not a threat to floral diversity in Britain. Further research is needed in island-like environments, but we question whether it is appropriate that more than three-quarters of taxa listed globally as invasive species are plants.invasive species | biodiversity | Anthropocene | ecology | botany T he Global Invasive Species Database (1) lists 3,163 plant (Plantae) and 820 animal (Animalia) species as invasive because they "threaten native biodiversity and natural ecosystems" in the regions to which they have been introduced. Given the relative numbers of animal and plant species that have been described (2-4), this implies that the per species likelihood of being listed as invasive is ∼25 times higher for plants than for animals. For the United Kingdom, 49 of 125 species (39%) categorized as invasive in the same database are plants (1), and a more detailed analysis included 102 plants in a list of 244 non-native species (∼42%, depending on taxonomic designations) that have negative ecological or human effects in Great Britain (5, 6). These numbers imply that non-native plants must be key threats to biodiversity both globally and in Britain. It is surprising, therefore, that examples of regional-scale or species-level extinctions associated with invasive plants are apparently rare (7-12).Most extinctions associated with introduced species have been caused by invasive predators and diseases encountering "naïve" prey and host species in distant and isolated parts of the world (13)(14)(15)(16)(17)(18)(19). Putative examples of competitive exclusion in the invasive species literature have usually turned out to be exam...

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