Spread of invasive species into Scotland: study

Executive summary

The number of invasive non-native species arriving in Scotland is increasing year on year.

Horizon scanning, to make predictions about the next potential invasive non-native species that could arrive, establish and impact on biodiversity and ecosystems, can inform strategy and action to reduce the threat of biological invasions.

Pathway analysis was used to rank the pathways of introduction of non-native species introduced into Scotland since 1950 and likely pathways of introduction for non-native species into Scotland in the next ten years.

Overall, 52 experts contributed to a horizon scanning study which assessed hundreds of potential species spanning freshwater, marine and terrestrial environments.

The scope of the study was to consider invasive non-native species that were currently absent in Scotland but that have the highest likelihood of arrival, establishment and the magnitude of their potential negative impact on biodiversity and ecosystems over the next 10 years. Preliminary lists, without impact scores, for invasive non-native species that could have a potential negative impact on the economy and human health were also reviewed.

Thirty invasive non-native species were agreed to have a high risk of arriving, establishing and impacting biodiversity and ecosystems in the next 10 years. From this a priority list of 10 invasive non-native species was extracted. Awareness raising was seen as critical, and likely to be effective, in preventing the arrival and spread of these top 10 invasive non-native species.

Five freshwater species were reported in the top 10 list of species: three molluscs (Asiatic clam (Corbicula fluminea), quagga mussel (Dreissena rostriformis bugensis) and zebra mussel (Dreissena polymorpha)) and two submerged plant species (floating pennywort (Hydrocotyle ranunculoides) and parrot’s feather (Myriophyllum aquaticum)). Freshwater environments are considered to be amongst the most vulnerable habitats to biological invasions and freshwater biodiversity is culturally and economically important in Scotland. One marine mollusc Crepidula fornicata was also listed in the top 10 invasive non-native species.

The remaining five invasive non-native species within the top 10 comprised two terrestrial plants, highbush blueberry (Vaccinium corymbosum and hybrids) and pheasant’s tail grass (Anemanthele lessoniana), two vertebrates (Reeve’s muntjac (Muntiacus reevesi) and raccoon (Procyon lotor) and flatworms (four species grouped together for communication purposes(Australoplana sanguinea, Caenoplana variegata, Kontikia andersoni and Obama nungara).

The most important pathways of arrival and spread associated with the long list of 171 invasive non-native species and 1096 established non-native species in Scotland are the horticultural and ornamental pathways. The pet pathway is important for introducing species on the horizon scanning long list but was not ranked as high for established non-native species. Pathways in the contaminant category have historically been important in introducing non-native species and this is likely to continue to be the case. Furthermore, the natural dispersal of species from an existing invaded range into Scotland is considered an important pathway of arrival.

The results from this horizon scanning study coupled with the pathway analysis provide detailed information to prioritise actions to prevent the establishment of new invasive non-native species in Scotland, including action plans for priority pathways of introduction and spread.