As Spring approaches and we await the glorious flowering of rhododendrons around Scotland, we should pause to reflect on the fact that they are really a dangerous alien invasive species, not native to our shores. Rhododendrons were first brought here in 1763 from Spain, to add a dash of colour to our gardens. They quickly spread, developing thick canopies that block the light from other species. They also carry diseases that are fatal to some native British trees. 

Invasive, non-native plants and animals are species that have been moved and introduced to places where they do not occur naturally, by deliberate or accidental human actions. They can often cause harmful environmental, social, or economic impacts. The planet’s natural barriers, like the great oceans, mountain ranges and deserts, enabled life to develop autonomously in different regions of the world, adapting to local conditions and interactions with other native species. When we move these plants and animals around, we effectively break down these natural barriers, introducing new diseases, new competitors for food and shelter and new predators, often causing major negative impacts. That’s why Scotland’s rare and treasured west coast temperate Celtic rainforest has suffered massive degradation from the rapid spread of non-native rhododendron, that has smothered the habitat and its unique wildlife.

The other dangerously harmful invasive plant, commonly found on riverbanks, roadsides and in railway cuttings in Scotland, is giant hogweed. First introduced in this country by private collectors in the 19thcentury, this massive plant has spread widely, forming impenetrable stands that drown out native plants and flowers and reduce species diversity. Giant hogweed produces a phototoxic sap, which when exposed to sunlight, can cause serious skin burns, creating a major hazard for walkers and ramblers. The plant also dies back in the winter, leaving riverbanks bare and prone to flood erosion.

Amongst the larger alien animal species is, of course, the American mink, a voracious predator, which preys on ground-nesting birds and water voles. Most mink found in the wild are descended from animals that escaped from fur farms, or were deliberately released, often by animal rights protesters, in the 60s and 70s. Parts of the Western Isles were particularly inundated, although successful eradication programmes have dramatically reduced their numbers. The grey squirrel has a similar alien pedigree. Shipped in from North America and released into the wild in Great Britain in 1876, they have become a major pest, carrying a pox virus to which our native red squirrel is susceptible. They also out-compete red squirrels for food.

Whilst there has been some progress with the eradication of American mink and grey squirrels, there has not been a similar success with the suppression of species like the signal crayfish. Introduced from America in 1975 as a potential food source, this lobster-like freshwater species spread rapidly across the UK, driving our native, white-clawed crayfish to near extinction through competition and contamination with the highly infectious crayfish plague. The signal crayfish feeds on fish eggs, as well as on young salmon, trout and aquatic plants. It also burrows into riverbanks causing erosion and flood risks. The threat it poses is so great that it is illegal to put even an accidentally caught signal crayfish back into the water. It must be killed immediately.

Of growing concern to Scottish anglers is the appearance of pink salmon in our lochs and rivers. Pink salmon are native to the Pacific but were introduced in the 1960s into rivers in Russia, gradually spreading to many parts of Northern Europe. Large numbers of the fish have been found in Scotland. Anglers are alarmed that the pink salmon will out-compete our native fish for food and habitat.

Other dangerous invasive aquatic species have been brought to our shores in ballast water, where the ballast tanks or cargo holds in ships are flooded with fresh or saltwater to provide stability to a vessel when it is not carrying cargo. The ballast water is then released in the next port-of-call where the ship picks up cargo, releasing non-native organisms into the marine environment. Although international controls were introduced in 2017 in the Ballast Water Management Convention, Scottish waters have seen an invasion of zebra mussels, which are native to freshwater lakes in Russia and Ukraine and cause major disruption to our local ecosystems. It is reckoned that pipe fouling and damage to harbour and water infrastructures caused by zebra mussels, costs the UK economy around £5 million annually.

NatureScot says that there are more than 180 alien species, including 122 plants and 60 animals, that pose a threat to our wildlife and habitats. The government agency, formerly known as Scottish Natural Heritage, is warning of the arrival of a new threat in the form of a 1 inch long, freshwater killer shrimp, native to the Black Sea and first recorded in the UK in 2010, but not yet seen in Scotland. The killer shrimp, which preys on other shrimps, damsel flies and aquatic insects, can out-compete rivals for food, causing devastating impacts. They are one of the most damaging invasive species in Europe.

Invasive Non-Native Species (INNS) pose a significant threat to our countryside, our wildlife, our economy and even, in some cases, to our health. According to NatureScot, they cost the Scottish economy around £246 million a year. As Spring arrives, we need to learn how to identify and report invasive non-native species and discover how to manage and prevent their spread. As the people of Ukraine have discovered, invaders make bad neighbours.