Nature Notes 25 May 2021 - Invasive Species

Rhododendron (c) Trish Fox

It's Invasive Species Week. Invasive species are species that have been introduced (deliberately or accidentally) by humans and have a negative impact on the economy, wildlife or habitat. This week in Nature Notes we are focussing on some of the most common ones we come across while we are undertaking conservation work across Northern Ireland.

Grey Squirrels

The red squirrel is one of our most iconic and much-loved native mammals. Unfortunately, red squirrel populations are threatened across the UK and Ireland by habitat loss and the presence of the invasive grey squirrel.

In Ireland the grey squirrel was originally introduced in 1911 in Co. Longford from North America. Since then it has spread throughout the island of Ireland having a devastating impact on our red squirrel populations. The grey squirrel is a threat to red squirrel populations due to competition and its ability to take advantage of food resources the red squirrels cannot. This means the red squirrel is driven out of these habitats in search for resources elsewhere. The grey squirrel can carry the Squirrelpox virus which is infectious to red squirrels causing lesions and eventually leading to a slow death from starvation. This virus can kill a red squirrel within two weeks. Red squirrels are starting to recover in Northern Ireland thanks to vital conservation work being carried out across the country.

Ulster Wildlife is developing a Red Squirrel Conservation Strategy which will help red squirrels to continue to recover in Northern Ireland and beyond. ~Katy Bell, Senior Conservation Officer

Grey Squirrel (c) Gillian Day

Grey Squirrel (c) Gillian Day

Carpet sea squirt

The carpet sea squirt is also know as 'sea vomit' due to its appearance and texture.

This invasive sea squirt forms large, thin colonies that drape over hard surfaces and cover other marine life, and from overhanging places it appears to 'drip' like candlewax. This dripping and lobed/lumpy appearance coupled with the colour - which ranges from tan or cream to yellows and oranges, is what gives rise to it's common name of sea vomit.

It's thought to have originated from East Asia and possibly arrived in ship's ballast water- it fragments and therefore spreads quite easily and is now quite widespread in Europe's shallow waters although only found so far in Strangford and Carlingford Loughs in NI. It can alter marine habitats and affect aquaculture and fishing activities, covering shellfish beds and fish nursery grounds. ~Annika Clements, Director of Nature, Climate & Environment

Didemnum vexillum - Strangford Lough 2012 (c) DAERA

Didemnum vexillum,  Strangford Lough 2012 (c) DAERA


Purple peril: its large purple flowers may be stunning, but rhododendron is one of the main threats to our internationally important peatlands. We are tackling it at five raised bogs and getting rid of it is not easy.

Introduced into the British Isles in the 18th century, Rhodendron ponticum is a member of the Ericaceae family (which includes heather and bilberry) and takes easily in the acidic peaty soils of our raised and blanket bogs. A 2m high bush that is 10m across can produce 1,000,000 seeds a year - 90% of which can germinate the next season! It is also very long-lived: sites occupied for 200 years show little sign of dying back. Chemicals in the leaves make it very resistant to insect attack and stop other plants growing around it. We’ve been tackling this thug by hand-pulling small plants, and cutting larger plants and painting the stem quickly with herbicide.

Contractors deal with the major invasions – sometimes drilling into the stem and injecting herbicide so the plants die in situ. Each site must be followed up every year for several years. We are trialling charcoal production as the wood is great for this. The cost of rhododendron control is eye-watering: but the cost of doing nothing is even higher. ~ Trish Fox, Senior Technical Officer

Rhododendron (c) Trish Fox

Rhododendron (c) Trish Fox

Sea Buckthorn

If I had to do a personal top ten of non-native invasive species Sea Buckthorn would feature high on my list. It's one of those funny things - you shouldn't feel that a plant is your enemy but I really can't stand this brute of a shrub and have often let my mind wander, planning its destruction.

This is a coastal shrub species native to the east coast of England and introduced to Northern Ireland in the 19th and 20th centuries to stabilise sand dunes. It grows quickly, reaching heights of around 8 to 10 metres, has silvery leaves, orange berries and particularly nasty thorns. Don't believe the hype - there are people who wax lyrical about this species - they tell you the berries contain more vitamin C than oranges, can be used in skin preparations and some local chefs use it in their dishes. That's all very true but none of those things can make up for the fact that this species rampantly colonises sand dunes, crowding out native habitats and species, fixing nitrogen from the atmosphere to change the soil composition and can actually be a cause of erosion on dune systems.

Added to that it is a nightmare to get rid of - it's grows fast and recolonises quickly, it can handle most types of pesticide, animals don't like to eat the plant, try cutting it out by hand and you end up feeling like a human pin-cushion, digging it out with a digger costs a fortune. There are few things to like about this plant, and I think the berries taste disgusting. ~ Andy Crory, Nature Reserves Manager

Sea buckthorn (c) Amy Lewis

Sea buckthorn (c) Amy Lewis

Get involved

You can help by reporting Invasive species to, find out more at