Ireland’s Top 5 Most Bizarre Invasive Species: Friend or Foe?

Introduction

Ireland’s lush landscapes are home to a rich tapestry of native flora and fauna, shaped by millennia of natural evolution. But in recent decades, the island has played host to some rather unusual guests: invasive species. These quirky intruders, often introduced through human activities, can have far-reaching consequences for Ireland’s ecosystems, agriculture, and even the economy.

While some might seem harmless or even beneficial at first, invasive species can rapidly become foes, wreaking havoc on native plants and animals. From strange deer to voracious plants, let’s take a closer look at five of Ireland’s most bizarre invasive species—and explore whether they’re friends or foes in the fight for balance in the environment.

1. Muntjac Deer: The Barking Intruder

Origin: Native to Southeast Asia
Status: Foe

One of the strangest animals to have infiltrated Ireland’s ecosystems is the muntjac deer, often nicknamed the "barking deer" due to the unusual sound it makes when alarmed. Introduced to the UK as ornamental animals in the early 20th century, muntjac have slowly made their way to Ireland, primarily through accidental or deliberate releases.

Why they’re a problem: These small, stocky deer might look cute, but they can cause significant damage to local ecosystems. Muntjac have a particular taste for young saplings, ground vegetation, and the bark of trees, leading to overgrazing. This poses a direct threat to Ireland’s woodlands, affecting native species and the regeneration of trees. In fact, muntjac deer can reduce forest regeneration by as much as 50%, particularly in areas where young tree saplings are vulnerable. They also pose a risk to agriculture by damaging crops and hedgerows.

Management: Efforts are being made to control their population through culling and monitoring. Although the muntjac population in Ireland is currently small, it is believed that numbers could rise quickly if not properly managed, as has been seen in the UK, where their population has reached 50,000. Their elusive nature and ability to reproduce year-round make them a challenging species to manage.

2. Japanese Knotweed: The Green Menace

Origin: Japan
Status: Foe

If there’s one invasive species that sends shivers down the spine of gardeners and developers alike, it’s Japanese knotweed. This seemingly innocent-looking plant is notorious for its ability to grow rapidly, destroying everything in its path. Introduced as an ornamental plant in the 19th century, it has since spread across Ireland, particularly along riverbanks, roadsides, and urban areas.

Why it’s a problem: Japanese knotweed is incredibly resilient, capable of growing through concrete, tarmac, and even the foundations of buildings. Its roots, or rhizomes, can grow up to 3 metres deep and spread up to 7 metres horizontally, making it almost impossible to eradicate without specialised treatment. This invasive plant can grow up to 10cm per day during the growing season, quickly dominating an area and outcompeting native plants. For farmers and landowners, knotweed can lead to a significant loss of usable land, as it reduces biodiversity and damages infrastructure. It is also notorious for lowering property values, often by 5% to 10%.

Management: The cost of eradicating Japanese knotweed across the UK and Ireland is estimated at a staggering €1.7 billion, with individual homeowners facing removal costs of between €2,500 and €5,000 per property. Controlling Japanese knotweed typically requires a combination of herbicide treatments and physical removal. Landowners are legally responsible for preventing its spread, making its management a top priority for affected areas.

3. Grey Squirrel: The Woodland Usurper

Origin: North America
Status: Foe (with a hint of friend)

The grey squirrel might seem like a charming addition to Ireland’s woodlands, but this North American import has a darker side. Introduced in the early 1900s, the grey squirrel quickly displaced its native counterpart, the red squirrel, from much of the country.

Why they’re a problem: Grey squirrels are larger and more aggressive than red squirrels, outcompeting them for food and habitat. They also carry the squirrelpox virus, which is deadly to red squirrels but harmless to themselves. As a result, red squirrel populations have declined by over 60% in areas where grey squirrels have established themselves. Grey squirrels are also notorious for causing damage to young trees by stripping their bark, which can lead to long-term consequences for Ireland’s forests.

A surprising twist: There is hope for the red squirrel, however. In areas where the native pine marten has made a comeback, grey squirrel populations have declined by as much as 40% to 50%. Interestingly, red squirrels seem to be less affected by pine martens, leading to their resurgence in these areas. While the grey squirrel has caused significant damage, the rise in pine martens may inadvertently help red squirrel populations recover.

Management: Conservationists are focusing on habitat restoration for red squirrels and encouraging the re-establishment of pine marten populations to naturally control grey squirrel numbers.

4. New Zealand Flatworm: The Soil Invader

Origin: New Zealand
Status: Foe

One of the more bizarre—and lesser-known—invasive species in Ireland is the New Zealand flatworm. First recorded in Northern Ireland in the 1960s, this small, slimy predator has since spread across the country.

Why it’s a problem: The New Zealand flatworm feeds on earthworms, a key component of healthy soil ecosystems. Earthworms play a critical role in aerating the soil, decomposing organic matter, and maintaining soil fertility. Studies show that in areas heavily affected by New Zealand flatworms, earthworm populations have decreased by 50% to 100%. By reducing earthworm numbers, flatworms threaten the long-term health of Ireland’s agricultural soils, leading to soil degradation and reduced crop yields.

Management: Unfortunately, there are no effective large-scale methods for controlling New Zealand flatworms. Farmers and gardeners are encouraged to monitor soil health and take action if flatworms are detected, but prevention is currently the best form of control.

5. Rhododendron: The Beautiful Invader

Origin: Southern Europe and Asia
Status: Foe

Despite its vibrant flowers, rhododendron is one of Ireland’s most problematic invasive plants. Introduced as an ornamental species in the 19th century, it has since escaped into the wild, particularly in the west of Ireland, where it has taken over vast tracts of land, including national parks such as Killarney.

Why it’s a problem: Rhododendron grows rapidly and forms dense thickets, blocking light from reaching the ground and preventing other plants from growing. In areas where rhododendron has taken hold, native plant diversity can drop by as much as 90%, as the dense canopy prevents other species from thriving. It is also toxic to grazing animals, making it a significant threat to livestock in areas where it has taken root.

Management: Rhododendron is a formidable opponent, and removing it is expensive and labour-intensive. It is estimated that rhododendron covers approximately 16,000 hectares in Ireland, with costs to remove it reaching up to €5,000 per hectare, depending on the severity of the infestation. Efforts to control rhododendron require a combination of mechanical removal (cutting and uprooting) and chemical treatments to prevent regrowth.

Conclusion: Managing Ireland’s Quirky Intruders

Invasive species like the muntjac deer and Japanese knotweed may seem like quirky, harmless additions to Ireland’s ecosystem at first glance, but their impact on native wildlife, agriculture, and biodiversity can be profound. Whether it’s plants taking over farmland or animals disrupting delicate ecosystems, these intruders require vigilant management to maintain balance in the environment.

Invasive species cost the Irish economy an estimated €261 million annually, including damage to agriculture, forestry, and natural ecosystems, as well as the costs associated with managing invasive species populations. While some species, like the grey squirrel, may have inadvertently aided in the recovery of native species like the red squirrel, most invasive species remain formidable foes to Ireland’s native biodiversity.

With sustained efforts, we can help protect the health of Ireland’s ecosystems for generations to come—before these quirky invaders become too firmly rooted to remove.

*By Anne Hayden MSc., Founder, The Informed Farmer Consultancy.