An invasive species (IS) is defined as a non-native organism that has been introduced by humans, and which has negative effects on the economy, environment, or health. Not all introduced species are invasive and some may be beneficial. However, certain non-native species can have a significant negative impact once introduced. Globally, IS rank second only to habitat destruction in terms of threats to biodiversity. Contracting parties involved in the Convention on Biological Diversity (CBD), Ireland included, have openly recognised this in decisions.

It is estimated that there are already over 12,000 alien species present in Europe, of which around 10–15% are invasive. They occur in all major taxonomic groups, ranging from mammals, amphibians, reptiles, fish, invertebrates and plants, to fungi, bacteria and other micro-organisms. They are also found in every type of habitat, both on land and in the surrounding seas. To a greater or lesser extent, all EU Member States have problems with IS in their territories. Studies estimate that the number of IS in Europe has increased by as much as 76% since the 1970s. The number is likely to continue to grow unless robust action is taken to combat this invasion.

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The ways in which IS negatively impact on native biodiversity are numerous. They (1) cause disease, (2) act as predators or parasites, (3) act as competitors, (4) alter habitat, or (5) hybridize with local species. Once introduced, control, management and eradication where possible of IS can be very difficult and costly; therefore early detection and reactive measures are desirable. IS are estimated to have cost the EU at least €12 billion per year over the past 20 years, and the figure is growing all the time.

In Ireland, the most prominent of the negative impacts appears to be direct competition with native biota, whilst alteration to habitats and the influence of parasites and pathogens are also consequential. Specific habitat types in Ireland currently under threat from invasive species include freshwater river systems, ponds, mesotrophic lakes, native woodland, lowland heath, coastal floodplain, coastal saltmarsh and coastal sand dunes. A variety of native species are also threatened by IS, including red squirrels, white-clawed crayfish, red deer and earthworms.

DSC01794According to Invasive Species Ireland, a risk assessment has been carried out to help establish which invasive species in Ireland are the most damaging or potentially damaging. This process was designed to take into account the economic and legislative obligations of both the Republic of Ireland and Northern Ireland. The most unwanted species have been selected from the highest scoring of all the species assessed. Information on each of these species is provided on the Invasive Species Ireland website.
Action is also being taken at an EU level with the passing of EU Regulation 1143/2014 on Invasive Alien Species. The regulation came into force on 1 January 2015, and seeks to address the problem of invasive alien species in a comprehensive manner so as to protect native biodiversity and ecosystem services, as well as to minimize and mitigate the human health or economic impacts that these species can have. The regulation foresees three types of interventions: prevention, early detection and rapid eradication, and management. A list of invasive alien species of Union concern will be drawn up and managed, with Member States using risk assessments and scientific evidence.

Inland Fisheries Ireland have put together a range of bio-security measures for field workers, boaters, anglers and recreational water users.

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According to Invasive Species Ireland the most problematic aquatic invasive species in Ireland are:

Freshwater: Asian clam (Corbicula fluminea), Bloody-red shrimp (Hemimysis anomala), Chinese mitten crab (Eriocheir sinensis), Chub (Squalius cephalus), Curly waterweed (Lagarosiphon major), Dace (Leuciscus leuciscus), Floating pennywort (Hydrocotyle ranunculoides), New Zealand pigmyweed (Crassula helmsii), Nuttalls waterweed (Elodea nuttallii), Parrot’s Feather (Myriophyllum aquaticum), Ruddy duck (Oxyura jamaicensis), water fern (Azolla filiculoides), Zebra mussel (Dreissena polymorpha).

Marine: Didemnum vexillum (Didemnum vexillum), Slipper limpet (Crepidula fornicate), Smooth cordgrass (Spartina anglica), Wire weed (Sargassum muticum).

Riparian: Giant hogweed (Heracleum mantegazzianum), Giant rhubarb (Gunnera tinctoria), Himalayan balsam (Impatiens glandulifera), Japanese knotweed (Fallopia japonica), Rhododendron (Rhododendron ponticum).

Coastal: Hottentot fig (Carpobrotus edulis).

For more information on invasive species and invasive species alerts, visit:

Invasive Species Ireland

The National Biodiversity Data Centre

NPWS Invasive Species in Ireland

Inland Fisheries Ireland Invasive Species 

European Commission Invasive Alien Species