Introduced & Naturalised Species

Introduced Species

Our island biodiversity has been vastly increased by species introduced by humans, and this process is still going on. Some of these introductions have been deliberate and some of them have been accidental. Nobody intended to bring in the brown rat but they hitched a ride on a wooden sailing ship and jumped off in the port of Dublin in or around 1722. Some plants were brought in to decorate gardens but then escaped and ran wild. Giant hogweed from the Caucasus did this about a 150 years ago and is now a nuisance in some parts of the country because it can cause severe skin irritation.

In general the longer an introduced species has been in the country the better we accept it. In the Middle Ages the Normans introduced rabbits, pheasants, fallow deer and probably mute swans. Most people now accept them as natural parts of the Irish countryside. However, the American Mink escaped from fur farms, or in some cases were deliberately released, only about fifty years ago and they are still very unpopular. Another American species, the Grey Squirrel, has been here about twice as long as the mink. They were introduced in 1911 and some people like them while others don’t.

Naturalised Species



Although we have less than thirty native tree species, well over 300 different kinds of tree actually grow in this country. Some of them depend on being planted by people but others have followed the example of the giant hogweed and have managed to plant themselves. Plants brought in from other countries that now manage to spread without human help are described as ‘naturalised’. Examples among trees include sycamore and beech.

In recent years, the growing popularity of the garden pond and the aquarium has meant that some foreign water-plants, imported as decoration and to provide oxygen for ornamental fish, have escaped into our rivers, lakes and canals. They have started to cause environmental problems, choking native plants, threatening fish life and making activities like swimming and boating difficult.

However, not all introduced species cause damage. In 1964 biologists identified a small, inoffensive, mouse-like creature found in north Kerry as a bank vole. Since then they have colonised about half the country, roughly south of a line connecting Waterford to Galway, and are spreading north-eastwards at a rate of between one and four kilometres a year. They do no damage but provide a valuable food source for predators like barn owls, foxes and stoats.


For a long time there was a mystery surrounding how and when the voles arrived in Ireland. A couple of years ago the mystery was elegantly solved by some Galway scientists. They proved that the voles arrived from north Germany in the 1920s in soil in the buckets of large earth-moving machines brought in by the Siemens company to the port of Foynes for the construction of the Ardnacrusha dam across the Shannon.

Much more recently another small mammal, the greater white-toothed shrew, was discovered in the midlands. They are a continental species, not found in Britain, but this time the mystery was solved much more quickly. A nest of shrews apparently travelled here from Holland in the root-ball of a large tree or shrub imported by a garden centre.

The growth in trade and global travel has speeded up the process of increasing our biodiversity.


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