Bridges & Species: Post-Glacial Colonisation

Kerry Slug

This black and white picture depicts The Kerry slug. It generally grows to approximately 7-8cm in length, has a dark brown body with light brown spots. The slug as two sets of tentacles, characteristic for all terrestrial slugs and snails. It is (apparently) the only slug species that will curl itself into a ball when it is disturbed. It has a Lusitanian distribution, which describes animals and plants that only occur along the western seaboard of Europe and not in the interior of counties. The Kerry slug likes wet weather and feeds on lichens living on rocks and lower tree trunks. The Kerry Slug is an important species and is well protected appearing under Annex II and Annex IV of the European Union Habitats Directive. Being an Annex II species means that the area in which the Kerry Slug lives is guarded under Irish and European law as a Special Area of Conservation. Being listed as an Annex IV species means that the Kerry Slug is an animal of European Community interest and requires strict protection.

Kerry Slug

Kerry Slug

This black and white picture depicts The Kerry slug. It generally grows to approximately 7-8cm in length, has a dark brown body with light brown spots. The slug as two sets of tentacles, characteristic for all terrestrial slugs and snails. It is (apparently) the only slug species that will curl itself into a ball when it is disturbed. It has a Lusitanian distribution, which describes animals and plants that only occur along the western seaboard of Europe and not in the interior of counties. The Kerry slug likes wet weather and feeds on lichens living on rocks and lower tree trunks. The Kerry Slug is an important species and is well protected appearing under Annex II and Annex IV of the European Union Habitats Directive. Being an Annex II species means that the area in which the Kerry Slug lives is guarded under Irish and European law as a Special Area of Conservation. Being listed as an Annex IV species means that the Kerry Slug is an animal of European Community interest and requires strict protection.

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Trout, salmon and Arctic char made their own way from the ocean into Ireland's thawing rivers at the end of the Ice Age. Along with species such as the freshwater eel, the lampreys, and the rare pollan of Lough Neagh, they are the native fish of Ireland. Among the species introduced after the 12th century, the pike has been a fierce predator on trout, and roach and dace have competed with trout stocks.

The history of Ireland's modern flora and fauna is one of 'natural' arrivals since the final end of the Ice Age, 10,500 years ago and of many species introduced, accidentally or on purpose, by people. One of the long-running arguments in Ireland's natural history has been the question of land-bridges between Britain and Ireland at the end of the Ice Age - where and if they existed, and for how long. A deep canyon down the middle of the Irish Sea has raised doubts among geologists, but the theory most favoured today is of a very slowly shifting land bridge - or perhaps several - travelling north from around Brittany as the huge weight of ice retreated towards the Arctic, and finally, after hundreds of years, vanishing beneath a rising sea.

Most botanists believe it was land bridges, rather than the spreading of seed by wind, birds or sea currents, that carried most of the 1,000 or so native plant species to Ireland. Some of the early trees, such as the pioneer willows and birches of the post-glacial tundra, arrived as tiny seeds floating in the wind. But the big forest species such as pine, elm and oak, crept north at a few metres a year, starting as far away as Spain. Pine arrived in the south-west of Ireland by 9,500 years ago, but oak and elm took several centuries longer. Birds such as pigeons and crows may have helped the spread of heavier seeds.

Otter (Madra Uisce) and Brown Trout (Breac)

Otter catches a brown trout in a South Dublin park

Original work carried out under contract for South Dublin County Council

Otter (Madra Uisce) and Brown Trout (Breac)
Original work carried out under contract for South Dublin County Council

Otter (Madra Uisce) and Brown Trout (Breac)

Otter catches a brown trout in a South Dublin park

Original work carried out under contract for South Dublin County Council

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Another long-running mystery in Ireland's natural history has been the real origin of our so-called 'Lusitanian' species - plants and animals living far from their natural home in the region of Spain and the Pyrenees that the Romans called Lusitania. Best known are the natterjack toads (Ireland's only toad species) that run around in some of County Kerry's sand-dunes; a big and beautifully-spotted slug of Kerry's mossy trees and boulders (Geomalacus maculosus); and several kinds of heather from Spain and Portugal that grow in Connemara and elsewhere on the western seaboard. Recent genetic research suggests that the toads, migrating north, probably crossed a land bridge from Britain and that the heathers came to Ireland packed around Spanish wine casks or in the baggage of returning medieval pilgrims.

Ireland has only 28 species of wild mammal compared with 55 in Britain. The comparison is distorted by the number of Britain's own introductions (of, for example, several deer that are absent from Ireland), but some of the 'vacancies' do seem to bear on the land bridge issue. We are missing moles, for example, along wth burrowing common shrews, neither of which might have fancied digging into soggy glacial silt lacking in earthworms. Tundra-going field voles, on the other hand, a staple prey of Britain's predatory mammals, could have crossed a bridge but did not, thus leaving a big question mark over the food of the predators in Ireland at that time.

The island's original 'ark' of species was quite limited after the Ice Age. Brown bear and wolf were among the early predators; fox, badger and wild cat were here by the stone age Neolithic period, and the pine marten soon after. The colour and coat-pattern of the Irish stoat marks it out as a very early native species, a distinction echoed in the notably darker fur of the Irish otter and Irish mountain hare. Among the introductions we can be certain of are the rabbit (brought in by the Normans), the grey squirrel (from America), the bank vole (in a 20th century shipload of timber) and the American mink , which began to escape from fur farms in the 1950s. The hedgehog is a medieval probable, brought in for food or its spiny skin (for combing the fibres of wool).