Various theories on how animals got here

IRISH scientists started to argue about how and when various species of animals and plants got to our island back when Queen Victoria was on the throne.

The arguments have rumbled on since. There are basically three schools of thought and they are neatly illustrated in the case of the red deer.

theory one

This is probably the most widely held. It states that red deer arrived here from either Britain or continental Europe over land bridges that existed after the end of the last Ice Age but which sank soon afterwards.

theory two

The basis of this is red deer were here before the Ice Age and managed to survive in warm refuges and then spread out again when the ice melted about 10,000 years ago.

theory three

This one has become a bit unfashionable, believes that red deer were introduced into Ireland by humans, probably Neolithic deer farmers, and then went wild.

Similar arguments rage over many other plant and animal species and are complicated by the puzzling existence of the Lusitanian and Armorican fauna and flora. Lusitania and Armorica were provinces of the Roman Empire. Lusitania covered most of modern Portugal and part of Spain, Armorica was in north-western France.

Today there are some plants and animals in the south-west of Ireland which really belong in the ancient province of Lusitania and a smaller number in the south-east that belong in Armorica. There are theories about why this should be the case, but no definitive explanations.

Personally I’ve always had a problem with the theory that relies on the existence of post-glacial land bridges. Part of the problem lies in the fact that I have quizzed a lot of geographers, oceanographers and geologists and none of them can tell me where these land bridges were or when they existed. You’d think they would have left some trace.

I’ve believed for a long time that the theory that there were warm refuges in the south of the country that allowed some animals and plants to survive the 12,000 years or so of the last Ice Age and then recolonise is the one that best fits the facts.

Records of tree pollen, for example, not only give us the date at which certain species arrived in this country after the ice melted, they also give us the direction they came from.

Some come from the north-east, the most likely location for a land bridge, but others colonise from the south-west, the most likely place for a warm refuge but the least likely for a land bridge.

But modern genetics has the tools to settle these long-running arguments. And the other day while I was doing some research on the internet I was delighted to come across a rather obscure and very technical paper published last year by some British geneticists who had been analysing the mitochondrial DNA from common frogs all over Europe, including a good sample from Ireland.

incorrect textbooks

All the textbooks will tell you frogs are not native to Ireland but were introduced comparatively recently. There’s even a date, 1696, when a fellow of Trinity College Dublin apparently imported live frogs from England so that medical students could dissect them.

What the new study has proved is that there are at least two, and possibly three, distinct genetic types of frog in Ireland and that frogs in the south and west have been a separate population since well before the last Ice Age. In other words, the textbooks are wrong and frogs are a native Irish animal.

Trinity students may have released the professor’s frogs in 1696 but they weren’t introducing a new species. There were already native frogs wild in Ireland.

* dick.warner@examiner.ie


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