Until now considered as migratory and evolutionary dead-ends, oceanic islands are nevertheless not a final resting place. Alain Vanderpoorten and his team have in fact discovered, using bryophytes as a model, that spore producing plants, (re)colonise the European continent from the Azores, the Canaries and Madeira, which hence appear, thanks to the stability of their sub-tropical climate, to have served as refugia during the Ice Age in Europe

Islands…Just by itself, the word has a flavour of holidays, readily evoking palm trees, turquoise waters and birds with iridescent plumage. Every year, millions of tourists from the four corners of the planet fly off to islands of the Pacific, Atlantic and Indian oceans, looking for sunshine and a relaxed way of life. Over the centuries, these parcels of land lost in the immensity of the oceans have never ceased to fascinate and inspire human beings. Scientists themselves do not escape from the islands’ fascinating power.

Since Charles Darwin’s and Alfred Russel Wallace’s times, islands have been recognized as natural laboratories for studying major evolutionary processes such as speciation. It was in fact by observing populations of island organisms that the two naturalists developed what rapidly became the most widely acknowledged theory of evolution by natural selection. ‘Oceanic islands are ideal models for studying phenomena such as speciation and evolution, as they are geographically isolated, are easy to circumscribe geographically and have a geological dynamic which is a source of environmental richness. On islands rates of speciation are typically much higher than on the continent,’ explains Alain Vanderpoorten, a F.R.S.-FNRS Research Associate at the University of Liège Botany Institute’s Biology of Evolution and Conservation Unit. Islands therefore constitute key models for an improved understanding of complex evolutionary mechanisms at the continental scale.

The theory of migratory and evolutionary dead-end islands

If islands are a source of biodiversity – given their high rate of speciation when compared to the continents – they are also considered, paradoxically, as migratory and evolutionary dead-ends. Why? Four main reasons account for this phenomenon. ‘First of all, organisms which reach islands quickly lose their dispersal power because they are not faced with competition or predation. In addition, good dispersers among the island colony are rapidly blown away, so that only poor dispersers eventually remain in the island setting. Furthermore, returning to the continent is very difficult for island organisms because the ecological niches are saturated in the continental setting. Finally, the fate of volcanic islands, and hence of their biota with limited dispersal abilities, is to progressively sink under the sea level by erosion’ points out Alain Vanderpoorten.

One of the most striking examples of island species with extremely reduced dispersal ability is the dodo. This large bird, belonging to the pigeon family and today extinct, populated the island of Mauritius until the end of the seventeenth century. Its ancestors had arrived on the island by air but over the centuries, the dodo lost the ability to fly owing to the absence of predators on the island. Unfortunately it is this evolutionary characteristic, as well as the absence of a fear of potential predators, which was fatal to it when humans began to colonise the island of Mauritius at the end of the sixteenth century.