On the mountainside there is a battle of the aliens. The once almost universal blue of the Alaskan Lupin is being smothered by the taller white mass of flowers from a tougher weed. Warmth is transforming Iceland. Short grass and tiny ground-hugging plants, typical of cold and rocky areas, are no longer the only plants that can survive. The Lupin, a tough, cold-resistant plant, was introduced deliberately by the government from similar climes in Alaska 50 years ago to reverse erosion and add nitrogen to the soil. It has been so successful it covers vast areas of open country, so much so that there is serious concern it is out of control. Now a new uninvited invader has entered the fray - but this is a weed of temperate climes - Cow Parsley. How it arrived is not known but it probably came from seeds trapped in the mud of a tourist's boot. This summer there are hundreds of thousands of plants smothering both the Lupin and all other local vegetation. Iceland's greatest asset, its fish stocks, are also on the move. The capelin on which the cod feed are disappearing northwards because the water is too warm. Monkfish, once a rarity because the sea was too cold, are now an important commercial catch. Shrimp are far fewer because cod, deprived of capelin to eat, and haddock in increasing numbers, are eating them before they can be caught for human consumption. The new warmth in Iceland is not just changing the sort of plants that can grow here, it is melting the glaciers and ice caps and changing lifestyles too. British tourists with winter coats arrive to find long-legged girls in hot pants and T-shirts sitting outside on the pavement cafes in the capital Reykjavik. The country's population is also on the increase, up to 280,000 from a low of 40,000 during the mini-ice age of the 18th century. Unlike most of the rest of the planet, climate change offers a lot of advantages for Iceland. Twenty years ago it was not possible to grow barley because it was too cold for grain to ripen. Ten years ago in the extreme south farmers succeeded in getting a crop for winter fodder - it was the first harvest since the time of the first Viking settlers. This year farmers are planting barley all over the country. The change is so rapid that the country's scientists cannot believe it will continue. For the last two years temperatures have been the highest since records began in the country in 1822 and, judging by the Greenland ice cores, any time since the medieval warm period when the Vikings arrived in Iceland. Everyone believes that soon there must be a swing back to colder times.