British plant census finds an invasion of foreign species is jeopardising natives

The first complete census of Britain's native plants for 40 years has found that thousands of species are disappearing.

The research, by thousands of amateur botanists, is now being prepared for publication after years of diligent work during which the whole of Britain was divided into 10-kilometre squares and every plant species in those areas was noted.

Simon Leach, a botanical adviser to English Nature, said: "There's been a decline of an almost tragic extent in native species. Species like the great water Parsnip pave declined hugely. It's gone from about one-third of the squares where it was present in 1962."

David Pearman, of the British Botanical Society, said: "There have been huge changes, especially to wet- condition plants; those have almost disappeared from large areas of the South-east such as Kent, Surrey and Sussex."

While there are roughly 1,500 native wild flower species in the UK, the number of alien species has risen to nearly 3,000. One reason is the arrival in the past 40 years of alien plant species such as Japanese knotweed, Spanish bluebells and Australian swamp stone crop.

Dr Martin Harper of the plant conservation charity, Plantlife, said: "The Australian swamp stone crop can grow up to 15 centimetres (6 inches) a day, hogging sunlight, killing fish and changing the ecology of an area."

Other alien species have also caused problems. One of the best-known is Japanese knotweed, which Swansea City Council has spent more than £1m trying to eradicate. The giant hogweed is also still too prevalent, despite efforts over the past 20 years to wipe it out.

The other force that is affecting native plants is climate change. With the weather becoming warmer, plants that survived best in cold conditions such as the north of the country or at altitude are being threatened as their habitat changes and is viable for outside species.

James Smart of Plantlife said: "It means plants will start colonising areas higher up hills or in the north. In some cases, there will be nowhere for the plants that are there to go – so invasive species might overwhelm them."

Mr Pearman said some native species were thriving. "Those that have a good strategy for coping do better. In particular, the ones that deal with stress do the best. It might sound a little odd, but plants, like humans, find the modern world stressful.

"Actually, although the number of wild flowers in particular fell in the mid-1970s, the outlook overall is still bright. The picture isn't nearly as bad as the doomsday people suggest. There's quite a nice amount of diversity out there."

One native plant that is thriving is Danish scurvygrass, which normally lives on coastal cliffs. But now it has been found along the edges of the A55, an expressway in north Wales that runs between the English border and Anglesey.