A milestone study of British birds, butterflies and wild flowers has revealed the strongest evidence yet that we are on the verge of a mass extinction of global wildlife - the sixth mass extinction in the history of life on Earth.
Scientists have accumulated the most detailed data to date indicating that human activity is systematically stripping the planet of its rich biodiversity. Nearly a third of native British plants have significantly decreased in 40 years, more than half of native birds have declined in just two decades and nearly three- quarters of British butterflies have fallen in numbers in 20 years.
The study involved about 20,000 naturalists who inspected the entire British landscape to compile three atlases of native birds, butterflies and wild plants. The information they gathered on the presence or absence of more than 1,500 species in each 10- kilometre (six-mile) square of countryside they surveyed was compared directly with similar atlases compiled 20 or 40 years previously.
In the relatively short period between the past and present surveys, the scientists found a dramatic decline of all three major groups of wildlife, with one-third of all species studied disappearing from at least one part of the UK they had occupied 20 or 40 years ago.
Jeremy Thomas, the leader of the study from the Centre for Ecology and Hydrology in Dorset, said the decline in butterflies was much worse than expected and far worse than that of birds or plants. "The results are appalling," he said. "In Britain 71 per cent of all butterfly species have declined in the last 20 years. "For the first time we can say that in the UK one group of insects has suffered as badly as birds or plants - this adds enormous strength to the hypothesis that the world is approaching its sixth major extinction event."
For more than a decade scientists have constructed computer models of the rate at which species are going extinct. Such models suggested a rate of anywhere between a hundred to many thousands of times greater than normal "background" rates. The information used for these models was based on the fossil record and what little was known about the rate of extinction within certain well-studied but rather unrepresentative groups, such as birds, fish, certain mammals and palm plants. but Britain has good records of wildlife that could help to fill many of the gaps.
"There are simply no data sets that approach this detail and scale anywhere in the world," Dr Thomas said.
"Even though UK butterflies are a tiny proportion of the world's insects, and although the UK is a small country, this is the first time it has been possible to compare for any group of insects with the better recorded groups [of animals and plants]. "The gloomy result is that this group has indeed declined as rapidly as plants and birds and it's because of this we believe it provides tentative support of the sixth mass extinction event," he added.
In 1999, Lord May of Oxford, the president of the Royal Society and the Prime Minister's former chief scientific adviser, estimated that the current extinction rate could be up to 10,000 times higher than it should be under normal circumstances. In a speech at the time to the World Conservation Union, he said: "This represents the sixth great wave of extinction, fully compatible with the big five mass extinctions of the geological past, but different in that it results from the activities of a single other species [humans] rather than from external environmental changes."
Yesterday, Lord May said the latest study, showing a 28 per cent decline of native plants, a 54 per cent decrease in abundance of native birds and a 71 per cent decline of butterflies, supported the belief that the world was on the cusp of another mass extinction. "These are dismaying trends," he said. "If this pattern holds more generally then estimates of global extinction rates - which are mainly based on birds and mammals - could err on the optimistic side."
The study, funded by the Natural Environment Research Council and published in the journal Science, is important because of its focus on a major group of insects.
Dr Thomas said: "Past assumptions about extinctions were based on just a small number of species studied, mainly birds. But birds make up only 0.6 per cent of all species on Earth. An obstacle to this conclusion [of sixth mass extinction] is that really no reliable information has existed for insects and insects comprise over half the species on Earth.
"Butterflies have declined by an order of magnitude greater than either birds or plants. This was an unexpected result and it has implications both nationally and globally. We tentatively suggest that this provides the first objective support for any group of insects for the hypothesis that the world is experiencing the sixth major extinction event in the history of life."
Since the last British butterfly survey 20 years ago, two species - the large blue and the large tortoisehell - have gone extinct in the UK. Of the 58 native species studied, the high brown fritillary has declined most, down by 71 per cent.
Dr Thomas said this was probably because of changes in the way woodlands are managed, which have made woodland floors shadier places, hampering the survival of caterpillars that live on forest violets. Lost of habitat is the overwhelming reason for the decline of both wild animals and plants.
The ploughing of heathland and the draining of wetlands have resulted in complete destruction of some habitats, while others have become degraded as a result of other forms of human activity, such as pollution.
Professor Georgia Mace, director of science at the Institute of Zoology in London, who has studied extinction rates, said that the latest study suggests the problem could be far worse than previously imagined. "According to the results here, we could be seriously underestimating the severity of the problem," Professor Mace said.
THE END OF THE WORLD, BACK THEN
Cretaceous-Tertiary extinction (about 65 million years ago) The last mass extinction wiped out the dinosaurs and nearly half of the main groups of marine animals. Most likely cause was a collision with a large asteroid.
Triassic extinction (199m to 214m years ago) Killed nearly half of the major groups of marine wildlife. Believed to have been caused by undersea volcanoes.
Permian-Triassic extinction (about 251m years ago) The worst mass extinction, wiping out up to 95 per cent of species. Probably caused by volcanic eruptions or an asteroid.
Late Devonian extinction (about 364m years ago) Death toll estimated to be 22 per cent of marine families and 57 per cent of marine genera. No one knows why it happened.
Ordovician-Silurian extinction (about 439m years ago) Killed a quarter of marine families, including some bizarre creatures such as Hallucigenia (right). Believed to have been caused by a fall in sea levels as glaciers formed, then rising sea levels as glaciers melted.