Butterfly experts from around the world gather in Reading, Berkshire, today as evidence mounts that butterfly numbers are declining rapidly all around the world.
Sir David Attenborough will tell delegates attending Butterfly Conservation's Sixth International Symposium that the declines are deeply worrying.
He said: "Butterflies are sensitive indicators. They decline when habitats are destroyed and when man harms the environment. We have known about butterfly losses in Britain for over 50 years. Now there is mounting evidence that it is a global problem."
Sir David, who is President of Butterfly Conservation, added: "If butterflies are disappearing, other wildlife will be declining too. Some will be facing extinction.
The Symposium is the biggest-ever gathering of international butterfly experts. It was called to assess the success of efforts around the world in meeting the United Nation's target of halting biodiversity declines by 2010. However, the agenda reflects fears in countries throughout Europe and as far apart as Japan and the United States that there is still a long way to go to achieve this goal.
Sir David said: "Halting biodiversity loss is the coming decade's great challenge. It's on a par with getting a man on the moon in the 1960s. An increase in butterfly numbers around the world could be the first indication that we've achieved this goal. Like that first step on the moon, it would be a giant leap for mankind."
Dr Martin Warren, Chief Executive of Butterfly Conservation, said: "There is increasing evidence that the problem is a global one. We have just published a new Red List of European Butterflies showing that around 10% of European butterfly species are facing possible extinction."
He added: "We are getting the same message from elsewhere. Symposium delegates come from all six continents. They are all saying the same. Declines on the scale of those occurring in Britain and in Europe are happening in their part of the world too. Habitats are being destroyed; butterflies and other wildlife are in decline."
Dr Warren said that it was already apparent that the 2010 deadline for halting biodiversity loss had not been met. But he added that trying to achieve it had been important. "I think we now know the enormity of the challenge. We must make it a top priority in the coming decade."
Monitoring of butterfly numbers started in the UK in 1976 and has spread around the world. The data is increasingly vital in assessing the success and failure of efforts to halt biodiversity loss.