One of the last two refuges of the great raft spider is the Redgrave and Lopham Fen in East Anglia, home to a rich variety of wildlife including fewer than 100 of the rare arachnids.
Suffolk Wildlife Trust officials say that too much water has been extracted from the fen - which is irrigated by a natural borehole - for use by the public and farmers.
Now that supplies in the region are low, the moisture on which the spider depends is drying up. Conservationists have started pumping emergency supplies of water into the fen, near Diss in Norfolk, in the hope of improving conditions. The spider's only other habitat is at Pevensey in East Sussex.
Trust assistant director Julian Roughton said: "We are pumping water into their pools to help them out. I suspect that without that they would not survive. They are surviving, but that's about all they are doing."
The European Union is backing a pounds 3m project which aims to revitalise Redgrave and Lopham Fen by re-siting a borehole by the Essex and Suffolk water company. EU officials hope that, if successful, the project can be repeated at other threatened wetland sites in Europe. However, the Suffolk Wildlife Trust fears that it may come too late for its particular colony.Reuse content