By David GreenEXPERTS are mystified why a colony of rare spiders is still on the verge of extinction, despite a £3.2million project to restore the fenland nature reserve where it is found.

By David Green

EXPERTS are mystified why a colony of rare spiders is still on the verge of extinction, despite a £3.2million project to restore the fenland nature reserve where it is found.

Habitat conditions for the great raft spider - known at only one other site in England - are now thought to be ideal at the Suffolk Wildlife Trust's Redgrave and Lopham Fen nature reserve.

As part of a major restoration programme, water levels were raised to enable the spider and the other rich wildlife on the reserve to thrive. The restored reserve was officially reopened in 1998 by the Duke of Edinburgh.

However, its population of great raft spiders shows no sign of increasing and at one of its two sites on the reserve numbers are at their lowest ebb since detailed records began to be taken in 1991.

Until the restoration project started in the late 1990s, the reserve had been drying out as a result of the demise of traditional fenland management, including reed and sedge cutting, and the operation of a nearby public water supply borehole.

Water had to be pumped into the reserve to maintain the pools used by the spiders during some of the “dry” years.

Dr Helen Smith, a spider expert who monitors the Redgrave and Lopham Fen colony on behalf of English Nature's species recovery programme, said the reason for the population's failure to respond to habitat improvements was a mystery.

“In its present size, the population is not at all stable. Now that water levels have been restored it has the potential for rapid increase, but it just hasn't happened” she added.

“It could be that the water quality is not quite good enough or it may be genetic problem from which they are unable to recover.”

The number of female great raft spiders at Redgrave and Lopham Fen is being counted in tens instead of the thousands which had been expected by now.

Each female can have two breeding cycles annually and each can carry between 500 and 600 eggs.

Dr Smith said a University of East Anglia student working on a Doctor of Philosophy degree was to carry out DNA fingerprinting of the spider colony to try to throw more light on the problem.

The fingerprinting exercise would also be useful if, in the future, plans were considered to introduce spiders from England's only other population, a thriving colony at Pevensey on the south coast.

A colony of the great raft spider has recently been found on the edge of a canal in south Wales, heightening speculation that there could be other undiscovered colonies elsewhere in the UK.

Julian Roughton, Suffolk Wildlife Trust director, said the raft spider situation was “disappointing” but although it had caught the public's imagination, the creature was by no means the only species of interest on the nature reserve.

“The European money we got for the restoration project was based on it being one of the most important lowland valley fens in England and of international importance for a range of plants and animals,” he added.

Mr Roughton said the Redgrave and Lopham Fen spider colony could turn out to be a “relic” - the remnant of a large colony which might have inhabited the adjacent River Waveney before the deep dredging and straightening programme of the 1960s and 70s.

“We may have to rethink what we imagine is the ideal habitat for raft spiders - the two other colonies discovered in the past 20 years are in very different habitat,” he added.