T H E DISTRIBUTION OF T H E GREAT RAFT SPIDER
21
However, the spiders appear to move readily between pools and it is unlikely therefore that these populations are completely isolated from each other. The populations on Little and Middle Fens appear to be in no immediate danger of extinction. That on Little Fen, where there are many old peatdiggers' pools, is extensive and in 1980 seemed encouragingly large. Much of the willow scrub which encroached there in recent years has been removed. D. plantarius was first seen on the deep machine-dug pits on Middle Fen in 1980, three years after they were made - and several were seen. The species is less easily f o u n d elsewhere on Middle Fen although the rapid colonization of the machine-dug pits suggests that the species was already present on this area of the reserve when the pits were dug. The population on Redgrave Fen gives most cause for concern. It is scattered over an area of less than 1ha. The survival of the species within this area almost certainly depends on the existence of several small pools. The water table is lower than on Little Fen and, as a result, there has been extensive invasion by willow, birch and hawthorn scrub and by fine-leaved grasses. T h e spider was presumably present on Redgrave and Lopham Fens when the pools were dug, but the water table was then much higher and the pools were unnecessary for the spider's survival. Because there was more open water and the Vegetation was mostly coarse-leaved the spiders would have been able to move more easily amongst the Vegetation than today. T h e pools are probably now more vital to the survival of the spider. In a dry summer the only open water, apart f r o m the rivers and dykes, in the parts of the reserve which D. plantarius inhabits, is contained in the peat-diggers' or machinedug pools. Spiders were not seen at pools amongst the willow scrub. Possibly such pools are unsuitable because they receive less sun, and hence are unfavourable to the sun-loving spiders, and because such pools tend to be stagnant and devoid of aquatic life, some of which could be food for the spiders. However, as pools at the edge of the scrub also have no or very few spiders a more satisfactory explanation may be that the replacement of reeds and sedges by grasses m a k e s dispersal difficult. T h e rapid invasion of the cleared area, in which the Vegetation between the pools was well-trampled, supports this hypothesis. If the spiders frequently travel between pools, then in drier, grassier areas they would spend less time at the pools. Where grasses are dominant D. plantarius can probably only survive if the pools are densely concentrated, as in the area it inhabits on Redgrave Fen. By cutting Vegetation D. plantarius could readily be encouraged into some areas of the reserve it does not currently inhabit. A programme of scrub clearance is in progress and the distribution of the spider may expand. However, to ensure its survival by this means would require continual reserve m a n a g e m e n t and a more satisfactory Solution would be to raise the water table to former levels.
Trans. Suffolk Nat. Soc. 21