The status and conservation of the fen raft spider (Dolomedes plantarius) at Redgrave and Lopham Fen National Nature Reserve, England Original Research Article

Dolomedes plantarius, a semi-aquatic spider, occurs at only two sites in the UK. At Lopham and Redgrave Fen NNR, ground-water abstraction between 1960 and 1999 dried out the pools on which the spider depended, reducing its range by over 80% and leaving two small, isolated populations. A population index, based on systematic monitoring from 1991 to 1999, showed significant variation between years and between the two populations, one of which showed a slight but significant positive trend. Changes in the density of spiders on the pools and the numbers of occupied pools contributed to variation in the index to differing extents in the two populations. The age structure of the populations was highly variable between years and there was no relationship between the abundance of spiders and the numbers of nursery webs. Factors contributing to the persistence of D. plantarius at this site, and obstacles to its recovery following hydrological restoration in 1999, are discussed.