According to the British Arachnological Society, there are some 654 different species of spider in the U.K. However, a recent survey (link opens a PDF) they undertook (in conjunction with Natural Resources Wales) suggests that 16% of our spider species are threatened. Eighteen of these species are critically endangered and eighty four species are classified as vulnerable or endangered. The terms used are based on those of IUCN Red List categories, so critically endangered means Extremely high risk of extinction in the wild.
One species described as vulnerable is the fen raft spider. The fen or great raft spider (Dolomedes plantarius) is a semi-aquatic spider. It lives in lowland fen and marshy areas; for example, it may be found at the nature reserve at Crymlyn Bog, Swansea and Pant y Sais.. Within such areas, it is to be found the edges of pools and ditches. Emergent vegetation is important to the spiders as it is used as a perch for hunting and basking in the sun. They avoid areas where the water surface is shaded as they like warmth.
These spiders are predatory and hunt from their their perches at the waters' edge. They feed on insects such as pond skaters and dragonfly larvae, plus the occasional small fish and smaller aquatic spiders. When prey is sensed, the spider is able to run across the surface of the water to reach it, using surface tension. The spider is quite large; adult females can have bodies of 2 cm in length, with a span of 7 cm including their legs. It is typically black or brown in colouration with cream or white stripes on the sides of the body. There are excellent images of these spiders here
U.K. spiders are not the only ones at risk - spiders face difficulties world wide. A recent meta-study by workers at King Juan Carlos University (URJC) looked at some 173 different research papers and concluded that spider species and numbers are affected world wide by farming practices and forest management, for example,
- through grazing,
- the intensive use of pesticides / insecticides,
- loss of ecosystem heterogeneity and
They also noted that woodland species were affected by habitat fragmentation.