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Dolomedes plantarius, Fen Raft Spider

Dolomedes plantarius, the fen raft spider, is "one of Britain's largest spiders and rarest animals, (Smith 2000)." While data is scarce, it is believed to be in decline throughout the calcareous Palearctic wetlands it inhabits. The first UK population was discovered at Redgrave and Lopham Fen in 1956. In 1960, 39 years of groundwater abstraction began, peaking at a rate of 3600 tons daily, and the drop in the water table was exacerbated by frequent droughts. Vast areas of the fen dried out and were replaced by scrub. The alkaline, nutrient poor water of the fen became acidified and nutrient rich, primarily due to waste water pollution from nearby swine production. Former vast areas of Schoenus and Juncus have been replaced by Molinia, and great fen sedge, Cladium mariscus is being outcompeted by invasive Phragmites australis, the common reed.

The fen raft spider, like all pisaurids, exhibits extended maternal care. Adult females carry their egg sacs in their chelicerae for several days, then construct a silken protective web around the egg sac amongst vegetation, guarding from the outside until the hatchlings complete their first molt and disperse. D. plantarius has only been observed at the fen to construct these nursery webs on great fen sedge, which is disappearing from the fen. While a number of critically endangered and endangered invertebrate species are believed locally extinct, a population of approximately 100 adult females survived by inhabiting the lone source of standing water during droughts, pools left behind by 19 th century peat digging. This is currently the only known habitat of the spider at the fen, and little is known of a population of approximately 3000 adult females at Pevensey Levels, the only other known UK locality.

The Redgrave and Lopham Fen National Nature Reserve was established in 1992, encompassing the entirety of the fen, and it was awarded additional EU conservation funds as an internationally important area for conservation. A five year program to remove scrub and restore water and habitat quality was not very successful at raising the water table, improving water quality, or controlling invasive species. A Species Recovery Programme drafted in 1991 by English Nature for D. plantarius has not made much progress towards the goals of increasing the population 10-fold, improving the habitat, and introducing spiders to 2 new sites by 2010. From 1991 to 1999, the population had not seen any growth, and is still restricted to peat holes and irrigated pools simulating peat hole conditions.


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