A Population Study of Spiders in Limestone Grassland

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Abstract
Three sampling areas near Wytham Wood, Berkshire are distinguished: a Brachypodium pinnatum sward: an area consisting mainly of Festuca rubra; and a sparse and diverse vegetation on loose Coral Rag limestone. Collecting and sampling methods included a platform temperature-gradient apparatus designed to extract the spider fauna from vegetation quadrat samples. The dense Festuca turf supported more spiders (up to 841.9/m2) than Brachypodium. and both recorded higher population densities than the more varied vegetation on the Coral Rag limestone. The most uniform vegetation (Brachypodium) recorded the lowest mean number of species; the highest number was recorded on the Coral Rag where floristic diversity was greatest. The proportion of adult spiders was never more than 48% during the year, being highest in the winter and lowest in late summer ( <7% in July). The seasonal trends in population density and proportion of adults are similar in all three habitats. A total of 141 species was recorded in the 2[degree]8 acres (113.3 m2) of which 63% could be assigned to the ground zone. This total, which included 58 Linyphiidae, is nearly a quarter of the known British fauna. Population interchange between the habitats appeared to take place easily, but within the same families the dominant species differed from habitat to habitat. Examples are given of cryptic coloration in those spiders which hunt on the vegetation surface, mainly in the lower part of the field-layer. The numbers of spiders taken in the traps in all 3 habitats showed a large peak in early summer (mainly Lycosidae) followed by a fall and subsidiary peak in the autumn (mainly Thomisidae), which is most pronounced in the Coral Rag vegetation and least in the Brachypodium. Habitat preference in a number of common spiders is demonstrated both by trapping frequency and numbers recorded per unit area; both methods give the same picture of habitat selection. Selective grazing and trampling by herbivores such as sheep influence population density and the micro-distribution of individual species.