Classification and evaluation of spider (Araneae) assemblages on environmentally sensitive areas in Northern Ireland

Spiders have been collected as part of a biodiversity monitoring programme on environmentally sensitive areas (ESAs) in Northern Ireland initiated in 1993. Work to date on spiders in Ireland has consisted mainly of the cataloguing of County records. This paper furthers this work by classifying spider assemblages according to the habitat on which they were trapped. A total of 64 205 individuals of 184 species have been trapped using pitfalls representing almost half of the known Irish species. Distinct spider assemblages were recognisable on heather moorland, in woodland and on species-rich and species-poor grasslands. Geographical differences in assemblage composition were discernible. The number of records for each species was used to construct a provisional set of species quality scores to rank sites accordingly. Heather moorland harboured the highest mean number of species per site whilst ash woodland in County Fermanagh had the highest species quality score per site based on the number and rarity of spiders. Species-poor grassland had the lowest numbers of spider species with the lowest species quality scores due to their relative abundances. The scoring system used in conjunction with traditional phytosociological studies adds an important dimension to the evaluation of biodiversity on a monitoring site. Scores are a valuable tool for the future evaluation of monitored sites. Our results highlight the importance of heather moorland and woodland as plant communities and as unique habitats for specialist spider species.