Spider Adaptation to Artificial Biotopes: The Fauna of Percolating Filter Beds in a Sewage Treatment Works

Abstract

The spider faunas of two filter beds at the Minworth Sewage Treatment Works in Birmingham, UK, were studied from June 1980 to June 1981 inclusive, to investigate their role in this specialized community. The spider fauna was dominated by only two species, Leptorhoptrum robustum and Erigone longipalpis (Linyphiidae), although two others were able to live permanently in the stone medium, in very low numbers. The zonation of spiders throughout the 1.8 m depth was studied by using vertical shafts containing 10-12 canisters of stones which could be removed and sampled. The filter bed environment has a constant humidity, small variation in temperature, abundant food, and is relatively free from spider predators and parasites apart from entomophagous fungi. The lowest numbers of spiders were recorded in the winter and spring, and very high values were recorded in mid-summer when each individual spider had only 3.2 x 3.2 cm of stone surface and 2.5 cm 3 of air space. Immature specimens were always more numerous than adults except in winter and spring. The highest numbers of egg cocoons of both dominant species were recorded in October, but there was a long delay before the majority of the eggs hatched in May/June. The very high populations in June of over 67 000 m -3 coincided with massive aerial dispersal movements. The influence of toxic metals in the crude sewage on the spider fauna is discussed but no evidence was found that the populations were affected. An estimate is made of the potential ability of the spider fauna to control nuisance flies which breed in the filter beds. Control could be achieved by reducing the interruptions to sewage flow and confining shutdowns for maintenance work to early spring when populations are low. This would also reduce the incidence of spider bites among maintenance staff.