The distribution and abundance of Wolf Spiders (Araneae: Lycosidae) on Chapel Fell in the Northern Pennines

Abstract

The study is introduced with some background information on the Pennine moors and their animal communities with particular reference to spiders and general information on w o lf spiders, in particular. The study was carried out by pitfall trapping at 20 sites on Chapel Fell, 14 selected for differing vegetation and soil type, at around 600m in altitude and 6 on an altitude transect, with a wet and a dry sites at each of three altitudes. Eight species of lycosid were caught. The numbers were approximately a quarter to a third of the all spiders caught during the early summer, but there was considerable variation (30%) between years. The catch of lycosids varied over the study period, rising from single figures in early May to a peak of 637 in 1991 and 980 in 1992 during June or July, after which the numbers dropped again. Catches from 1991 were examined for a six month period. These showed three peaks in abundance/ activity; the major one in July and two smaller ones in June and September. The total numbers of each individual species caught varied between 65% of all lycosids to 0.1%. There were significant changes in the numbers of most species, between years. Species richness for each site was worked out and a maximum of six and minimum of one species per site was found. Species diversity was calculated using Simpson's Index. Both of these measurements showed a tendency to increase between 1991 and 1992. Their distribution between the sites was analysed by Detrended Correspondence Analysis (DECORANA) which suggested that species show preferences between sites, which are based on soil moisture variation and vegetation differences. Altitude is also shown to affect species distribution; some species only occur in very small numbers at high altitudes, which are caught in hundreds lower down. Other species show no significant variation. The wet and dry site on the altitude transect demonstrate that five out of seven show significant preferences with regard to moisture. The lycosid Alopecosa pulverulenta was shown to have varying preferences with regard to soil moisture, depending on the soil type. The ratio of males to females caught was approximately 9:1. There was an increase in the proportion of males caught in 1992 for most species, which could be related to activity occurring earlier in the seas that year. The timing of the appearance of juvenile forms suggests two spiders Alopecosa pulverulenta and Pirata piraticus could have two year life-cycles in the uplands In view of controversy, the value of using pitfall traps is discussed and it is concluded from studies that have been carried out that it was the best method available. Suggestions are made to account for the patterns of distribution and abundance of lycosids on Chapel Fell, particularly interspecific competition or alternatively climatic factors. A longer study would be needed to reach any firm conclusions, because neither explanation can be ruled out by the resuUs obtained.