Notes on Arctosa alpigena

From The Newsletter No. 54 March 1989

When Ashmole (1980a) reported the capture in 1979 of Tricca alpigena on the Cairngorms, this large and strikingly marked species was regarded as one of Britain's rarest spiders. It was then known only from the Cairngorms, where there had been four earlier captures, the three published records being of single specimens.

The first find of T. alpigena in Britain was reported by Traill (1873) from 'near Braemar' where it was taken in 1872. The single male was described by Pickard- Cambridge (1873) as new to science and named Trochosa biunguiculata, and it was only later that Braendegaard (1939) showed that it was identical to Tricca alpigena (Doleschall, 1852).

The first capture to be more precisely localised was by Mr Alexander Robertson, who found a female on Cairn Toul in June 1893 at an altitude of about 1,040m (Carpenter & Evans, 1894). In 1914 Jackson (1915) found a male in grassland, also on Cairn Toul, at 940m. Ashmole (1980b), giving Merrett (pers. comm.) as the source, also mentions that A.A.D. La Touche had found the species 'on Cairngorm'. The find by Ashmole was in July 1979 at 1,120m on Feithe Buide, which lies between Cairn Gorm and Ben Macdui.

During 1983 and 1985 D. H. trapped T. alpigena in pitfalls and water-bowls on the summit plateau of the Creag Meagaidh National Nature Reserve (NN 48) (Table 1). Traps in Nardus stricta (mat-grass) grassland were at 975m a.s.l. and those in Racomitrium lanuginosum (woolly-fringe moss) heath were at 1,000m a.s.l. There were six pitfalls and two water-bowls in each plant community. The catch of T. alpigena included 5.7% immatures. Adult T. alpigena made up 19.2% of the total adult spiders taken (n = 1,035), whereas other nonlinyphiids made up only 1.0%.

In 1987 S.M. and H.G. caught T. alpigena in pitfall traps on the Cairn Gorm plateau (NJ 00, 1,050m a.s.J.) between May and September (Table 2). There were ten pitfalls in operation in each plant community for between 7 and 17 days each month. Immatures at 35.5% made up a much larger proportion of the catch than on Creag Meagaidh. Mature T. alpigena made up 20.8% of the total catch of adult spiders (n = 437), whereas other non-linyphiids made up 5.5%.

The numbers per trap day for Creag Meagaidh (Table 1) have had to be modified to make them comparable with Cairn Gorm because the traps included water-bowls, and six pitfalls were used instead of ten. The catch in the six pitfalls was therefore multiplied by 1.67, to give figures for the whole trapping period roughly comparable with those from Cairn Gorm. Although the catch on Creag Meagaidh was much higher in N. stricta grassland than in R. lanuginosum heath, on Cairn Gorm the catch was almost equal in these plant communities. Only two specimens were caught in Juncus trifidus heath on Cairn Gorm. This heath consists of scattered tussocks of J. trifidus among bare gravel, and such open conditions may be inhospitable to T. alpigena. The other plant communities, on both sites, have only scattered rock. On both sites the numbers caught dropped off in the later trapping periods. Though the results are not strictly comparable, the early high numbers were apparently more prolonged on Creag Meagaidh than on Cairn Gorm.

Pitfall traps were in operation on two other high altitude plateaux. On the Drumochter Hills (NN 68) at 880m a.s.l. no T. alpigena were caught in 1987 or 1988, although the range of plant communities in which the traps were set and the trapping effort were similar to those on Cairn Gorm. Similarly on Glas Maol (NO 17) in 1988, no T. alpigena were caught by pitfalls in N. stricta grassland or R. lanuginosum heath, although the altitude at 1,050m was similar to that at the Creag Meagaidh and Caim Gorm sites. The trapping effort was about half that on Caim Gorm.

On the 26th June 1987 D.H. captured a single male T. alpigena, which was running about in N. stricta grassland at 850m a.s.l. on Sgurr nan Clach Geala in the Fannich Hills, Wester Ross (NH 17). This is the only find known to us from north of the Great Glen.

These recent captures suggest that T. alpigena is not as rare as was once thought and may yet be found on other high-altitude massifs in the Scottish Highlands.

Table 1. Catch of Tricca alpigena in traps on the summit plateau of Creag Meagaidh (catch in pitfalls + water-bowls)

Plant community------------------------------Dates-------------------Estimated
-----------------------------------------------------------------------no- per
------------------------16 May-----18 Jun-----14 Aug------------------trap day
-----------------------to-18-Jun---to-10 Jul---to-09-Oct--------------(10 traps)
--------------------------1983--------1983---------1985----------------

Racomitrium heath--------19+5---------9+1---------0+0------------------0.42
Nardus grassland---------66+8--------64+14--------25+0-----------------2.31

Total catch---------------98-----------88---------25-----------------211
Trap days-----------------33-----------23---------56-----------------112

Table 2. Catch of Tricca alpigena in traps on the summit plateau of Cairn Gorm
--------------------------------- Date ---------------------------
Plant community ----May--June--July--Aug--Sept--No. per trap day (10 traps)
Racomitrium heath---40-----2-----3----7-----6--------0.97
Nardus grassland----43-----2-----3----6-----0--------0.9
Empetrum heath-----10-----6-----6----5-----0--------0.45
Juncus trifidus--------0-----0-----0----2-----0--------0.03

Total catch----------93----10----12----20----6-------141
Trap days------------7-----11----11----14----17------60

 
Added by John Partridge at 18:11 on Wed 11th Jan 2012. Return to Summary for Arctosa alpigena