SPIDERS AND PHALANGIDS OF INCHCAILLOCH, LOCH LOMOND III - COMPARISON WITH THE MAINLAND

By DAVID J. CURTIS, G.S.LANGLEY, H.G. STEWART, J. BOWDITCH and S. THOMPSON.
Department of Biology, Paisley College of Technology

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Introduction

The Loch Lomond National Nature Reserve comprises Inchcailloch, with other islands, and a mainland portion. The mainland part of the Reserve, approached from Gartocharn, lies beside the River Endrick which flows into the south-east corner of Loch Lomond (Fig. 1). Aspects of the arachnid communities of Inchcailloch have been considered earlier (Curtis, 1973, 1975, 1978) and this paper compares the island fauna with that of the mainland.

The island of lnchcailloch is covered mainly with deciduous woodland, primarily of oak (Quercus spp.), with areas of mixed woodland including birch, hazel and other species. Alder carr occurs in low-lying areas, some Scots pine stands on the higher ridge, with a few clear areas. More details of the island vegetation are available (Horrill, Sykes and Idle, 1975), but in broad terms the dominant biotope is deciduous woodland. In contrast the mainland part of the Reserve includes areas of markedly different character. Deciduous woods with tree species and ground flora similar to those of Inchcailloch are present in the Entrance, Shore, Pentagonal and Ring Woods. Within the Ring Wood East is a distinct area of birch woodland with well-developed moss lawns and hummocks involving Sphagnum and Polytrichum species. Areas of open grassland are the Limehill Field, Limehill Rough, Pentagonal Field and Ring Point, with boggy areas such as the Whin Park (wet/dry heath mosaic), Ring Bog, Aber Bog, the Twenty Acres and, to the north-east of the River Endrick, Crom Mhin. The relative areas of these wooded and open habitats are apparent in the sketch map (Fig. 1).

The spider and phalangid communities have been studied since 1971 and the data on which this comparison is based are derived mainly from the following work:-
Inchcailloch, 1971-73: four sites sampled by Dr. Curtis;
mainland, 1974-76: seven sites sampled by Dr. Curtis;
Inchcailloch and mainland, 1974-75: four sites sampled by Mr. Langley;
island and mainland1 1976-77; three sites, Dr, Curtis and Mr. Stewart; island, 1977-78: 16 sites, Dr. Curtis and Mr. Thompson;
mainland, 1977-78: 15 sites sampled by Mr. Bowditch;
Crom Mhin, 1977: one site, Dr. Curtis. Additional data have come from work by Mr. P. Lerpiniere and also by Mr. E. Morton (Curtis and Morton, 1974).
These studies have concentrated on the ground-active fauna, using pitfall traps of various types. Complementary sampling with corrugated paper traps on tree trunks, sweep netting, vacuum extraction and hand sorting have added a few species from other vegetation strata. These species from higher vegetation are usually taken, albeit in small numbers, in pitfall traps when a prolonged sampling programme is employed.

Species Complements

Twelve species of harvestmen (Phalangida) and 184 species of spiders have been recorded from the Reserve. While the phalangids are relatively ubiquitous over both island and mainland, the spiders show more variation in distribution. The occurrence of the 196 species in four categories is listed in (Table 1), which uses the nomenclature of Locket, Millidge and Merrett (1974) (below referred to as LMM) for spiders. The four categories used are (1) the woodland of Inchcailloch, (2) "grassy" woodlands of the mainland which have ground floras similar to those of Inchcailloch, (3) "mossy" woodlands of the Ring Wood East, and (4) open habitats of the mainland.

Spider species include 27 confined to Inchcailloch, seven restricted to grassy mainland woods, 13 to the mossy Ring Wood East and 16 to the open areas of the mainland. The total species richness of the spider faunas of the four areas are respectively 129, 102,108 and 83 species. This would appear to indicate greater richness for the island, or a more diverse fauna, but a critical examination of the data does not confirm this. Considering a single year of sampling, 1976-77, when the woodlands were sampled concurrently using the same method (pitfall traps with formalin/teepol trapping fluid), we find richness values of 66, 71 and 59 species for Inchcailloch, mainland grassy and mossy woods respectively. The numbers of individuals corresponding to the last three species richness values are 1833, 1234 and 777, indicating a more active and/or dense population on Inchcailloch.

One can calculate the estimated number of species for a given sample size. Thus, for a collection of 100 specimens per site over the year, one would expect 14 species from Inchcailloch, 33 from the grassy wood and 28 from the mossy wood on the mainland. Resolution of these apparently conflicting data lies in the relative abundances of the various species in the communities. These are illustrated in Fig. 2, where it can be seen that the Inchcailloch community contains a large number of rare species, each represented by a single specimen, whereas the grassy wood of the mainland, and to a lesser extent the mossy site, has many species which are moderately abundant. Accordingly, for a standard sample size (or sample period) one expects more species from the mainland woods than from the island.

Community Comparisons

Irrespective of relative abundances, the spider data show no significant similarity, nor dissimilarity, between Inchcailloch and the combined mainland woods (  χ² = 0.46, d.f. = l, p = 0.5 approx.). However, in spite of a strong likeness between the two types of woodland on the mainland (  χ² = 18.87; p < 0.00l), the spider fauna of the grassy mainland woods is similar to that of Inchcailloch (  χ² = 11.25; p < 0.00l). The fauna of the open areas of the Reserve shows no strong similarity to any of the woodlands.

In broad terms, phalangids are the dominant cursorial hunters on Inchcailloch and lycosids (wolf spiders) on the mainland. The phalangids N. bimaculatum, M. morio, 0. tridens, 0. palpinalis and L. ephippiatus are all more active in the woods of the island than those of the mainland and, with the exception of M. morio, even less prominent in open land. 0. agrestis and 0. hanseni are more active in mainland woods.

Effectively replacing these harvestmen on the mainland are the lycosids: P. pullata, P. amentata, A. pulverulenta, T. spinipalpis and P. hygrophilus. Only the wolf spider P. lugubris is more active on Inchcailloch than in the Gartocharn woods. Tetragnathid spiders in the genus Pachygnatha, like lycosids, spin no webs when adult, and show a contrast between P. clercki which is more active in open and wooded mainland than on Inchcailloch and the converse pattern for P. listeri.

The theridiid and linyphiid spiders spin more or less horizontal sheet webs with varying amounts of vertical threads attached and also show contrasts between the different areas, though not as marked as the cursorial hunters. The theridiid, R. lividus, is more active on Inchcailloch than in the mainland woods and is relatively infrequent in the open areas.

Relatively abundant linyphiid species which are more numerous on Inchcailloch include W. acuminata, D. tibiale, D. picinus, H. excisa, C. sylvaticus, M. rufus, L. zimmermanni, L. tenebricola and especially M. fuscipes. G. rubellum is found only on Inchcailloch and appears to be replaced by its congener G. rubens on the mainland. D. latifrons is more abundant in the mainland woods than in either open areas or on Inchcailloch, while other species with mainland predominance tend to be most abundant in mossy woodland or open areas: C. obscurus, T. pallens and especially E. dentipalpis.

These comparisons are based on data over the years, and one must realise that in addition to seasonal variations there can be marked fluctuations from year to year. This has been noticed for species such as N. bimaculatum, O. tridens, O. palpinalis, W. acuminata, O. rubellum and C. dilutus.

Rare Species

The term 'rare' may be applied to (a) species which are of infrequent occurrence over the country as a whole, and (b) those which were recorded infrequently on the Reserve but are more common elsewhere. The latter situation is most readily construed as a population which may be ephemeral and not likely to persist. However, this may not be the case as such infrequent species may be captured again after several years, indicating a permanent population which is relatively sparse or inactive and thus unlikely to be recorded in large numbers in pitfall traps. An outstanding example is W. incisa, which is rare in sense (a) being known elsewhere from a few scattered localities in England and Wales (LMM, pp.126, 200). A single male was taken on the North Hill of Inchcailloch in 1972 and another in 1977, again on the North Hill. S. cornigera is another example, with a few males being captured at sporadic intervals on Inchcailloch.

Other rare (sense (a)) species are indicated in (Table 1) and interesting examples of these are:- T. spinipalpis which occurs in large numbers in the mossy places of the Ring Wood East; M. subopaca: Inchcailloch, only Scottish record, known from south-east England and a Liverpool factory (LMM, pp. 122, 147); B. pratensis: in boggy areas of mainland, first Scottish record; M. subaequalis: mossy wood (LMM, pp. 128, 246); E. ignobilis: mossy wood, first Scottish record (LMM, p.460); A. crassiceps: Ring Wood and Crom Mhin (LMM, p.250); C. distincta: Crom Mhin, few Scottish records (LMM, p.252); M. sublestus, Ring Wood East, elsewhere only Wicken and Woodwalton Pens, East Anglia, and Dubh Lochan, Stirlingshire (LMM, p.265); L. pinicola: Ring Bog, scattered distribution (LMM, p.280); L. angulatus: North Hill, Inchcailloch and mossy Ring Wood East, a northern species (LMM, p.280); A. warburtoni: mainland Reserve, only other Scottish record is at Loch Insh, Inverness-shire (LMM, pp.131, 284).

The spider faunas of these habitats include many species otherwise not recorded from the counties of Dunbartonshire and Stirlingshire (indicated by Curtis, 1976). The woodland arachnofaunas of the Reserve are matched in importance by those of the wetter open areas of the mainland; the Crom Mhin community ranks fairly well in comparison with other wetland locations in Strathclyde (Curtis, 1977).

Niches and Species

Why should there be so many rare (sense (b)) species on the island of Inchcailloch?

(1) The number of species in a community depends on the range of resources available for different species, and this is related to habitat diversity, or alternatively to 'niche availability'. The island appears to support a spider/phalangid community with higher numbers of individuals and of infrequent species than the mainland woods, which at first sight appear to offer as much variation in microhabitats. The environment on Inchcailloch certainly is heterogeneous; Horrill, Sykes and Idle (1975) have distinguished ten vegetation types and indicate that the mosaic of these types is on a relatively small scale. Thus one could argue that given the relative heterogeneity of environmental conditions, many species with slightly different ecological requirements areable to coexist. This may be affected by the relatively greater area of continuous woodland on Inchcailloch compared with the mainland woods, which could be regarded as 'habitat islands'.

(2) An alternative to the range of diverse habitats or possible niches is that the species present have narrow niches, or specialise in small ranges of environmental parameters. Consequently more of them can coexist. An important difference between the island and mainland could be the relative ease of access for spiders, although it is difficult to assess the effectiveness of the narrow water barrier between the island and the mainland. Nevertheless, it is possible that spiders may move more easily into the woodlands of the mainland because of the land links of these woods with others. This could explain the higher species diversity of the Gartocharn woods and possibly also the species frequencies.

Because of the higher probability of immigration into the mainland woods, the species there are likely to be 'better' competitors than their counterparts on Inchcailloch, in order to survive rather than be replaced by immigrants. Accordingly they tend to occur in larger numbers, perhaps utilising a wider range of resources. Thus the range of resources may be roughly equal in island and mainland woods, but because of differing degrees of competition the species of the mainland have broader niches.

Either explanation (1) or (2) could be appropriate. One could even argue the opposite of (2): spiders cannot escape readily from Inchcailloch and so they are forced into a more competitive situation which is eased by narrower niches, leading to higher proportions of rare species. These concepts hint at the complex relationships which exist between the species and suggest intriguing lines of study of these communities.

Acknowledgements

The assistance and cooperation of the Nature Conservancy Council throughout these studies is gratefully appreciated.

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