Community Parameters of the Ground Layer Araneid-Opilionid Taxocene of a Scottish Island

D.J. CURTIS
Department of Biology, Paisley College of Technology, Paisley, Scotland

Back to Arachnologia Return to Arachnologia

Adapted, February 2009, for presentation as a web page, 
from the original paper which should be cited as:
Curtis, D.J. (1978).
Community parameters of the ground layer
araneid-opilionid taxocene of a Scottish island.
Symp. Zool. Soc. Lond., No.42, 149-159.


SYNOPSIS

Ground fauna was sampled at four sites in woodland on the island of Inchcailloch. About 15000 adults of 11 species of harvestmen and 101 species of spiders were recorded.
Relative abundance at four sites was in order: 1,3,4,2. Species richness was in order: 3, 1, 2,4. Species diversity (PIE) was in order: 2, 3, 4, 1.
Number of individuals (N) caught were maximal in late summer and early autumn, when PIE values were lowest. PIE and N were negatively correlated, but species richness independent of N.
Taxocenes at four sites were most similar when the dominant species (Nemastoma bimaculatum) was most abundant.
Winter spiders had stenochrone males and eurychrone females. Spiders with both sexes stenochrone occurred in late spring/early summer; summer species were eurychrone.
Harvestmen tended to have females slightly more stenochrone than males.

INTRODUCTION

The term araneid-opilionid taxocene simply denotes the spider and harvest-spider members of a community. These are regarded as a coherent unit, having fairly similar ecological roles and relationships, especially in terms of prey (Todd, 1950; Turnbull, 1960a; Edgar, 1970). Hurlbert (1971) considers the utility of the taxocene concept and provides comprehensive criticism of community parameters.

This paper is concerned with surface-active species in woodland on Inchcailloch. Some aspects have been described previously (Curtis, 1973, 1975).

STUDY AREA AND METHODS

Inchcailloch is an island in the southeastern corner of Loch Lomond, western central Scotland. The 56-hectare island, forming part of the Loch Lomond National Nature Reserve, has a rich deciduous woodland dominated by oak (Quercus spp.) with much birch (Betula pubescens). The Highland Boundary Fault passes along the island's main axis from northeast to southwest. The summit is only 86 m high, but along the main ridge is contrasting woodland of Scots pine (Pinus sylvestris). Vegetation has been described by Horrill, Sykes & Idle (1975), soils by Hornung & Mew (1970) and solid geology by Rosen (1968).

Sampling Sites

Four sites were studied. Some of the plants at the stations have been indicated by Curtis (1973) and the vegetation may be referred to the Interpreted Groups of Horrill et al. (1975). Underlying rocks are old red sandstone and conglomerates.

Site 1, at the north end of Church Ridge, is fairly dry with an acid brown earth. The main tree cover is Quercus robur. Ground flora is in Group I, dominated by bracken (Pteridium aquilinum) and woodrush (Luzula sylvatica) with some bramble (Rubus fruticosus).

Site 2 is in an alder carr with a poorly drained acid peaty soil. Ground flora fits into Group V, having some R. fruticosus, the mosses Eurhynchium praelongum and Thuidium tamariscinum, honeysuckle (Lonicera periclymenum) and various grasses, with smaller amounts of bracken and woodrush.

Site 3 is near the summit of the North Hill with tree cover mainly P. sylvestris with some birch and rowan (Sorbus aucuparia). Ground flora is of Group X including mainly heather (Calluna vulgaris), blaeberry (Vaccinium myrtillus) and various mosses. The soil is a peaty ranker.

Site 4, in the Central Valley, has tree cover of Q. robur with other species including hazel (Corylus avellana) and birch. R. fruticosus is dominant in the field layer, beneath which is much bare earth. Ground flora is of Group III including Pteridium aquilinum, Endymion non-scriptus (bluebell), grasses such as Holcus mollis, with Eurhynchium praelongum. The soil is an acid brown podzol, much wetter than site 1.

Sampling Procedure

At each site 20 pitfall traps - plastic jars 6 cm in diameter and 8 cm deep - were used within a 1 m2 quadrat. These were lifted approximately weekly from August 1971 to August 1972, catches being pooled to give monthly data, and monthly during 1972-73. No killing fluid or preservative was used, to avoid damage to surrounding vegetation; using 20 traps at lower efficiency compensated for this.

There have been criticisms of pitfall sampling (e.g. Turnbull, 1973: 324). Catches depend on both numerical density and degree of activity, i.e. are a function of activity/density of the animals; this activity/density function probably reflects fairly well the relative ecological importance of the species. Low efficiency of the traps will cause acceptably small disturbance to the sampled population. Possible variation in efficiency is a serious drawback, but if results are treated cautiously they can provide useful indications of community properties. Others have expressed similar opinions, e.g. Williams (1962), Merrett (1976), Uetz & Unzicker (1976). Throughout this paper, phrases such as "numbers of spiders" should be taken as referring to activity/density function, not absolute numerical abundance.

Spiders were identified according to Locket & Millidge (1951, 1953); nomenclature follows Locket, Millidge & Merrett (1974). Opiliones (= Phalangida) are according to Sankey & Savory (1974).

OBSERVATIONS

Over the two-year period, 14,683 adults were taken, representing 11 species of harvest-spider and 101 spider species. Sites 1 and 3 yielded many more specimens (4158 and 5135) than sites 2 and 4 (2751 and 2639), but had fairly similar numbers of species (= species richness), with 65, 57, 69 and 54 at sites 1-4, especially in view of temporal variations in species richness. Frequency distributions of species abundance were similar at all sites (tested by χ2 ) with many species represented by few individuals.

Species Composition

The terms dominant, influent and accessory species, as used by Łuczak (1963) are convenient. The dominant species was the opilionid Nemastoma bimaculatum (Fab.) with captures at sites 1-4 of 2887, 1381, 2648 and 1820.

Influent species, contributing 5-50% of site total, occurred as follows at sites 1-4:
Opiliones:-
Oligolophus tridens (C.L.K.)297390257110
O. (Odiellus) palpinalis (Herbst)502219833
Lacinius ephippiatus (C.L.K.)105138031
Araneida, Linyphiidae:-
Monocephalus fuscipes (Blackw.)17310816238

Associate species each represent less than 5% of site total. The more common of these are:
Opiliones:-
Mitopus mono (Fab.)26527911
Oligolophus agrestis (Meade)411815926
O. hansenii (Kraepelin)56453
Araneida, Linyphiidae:-
Walkenaera acuminata Blackwall62276121
Gonatium rubellum (Blackwall)15413011
Tapinocyba pallens (O.P. Camb.)38595
Diplocephalus picinus (Blackwall)423015
Hilaira excisa (O.P. Camb.)03400
Centromerus sylvaticus (Blackw.)36152230
C. dilutus (O.P. Camb.)1511507
Macrargus rufus (Wider)5233518
Lepthyphantes alacris (Blackw.)10188219
L. zimmermanni Bertkau807318282
L. tenebricola (Wider)41236544

Differences between the four communities are apparent in terms of individual species. A full species list is given by Curtis (1973).

Species diversity may be expressed as the probability of interspecific encounter (Hurlbert, 1971), calculated as PIE = [N/(N - 1)][1 - Σ (ni/N)2], where N is total number captured, ni the number belonging to species i. For sites l-4, values were 0.509, 0.7l2, 0.693 and 0.512. Site 2 had highest PIE value in spite of lower species richness than 1 and 3.

Seasonal Variations

Numbers of active individuals each month varied markedly (Fig. la), being maximal in late summer. This reflects activity of N. bimaculatum; except the high peak due to O. palpinalis at site 3 in November 1972.

Species richness (Fig. lb) showed a slight increase in spring, contrasting with clear fluctuations in PIE. In spite of fairly constant or declining species richness, PIE rose sharply through autumn owing to decreasing dominance by N. bimaculatum, to maximal levels in winter. Diminished activity led to some low diversity values in early spring, especially at sites 1, 2 and 4. Increasing species richness gave higher diversity in early summer, thereafter declining as N. bimaculatum numbers rose.

Over the two annual cycles, PIE varied with number of individuals trapped (N) according to the following regression equations, all significant:

site 1: PIE=0.76 - 0.001(±0.0002)N; -0.70 (corr. coeff.)

site 2: PIE = 0.78 - 0.001(±0.0003)N; -0.54

site 3; PIE=0.80 - 0.001(±0.0002)N; -0.52

site 4: PIE = 0.72 - 0.002(±0.0005)N; -0.61.

In contrast, species richness appeared independent of N with correlation coefficients -0.16, 0.22, -0.08, -0.04 at the four sites.

Similar conclusions held for logarithmic transformations of N.

Species richness ranged from 4 at site 2 in autumn to 33 at site 3 in early summer. Broadly similar seasonal patterns have been observed in other woodlands, e.g. oak, Wytham Wood (Turnbull, 1960b), beech-maple (Elliott, 1930), and pine (Łuczak, 1959).

Patterns of Seasonal Activity

The community variations depend upon the seasonal variations in species' activity. From the proportion (pj) of its total annual catch which is taken in each month j for a species, an index of seasonality can be calculated as Is = 1/ Σpj2. For a completely eurychrone species with equal numbers in each month, Is = 12; Is = 1 for a completely stenochrone species found entirely in one month. Is values and related temporal patterns in pj are shown in Figs 1d and 2a.

Among the common spiders of Inchcailloch, the following sequence appeared. Winter species, e.g. W. acuminata, M. rufus, had stenochrone males and eurychrone females, although the rather less stenochrone males of C. dilutus persisted into spring. In spring, similar patterns were shown by M. fuscipes and T. pallens, but in later spring and early summer both sexes of D. picinus and L. tenebricola were stenochrone. No summer species were stenochrone, L. zimmermanni and L. alacris being eurychrone in both sexes. G. rubellum in autumn showed a trend back towards stenochrone males and had the most eurychrone of females (Is = 11). Progressing into winter, C. sylvaticus showed the typical winter pattern.

The opilionids also showed seasonal succession, but all tended to be stenochrone in activity. Females of L. ephippiatus, N. bimaculatum, 0. tridens and 0. agrestis appeared slightly more stenochrone than males. Only in O. palpinalis were males distinctly more stenochrone than females, while in O. hansenii both sexes have a short period of activity on the ground. The most eurychrone harvestman was M. morio (both sexes).

N. bimaculatum at the four sites showed similar rank order of activity over the months; values of Spearman's rank correlation coefficient comparing sites all exceed 0.87. However, in terms of actual numbers trapped there are significant differences (by χ2) with site 3 showing a greater spread of activity, particularly in early winter.

Comparisons Between Sites

Average overlap between taxocenes at the four sites increased during late summer and early autumn when N. bimaculatum numbers are highest
(Fig. 2b). Overlap measures were calculated as Cik = 100 (1 - ½∑| rij - rik | ), where rij is proportional representation of species i at site j and rik that at site k, the sign of the difference being ignored. These were used in single-linkage cluster analysis to give the dendrograms shown in Fig. 2c.

For the pooled catch, taxocenes of sites 1 and 4 appeared most similar, with 2 and 3 distinct. The relationships vary through the year as shown. Increased similarity from August through October is apparent. During March and April site 3 taxocene is distinct with T. pallens the most numerous species in contrast to M. fuscipes at 1, 2 and 4. During July and August 1972, large numbers of L. ephippiatus at site 2 distinguished that taxocene from the others, but in 1973 this was not so. Increased dissimilarity during winter and spring (December - May) reflects higher species diversity at all sites.

Climatic Relationships

Climatological data were obtained from a meteorological station at Arrochymore, only 1 km from the island. From the daily records, summary statistics for the various climatic features were calculated for each sample month. Cross correlations between these series and the temporal series of species' activity have been calculated, particularly for N. bimaculatum. This species is positively correlated with the various temperature parameters. Most consistent are the correlation coefficients for the four sites of 0.72, 0.74, 0.7l and 0.75 with minimum values of earth temperature one month previous to sample. N. bimaculatum activity also shows positive correlation with the mode for hours of sunshine per day (correlation coefficients of 0.87, 0.75, 0.85, 0.77) for the month five months prior to sample; daily sunshine varied greatly between sample periods. Lags of 10-11 months held for the lower correlations (0.61, 0.69, 0.55, 0.70) with relative humidity. Rainfall per day was very variable within sample months and showed lowest correlations (0.50, 0.50, 0.41, 0.49) with lags of 9 - 11 months for sites 1, 2 and 4 but only seven months for site 3.

Such correlations may be fortuitous, both sets of data following annual cycles. Much more investigation is required to indicate their significance, if any. Correlations with these factors may be quite complex (Pearson & White, 1964a). Insect populations have been shown to be sensitive to large-scale weather conditions (Wellington, 1957).

Periods of maximal species diversity correspond with low temperature and sparse sunshine. The taxocenes include no species which are stenochrone in summer; summer species tend to be eurychrone. As expected, the taxocenes comprise species adapted for the relatively cool and wet conditions encountered adjacent to the Scottish Highlands.

DISCUSSION

Other studies on the phenology of these animals have been reported; conversion of other results to monthly data makes comparison of IS values feasible, noting that the transformation may tend to flatten peaks. The phenology of N. bimaculatum, in particular, at Inchcailloch is broadly similar to that in Argyll woodlands (A. H. Lavery, pers. comm.) with IS from l.9 to 6.0 peaking September-November, and on Welsh moorland (Pearson & White, 1964b: IS = 4.O, October). This differs from eurychrone pictures presented by Sankey (1949), Todd (1949) and Phillipson (1959). The latter described an 18-month life cycle and Williams (1962) considered his data, with two peaks in activity (IS = 4.6, main peak July) consistent with this. Phalangids such as O. agrestis and O. hansenii show vertical migration during their life cycle; pitfall trap data only give an indication of their activity on the ground surface - a valid approach as long as only the ground-layer taxocene is being considered rather than the entire life-cycle of a particular species.

Spider activity patterns described here fit approximately to the types of phenology observed by Merrett (1969), Pearson & White (1964b), Williams (1962) and A. H. Lavery (pers. comm.) with consistent IS values. Three of the four categories described by Merrett (1969) were seen on Inchcailloch, but no summer stenochrone species, possibly reflecting different climatic conditions at Loch Lomond compared with Dorset.

These patterns of seasonal activity may benefit the community by reducing temporal overlap of otherwise similar species and thus preventing over-exploitation of resources. Climatic factors to which the species are adapted are modified by the annually and spatially varying effect of vegetation, as are actual physical structures afforded to the animals. The moderating effect of vegetation on climate is well known (Geiger, 1965; Cloudsley-Thompson, 1962) and the role played by specific plant structures in affording shelter to invertebrates is considered by Bossenbroek, Kessler, Liem & Vlijm (1977a,b). The physical structure of the vegetation is also important to spiders (e.g. Duffey, 1966). Low shrubs (C. vulgaris, V. myrtillus) at site 3 persist throughout the year, moderating microclimatic changes, and could be partially responsible for the relatively extended period of activity of N. bimaculatum here. At the other sites, most plants die down during late autumn. Some work has examined climatic conditions on Inchcailloch (G. S. Langley, pers. comm.) with fairly predictable results.

The synoptic parameters of species richness and diversity provide some information about taxocene structure, and it is tempting to attempt to relate these to stability, etc. However, for a taxocene which varies so much over the year stability is rather a difficult concept to apply.

PIE may be calculated for a particular species. For N. bimaculatum PIE rises as its abundance declines and falls again as numbers increase. Conversely, the probability of intraspecific encounter rises with increasing activity/density. Cynics may argue that PIE is merely a reflection of the numerical variations and has no biological meaning. However, the probability of intraspecific encounter reaching a maximum may be a partial cause of the subsequent decline in N. bimaculatum as a result of excessive competition, shortage of resources, etc. Clearly, further studies are necessary to clarify this.

ACKNOWLEDGEMENTS

Grateful thanks go to Prof. 1 C Smyth and Mr G. S. Langley, Dept of Biology, Paisley College of Technology; to Mr A. H. Lavery, rdentinny Outdoor Education Centre for data; to Dr A. F. Millidge for advice, particularly about Lepthyphantes; to Nature Conservancy ouncil for co-operation and also to Clyde River Purification Board for weather data.

REFERENCES

Bossenbroek, Ph., Kessler, A., Liem, A. S. N. & Vlijm, L. (1977a). The significance of plant growth forms as "shelter" for terrestrial animals. J. Zool., Lond. 182: 1-6.
Bossenbroek, Ph., Kessler, A., Liem, A. S. N. & Vlijm, L. (1977b). An experimental analysis of the significance of tuft structures as a shelter for invertebrate fauna, with respect to wind velocity and temperature. J. Zool., Lond. 182: 7-16.
Cloudsley-Thompson, J. L. (1962). Microclimates and the distribution of terrestrial arthropods. A. Rev. Ent. 7: 199-222.
Curtis, D. J. (1973). Spiders and phalangids of Inchcailloch, Loch Lomond. I. General considerations. West. Nat. 2: 29-39.
Curtis, D. J. (1975). Spiders and phalangids of Inchcailloch, Loch Lomond. II. Seasonal activity of harvestmen. West. Nat. 4: 114-119.
Duffey, E. (1966). Spider ecology and habitat structure. Senckenberg. biol. 47: 45-49.
Edgar, W. D. (1970). Prey of the wolf spider Lycosa lugubris (Walck.). Entomologist's mon. Mag. 106: 71-73.
Elliott, F. R. (1930). An ecological study of the spiders of the beech-maple forest. Ohio J. Sci. 30: 1-22.
Geiger, R. (1965). The climate near the ground. Cambridge, Mass.: Harvard University Press.
Hornung, M. & Mew, G. (1970). Report on the soils of the island of Inchcailloch, Loch Lonond National Nature Reserve. Bangor: Internal report to Nature Conservancy Council.
Horrill, A. D., Sykes, J. M. & Idle, E. T. (1975). The woodland vegetation of Inchcailloch, Loch Lomond. Trans. bot. Soc. Edinb. 42: 307-334.
Hurlbert, S. H. (1971). The nonconcept of species diversity: a critique and alternative parameters. Ecology 52: 577-586.
Locket, G. H. & Millidge, A. F. (1951, 1953). British spiders 1 & 2. London: Ray Society.
Locket, G. H., Millidge, A. F. & Merrett, P. (1974). British spiders 3. London: Ray Society.
Łuczak, J. (1959). The community of spiders of the ground flora of pine forest. Ekol. pol. A 7: 285-313.
Łuczak, J. (1963). Differences in the structure of communities of web spiders in one type of environment (young pine forest). Ekol. pol. A 11: 159-221.
Merrett, P. (1969). The phenology of linyphiid spiders on heathiand in Dorset. J. Zool., Lond. 157: 289-307.
Merrett, P. (1976). Changes in the ground-living spider fauna after heathiand fires in Dorset. Bull. Br. arachnol. Soc. 3: 214-221.
Pearson, R. G. & White, E. (1964a). Factors contributing to the annual cycles of surface-active arthropods in moorland country. Entomologist's mon. Mag. 100: 201-206.
Pearson, R. G. & White, E. (1964b). The phenology of some urface-active arthropods of moorland country in North Wales. J. Anim. Ecol. 33: 245-258.
Phillipson, J. (1959). The seasonal occurrence, life histories and fecundity of harvestspiders (Phalangida, Arachnida) in the neighbourhood of Durham City. Entomologist's mon. Mag. 95: 134-138.
Rosen, B. R. (1968). The solid geology of Inchcailloch. Edinburgh: Internal report to Nature Conservancy Council.
Sankey, J. H. P. (1949). British harvest-spiders. Essex Nat. 28: 181-191.
Sankey, J. H. P. & Savory, T. H. (1974). British harvestmen. Synopses Br. Fauna No.4.
Todd, V. (1949). The habits and ecology of rare British arvestmen (Arachnida, Opiliones), with special reference to those of the Oxford district. J. Anim. Ecol. 18: 209-229.
Todd, V. (1950). Prey of harvestmen (Arachnida, Opiliones). Entomologist's mon. Mag. 86: 252-254.
Turnbull, A. L. (1960a). The prey of the spider Linyphia triangularis (Clerck) (Araneae, Linyphiidae). Can. J. Zool. 38: 859-873.
Turnbull, A. L. (1960b). The spider population of a stand of oak (Quercus robur L.) in Wytham Wood, Berks., England. Can. Ent. 92: 110-124.
Turnbull, A. L. (1973). Ecology of the true spiders (Araneomorphae). A. Rev. Ent. 18: 305-348.
Uetz, G. W. & Unzicker, J. D. (1976). Pitfall trapping in ecological studies of wandering spiders. J. Arachnol. 3: 101-111.
Wellington, W. G. (1957). The synoptic approach to studies of insects and climate. A. Rev. Ent. 2: 143-162.
Williams, G. (1962). Seasonal and diurnal activity of harvestmen (Phalangida) and spiders (Araneida) in contrasted habitats. J. Anim. Ecol. 31: 23-42.

Community parameters of araneids & opilionids on Inchcailloch

Figure 1.



Inchcailloch spiders and harvestmen: seasonal variations in

(a) number of specimens (N) taken per month,
(b) species richness (S) and
(c) PIE.

Symbols relate to sites as indicated by final points in (a).

(d) Seasonal activity of more common spiders with males above (shaded) and females below axis; showing proportion of annual total taken in each month. Is values indicated.

Back to text

Community parameters of araneids & opilionids on Inchcailloch

Figure 2.



(a) Seasonal abundance of harvestmen with males above and females below axis: showing proportion of annual total taken in each month. IS values indicated.

(b) Variations in overlap between the four taxocenes; average of six values between the four sites.

(c) Single-linkage cluster analysis dendrograms showing varying relationships of taxocenes at the four sites.

Back to text

Return to..?
Back to Arachnologia Arachnologia
Ariadne's thread Ariadne Home Page