the effects of varied grazing management on epigeal spiders, harvestmen and pseudoscorpions of...

Agriculture, Ecosystems and Environment 86 (2001) 39–57

The effects of varied grazing management on epigeal spiders,harvestmen and pseudoscorpions of Nardus stricta grassland in

upland Scotland

Peter Dennis a,∗, Mark R. Young b, Christopher Bentley a

a Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UKb Culterty Field Station, University of Aberdeen, Newburgh, Ellon, Aberdeenshire AB41 0AA, UK

Received 8 November 1999; received in revised form 25 July 2000; accepted 29 August 2000

Abstract

A hypothesis that epigeal arachnid assemblages benefit more from greater vegetation structure than botanical speciescomposition in upland grasslands was tested. The test was carried out within a grazing experiment, initiated in 1991, toinvestigate vegetation dynamics in response to stocking with mixed livestock at varied rates. The experimental treatmentscomprised: no livestock, sheep only or sheep with cattle. Livestock treatments were grazed to maintain either 4.5 or 6.5 cmaverage sward heights between tussocks. Two replicates of each treatment were used and allocated to 10 plots across 22 ha ofNardus stricta-dominated grassland.

The effects on epigeal arachnids (excluding acarines) of the botanical and structural differences of the grassland betweentreatments during April–October 1993 and 1994 were assessed. Epigeal arachnid species composition was estimated usingcontinuous pitfall trapping and the densities of mainly money spiders (Araneae: Linyphiidae) were estimated from monthlysuction sampling and visual counts of spider webs in micro-habitats. These data were later compared with stocking rate,botanical species composition and vegetation structure.

Forty of the 84 sampled species occurred in all experimental treatments. There was a significant effect of treatment on thenumber of arachnid species in suction but not in pitfall samples. There was also a significant effect of treatment on the relativeabundance of 26% of these arachnid species. For most species of spider, harvestmen and pseudoscorpion, abundance wasgreater in the ungrazed and taller, grazed swards although a few species were captured in greater numbers in the treatmentswith shorter swards. Botanical composition, mean vegetation height and grazing intensity accounted for 48.5–53.2% of thevariability in the species composition/relative abundance of these arachnids, calculated by direct gradient analysis. Almosthalf of the species were randomly distributed across the experimental treatments and are recorded as widespread in uplandheathland or grassland habitats and lowland grassland. More spider webs were counted during July–September 1993–1994,with greater numbers (dominated by the linyphiid species, Lepthyphantes mengii) counted in tall, ungrazed swards comparedwith taller grazed swards created by sheep alone or sheep with cattle. In the treatments with fewer webs, these were occupiedby more linyphiid species. Suction sampling detected greater diversity of arachnids in the ungrazed N. stricta. This was relatedto increased plant litter below the leaf stratum where webs were counted.

∗Corresponding author. Tel.: +44-1224-318611; fax: +44-1224-311556.E-mail address: [email protected] (P. Dennis).

0167-8809/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0 1 6 7 -8 8 09 (00 )00263 -2

40 P. Dennis et al. / Agriculture, Ecosystems and Environment 86 (2001) 39–57

Vegetation structure and not botanical species composition within the N. stricta plant community determined arachnidspecies composition and abundance. Furthermore, no single grazing treatment supported the total number of arachnid speciesrepresented across the entire grazing experiment. It is concluded that varied grazing management, including some temporaryungrazed areas, is necessary to maintain the structural variability of grassland patches so as to maintain a spatial mosaic thatfavours the optimum arachnid fauna of upland grasslands. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Upland grassland; Habitat structure; Harvestmen; Grazing; Plant architecture; Spiders; Species composition; Scotland

1. Introduction

The majority of upland Britain comprises a fewplagio-climax communities, in particular heathlandsand grasslands, valued for the typicalness of theirbiota despite being poor in terms of overall plantand invertebrate species numbers (Ratcliffe, 1977).Arachnids (spiders and harvestmen) are an exceptionand have a high diversity (Ratcliffe, 1977; Coulsonand Whittaker, 1978; Downie et al., 1995). Thespiders in northern Britain, represented by 105 species(Ratcliffe, 1977), form nine distinct assemblages,six upland and three montane, characterised by al-titude, organic or mineral properties of the soil andvegetation structure (Coulson and Butterfield, 1986;Rushton and Eyre, 1992; Downie et al., 1995). This iscompared with spider assemblages of 104 species inwoodland and 73 species of grassland-heath of low-land Britain (Ratcliffe, 1977). Harvestmen (Arach-nida: Opiliones) are represented by five species in theuplands but despite the small number of species com-pared with the Araneae, Mitopus morio (Fabricius)is widely distributed and is a numerically importantarachnid species (Jennings, 1983). Little informationis available on the distribution of pseudoscorpions inthe uplands although their populations are favouredby increased plant litter (Bell et al., 1999), a typicalcomponent of upland, indigenous grasslands.

Habitat structure exerts a major influence on the dis-tribution and abundance of arachnids but it is not clearwhat relative contribution is made to spider populationparameters by plant species composition and vegeta-tion structure (Uetz, 1991; Wise, 1994). Arachnids areexclusively predators but the varied foraging strategiesof different species indicate a requirement for con-trasting architectural features in upland habitats. Webconstruction by spiders demands points of anchorageat different heights, whereas diurnal species, that usesight to pursue prey, are associated with patches of

low vegetation. Tall, rigid plant species associated withsuccessional changes contrast with existing vegetationand encourage the colonisation of web-building spi-ders (Gibson et al., 1992). The indirect evidence ofdependence of spiders on plant species’ compositionas a result of the contribution of each plant’s architec-ture to the general structural heterogeneity of vegeta-tion, could be related also to the host–plant specificityof their major prey species, the planthoppers (Homo-ptera: Auchenorrhyncha) (Waloff, 1980).

Grazing herbivores alter the structural heterogene-ity of vegetation within a particular successional seremore than the plant species composition, dependingon the intensity of grazing management of a site.Large-scale surveys of spiders in different sites haveindicated differences in relative abundance relatedto vegetation height and the intensity of grazingmanagement (Rushton et al., 1989). Cherrett (1964)suggested, however, that structural heterogeneity ofupland grasslands maintained by indigenous tussockgrass species buffered the effect of increased stockingrates of grazers on the number of spider species. Lit-tle is known about the implications of these grazingherbivore–plant interactions for the species composi-tion of arachnids in heathlands (Gimingham, 1988).

An experiment, originally established to investigatemixed-livestock grazing and vegetation interactions,provided the opportunity to test a major and two sub-sidiary hypotheses, namely: (a) that within a particularsuccessional sere, vegetation structure has a greatereffect on the number of species of arachnids thanbotanical species composition; (b) that intermedi-ate rates of grazing enhance the structural diversityof grasslands and maintain a higher diversity ofarachnids; (c) that sheep will have less effect onthe distribution of web-building spiders than cattleand sheep together because trampling within tussockclumps will be less for a particular sward height.Sheep or sheep and cattle were stocked at a level

P. Dennis et al. / Agriculture, Ecosystems and Environment 86 (2001) 39–57 41

to maintain two-sward heights in summer on a Nar-dus stricta–Galium saxatile grassland, a species poorsub-community (National Vegetation Classification(NVC) class U5a) (Rodwell, 1992). Nardus stricta isthe dominant species, has a tussocky structure and isnot eaten by sheep unless the availability of Agrostisand Festuca species is scarce (Grant et al., 1997). Oneeffect of the removal of or a low intensity of grazingis a short-term increase in the area covered by N.stricta. As a result, structural complexity increasesbecause N. stricta forms large tussocks.

2. Materials and methods

2.1. Study area

The grazing experiment was established in 1991 onan area of N. stricta-dominated grassland on the south-ern and western aspects of Blackdean Curr (NGR NT8321/8421-22) at 450–500 m above sea level at theMacaulay Land Use Research Institute’s SourhopeResearch Station in the Cheviot Hills, UK. The ex-periment was situated on well drained, basalt soils(Sourhope series). The characteristics and origins ofthe vegetation and full details of the experimental de-sign are described elsewhere (Dennis et al., 1997): fourgrazed treatments were stocked either by sheep aloneor by sheep and cattle. Six cattle were grazed fromJune to August in the sheep and cattle treatments andin all plots adjustment of the sheep numbers was usedto maintain one of the two average heights, namely 4.5and 6.5 cm, of the Agrostis–Festuca sward betweenthe Nardus tussocks. These sward heights were main-tained by continuous but varied stocking rates of thesheep between May and October from 1991 to 1994.A control was used which had no grazing manage-ment for 2 years and, in 1994, the Agrostis–Festucasward had grown to an average height of 8–10 cm.Two replicates of each treatment were used and allo-cated to 10 plots enclosed with post-and-wire fencingacross 22 ha of N. stricta-dominated grassland lyingon the summit and adjacent ridges of Blackdean Curr.

2.2. Arachnid sampling

From April to October in 1993 and 1994, spidersand harvestmen were sampled across the experiment

using a number of different sampling methods. Con-tinuous sampling was carried out using pitfall traps,with traps being emptied once a month. At thesemonthly intervals, alternative sampling methods wereused; suction sampling to estimate the distribution anddensity, mainly of money spiders (Linyphiidae), andvisual searches in quadrats to record the distributionof spiders webs in different micro-habitats.

2.3. Continuous sampling with pitfall traps

The epigeal arachnid assemblage (excludingacarines) was sampled with 12 pitfall samples perplot located randomly along existing transects markedacross each of the 10 plots for the purpose of botanicalsurvey. This maintained a link between the arachniddata and inclined-point quadrat data collected at eachbotanical sample location. Traps were plastic cups of7.5 cm diameter and 10 cm depth containing a 50%solution of ethylene glycol as a preservative. Plywoodcovers were placed over each trap to limit the effectof trampling and reduce the risk of traps floodingduring heavy rain. Traps were operated from Aprilto October in both 1993 and 1994; captured materialwas emptied monthly and arachnids were identifiedand sorted with the aid of taxonomic keys and ex-pert assistance (Sankey and Savory, 1974; Roberts,1985, 1987; M. Davidson, personal communication,1996).

2.4. Monthly suction samples

Suction samples (D-vac; Dietrick, 1961) weretaken from all 10 plots between 10.00 and 16.00 hGMT, monthly from May to October during 1993and 1994. Six samples were taken from each plot ata standard distance and direction from half the pitfalltrap locations, selected at random. Each sample com-prised five separate suction samples of 40 s durationusing the standard nozzle (combined area of sam-ple: 0.43 m2) positioned in each of five sub-divisionsof a pre-placed quadrat. The suction samples weretransferred to polyethylene bags and frozen at −30◦Cuntil sorting. Each sample was sieved through 5, 2and 1 mm mesh sizes onto filter paper to remove leaflitter and dried dung from the samples. Each layerwas searched for arachnid specimens and, finally,dust and remaining arachnids were separated by hand


on the filter paper and all were transferred to 70%ethanol and glycerol solution. All Araneae, Opilionesand Pseudoscorpiones were identified to species leveland counted for each sample.

2.5. Visual search for spider webs

At the same monthly intervals, a 1 m2 rectangle(0.447 × 2.236 m2), divided into five square sectionsof equal area, was placed on the grass surface forthe purpose of counting spider webs. The countswere deliberately carried out in the early morningwhen condensation from morning mist highlightedthe webs. When the night-time air had been dry orstrong sunshine quickly dried the moisture from webs,a standard backpack sprayer was carried to spray amist across the quadrat and reveal the fine silk of thewebs. In 1993, the quadrat was placed at a standarddirection and distance from the six pitfall trap loca-tions that were not used as reference locations forthe suction sampling. In 1994, quadrats were placedat all 12 pitfall trap locations in each plot. A recordwas made of the number of spider webs that occupiedsix micro-habitats: suspended in leaves of N. strictatussock, anchored between tussocks and on culm(tussock stump), horizontally on the Agrostis–Festucasward, poached areas of sward and on dried cattledung. For the purpose of the data analysis, the web lo-cation categories of Agrostis–Festuca sward and dungpats were merged into a single category and giventhe name of the former. In July 1994, a repeat of themonth’s web counts was made in 12 randomly placedquadrats in each plot to test for an effect of repeatedsampling disturbance on the web densities of eachmicro-habitat.

During July 1994, a duplicate survey was carriedout in which webs were located on a random walkthrough each experimental plot. The purpose was tocheck that there was no bias for the samples repeated atparticular sample locations through May to September1994. It was also possible to collect individual spidersfrom webs in the duplicate sample for later identifica-tion to determine the species composition of the websof each micro-habitat, under each grazing treatment.Collection of spiders from webs in the main surveywas not possible because such destructive samplingwould certainly have affected later counts at theselocations.

2.6. Vegetation structure

Data on botanical composition were available frominclined-point quadrat data (Dennis et al., 1997). Dataon vegetation structure was collected at short andintermediate distances around the location of eacharachnid sample. Vegetation structure was estimatedby measuring vegetation height at eight points in a ra-dial sampling design, stratified at five distances fromthe sample location. Mean and variance in vegetationheight was calculated at radii of 2.82, 3.99, 4.89, 5.64and 6.31 m from the pitfall trap/quadrat locations.An effect of changes in spatial scale on the magni-tude of these structural variables was investigated toensure the relationship between vegetation structureand arachnid species composition was consistent. Theanalyses tested the relationship of arachnids with eachof these values of vegetation height and, if no differ-ence was found, the main analyses were based on allavailable height measurements (an area of 125 m2).

2.7. Analyses

Variations in capture efficiency attributable to theindividual locations of pitfall traps was avoided bycombining the catches from the 12 traps in each plot,an adaptation of the method used by Luff and Rushton(1989). Catches were accumulated for each plot todeal with variations in the period of activity of differ-ent arachnid species through each season. In this way,a more accurate measure of the number of speciesin the habitat could be achieved (Curtis, 1980). Theabundance of arachnids in pitfall traps was accumula-ted over 2 years and ranked by decreasing proportionof the total individual catch. No further analysis wascarried out on species represented by five or lesstrapped individuals because no normal distribution ispossible with one or less individuals per treatment.The grazed treatments formed a two-livestock bytwo-sward height factorial design and were analysedusing an appropriate factorial ANOVA (SAS, 1998) totest for effects of livestock or target sward height oncaptures of arachnid species. A one-way ANOVA wasapplied to data on each of the arachnid species withan overall trap abundance greater than five within all10 plots, to test for differences between the treatmentsthat were both grazed and ungrazed. The multivariatetechnique canonical correspondence analysis (CCA;


ter Braak, 1988) was used to calculate the variabilityin arachnid species/abundance accounted by botani-cal composition, vegetation height and stocking rate.Ordination biplots were prepared to represent the vari-ance in the arachnid assemblage associated with thesehabitat variables, a procedure used in the context ofan analysis of the Coleoptera (Dennis et al., 1997). Asimilar analysis was applied to the data from the suc-tion samples, limited to July and August 1994 whena higher proportion of adults aided identification.

Monthly counts of webs, averaged per plot andacross the 2 years were analysed for treatment effectsusing ANOVA. A two-way ANOVA tested for aneffect of web location in addition to grazing treat-ment on the web counts taken in 1994. An effect ofsampling disturbance on web counts was tested bycomparison of the web counts in monthly samplelocations and in an equivalent number of randomlylocated quadrats using a paired t-test (SAS, 1998).Differences in the species composition of webs in dif-ferent treatments and micro-habitats was tested witha two-way ANOVA. Finally, the effect of the scale ofmeasurement of vegetation height was tested. Each ofthe five values of mean vegetation height per plot weresubstituted into different runs of the CCA. A compari-son was made of changes in the variance accounted bythis environmental variable at the five spatial scales.

3. Results

3.1. Continuous sampling with pitfall traps

A total of 21 758 individuals of 83 species wascaptured, identified and sorted during the two periodsof trapping in 1993 and 1994. Table 1 gives the tax-onomic Order, Family and authority of all arachnidspecies named below. Twenty-three and 44 arachnidspecies accounted for 95 and 99% of the total indi-viduals captured, respectively. Eleven species wererepresented by more than 500 individuals each and23 species were represented by less than five individ-uals each (Table 1). There was no significant effectof treatment on the total number of arachnid specieseither calculated with all species (F4,9 = 0.49, notsignificant) or species represented by greater than fiveindividuals (F4,9 = 1.19, not significant). Similarly,there was no experimental effect on the numbers of

species in the family with most species, the Linyphi-idae (F4,9 = 0.44; not significant). There was aneffect of livestock treatment on total individuals ofall species combined for grazed treatments analysedby factorial ANOVA (F1,4 = 9.03, P < 0.05). Therewas a significant effect of treatment on 10 of the 55arachnid species tested. The numbers in parenthesesrefer to the sequence of species in the table. The fac-torial ANOVA revealed a significant livestock effecton M. morio (1) (F = 16.77, P < 0.05), Silometopuselegans (6) (F = 12.96, P < 0.05), Lepthyphantesericaeus (11) (F = 27.50, P < 0.01) and Neobisiummuscorum (48) (F = 514.61, P < 0.001) with con-sistently larger captures in sheep grazed as opposedto mixed livestock-grazed treatments (Table 1). Fivespecies of arachnid were significantly effected bysward height, captures of M. morio (1) (F = 19.32,P < 0.05), Alopecosa pulverulenta (9) (F = 23.51,P < 0.01) and Walckenaeria antica (30) (F = 8.91,P < 0.05) were greater in the short swards, whereascaptures of S. elegans (6) (F = 8.10, P < 0.05) andL. ericaeus (11) (F = 16.48, P < 0.05) were greaterin taller swards. There was a significant interactionbetween these factors for the species, L. ericaeus (11)(F = 13.71, P < 0.05), W. antica (30) (F = 15.56,P < 0.05) and Xysticus cristatus (31) (F = 11.37,P < 0.05). The one-way ANOVA applied to alltreatments demonstrated significantly greater capturesof Pardosa pullata (2), A. pulverulenta (9), Lepthy-phantes mengii (13), Bathyphantes parvulus (29) andN. muscorum (48) in the ungrazed treatment (Table 1).

3.2. Canonical Correspondence Analysis (CCA)

A matrix of abundance values for 55 arachnidspecies across the 10 plots was used for the CCA alongwith the treatment and vegetation variables: stockingrate, vegetation height and botanical diversity. Meanand variance in vegetation height remained constantand, therefore, scale-independent when calculated forthe five quadrat sizes of 25, 50, 75, 100 and 125 m2

at each sampling location. The variable, Shannon H,was calculated from inclined-point quadrat data andused as the measure of botanical diversity. The ex-periment was restricted to the N. stricta NVC classU5a and the coefficient of variation for botanicaldiversity across the experiment was correspondinglysmall (CV = 0.09) compared with that of vegetation

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Table 2(a) Inter-set correlation coefficients (italic numbers indicate largest coefficients) and (b) regression coefficients and t-values (CANOCO, terBraak, 1988) between the environmental variables and the three ordination axes for arachnid species scores obtained using direct gradientanalysis, CCA (n = 10) on pitfall trap captures of arachnids on N. stricta grassland, 1993–1994a

Variable Axis 1 Axis 2 Axis 3

(a) Correlation coefficients (range as mean ± 1 S.D.)MEANVEG (cm, 14.4 ± 3.68) −0.73 0.52 −0.15GLU (LU ha−1 yr−1, 63.0 ± 42.55) 0.37 −0.52 0.62SHANNON (H ′ = 1.84 ± 0.192) 0.34 0.82 0.09% Variance/axis 24.3 18.2 10.7

(b) Regression coefficients (t-values)MEANVEG −1.20 (−4.26∗∗) 0.36 (1.07) 1.00 (2.91∗)GLU −0.37 (−1.26) −0.30 (−0.09) 1.62 (4.55∗∗)SHANNON 0.52 (2.80∗) 0.83 (3.70∗) 0.40 (1.76)Eigenvalue 0.075 0.056 0.033% Contribution to explained variance 45.6 34.1 20.3

a MEANVEG: mean height of vegetation (cm); GLU: livestock grazing rate converted to dairy cow equivalents (LU ha−1 yr−1);SHANNON: Shannon botanical diversity index, H′ (Magurran, 1988).

∗ P < 0.05 for n−q−1 degrees of freedom (n: number of samples; q: number of treatments).∗∗ P < 0.01 for n−q−1 degrees of freedom (n: number of samples; q: number of treatments).

height (CV = 0.26). Stocking rate was calculatedas the accumulated number of livestock grazed perhectare each day converted to livestock units (equiv-alent to one dairy cow). The CCA was calculatedwith all three variables and 53.2% of the variancewas accounted for by the first three ordination axes(Table 2). Axis 1 was significantly negatively corre-lated with mean vegetation height, axis 2 significantlypositively correlated with botanical diversity and axis3 was significantly positively correlated with grazingintensity. The slope of the regression (or canonical)coefficients and their t-values reflected the trend inthe correlation coefficients. The first axis accountedfor 45.6% of the explained variance and was corre-lated with vegetation height to twice the magnitudeof botanical diversity (Table 2). Scores for axes 1 and3 revealed significant treatment effects (F4,9 = 32.6,P < 0.001 and F4,9 = 9.76, P < 0.05, respectively)and the factorial ANOVA on scores of axis 2 of thegrazed treatments revealed a significant sward heighteffect equating to larger numbers of species in tallerswards (F1,4 = 12.21∗) (Table 1).

3.3. Biplot of arachnid species, experimental plotsand treatment variables

A biplot of axes 1 and 2 scores from the CCA ofarachnid species, experimental plots and treatment

variables indicated that the responses of the arachnidspecies were to three different environmental gradi-ents (Fig. 1). The main variability in the distributionof the spider species in the assemblage sampled onthe N. stricta-grassland occurred along the line of theenvironmental gradients, vegetation height and graz-ing intensity (Fig. 1). The spiders (their position inTable 1 given in parenthesis) Meioneta rurestris (52),Gonatium rubens (38), B. parvulus (29), A. pul-verulenta (9) and Tapinocyba praecox (22) and theharvestmen, Oligolophus tridens (42) and Oligolo-phus agrestis (20) correlated with the taller vegetationassociated with ungrazed plots (Fig. 1). There wasa cluster of aranean species associated with shorterswards grazed by both sheep alone and sheep andcattle, namely X. cristatus (31), Erigone atra (49),Ceratinella brevipes (35), Savignya frontata (40),Erigone dentipalpis (26), Pardosa palustris (17),Centromerita concinna (25) and Dicymbium bre-visetosum (41). The araneans Dicymbium nigrum(23), Tiso vagans (10), Pacygnatha degeeri (14) andAgroeca proxima (34) correlated with heavily grazedtreatments that had a higher botanical diversity, prob-ably because of a reduction in the cover of N. stricta(Fig. 1). Porhomma montanum (50), Bathyphantesgracilis (45) and Maro minutus (39) were associatedwith the lower botanical diversity, represented bysheep grazed, short swards, a habitat represented by


Fig. 1. Biplot scores for spider species (�), experimental plots (�) and habitat variables (→) calculated with CCA of the spider assemblageof variably grazed, upland N. stricta grassland as determined from captures in pitfall traps. Numerical labels refer to spider speciesin the order presented in Table 1. Experimental plots are indicated by combined letter and numerical labels: S, sheep; C, cattle andUNG, ungrazed, where numbers indicate the target Agrostis–Festuca sward height between Nardus tussocks. Environmental variables are:MEANVEG, mean vegetation height; GLU, grazing intensity and SHANNON, botanical species diversity.

large patches of undisturbed tussocks of N. strictaamong the short sward patches (Fig. 1). Many of thespider species were not differentiated along these en-vironmental axes, hence the cluster with scores lessthan 1.0 in the central area of the biplot.

3.4. Monthly suction samples

Samples taken with a suction sampler in July andAugust 1994 detected about half the species recordedusing the pitfall sampling method, 317 individualsof 36 identifiable species (Table 3). This was partlybecause of the number of unidentifiable, immaturespiders in the suction samples and also the bias to-wards sampling smaller species, located higher in the

vegetation. There was a significant effect of grazingtreatment on the total number of arachnid species andlinyphiid species and total individuals (Table 3), allshowing the same trend of most species in ungrazedand taller, sheep grazed swards. Fewest species wereassociated with both the sheep with cattle grazed treat-ments. For example, for total arachnids and linyphiids,respectively, there were 30.5±3.50 (mean±S.E.) and18.0 ± 2.00 species in the ungrazed and 14.5 ± 1.50and 8.5 ± 0.50 species in the short, sheep and cattlegrazed sward. There was a significant difference inthe proportion of linyphiid species among treatments(F4,5 = 6.38, P < 0.05), range 58.9–73.1%, no arc-sine transformation was carried out because valuesfell between 20 and 80% (Crawley, 1998). There was


Table 3Mean number 0.46 m−2 of arachnids in suction samples taken from varied grazing treatments imposed on a N. stricta–G. saxatile plantcommunity during July and August 1994a

Ref. No. Species of Arachnida Mean abundance per treatment in July and August 1994 F

Ungrazed Sheep (cm) Sheep and cattle (cm)

8–12 6–7 4–5 6–7 4–5

1 Lepthyphantes mengei 21.02 6.88 6.48 4.67 4.68 9.73∗

2 Bolyphantes luteolus 14.67 1.48 2.10 2.67 2.10 20.53∗∗

3 Imm. Linyphiidae 7.90 3.67 1.94 0.50 0.33 7.14∗

4 Agyneta decora 1.21 4.63 3.69 1.15 0.71 6.14!b

5 Lepthyphantes ericaeus 2.58 2.42 0.96 1.60 0.96 1.59 NSc

6 Silometopus elegans 2.92 0.79 0.81 2.15 0.96 1.32 NS7 Oedothorax retusus 3.85 1.27 0.25 0.63 0.50 2.13 NS8 Oligolophus Agrestis 4.25 0.31 0.31 0.27 0.35 117.54∗∗∗

9 Lepthyphantes tenuis 2.71 0.77 0.19 0.52 0.00 6.63∗

10 Xysticus cristatus 1.33 0.79 1.46 0.38 0.21 1.00 NS

11 Agyneta olivacea 3.33 0.00 0.00 0.10 0.00 9.99∗

12 Allomengea scopigera 2.50 0.15 0.27 0.35 0.04 4.37 NS13 Tiso vagans 1.06 0.85 0.73 0.42 0.10 7.83∗

14 Imm. Pardosa sp. 1.98 0.10 0.42 0.10 0.10 2.19 NS15 Porrhomma campbelli 1.69 0.08 0.35 0.10 0.15 58.98∗∗∗

16 Pardosa pullata 1.75 0.08 0.04 0.08 0.13 1.55 NS17 Meioneta rurestris 1.48 0.15 0.10 0.00 0.00 108.64∗∗∗

18 Mitopus morio 0.60 0.29 0.04 0.25 0.31 1.03 NS19 Micrargus herbigradus 0.65 0.40 0.19 0.21 0.00 9.18∗

20 Gonatium rubens 0.92 0.04 0.08 0.00 0.00 11.10∗

21 Drassodes cupreus 0.73 0.21 0.04 0.00 0.00 10.56∗

22 Neobisium muscorum 0.56 0.00 0.21 0.00 0.04 2.42 NS23 Nematostoma bimaculatum 0.73 0.00 0.00 0.00 0.00 1.00 NS24 Erigonella hiemalis 0.00 0.10 0.23 0.10 0.10 0.39 NS25 Gongylidiellum vivum 0.13 0.21 0.00 0.00 0.00 0.78 NS26 Pacygnatha degeeri 0.33 0.00 0.00 0.00 0.00 4.00 NS27 Monocephalus fuscipes 0.21 0.10 0.00 0.00 0.00 4.00 NS28 Trochosa terricola 0.31 0.00 0.00 0.00 0.00 9.00∗

29 Lepthyphantes zimmermanni 0.25 0.00 0.00 0.00 0.00 36.00∗∗∗

30 Bathyphantes gracilis 0.08 0.04 0.00 0.00 0.10 0.58 NS

31 Robertus lividus 0.21 0.00 0.00 0.00 0.00 1.00 NS32 Walkenaeria acuminata 0.00 0.21 0.00 0.00 0.00 1.00 NS33 Centromerita concinna 0.00 0.08 0.10 0.00 0.00 0.76 NS34 Walckenaeria antica 0.04 0.04 0.10 0.00 0.00 0.64 NS35 Dicymbium nigrum f. brevisetosum 0.15 0.00 0.00 0.00 0.00 5.44∗

36 Erigone dentipalpis 0.00 0.00 0.08 0.04 0.00 0.80 NS37 Enoplognatha ovata 0.04 0.00 0.00 0.00 0.00 1.00 NS38 Tapinocyba praecox 0.04 0.00 0.00 0.00 0.00 1.00 NS

Total individuals 82.21 26.15 21.19 16.40 12.46 40.17∗∗∗

Species number 30.50 20.50 19.00 16.00 14.50 12.52∗∗

Linyphiid species 18.00 15.00 12.50 10.50 8.50 12.11∗∗

CCA axis 1 scores −0.95 1.48 1.37 0.52 0.42 7.57∗

CCA axis 2 scores −0.29 0.08 −1.47 1.59 1.50 5.70∗

CCA axis 3 scores 0.24 1.58 −1.00 −0.25 −1.49 1.13 NS

a Tested with ANOVA (degrees of freedom, n1 = 4, n2 = 9). F values given in italics are derived from the two-by-two factorial ANOVA applied tograzed-only treatments. See Table 1 for taxonomic information on each species.

b Not significant but 0.05 < P < 0.075.c Not significant.∗ P < 0.05.∗∗ P < 0.01.∗∗∗ P < 0.001.


a higher percentage of linyphiid species representedin the taller sheep-grazed treatment. The factorialANOVA revealed a livestock effect on total speciesnumber (F = 23.14, P < 0.01), total linyphiidspecies (F = 41.29, P < 0.01) and on the abun-dance of two species (numbers in parentheses referto sequence in Table 3) Micrargus herbigradus (19)(F = 12.24, P < 0.05) and T. vagans (13) (P = 7.83,P < 0.05), and the unidentified, immature linyphiids(3) (F = 7.14, P < 0.05). These were all sampledin greater numbers in sheep grazed compared withmixed-grazed treatments (Table 3). The linyphiid, M.herbigradus (19), was the only species for which asignificant sward height effect was found (F = 15.81,P < 0.05), although an effect of sward height wasfound for total linyphiid species (F = 11.57, P <

0.05), with greater numbers found in taller swards inall instances. In addition, the one-way ANOVA ap-plied to each species revealed that 14 species and theunidentified, immature linyphiids were affected by theexperimental treatments (Table 3). Twelve of these 14species, including the most abundant, L. mengii (1)and Bolyphantes luteolus (2), occurred in significantlyhigher densities in the tall, ungrazed N. stricta.

A similar multivariate, direct gradient analysis(CCA) was carried out on the assemblage data usingthe same experimental and environmental inputs as

Table 4(a) Inter-set correlation coefficients (italic numbers indicate largest coefficients) and (b) regression coefficients and t-values (ter Braak,1988) between the environmental variables and the three ordination axes for arachnid species scores obtained using direct gradient analysis,CCA (n = 10) on suction samples of arachnids on N. stricta grassland, 1994a

Variable Axis 1 Axis 2 Axis 3

(a) Correlation coefficients (range as mean ± 1 S.D.)MEANVEG (cm, 16.6 ± 3.82) −0.83 −0.41 −0.03GLU (LU ha−1 yr−1, 39.0 ± 44.52) 0.71 0.48 −0.30SHANNON (H ′ = 1.93 ± 0.176) −0.78 0.47 0.18% Variance/axis 33.8 8.2 6.5

(b) Regression coefficients (t-values)MEANVEG −0.83 (−2.14) −0.39 (−1.30) −2.12 (−2.69∗)GLU −0.22 (−0.59) 0.66 (2.26) −2.13 (−2.79∗)SHANNON −0.50 (−2.51∗) 1.02 (6.64∗∗∗) 0.33 (0.82)Eigenvalue 0.173 0.042 0.033% Contribution to explained variance 69.7 16.9 13.3

a MEANVEG: mean height of vegetation (cm); GLU: livestock grazing rate converted to dairy cow equivalents (LU ha−1 yr−1);SHANNON: Shannon botanical diversity index, H′ (Magurran, 1988).

∗ P < 0.05 for n−q−1 degrees of freedom (n: number of samples; q: number of treatments).∗∗∗ P > 0.001 for n−q−1 degrees of freedom (n: number of samples; q: number of treatments).

for the analysis of pitfall trap data. The first threeordination axes produced a combined eigenvalue of0.248 compared with 0.164 for the CCA on pitfallcaptured specimens and these three axes accountedfor 48.5% variance of the species/abundance data,4.7% lower than for the pitfall captured arachnids(Tables 2 and 4). The variable, Shannon H, was sig-nificantly correlated with both axes 1 and 2. Thevariables, mean vegetation height and number of live-stock grazed per hectare, were also correlated withaxes 1 and 2, respectively. The variable, number oflivestock grazed per hectare, was correlated with axis3 (Table 4). The regression (canonical) coefficientswith the largest t-values indicated that the variablemost strongly associated with axes 1 and 2 was botan-ical diversity, although inversely in the former case.The strongest association with axis 3 was with thenumber of livestock grazed per hectare (Table 4).

Scores along axis 1 of the biplot were inversely cor-related with vegetation height and botanical diversity,whereas grazing intensity and botanical diversity werepositively correlated and vegetation height inverselycorrelated with axis 2 scores (Fig. 2). Essentially,variance in the species data in relation to the axesreflected their response to differences in the structuraland botanical composition of the vegetation. Outliersof the arachnid species were situated in three areas:


Fig. 2. Biplot scores for spider species (�), experimental plots (�) and habitat variables (→) calculated with CCA of the spider assemblageof variably grazed, upland N. stricta grassland as determined from suction samples of August 1994. Refer to Table 3 for a key of speciesrepresented by the numerical labels on the biplot, and Fig. 1 for an explanation of the environmental and plot codes.

six species on a gradient of positive axis 1 and negativeaxis 2 scores, a cluster of 11 species with negative axis1 and 2 scores, and three species with positive axis 1and 2 scores (Fig. 2). The largest cluster was closelyassociated with taller vegetation, e.g., the araneans,B. luteolus (2), Oedothorax retusus (7), Allomengeascopigera (12), M. rurestris (17), P. degeeri (26) andTrochosa terricola (28), the harvestmen, M. morio (18)and O. agrestis (8), and the pseudoscorpion, N. musco-rum (22). The outlier species corresponding to lowerstocking rates, correlated negatively with both axes,namely E. dentipalpis (36), D. nigrum (35), L. mengii(1), X. cristatus (10) and S. elegans (6), Agyneta oli-vacea (11) and T. vagans (13). W. antica (34) formeda cluster associated with the treatments of both live-stock types grazed to the taller 6–7 cm sward.

3.5. Visual search for spider webs

The mean density of spider webs counted in eachtreatment increased through May and June in eachyear and reached their maxima in July and Septemberin 1993 and July and August in 1994 (Fig. 3). Webcounts were similar (t9 = 0.467, not significant) inquadrats at the standard locations used each monthcompared with an equivalent number of randomlypositioned quadrats in July 1994. Significantly morewebs were counted in ungrazed than other treatmentsin both 1993 (F4,5 = 30.2; P < 0.01) and 1994(F4,9 = 25.3; P < 0.01). For 1994, there was a highlysignificant effect of both treatment (F4,35 = 31.5;P < 0.001), web position in the grassland (F6,35 =34.0; P < 0.001) and an interaction between these


Fig. 3. Mean monthly density of spider webs in 1 m2 quadrats placed in treatments of a controlled grazing experiment on N. strictagrassland during 1993 and 1994. See Fig. 1 legend for an explanation of the treatment codes in the legend.

factors (F24,35 = 11.3; P < 0.001) (Table 5). Signif-icantly more webs were recorded on N. stricta leavesthan all other categories, and also for between-Nardustussocks than remaining categories (Fig. 4). Higherproportions of webs were counted in the sward underhigher stocking rates because of the increased area ofsward, extent of poaching by livestock and amount ofdung in the N. stricta grassland. The greater numberof webs were observed in apertures of hoof prints andassociated with dung. There was also a proportionalincrease in the density of webs on the culm of N.stricta tussocks (Fig. 4), caused by the higher grazingpressure exerted by a combination of sheep and cattlewhich resulted in cattle feeding on leaves of N. strictato produce characteristic stumps.

There was no difference in the web densities be-tween the standard and random quadrats sampled inJuly for each of the categories: N. stricta leaves, culm,Agrostis–Festuca sward, poached sward and dung

Table 5Mean density m−2 of spider webs associated with different grazing treatments and microhabitats in an upland N. stricta-dominated grasslanda

Sheep (cm) Sheep/cattle (cm) Ungrazed (cm)

4.5 6.5 4.5 6.5 8.5

N. stricta leaves 0.8 ± 0.33 0.6 ± 0.73 0.4 ± 0.16 0.6 ± 0.89 4.7 ± 2.60Between tussock 0.9 ± 0.08 1.3 ± 1.71 0.7 ± 0.41 1.8 ± 0.16 3.3 ± 2.36N. stricta culm 0.2 ± 0.24 0.1 ± 0.24 0.5 ± 0.41 0.2 ± 0.16 0.04 ± 0.08Agrostis–Festuca sward 0.1 ± 0.24 0.6 ± 0.24 0.3 ± 0.65 0.3 ± 0.00 4.1 ± 2.84Dung 0.1 ± 0.16 0.0 ± 0.00 0.1 ± 0.08 0.1 ± 0.16 0.0 ± 0.00Poach 0.3 ± 0.24 0.3 ± 0.57 0.2 ± 0.24 0.3 ± 0.49 0.0 ± 0.00

a Mean of 24 counts per treatment taken at random positions in July 1994.

(1.76 > t9 > 0.52; not significant). However, therewas a significant difference in the web densities forthe between-tussocks category (t9 = 3.64; P < 0.01,mean difference of 0.99) (Table 5, Fig. 4). The datafrom the random quadrats in July showed a similar,highly significant effect of treatment (F4,30 = 12.7;P < 0.001), web position in the grassland (F5,30 =10.0; P < 0.001) and an interaction between thesefactors (F20,30 = 3.8; P < 0.001).

Spiders collected from the webs at the repeat sam-ple locations in each grazing treatment were domi-nated by immatures and adults of L. mengii (range ofproportions, 0.30–0.44 and 0.45–0.60, respectively).Other species comprised a small proportion of theindividuals collected (range of proportions, 0–0.21).There was no significant difference in the arcsinetransformed-proportions for the categories: immatureLinyphiidae (F4,9 = 1.85), L. mengii (F4,9 = 1.26)and other species combined (F4,9 = 2.64). However,


Fig. 4. Mean density of spider webs associated with different graz-ing treatments and microhabitats in an upland N. stricta-dominatedgrassland. Mean for 24 counts per treatment taken at the samepositions each month, May–September 1994. Data for spider webson cattle dung and moss clumps have been excluded from the fig-ure due to small values. See Fig. 1 legend for an explanation ofthe treatment codes in the legend.

Table 6Web-building spiders (mean number of species ± 95% confidencelimits) collected from webs in grazing treatments imposed on N.stricta-dominated grassland (F4,20 = 3.90; P < 0.05)

Treatment Mean numberof species

95% confidencelimits

Sheep, 4.5 cm 4.5 ±0.98Sheep, 6.5 cm 2.0 ±0Sheep/cattle, 4.5 cm 2.5 ±2.93Sheep/cattle, 6.5 cm 6.0 ±1.95Ungrazed 2.5 ±0.98

there was a significant treatment effect on the numberof web-building species collected from the spider webs(Table 6). Significantly more linyphiid species weresampled from webs in the taller sheep/cattle grazedsward and the shorter sheep grazed sward than in thetaller sheep grazed sward, ungrazed N. stricta or con-trasting shorter sheep/cattle grazed sward (Table 6).

4. Discussion

The results of this experiment on the epigealspiders, harvestmen and false scorpions of upland,N. stricta-dominated grassland support the main

hypothesis that vegetation structure has a greater ef-fect on the species composition and abundance ofarachnids than botanical species composition. Mostthe epigeal arachnid species were sampled in highernumbers in the ungrazed and the less intensivelygrazed treatments with a taller average vegetationheight. This corroborates the findings of experimentalstudies on spiders from lowland, calcareous grass-lands (Gibson et al., 1992) and inferential studies ofspider assemblages in grassland or heathland biotopes(Rushton and Eyre, 1992; McFerran et al., 1994;Downie et al., 1995; Rypstra et al., 1999). In low-land grasslands under varied grazing management,assemblages of spiders were typically impoverishedversions of the fauna in ungrazed grassland and theseeffects were also related to vegetation height, a func-tion of structural complexity rather than botanicaldiversity (Gibson et al., 1992). In the present study,differences in vegetation structure caused by the treat-ments significantly affected both species composition,although 40 of 84 arachnid species were commonto all treatments, and the relative abundance of 26%of the arachnid species. Of these species, just threespecies were significantly related to short, intensivelygrazed swards although a total of 12 species associ-ated with shorter swards in the direct gradient anal-ysis (CCA). No individual treatment had a speciescomposition representative of the mosaic producedby the combination of experimental treatments. Thediffering biology of the various arachnid species ac-counts for the contrasting response of the differentspecies to vegetation height. Ambushing hunters werecaptured in vegetation with contrasting height oversmall distances (e.g., X. cristatus), running huntersand thermophilic web builders were captured mainlyon the shorter swards (e.g., E. dentipalpis). Most webbuilders were on leaves within or between closelyspaced N. stricta tussocks, which satisfied the require-ments of web builders for anchorage (e.g., L. mengii).Increased plant litter, known to increase refuges forspiders (Rypstra et al., 1999), is associated with tallervegetation that resulted from no or minimal grazing,and was shown to support a larger number of arachnidspecies in this study. For example, pseudoscorpionsare sensitive to the quantity of plant litter (Bell et al.,1999) and the numbers captured in pitfall traps re-flected the relative accumulation of plant litter causedby the intensity of grazing of the treatments.


The effect of plant structure on arachnids could actindirectly through food supply. Some arachnid speciescould be enhanced by plant species composition if theyhave particular prey requirements, for instance, sev-eral linyphiids are specialist predators of planthoppersthat require specific host plants (Waloff, 1980). Evengiven limited changes to botanical species composi-tion in Nardus-dominated grasslands, prey abundancecould not be entirely excluded as a factor becausethis too can respond to the architecture of individualplant species (Waloff, 1980) or the accumulation ofdead plant material, typical of upland organic soils.This was certainly the explanation given to accountfor the dominance of money spiders (Linyphiidae) athigher altitudes in the uplands. Upland organic soilsfavoured springtails (Collembola), prey with a rela-tively aseasonal distribution when compared with theinsect prey required by many of the larger arachnidspecies (Coulson and Butterfield, 1986).

In general, the experimental results refute thesecond hypothesis that intermediate rates of grazingenhance the structural diversity of grasslands andmaintain a higher diversity of arachnids, with twoexceptions. Firstly, adults and juveniles of L. mengiidominated the significantly higher density of spiderwebs counted by eye within N. stricta leaves of theungrazed treatments, whereas a larger number oflinyphiid spiders were represented within the lowerdensity of webs in the moderately grazed treatments.However, this effect was overwhelmed by the overalllarger number of arachnid species detected by pitfalltraps and suction sampling in the ungrazed, tallest N.stricta, a consequence of the greater accumulation ofplant litter. Secondly, the ambush spider, X. crista-tus, was captured in greater numbers in treatmentsof intermediate grazing intensity because this speciesfavoured fine-grain heterogeneity in plant structure forits hunting strategy. Within the grazed treatments gen-erally, where there were significant effects of livestockand sward height, the general trend was for greaterabundance of species in the sheep grazed and tallersward heights, lower not intermediate rates of grazing.Indigenous upland grasslands typically include specieswith tussock architecture that are not readily consumedby livestock, e.g., N. stricta, which forms distinctivepatches under grazing because of the selection by graz-ers of other more preferred species (Grant et al., 1997)or can form a blanket cover with no grazing. Structural

heterogeneity is increased by a higher grazing intensityalthough average vegetation height is decreased. Thisis contrary to the situation in lowland calcareous grass-lands where it is the relaxation of grazing pressure thatleads to a deeper litter layer and the recruitment of thetall, rigid, herbaceous species (successional change)that increase structural heterogeneity which is bene-ficial to web spinners (Gibson et al., 1992). Arachnidassemblages may also depend on the spatial scale ofhabitat variability, such as the distribution of differentplant species in the sward over short distances (Gib-son et al., 1992). At intermediate distances, speciesmay be sensitive to changes in the mean height of thevegetation within a vegetation community created bycontrasting grazing intensities (Rushton et al., 1989).Over larger distances, arachnids may respond to thedifferences in vegetation density produced by vari-ability in vegetation community (Downie et al., 1995).

The final hypothesis that sheep will have less effecton the distribution of web-building spiders than cattleand sheep together was supported, in particular, byspecies collected by suction sampling and by the totalindividuals captured by pitfall trapping that revealeda significant factorial ANOVA for livestock species.The most likely mechanism was the more widespreadtrampling by cattle compared with sheep within tus-sock clumps for a particular sward height. Sheeptended to avoid substantial patches of tussock suchthat webs would not be damaged. Tussocks buffer theeffect on arachnids of increases in grazing pressure inupland indigenous grasslands compared with lowlandgrasslands (Cherrett, 1964; Dennis et al., 1998). Tram-pling also affects the structure of plant litter and thefrequency of trampling has been shown to reduce theabundance and diversity of spiders in standard-sizedbags of hay by reducing the open spaces availableto spiders within the litter (Duffey, 1975). The directgradient analyses suggested that livestock variables ingeneral were weaker estimators in comparison withthe direct measures of vegetation. Also, stocking ratedid not strongly correlate with the vegetation variables.Species of grazer affected web densities and arachnidspecies number less than average sward height basedon data from web counts and suction sampling. Thismay reflect the patchy grazing pattern that mammalianherbivores impose on upland vegetation.

The arachnid assemblage of this upland, N.stricta-dominated grassland community was made up


of 84 species, 40 of which were common to all treat-ments. Most of these species were probably randomlydistributed as a function of the regional diversity ofspecies in the surrounding biotope mosaic. Certainly,aranean species sampled along altitudinal and vegeta-tion gradients in western Norway proved to be widelydistributed and easily dispersed (Otto and Svensson,1982). Between 27 and 35 aranean species of this N.stricta-dominated site were recorded at several otherupland UK locations, namely, the North Pennines,Durham and Northumberland in northern England,County Antrim in northern Ireland and the Grampianmountains of northeast Scotland (Rushton and Eyre,1992; McFerran et al., 1994; Downie et al., 1995). Theassemblage also comprised 75% of the 105 species ofspiders recorded in grasslands and heaths of uplandBritain by Ratcliffe (1977) although this assemblageincluded 28 montane and 45 sub-montane species thatwould not have been expected at the 500 m altitudeof this experiment (Ratcliffe, 1977; Downie et al.,1995). There is uncertainty about the habitat affinityof arachnid species sampled by pitfall traps in lowabundance. Of the 83 species sampled in pitfall traps,there were 17 species not listed in Table 1 becausetwo or less individuals represented them. Each ofthese species could represent one of three categories:abundant species of N. stricta-grassland that are ableto evade capture, rare species of the habitat or speciesthat have accidentally dispersed from other habitats(tourists).

Differences in the results derived from pitfall trapcompared with suction sample data were expectedbecause suction samplers sample spiders associatedwith the varied, aboveground structures of differ-ent plant species, whereas pitfall traps sample thosemoving over the ground. Data on arachnids collectedin pitfall traps must be treated with caution becauseof the variability in efficiency with which differentspecies are captured (Topping and Sunderland, 1992;Dinter, 1995; Sunderland et al., 1995; Sunderland andTopping, 1995). Hence, the emphasis placed on the di-rect gradient analysis of species assemblages derivedfrom the pitfall trap survey. Pitfall catches generallymisrepresent the true population structure of spiders,reflecting activity, density and trapability differencesbetween species. Hence, the number of males andspecies of Lycosidae and Erigoninae are overesti-mated in comparison with other species when pitfall

traps are used (Dinter, 1995). The most abundantarachnid, the harvestmen, M. morio, a nocturnal car-nivore that feeds on a wide range of invertebrate prey(Sankey and Savory, 1974), is a species frequentlyabundant over a broad range of habitats (Jennings,1983). Significantly more were caught in pitfall trapsin shorter swards but the pattern of captures may havebeen an artefact of its behaviour of walking over thegrass canopy, above traps, in the treatments with tallervegetation. Suction sampling collected a subset of 36of the arachnid species detected by pitfall trappingexcept for Nematostoma bimaculatum (Fabricius)(Opiliones: Nemastomatidae) that was not sampledby pitfall traps. There was a smaller number anddifferent sequence of species by relative abundancefor suction samples, which reflected the fundamentaldifferences in these two sampling methods. A reason-able estimate of the population structure of surfaceactive, dominant species can be achieved with suc-tion samples, particularly if samples are taken only infavourable, dry conditions (Dinter, 1995). However,species composition is generally under-estimatedin taller vegetation because of the bias towardssmaller species situated higher in the vegetation andby the short duration and frequency of samplingthrough time (Gibson et al., 1992). The monthlydifferences in web counts illustrated the seasonalchanges in the population density of web-spinningspiders.

Different conclusions have been drawn from stud-ies of the response of arachnids to grazing manage-ment in lowland and upland grasslands (Gibson et al.,1992; Treweek et al., 1997). The responses of spidersto controlled stocking rates of sheep have been mea-sured on lowland calcareous grasslands (Gibson et al.,1992; Treweek et al., 1997) but there may be manydifferences between these fertile, lowland grasslandsand the typical, tussocky grasslands of the cold, acidicsoils of upland Britain. Aranean assemblages wereless diverse in lowland, calcareous grasslands underan increased intensity of winter and spring grazingcompared with areas grazed only in summer at con-ventional stocking rates (Treweek et al., 1997). In onestudy of grassland and heathland spiders, the relativeabundance of species was recorded as lower underno grazing than under commercial stocking rates(McFerran et al., 1994) although this may have beenan artefact of the inappropriate use of pitfall traps.


Grazing “management” is implicated as a major in-fluence on spider assemblages by various studies butdirect effects (disturbance of webs by treading) areclearly less important than indirect effects (alteringvegetation structure). Gibson et al. (1992) suggesteda strong link between spider assemblages and habitatstructure in lowland grasslands, leaving little needto invoke other mechanisms to account for those as-semblages. However, vegetation structure is clearlydetermined by a range of factors, namely the produc-tivity of the location, the plant community, season andintensity of grazing. Management through high graz-ing intensities alone was the most detrimental optionbecause this encouraged a small number of nationallywidespread species. Therefore, two approaches wouldbe favourable: (a) for nature conservation managementto provide areas of low or no grazing whilst commer-cial grazing remains predominant, or (b) for the in-tegration of agricultural and environmental objectiveswithin upland grazing management by incorporatinga spatial mosaic composed of different patches ofgrassland under varied management regimes, achievedthrough rotational grazing with some rested plots. Thiswould best allow maximum species richness of arach-nid assemblages in upland habitats rather than a regimebased on a single grazing prescription. This recom-mendation is compatible with conclusions from stud-ies of botanical diversity in prairie (Collins and Barber,1985), arthropod diversity in lowland, chalk grasslands(Morris, 1978), coleopteran diversity on the sameupland N. stricta grassland (Dennis et al., 1997) andarthropods of upland heathland (Usher, 1992). Gen-eral arthropod diversity can be maintained or continueto rise even when plant taxonomic diversity is reducedon account of structural heterogeneity (Gibson et al.,1992). Furthermore, it is important to adopt a flexibleapproach in grassland management for biodiversitybecause practices that favour botanical diversity maynot produce a corresponding benefit for arachnid andother arthropod diversity (Treweek et al., 1997).

Acknowledgements

This project was supported by the Scottish OfficeAgriculture, Environment and Fisheries Department(now Scottish Executive Rural Affairs Department)through their Flexible Fund scheme. Thanks are

expressed to staff at the Sourhope Experimental Sta-tion, in particular, Harry Sangster and Gordon Com-mon for their assistance and cooperation; to MikeDavidson for verification and assistance in the identifi-cation of spiders; to Paola Sabbatini and Jean-DamienCagnard for their help with the vegetation survey; andto anonymous referees for improvements to an earlierversion of this paper.

References

Bell, J.R., Gates, S., Haughton, A.J., Macdonald, D.W., Smith,H., Wheater, C.P., Cullen, W.R., 1999. Pseudoscorpiones infield margins: effects of margin age, management and boundaryhabitats. J. Arachnol. 27, 236–240.

Cherrett, J.M., 1964. The distribution of spiders on the MoorHouse National Nature Reserve, Westmorland. J. Appl. Ecol.33, 27–48.

Collins, S.L., Barber, S.C., 1985. Effects of disturbance on diversityin mixed-grass prairie. Vegetatio 64, 87–94.

Coulson, J.C., Butterfield, J.E.L., 1986. The spider communitieson peat and upland grasslands in northern England. HolarcticEcol. 9, 229–239.

Coulson, J.C., Whittaker, J.B., 1978. Ecology of moorland animals.In: Heal, O.W., Perkins, D.F. (Eds.), Production Ecologyof British Moors and Montane Grasslands. Springer, Berlin,pp. 52–93.

Crawley, M.J., 1998. GLIM for Ecologists. Blackwell, Oxford,p. 379.

Curtis, D.J., 1980. Pitfalls in spider community studies (Arachnida:Araneae). J. Arachnol. 8, 271–280.

Dennis, P., Young, M.R., Howard, C.L., Gordon, I.J., 1997. Theresponse of epigeal beetles (Col.: Carabidae; Staphylinidae) tovaried grazing regimes on upland Nardus stricta grasslands. J.Appl. Ecol. 34, 433–443.

Dennis, P., Young, M.R., Gordon, I.J., 1998. Distribution andabundance of small insects and arachnids in relation to structuralheterogeneity of grazed, indigenous grasslands. Ecol. Entomol.23, 253–264.

Dietrick, E.J., 1961. An improved backpack motor fan for suctionsampling of insect populations. J. Econ. Entomol. 54, 394–395.

Dinter, A., 1995. Estimation of epigeic spider population densitiesusing an intensive D-vac sampling technique and comparisonwith pitfall trap catches in winter wheat. In: Toft, S., Riedel, W.(Eds.), Arthropod Natural Enemies in Arable Land. I. Density,Spatial Heterogeneity and Dispersal. Acta Jutlandica 70 (2),23–32. Aarhus University Press, Aarhus, Denmark.

Downie, I.S., Butterfield, J.E.L., Coulson, J.C., 1995. Habitatpreferences of sub-montane spiders in northern England. Eco-graphy 18, 51–61.

Duffey, E., 1975. Habitat selection by spiders in man-made envi-ronments. In: Proceedings of the Sixth International Arachno-logical Congress, Amsterdam, the Netherlands, pp. 58–67.


Gibson, C.W.D., Hambler, C., Brown, V.K., 1992. Changes inspider (Araneae) assemblages in relation to succession andgrazing management. J. Appl. Ecol. 29, 132–142.

Gimingham, C.H., 1988. What’s going on in the uplands. In:Usher, M.B., Thompson, D.B.A. (Eds.), Ecological Changein the Uplands. Blackwell Scientific Publications, Oxford,pp. 1–6.

Grant, S.E., Torvell, L., Sim, E.M., Small, J.L., Armstrong, R.H.,1997. Controlled grazing studies on Nardus grassland: effects ofbetween-tussock sward height and species of grazer on Nardusutilization and floristic composition in two fields in Scotland.J. Appl. Ecol. 33, 1053–1064.

Jennings, A.L., 1983. Biogeographical variation in the harvestmanMitopus morio (Opiliones: Arachnida). J. Zool., London 200,367–380.

Luff, M.L., Rushton, S.P., 1989. The ground beetle and spider faunaof managed and unimproved upland pasture. Agric. Ecosyst.Environ. 25, 195–205.

Magurran, A.E., 1988. Ecological Diversity and its Measurement.Croom Helm, London.

McFerran, D.M., Montgomery, W.I., McAdam, J.H., 1994. Theimpact of grazing on communities of ground-dwelling spiders(Araneae) in upland vegtation types. In: Proceedings of theRoyal Irish Academy on Biology and Environment, Vol. 94B,pp. 119–126.

Morris, M.G., 1978. Grassland management and invertebrateanimals — a selective review. In: Scientific Proceedings of theRoyal Society of Dublin, Series A, Vol. 6, pp. 205–247.

Otto, C., Svensson, B.S., 1982. Structure of communities ofground-living spiders along altitudinal gradients. Holarctic Ecol.5, 35–47.

Ratcliffe, D.A. (Ed.), 1977. A Nature Conservation Review, Vol.1. Cambridge University Press, Cambridge, p. 417.

Roberts, M., 1985. The Spiders of Great Britain and Ireland, Vol.1. Atypidae to Theridosomatidae. Harley Books, Colchester,Essex, p. 229.

Roberts, M., 1987. The Spiders of Great Britain and Ireland, Vol.2. Linyphiidae. Harley Books, Colchester, Essex, p. 204.

Rodwell, J.S. (Ed.), 1992. British Plant Communities, Vol. 3.Grasslands and Montane Communities. Cambridge UniversityPress, Cambridge, p. 540.

Rushton, S.P., Eyre, M.D., 1992. Grassland spider habitats innorth-east England. J. Biogeography 19, 99–108.

Rushton, S.P., Luff, M.L., Eyre, M.D., 1989. The effects of pastureimprovement on the ground beetle and spider communities ofupland grasslands. J. Appl. Ecol. 26, 489–503.

Rypstra, A.L., Carter, P.E., Balfour, R.A., Marshall, S.D., 1999.Architectural features of agricultural habitats and their impacton the spider inhabitants. J. Arachnol. 27, 371–377.

Sankey, J.H.P., Savory, T.H., 1974. British harvestmen. Synopsesof the British Fauna (N.S.), Vol. 4, pp. 1–76.

SAS, 1998. Statview Reference. SAS Institute Inc., USA.Sunderland, K.D., Topping, C.J., 1995. Estimating populations

of spiders in cereals. In: Toft, S., Riedel, W. (Eds.), Arthro-pod Natural Enemies in Arable Land. I. Density, SpatialHeterogeneity and Dispersal. Acta Jutlandica 70 (2), 13–22.Aarhus University Press, Aarhus, Denmark.

Sunderland, K.D., De Snoo, G.R., Dinter, A., Hance, T., Helenius,J., Jepson, P.C., Kromp, B., Lys, J.-A., Samu, F., Sotherton,N.W., Toft, S., Ulber, B., 1995. Density estimation forinvertebrate predators in agroecosystems. In: Toft, S., Riedel, W.(Eds.), Arthropod Natural Enemies in Arable Land. I. Density,Spatial Heterogeneity and Dispersal. Acta Jutlandica 70 (2),133–162. Aarhus University Press, Aarhus, Denmark.

ter Braak, C.J.F., 1988. CANOCO. A Fortran program forcanonical community ordination by partial, detrended, canonicalcorrespondence analysis, principal components analysis andredundancy analysis, Version 2.1. Agricultural MathematicsGroup, Wageningen, the Netherlands.

Topping, C.J., Sunderland, K.D., 1992. Limitations to the use ofpitfall traps in ecological studies exemplified by the study ofspiders in a field of winter wheat. J. Appl. Ecol. 29, 485–491.

Treweek, J.R., Watt, T.A., Hambler, C., 1997. Integration of sheepproduction and nature conservation: experimental management.J. Environ. Mgmt. 50, 193–210.

Uetz, G.W., 1991. Habitat structure and spider foraging. In: Bell,S.S., McCoy, E.D., Mushinsky, H.R. (Eds.), Habitat Structure:The Physical Arrangement of Objects in Space. Chapman &Hall, London, pp. 325–348.

Usher, M.B., 1992. Management and diversity of arthropods inCalluna heathland. Biodivers. Conserv. 1, 63–79.

Waloff, N., 1980. Studies on grassland leafhopper (Auchenor-hyncha: Homoptera) and their natural enemies. Adv. Ecol. Res.11, 81–215.

Wise, D.H., 1994. Spiders in Ecological Webs. CambridgeUniversity Press, Cambridge, p. 328.

the effects of varied grazing management on epigeal spiders, harvestmen and pseudoscorpions of...

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