Impacts of grazing and burning on spider assemblages in dry eucalypt forests of north-eastern New South Wales, Australia

Austral Ecology

(2003)

28

, 526–538

Impacts of grazing and burning on spider assemblages in dry eucalypt forests of north-eastern New South Wales, Australia

REBECCA HARRIS,* ALAN YORK

†

AND ANDREW J. BEATTIE

Key Centre for Biodiversity and Bioresources, Department of Biological Sciences, Macquarie University, Sydney, New South Wales, Australia

Abstract

In the dry eucalypt forests of north-eastern New South Wales, Australia, cattle grazing occurs at lowintensities and is accompanied by frequent low-intensity burning. This study investigated the combined effects ofthis management practice on the ground-dwelling and arboreal (low vegetation) spider assemblages. Spiders weresampled at 49 sites representing a range of grazing intensities, using pitfall trapping, litter extraction and sweepsampling. A total of 237 spider morphospecies from 37 families were collected using this composite samplingstrategy. The abundance, richness, composition and structure of spider assemblages in grazed and ungrazed forestsites were compared and related to a range of environmental variables. Spider assemblages responded to a range ofenvironmental factors at the landscape, habitat and microhabitat scales. Forest type, spatial relationships and habitatvariability at the site scale were more important in determining spider assemblages than localized low-intensitygrazing and burning. However, it is possible that a threshold intensity of grazing may exist, above which spidersrespond to grazing and burning. Although low-intensity grazing and burning may not affect spider assemblagesbelow a threshold stocking rate, that stocking rate has yet to be established.

Key words:

assemblage structure, grazing, pitfall trapping, spiders, sweep sampling.

INTRODUCTION

Cattle grazing is widespread throughout the publicforests of Australia, and in many forests forms animportant part of the forest management strategy.Cattle grazing is considered to have significant benefitsthrough the reduction of fuel loads and provision ofgrazing lease revenue (SFNSW 1995a). In someregions, the holders of grazing leases may also beprovided with a burning permit to promote grassgrowth and improve the quality of fodder for cattle.Grazing in these regions is therefore linked to frequentlow-intensity (< 500 kW m

–1

) burning regimes (Moore& Floyd 1994; SFNSW 1995b).

Much of the research on cattle grazing in Australiahas concentrated on the impacts of heavy grazing inagricultural lands or on low to moderate grazing insemiarid grasslands (King & Hutchinson 1983;Scougall

et al

. 1993; Fensham

et al

. 1999). Whereresearch has been carried out on low-intensity grazingin forests, it has focused on the response of vascularplants and vertebrates. These studies have shown that

cattle grazing affects habitat structure in Australianwoodlands and forests through a reduction in under-storey vegetation (Williams 1990; Pettit

et al

. 1995;Henderson & Keith 2002), leading to changes invertebrate species richness and composition (Smith

et al

. 1994) and modifications to the litter and groundmicroclimate (Friedel & James 1995; Yates

et al

. 2000).All of these effects may be exacerbated by associatedchanges to the fire regime, as frequent burning has alsobeen shown to cause a reduction in vegetation diversityand structure (Gill 1981; Fox & Fox 1986; Moore &Floyd 1994) and to affect the physical and chemicalproperties of soil and litter (Springett 1976; Majer1984; York 1996, 1999a).

The impact of grazing and burning on forest-dwelling invertebrate communities remains largelyunknown. Studies in heavily grazed grasslands andwoodlands have linked reduction in vegetation andlitter cover to changes in the structure and compositionof invertebrate communities (Abensperg-Traun

et al

.1996; Bromham

et al

. 1999). These effects are knownto increase with grazing pressure (King & Hutchinson1980, 1983), but the impact of low-intensity grazing oninvertebrates is less well understood.

Spiders have been suggested as a potential prioritygroup for the assessment of ecological disturbancebecause they are diverse, abundant and sensitive torelatively small changes in habitat structure (reviews byTurnbull 1973; Uetz 1991; Wise 1993). Several studieshave shown that the abundance, species richness and

*Corresponding author. Present address: Centre for Bio-diversity and Conservation Research, The Australian Museum,6 College Street, Sydney, New South Wales 2010, Australia(Email: [email protected]).

†

Present address: Forest Science Centre, University ofMelbourne, Natural Resources and Environment, Creswick,Victoria, Australia.

Accepted for publication April 2003.

EFFECTS OF GRAZING AND BURNING ON SPIDER ASSEMBLAGES 527

composition of spider assemblages are strongly influ-enced by vegetation density (Hatley & MacMahon1980; Gunnarsson 1990), vertical and horizontalvegetation diversity (Robinson 1981; Greenstone1984), and the depth and complexity of the leaf litterlayer (Uetz 1979, 1991).

Burning and very heavy grazing have been investi-gated separately and have been shown to alter thecomposition and structure of spider assemblages(grazing:

de

Keer

et al

.

1989;

Gibson

et al

.

1992;Zulka

et al

. 1997; burning: Little & Friend 1993; York1999a; Moretti

et al

. 2002). However, there have beenno studies investigating the combined effects of low-intensity grazing and burning on spider assemblages inAustralian forests.

The aim of the present study was to investigate theimpacts of low-intensity cattle grazing and associatedburning on ground-dwelling spider assemblages in dryeucalypt forests of north-eastern New South Wales(NSW), Australia. The forests have a long history ofclearing, burning, grazing and logging, with cattle

grazing and integrated logging currently the pre-dominant land uses. Because grazing is linked to afrequent low-intensity burning regime in this region(Moore & Floyd 1994; SFNSW 1995b), our studyconsiders grazing and burning as a composite manage-ment practice and we do not attempt to isolate theimpacts of each. The term ‘grazing’ will be used in thispaper to refer to the practice of grazing and burning,although burning may not be explicitly mentioned.

METHODS

Study location

The study area was located in north-eastern NSW,approximately bordered by latitudes 29

�

00

�

–29

�

30

�

Sand longitudes 152

�

35

�

–153

�

20

�

E (Fig. 1). The area issituated in the overlap of two major biogeographicalsubregions, the Torresian (subtropical) and Bassian

Fig. 1.

Location of study sites in grazed and ungrazed forest patches in north-eastern New South Wales.

528 R. HARRIS

ET AL.

(cool temperate), and is characterized by steep altitu-dinal and climatic gradients. The climate is warmsubtropical with a well-defined summer–autumn rain-fall peak (January–March), and a relatively dry winterand spring. Casino, a town in the north of the studyarea, has a mean annual rainfall of 1107 mm and anaverage annual temperature range of 13–26

�

C. Theclimate, geology and topography of the region aredescribed in SFNSW (1995a).

Study design

A stratified random sampling design was employed,based on two primary criteria: geological type (mixedsediments/sandstones/unconsolidated materials) andmanagement history (grazed/ungrazed). Study siteswere selected from the broad forest types of ‘grassyopen’ and ‘heath forest/woodland’, the drier foresttypes in which grazing predominantly occurs (Binns1995).

These

forests

are

characterized

by

an

over-storey dominated by

Angophora woodsiana

,

Eucalyptuspilularis

,

Corymbia gummifera and Eucalyptus plancho-niana

, with an understorey of moderately dense todense grasses, usually with a sparse to dense (1–2 m)sclerophyllous shrub stratum. Areas logged within thelast 10 years were excluded from the design. Forestareas satisfying these criteria were selected with the aidof the State Forests spatial GIS database, with siteslocated using a randomization process. To ensure thatsites were accessible to cattle but not subject to extremegrazing pressure (as would occur near wateringpoints), sites were constrained to being on a slope lessthan 10

�

, within 100 m of a road and more than 50 mfrom a watercourse. Initially, 20 sites were locatedwithin each geological type, with 10 grazed and 10ungrazed sites, giving a total of 80 sites. Paired grazedand ungrazed sites were matched across comparablesoil-type, land-use history and broad forest types.Because the study area extended over approximately50 km

�

60 km and sites were separated by a mini-mum distance of 500 m, sites were considered inde-pendent. Forest invertebrate communities have beenshown to exhibit rapid turnover at this scale (Ferrier &Watson 1997; Oliver

et al

. 1998). The original designof 80 sites was modified after heavy rain and floodingin February 1997, which prevented access to a numberof areas (primarily sites on carbonate sediments). Datafrom 49 sites across three geological types (13 mixedsediment sites, 18 sandstone sites and 18 unconsolid-ated materials sites) are presented here (Fig. 1).

Measurement of environmental variables

At each site, the environmental variables consideredmost likely to affect spider assemblages were quanti-

tatively assessed within a 50 m

�

20 m quadrat, asfollows.

Site characteristics

Aspect, slope and altitude were recorded for each site.Sites were allocated to one of five broad forest typegroups, based on the Baur forest types (ForestryCommission of New South Wales 1989).

Habitat structure

Vegetation density was calculated for five levels of theunderstorey vegetation using the coverboard techniqueof MacArthur and MacArthur (1961). The levels were:ground herbs (0–20 cm), small shrubs (20–50 cm),medium-sized shrubs (50–100 cm), tall shrubs (100–150 cm) and very tall shrubs (150–200 cm). Twentymeasurements were systematically made within eachquadrat for each level. An index of the overall verticalcomplexity (foliage height diversity, FHD) was calcu-lated utilizing the Shannon Index, –

�

i

p

i

log

e

p

i

, where

p

i

is the proportion of the total foliage that lies in eachof the chosen vegetation layers (see MacArthur &MacArthur 1961). The diameter at breast height overbark (d.b.h.o.b.) of all woody vegetation with a dia-meter greater than 5 cm was measured and convertedto basal area, the sum of which was used to representone component of the overstorey vegetation structureat each site (Binns 1995). Leaf litter depth was meas-ured to the nearest 0.5 cm at 20 points within eachquadrat using a graduated probe (see York 1998). Tensoil cores (5 cm diameter

�

10 cm deep) were collec-ted and bulk density and organic matter content(expressed as percentage loss on ignition) were meas-ured using the procedures of Corbett (1969) and Pitty(1978).

Disturbance history

Reliable logging histories were not available for all sites,so indices of past disturbance were based on thenumber of small (<10 cm diameter) and large (

≥

10 cmdiameter) logs and the number of cut stumps withineach quadrat. Site inspections revealed that simplegrazed/ungrazed comparisons were inadequate todescribe grazing impact because of the highly variablelevels of grazing activity on both ‘grazed’ and‘ungrazed’ sites. To compensate for this, an index wasdeveloped to describe recent grazing intensity based onthe number of cowpats within the site, around theperimeter of the site and along a 100 m length of theadjacent road (York 1998). For the broad grazed/ungrazed comparisons, sites with cowpats within thequadrat and/or the perimeter were classified as ‘grazed’and those without were ‘ungrazed’. Based on the index,sites were also allocated to one of five grazing categ-


ories, ranging from ungrazed sites (1), through to siteswith a relatively high grazing intensity (5) (see York1998). This enabled the initial factorial design to becomplemented by the use of a gradient analysisapproach (Krebs 1985) to investigate the effect ofgrazing intensity on measured habitat variables andspider communities.

Spider sampling

Spider assemblages were sampled in February 1997using pitfall trapping, extraction from leaf litter andvegetation sweeping. Nine pitfalls (8 cm diameter

�

12 cm deep) were located at 5-m intervals along acentrally located transect within each quadrat. Thepitfalls were half filled with a nonattractive preservativesolution of alcohol and ethylene glycol (50 : 50) (Weeks& McIntyre 1997) and left open for a 10-day period.To prevent flooding, traps were covered with a plasticroof supported approximately 20 cm above the soil bymetal

pegs.

Leaf

litter

extraction

was

used

to

targetthe non-cursorial component of the ground-dwellingspider assemblage. Twenty leaf litter samples (each1000 cm

3

) were collected within each site, bulked andconsolidated using a field extractor (Upton 1991).Spiders were extracted using modified Tullgren funnels(see Tanton

et al

. 1983). At each site the understoreyvegetation from ground level to 1 m was sampled by a100-m walked sweep. All samples were taken by thesame person (A. York) between 10.00 and 14.00 hoursover a period of 4 days.

Adult spiders from all samples were identified tofamily, and then assigned to morphospecies. Juvenilespiders were excluded from the analyses as very fewcan be reliably identified to species (Coddington

et al

.1991; Norris 1999). In Australia, only a small propor-tion of spider species has been named, so the use ofmorphospecies

as

a

surrogate

for

taxonomic

speciesis widespread (New 1999; Oliver

et al

. 2000). Oliverand Beattie (1996) have demonstrated that spidermorphospecies richness provides an accurate estimateof species richness in forest habitats. A referencecollection has been lodged at the Australian Museum,Sydney.

Data analysis

Environmental variables

A full analysis of environmental variables is presentedin York (1998) and Harris (2000). Inter-relationshipsbetween environmental variables were examined byprincipal components analysis (PCA), to identifygroups

of

variables

with

similar

responses

for

inclu-

sion in subsequent analyses (see ‘Environmentaldeterminants of spider assemblages’).

Descriptors of spider assemblages

Analyses were performed separately on the ground-dwelling spiders (pitfall and litter samples combined)and those from the low vegetation (sweep samples),because preliminary results showed very little overlapin the composition of these assemblages. All analyseswere carried out on data from adult spiders.

Species richness and abundance were used todescribe the spider assemblage at each site. Differencesrelated to environmental variables and grazing intensitywere investigated graphically and by analysis ofvariance

(

ANOVA

type

II,

because

of

unequal

levelsof

replication). The similarities of each site, basedon the species composition of the spider assemblages,were investigated using non-metric multidimensionalscaling (NMDS) (StatSoft 1997). Given the quanti-tative uncertainty inherent in pitfall trapping for spiders(Curtis 1980; Coyle 1981; Churchill & Arthur 1999),presence/absence data were considered the most reli-able (Clarke 1993). Analysis of similarity (ANOSIM,5000 permutations) was performed to test for differ-ences in species composition between the two a priorigroupings of grazed and ungrazed sites.

Differences in the species richness of each spiderfamily in grazed and ungrazed forests were assessed toinvestigate changes correlated with grazing and burn-ing. Because many of the habitat requirements andmodes of foraging of spiders are reflected at the taxon-omic division of family (Davies 1986; Canard 1990;Churchill 1997), this approach provided a qualitativeassessment of disturbance-related change in the struc-ture of spider assemblages. Only pitfall data were usedfor this comparison because of the emphasis on pitfallsampling inherent in the design. Families were alloc-ated to one of four groups based on known habitatrequirements (Clyne 1981; Main 1981, 1984; M. Gray,pers. comm., 1998; York 1999a):1. Moist habitat specialists: Anapidae, Cyatholipidae,

Micropholcommatidae, Mysmenidae, Oonopidae,Theridiidae, Hahniidae and Liocranidae.

2. Generalists, tolerant of dry habitats: Ammoxeni-dae, Corinnidae, Gnaphosidae, Lamponidae, Salti-cidae and Trochanteridae.

3. Open or disturbed habitat specialists: Desidae,Linyphiidae, Lycosidae, Zodariidae, Zoridae andProdidomidae.

4. Others, including: (i) families not specificallytargeted by pitfall trapping (e.g. Thomisidae,Araneidae, Deinopidae); (ii) families representedby only one individual (e.g. Heteropodidae,Hexathelidae); or (iii) generalized hunters withno known habitat preferences (e.g. Ctenidae,Cycloctenidae).

530 R. HARRIS ET AL.

Species richness of families on grazed and ungrazedsites, for each geology, was compared by multivariateanalysis of variance (MANOVA) and interpreted withinthe context of the known biology of each family.

Environmental determinants of spider assemblages

Canonical correspondence analysis (CCA) (ter Braak1986) was used to investigate the relationship betweenthe sites based on the composition of their spiderassemblages, and to determine how the assemblagesresponded to gradients in environmental variables. Theprogram CANOCO Version 4 (ter Braak & Smilauer1998) was used. To identify the local scale effects ofgrazing on spider assemblages, the effects oflandscape-scale factors such as geology and broadforest type were first tested by the forward selectionprocedure (Legendre 1990; ter Braak & Smilauer1998). Similarly, the presence of large scale spatialtrends within the data was investigated by using ageographical data matrix based on the geographicalcoordinates (x, y) of each site. This matrix containedthe terms for a cubic trend surface regression of theform: z = b1x + b2y + b3x2 + b4xy + b5y2 + b6x3 + b7x2y+ b8xy2 + b9y3 (Legendre 1990; Borcard et al. 1992).The forward selection procedure in CANOCO was usedto select those spatial terms that explained a significantamount of the variation in the species data. These termswere then included in the environmental dataset, andtheir effect partitioned out (controlled for) by includingthem as covariables in subsequent analyses.

For each of the major components identified by thePCA of habitat variables (see York 1998; Harris 2000),one representative variable from each of the indepen-dent groups was chosen for inclusion in the CCA.These were the variables with the strongest correlation,as follows: (i) percentage cover of tall shrubs to repre-sent the understorey vegetation; (ii) the number of cutstumps to indicate past management history; (iii) litterdepth; (iv) bulk density (representing topsoil struc-ture); (v) ground herb patchiness; (vi) basal area; (vii)bulk density patchiness. The grazing index wasincluded as a separate variable because it was ofprimary concern. A subset of environmental variableswas used for two reasons. First, the power of thisanalysis is improved if the number of environmentalvariables is small compared with the number of sites(ter Braak & Prentice 1988). Additionally, interpre-

tation and presentation of the CCA results issimplified with a reduced set of environmentalvariables (ter Braak & Verdonschot 1995). A MonteCarlo permutation test (300 permutations) was usedto test the statistical significance of the relationshipbetween the species and each of the environmentalvariables.

RESULTS

Ground dwelling spider assemblages (pitfall and litter samples)

A total of 3958 spiders was collected from all sites inthe pitfall and litter samples, with the litter samplescontributing only 539 spiders. Of the total, 67% werejuvenile and were excluded from the analyses. Pitfalltraps caught a greater proportion of adult spiders, at29% of the total pitfall catch, compared with 13% in thelitter samples. Very few spiders were caught in the littersamples on any site, reflecting the dry, shallow litterlayer, which ranged from 0.5 to 3.5 cm (mean depth)across all sites (see York 1998).

Spider abundance and richness

Adult spider abundance ranged from a minimum of 14to a maximum of 146 per site, with a mean ± SE of78.9 ± 4.7. There was no significant difference betweengrazed (77.0 ± 6.2) and ungrazed (81.8 ± 7.2) sites(F1,43 = 0.38, P = 0.54). Abundance was significantlydifferent between the different geological types(F2,43 = 4.01, P = 0.03); however, there was no clearseparation of geological types in the post hoc test(Student–Newman–Keuls procedure). The unconsoli-dated sites had the most abundant spider assemblagesand the mixed sediment sites the least abundant. Therewas a significant interaction between grazing andgeology (F2,43 = 1.42, P = 0.001).

Richness values ranged from five to 27 across allsites, with a mean of 14.0 ± 0.8. Mean species richnessdid not differ significantly between grazed (13.5 ± 1.0)and ungrazed (14.9 ± 1.3) sites (F1,43 = 0.87, P =0.36), or between geological types (F2,43 = 1.26,P = 0.30). There was no significant interaction betweengrazing and geology (F2,43 = 0.60, P = 0.55).

Table 1. Spearman’s Rank Correlations between habitat variables and spider abundance and richness (n = 49)

% Cover ground herbs FHD SE Bulk density LOI Basal area Grazing index

Spider abundance 0.202* –0.327* –0.349** 0.435** 0.385** –0.418Morphospecies richness 0.1800 –0.2720 –0.21400 0.21900 0.287*0 –0.087

*P < 0.05; ** P < 0.01. FHD, foliage height diversity; LOI, loss on ignition.


Spearman’s rank correlation showed that total spiderabundance was positively correlated with the percent-age cover of herbs, percentage organic matter contentand basal area, and negatively correlated with thevariability in overall vegetation structure (FHD SE)and bulk density (Table 1). The only habitat variablewith which species richness was correlated was basalarea. The intensity of grazing was not significantlycorrelated with either spider abundance or speciesrichness.

Species composition

A total of 36 families were caught across all sites,comprising 171 species. Most species (75%) wererepresented by fewer than five individuals, and many(35%) were represented by singletons. The mostspecies-rich families were the Salticidae (25 species),Theridiidae (21 species), Lycosidae (14 species),Zoridae (11 species) and Corinnidae (11 species).Thirteen (36%) families were represented by fewerthan five individuals. The most abundant families werethe Oonopidae (160 adults), Zoridae (155 adults),Corinnidae (120 adults), and the Zodariidae andLinyphiidae (each with 93 adults). Very few mygalo-morph spiders were caught, with a total of only 18individuals belonging to four families (Actinopodidae,Hexathelidae, Idiopidae and Nemesiidae).

The MDS ordination did not reveal any significantdifference in the composition of ground-dwellingspider assemblages between grazed and ungrazedsites (Fig. 2; ANOSIM global R = 0.012, P = 0.37).However, of the 81 species that occurred on bothgrazed and ungrazed sites, more than 50% were absentfrom sites with the highest grazing intensity.

Structure of spider assemblages

The structure of the spider assemblages based on themean number of morphospecies within habitat categ-ories (moist, open, generalist or other) is presented inFig. 3. There was no significant difference between the

Fig. 2. Multidimensional scaling ordination based onground-dwelling spider presence/absence data. (�), un-grazed; (�) grazed sites. For ease of interpretation, a 2-Drepresentation of the 3-D ordination is shown.

Fig. 3. Structure of ground-dwelling spider assemblages on grazed and ungrazed sites based on the mean number ofmorphospecies in each family. G, grazed; U, ungrazed; (�), moist habitat specialists; ( ), generalists; ( ), open/disturbed habitatspecialists; ( ), others.


grazed and ungrazed sites on any geological type(MANOVA: P > 0.05 in each case).

Environmental determinants of ground dwelling spider assemblages

The forward selection procedure in CANOCO indicatedthat the variability in the composition of ground-dwelling spider assemblages was significantlyinfluenced by broad forest type (F = 1.25, P = 0.01).Geographical position was close to significance at the0.05 level (x3: F = 1.16, P = 0.06). A plot of geo-graphical coordinate x (east–west location) againstspecies richness (Fig. 4) showed a curvilinear trendfrom east to west, with the highest species richness onthe coast, declining in a westerly direction until increas-ing again in the more elevated parts of the study area.The pattern was best explained by a quadratic functionof the form y = a + bx2 (r = 0.455; P = 0.005). Thisrelationship was, however, independent of altitude(Spearman’s rank correlation: r = 0.083, P = 0.568).

The results of the CCA are shown in Fig. 5. Thearrows depict the relative influence of the habitatvariable on the composition of the spider assemblage,with the line length relative to the other variables,rather than an absolute degree of influence (ter Braak& Verdenschot 1995). Percentage cover (F = 1.39,P = 0.047) and patchiness (F = 1.23, P = 0.030) of theunderstorey vegetation were the only statisticallysignificant variables, although patchiness of the herblayer (F = 1.2, P = 0.056) was close to significance atthe 0.05 level. The influence of grazing intensity(F = 1.05, P = 0.359) and logging history (F = 0.93,P = 0.701) were not statistically significant.

Spiders of the low vegetation (sweep samples)

A total of 3424 individuals were collected in the sweepsamples (n = 48), of which 18% (412) were adult. The82% juveniles were excluded from the analyses.

Spider abundance and richness

Adult abundance in the low vegetation ranged fromzero to 25 per site, with a mean ± SE of 8.6 ± 1.0.There was no significant difference between grazed(8.3 ± 1.4) and ungrazed (8.8 ± 1.4) sites(F1,42 = 1.11, P = 0.30). Abundance was significantlydifferent among the different geological types(F2,42 = 3.27, P = 0.04). There was no significant inter-action between grazing and geology (F2,42 = 2.54,P = 0.09).

Adult richness values ranged from zero to 12 acrossall sites, with a mean of 5.3 ± 0.4. Mean species rich-ness did not differ significantly between grazed(5.3 ± 0.6) and ungrazed (5.3 ± 0.5) sites(F1,42 = 0.35, P = 0.56), or among geological types(F2,42 = 2.2, P = 0.12). However, there was a signifi-cant interaction between grazing and geology(F2,42 = 3.18, P = 0.05). On unconsolidated sediment,species richness was higher on ungrazed sites, and onsandstone it was higher on grazed sites.

Total spider abundance was negatively correlatedwith altitude (Spearman’s rank correlation r = –0.326,P = 0.02) and positively with basal area (r = 0.310,P = 0.03). Species richness was negatively correlatedwith altitude (r = –0.372, P = 0.01). There were nosignificant relationships between abundance and rich-ness and grazing intensity or any other habitat variable.

Fig. 4. Relationship between site species richness and east–west location. (�), ungrazed; (�) grazed sites.

Fig. 5. Canonical correspondence analysis ordination ofground-dwelling spider presence/absence data, controlling forforest type and east–west location. (–––), variables with astatistically significant influence; (�), ungrazed; (�) grazedsites.


Species composition

In total, 87 morphospecies from 13 families werecaught in the low vegetation. Of these, 70% (61) ofmorphospecies were represented by 1–2 individuals,and 85% (74) by less than 10 individuals. Eighty-twoper cent of morphospecies were found only in thesweep samples and 18% were also caught in the pitfalltraps. The sweep and litter samples had no species incommon.

The most species-rich families were the Theridiidae(24 species), Salticidae (21 species) and Araneidae (18species). The most abundant families were the Theri-diidae (143 adults), Salticidae (102 adults), Araneidae(62 adults) and Tetragnathidae (62 adults). Fourfamilies (31%) were represented by five or less indi-viduals (the Clubionidae (five adults), Deinopidae (oneadult), Theridiosommatidae (two adults), and Ulobor-idae (three adults)). Two families (Tetragnathidae andTheridiosommatidae) were only found in the sweepsamples, whereas the number of species in several otherfamilies differed from that found in the other collectingmethods. For example, the Araneidae, one of the mostspecies-rich and abundant families collected in thesweep samples (18 species, 62 adults), was representedby only three species (and three individuals) in thepitfalls, and none in the litter samples.

There was no significant difference in the compo-sition of spiders inhabiting the low vegetation of grazedand ungrazed sites (MDS ordination on presence/absence data: ANOSIM global R = –0.051, P = 0.80), oracross geological types (ANOSIM global R = 0.028,P = 0.28). Three sites were excluded from theseanalyses as no adult spiders were caught.

Environmental determinants of spiders inhabiting the low vegetation

The CCA of the sweep data showed no patterns in thecomposition of the spider assemblages on grazed andungrazed sites. Unlike the ground-dwelling spiderassemblages, there was no influence of geographicalposition or forest type, but geology was found to be asignificant influence (F = 1.30, P = 0.015). The depthof the leaf litter layer was close to being statisticallysignificant at the 0.05 level (F = 1.33, P = 0.055). Noother variable was found to have a significant influenceon the species composition.

DISCUSSION

This study investigated the combined effect of grazingand burning in a region where grazing occurs at verylow stocking rates and is unevenly distributed acrosslarge areas of forest. Even at these levels, grazing has

been linked to changes in the vegetation structure, aprimary potential determinant of the composition anddiversity of spider assemblages. Exclusion experimentsin the dry forests of the study area have shown thatgrazing, in isolation from burning, leads to a significantreduction in the shrub and herb layers (Moore & Floyd1994). Frequent burning, however, causes a reductionin shrubs and an increase in the cover of the herb andgrass layer (Moore & Floyd 1994). Grazing thereforeexacerbates the impacts of frequent burning, causing areduction in the ground herb layer as well as the shrublayer (see also Pettit et al. 1995; Fensham et al. 1999).The results of the present study site supported this,with the main effect of grazing and associated burningbeing a reduction in the cover of the understoreyvegetation, an effect that increased with increasinggrazing intensity (York 1999b).

Effects of grazing

With the grazing-related reduction in the structure ofthe small and medium shrub layers (20–100 cm), it wasexpected that the arboreal spiders inhabiting the lowervegetation strata would be most affected by grazing andburning. Spider species richness has been shown to bepositively correlated with shrub volume and shrubfoliage diversity in a number of experimental manipu-lations (Colebourn 1974; Hatley & MacMahon 1980;Greenstone 1984; Gunnarsson 1990). However, in thepresent study, grazing and burning had no significantimpact on the abundance, richness or composition ofthe spiders inhabiting the low vegetation.

The ground-dwelling spider assemblages were alsounchanged by grazing and burning, in contrast to thebeetle community at the same sites (York 1999b).Although beetle abundance and richness did not differbetween grazed and ungrazed sites, species compo-sition was substantially different.

Other Australian investigations have similarly shownthe abundance and species richness of spiders tobe unaffected by grazing in woodland remnants(Abensperg-Traun et al. 1996; Bromham et al. 1999).However, in those studies the composition of the spiderassemblages differed between grazed and ungrazedtreatments, with different families responding differ-ently to grazing, depending on their habitat require-ments. York (1999a) reported that change in therepresentation of spider families was a better indicatorof disturbance than abundance or species richness. Heshowed that frequent burning in dry eucalypt forestswas associated with an 88% decrease in the number ofmoist habitat specialists and a 35% increase in spiderspecies with known dry or disturbed habitat prefer-ences (York 1999a). In the present study, a similartrend was found on the grazed unconsolidated sedi-ment sites, which had fewer moist habitat specialists


and more species characteristic of open or disturbedhabitats. However, this trend was not statisticallysignificant, and was not apparent on the other geo-logical types.

The intensity of grazing on each of the three geo-logical types at the study site was not equal. Theunconsolidated sediment geology included a greaternumber of sites with the highest grazing intensity,whereas the mixed sediment geology did not includeany heavily grazed sites. Using the mean spider speciesrichness across sites with a range of grazing intensitiesmay obscure trends in the structure of the spiderassemblage that occur only above a certain level ofgrazing. Gibson et al. (1992) suggested that a thresholdlevel of grazing may exist, below which spider assem-blages are unaffected. In their study, only very heavygrazing produced an altered spider assemblage. Theshift towards spider species tolerant of dry openhabitats on the unconsolidated sediment geologicaltype may reflect the impact of the higher grazingintensity on these sites. The intensity of grazing maydetermine the long-term impact, as supported by thespecies composition of the ground-dwelling spiderassemblages on sites with the highest intensity ofgrazing. Of the 81 species found to be common onboth grazed and ungrazed sites, more than 50% wereabsent from sites with the highest grazing intensity. Itmay only be above a certain intensity of grazing that thecomposition of spider assemblages is affected. Furtherresearch is required to demonstrate that such a thres-hold intensity exists, and how it translates to actualstocking rates.

More rigorous analysis would have been possible ifall geological types had the same number of sites withineach grazing category. However, this was not possiblewith this experimental design, as the intensity ofgrazing cannot be fully separated from geology in thisregion. Topography and proximity to private land havedetermined past and current land uses, includinggrazing. The unconsolidated sediment sites were sub-jected to the most intense level of grazing in the region,as they were situated on the low-lying, wetter areas,adjacent to cleared private land (SFNSW 1995a). Thesandstone sites were characterized by a steeper topo-graphy (York 1997) and were therefore less accessibleto cattle, so grazing was less intense on these sites. Siteson mixed sediment were situated between these twoextremes, and were therefore subjected to an inter-mediate intensity of grazing. For this reason, geologicaltype was used as a primary stratification criterion, in anattempt to provide a true reflection of land use in theregion.

The differing results on the three geological typeshighlights the importance of studying spider assem-blages at the landscape scale over a range of habitattypes before generalizations can be made about theeffects of disturbance. This is particularly important

because the distribution of spiders is highly variable inspace, in response to a range of environmental factorsat the landscape, habitat and microhabitat scale. Theability to highlight responses to grazing and associatedburning is therefore strongly influenced by the scale ofmeasurement.

Environmental determinants of spider assemblages

At the landscape scale, broad forest type and distancefrom the coast (E–W spatial component) had a signifi-cant influence on the richness and composition ofspider assemblages. Similar coarse relationshipsbetween forest type and assemblage compositionhave been described for other invertebrate taxa (Oliveret al. 1998; York 1999b). Spatial patterning of foresttypes in the study area was related to climate, topo-graphy, disturbance regime and underlying patternsof geology and soil type (Forestry Commission ofNew South Wales 1989). Although geological typewas not identified as a primary determinant of ground-dwelling spider assemblages, there was a clear associ-ation between geological type and broad forest type.Sixty-nine per cent of mixed sediment sites wereclassified within the Spotted Gum/Spotted Gum –Ironbark/Grey Gum League B(e). Similarly, sandstonesites were predominantly (50%) from the Blackbutt –Bloodwood/Apple League B(b), with unconsolidatedmaterial sites tending to support forests from thePaperbark/Forest Red Gum B(a)(g) and Grey Gum –Grey Ironbark – White Mahogany B(d) Leagues (44%and 22%, respectively). The importance of geology wasevident in the analyses of the species richness of thespiders of the low vegetation and the richness andabundance of ground-dwelling spiders, in which therewas an interaction between grazing and geological type.

Given the extent of the study area (60 km � 50 km)and the rapid species turnover usually shown byinvertebrates, a certain amount of variation could beexpected purely on the basis of the geographicalseparation between the sites (Ferrier & Watson 1997;Ferrier et al. 1999). In the present study, the spatialcomponent of the variability between the spider assem-blages was related to distance from the coast, which ismost likely to correspond to a trend in rainfall andproductivity, with the highest rainfall on the coast andin the mountainous area in the west of the study area(SFNSW 1995a).

Within this broad framework, the cover and patchi-ness of the understorey vegetation were found to be themost significant influences on the composition of theground-dwelling spider assemblages at the habitatscale. This result is consistent with the widely held viewthat vegetation structure is of primary importance indetermining the composition of spider assemblages(see reviews by Uetz 1991; Wise 1993). Vegetation


structure is important because it provides web-attach-ment sites (Greenstone 1984; Rypstra 1986), foragingperches (Greenquist & Rovner 1976; Döbel et al.1990) and protection from predators and extremeweather conditions (Hatley & MacMahon 1980;Riechert & Gillespie 1986). In addition, the densityand patchiness of the understorey vegetation affectstemperature, humidity and light intensity at the groundlevel, factors that influence spider habitat selection andactivity (Cloudsley-Thompson 1962; Turnbull 1973;Uetz 1991).

Surprisingly, these variables had no detectableinfluence on the composition of spiders of the lowvegetation. A more comprehensive sweep samplingprogram may be necessary to detect any relationshipbetween the spiders and the measured habitat variables.Although many spiders were caught in the sweepsamples (3424 individuals), the large proportion ofrare species (70% morphospecies represented by 1–2individuals, 85% by less than 10 individuals) suggestedthat the intensity of sampling may have been insuffici-ent to provide a representative sample of the spiderspresent. Additionally, standardization of samples acrossall sites is difficult with this technique because ofcollector bias (e.g. avoidance of spiky plants or those invery dense stands), and differences in the area beingsampled in sites with more or less vegetation (DeLong1932; Churchill 1993; Churchill & Arthur 1999).

In several studies the depth of the leaf litter layerwas an important determinant of spider assemblagecomposition, as litter affects prey abundance, reducestemperature and humidity fluctuations, and providesstructural retreats from predation (Uetz 1991; Bultmanet al. 1982; Bultman et al. 1982). In the dry eucalyptforests of the present study, however, litter depth didnot significantly influence the spider species compo-sition. All of the forests in the present study had a veryshallow leaf litter layer, with very little variation amongsites. The spider assemblage reflected this, with veryfew spiders caught in the leaf litter samples. This couldbe a result of frequent burning, which is common inthis region (SFNSW 1995b). Frequent burning signifi-cantly reduces the abundance of litter-dwelling spidersin dry eucalypt forests in northern NSW (York 1999a).

Grazing effects

The range of grazing intensities being investigated inthe present study was not an important influence onspider assemblages. Grazing intensity was very low,with an average of one animal per 20–70 ha, dependingon the season (SFNSW 1995a). At this stocking rate,grazing will have localized effects, concentrated aroundwatering points and trails, areas that were deliberatelynot sampled. Even in a relatively homogenous environ-ment such as pasture, landscape use by cattle has been

shown to be spatially heterogeneous. Slope, distancefrom water, the density of palatable vegetation, andeven the direction of the prevailing wind all affect theutilization of an area by cattle (Senft et al. 1983; Friedel& James 1995). In a forest environment, where theheterogeneity may be far greater because of variabilityin topography, the density of vegetation and the pres-ence of logs and stumps, the spatial variability in theintensity of grazing is likely to be even more marked.Overall, as there was no clear separation of grazed andungrazed sites, habitat variability at the site scalewas probably more important in determining spiderassemblages than localized, low-intensity grazing andburning.

Study limitations

This study was limited by a lack of historical inform-ation about grazing and fire. However, combining theindex of current grazing intensity with historicalrecords of grazing leases and occupation permits pro-vided estimates of grazing pressure at each site thatwere related to reductions in understorey vegetationcover (York 1998).

Our study provided a ‘snapshot’ comparison ofspider assemblages in grazed and ungrazed forests atone point in time. The final species list probably under-estimated the true spider species richness in theseforests. Given that adult spiders are needed for identifi-cation, medium-sized and large spiders with annuallife-cycles (e.g. mygalomorph spiders) will be less wellrepresented than smaller species that reproduce non-cyclically whenever conditions are favourable (Huhta1971). Depending on the aim of a study, however, itmay not be necessary to sample over long periods oftime. Maelfait et al. (1989), investigated the effect ofthe timing of sampling spiders on the ability to distin-guish between two grassland (heavily grazed andgrazed but not trampled) and two woodland types(open and closed). Short sampling periods weresufficient to distinguish between habitats that differonly slightly from each other, although sampling timewas important. In spring and summer, short samplingperiods were adequate, but in winter longer samplingperiods were necessary to distinguish between the sites.In comparative studies where the impact of disturbanceis being investigated, ‘snapshot’ studies are thereforelikely to be sufficient if the timing is taken into account.

Sampling considerations

This study showed that different sampling methodstarget particular components of the spider assemblageand may provide very different estimations of spiderabundance and species richness and composition.


Spider communities exhibit strong vertical stratifi-cation among different vegetation layers (Coddingtonet al. 1991; Churchill 1993; Edwards 1993; Churchill& Arthur 1999). There was very little overlap in thespecies caught by each of the three sampling tech-niques, with different families dominating the groundand low vegetation assemblages. A composite samplingstrategy is therefore essential to sample the range ofmicrohabitats likely to be affected by disturbance, toallow conclusions about the response of spiders todisturbances such as grazing and burning. This isparticularly important when results are ofteninterpreted as being representative of the wholecommunity.

ACKNOWLEDGEMENTS

This project was funded by the Resource and Conserv-ation Assessment Council of NSW and the Key Centrefor Biodiversity and Bioresources, Macquarie Univer-sity. This paper represents contribution number 381 ofthe Key Centre for Biodiversity and Bioresources.Thanks to Jaynia Tarnawski and Helen Doherty forlaboratory sorting and assistance in the field, and themany student volunteers who also helped with thefieldwork. Mike Gray provided valuable advice andhelp with spider identifications.

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Impacts of grazing and burning on spider assemblages in dry eucalypt forests of north-eastern New South Wales, Australia