the influence of conservation field margins in intensively managed grazing land on communities of...

The influence of conservation field margins inintensively managed grazing land on communities offive arthropod trophic groups

ANNETTE ANDERSON,1 TIM CARNUS,1 ALVIN J. HELDEN,1 ,2 HELENSHERIDAN1 and GORDON PURVIS1 1UCD School of Agriculture and Food Science, Agriculture

and Food Science Centre, University College Dublin, Dublin, Ireland and 2Department of Life Sciences, Animal and Environ-

mental Research Group, Anglia Ruskin University, Cambridge, UK

Abstract. 1. Arthropods, a major component of functional biodiversity withinagro-ecosystems, contribute to sustainability through processes including nutrientcycling and pest control. Extensively managed field margins can help protect thisfunctional biodiversity by providing habitat for beneficial species.2. This 2 year study investigated the relative benefits of grassland field margin

treatments (Fenced only, Rotavated, and Reseeded (with a grass and wildflower mix-ture)) on the abundance and taxon richness of five arthropod trophic groups (detriti-vores, herbivores, predators, parasitoids and hyperparasitoids) from the ordersColeoptera, Hemiptera, Hymenoptera, Diptera and Araneae.3. The taxon richness and abundance of all trophic groups (with the exception of

herbivore abundance) was greater in fenced field margin treatments than in the adja-cent grazed field, particularly by the final sampling occasion. However, there wereseasonal differences, with abundance and taxon richness generally greater in Augustthan in June. Only detritivores and herbivores responded to the individual fencedfield margin treatments. The botanically more species rich, rotavated and reseededtreatments, had greater detritivore and herbivore richness and abundance than thefenced only treatments.4. Community structure analysis indicated that the grazed (within) field and fenced

field margins had two distinct communities for all trophic groups, demonstrating theconservation value of themargins within intensivelymanaged agricultural grasslands.5. The current Irish agri-environment scheme requires fencing of field margins.

Our results highlight how this relatively simple measure can benefit arthropod con-servation and functional biodiversity. This may in turn benefit farm productivitythrough potentially improved nutrient cycling and natural pest control.

Key words. Agri-environmental scheme, agro-ecosystem, biodiversity, detriti-vore, functional, herbivore, Ireland, parasitoid.

Introduction

Arthropods are a major component of functional biodiversityin most terrestrial ecosystems and in agriculture contribute tosustainability through processes such as pollination, nutrientcycling and pest control (Weisser & Siemann, 2004; Haaland

et al., 2011). Unlike arable crops, grasslands in agro-ecosys-tems are not subject to the annual disturbance of soil cultiva-

tion, and so are potentially relatively species rich habitats(Plantureux et al., 2005). However, intensive pasture manage-ment in livestock farming, involving the use of frequentlyreseeded swards of limited botanical diversity mainly perennial

ryegrass (Lolium perenne L.), and increased fertiliser inputsand stocking rates, reduce this potential (McDowell, 2008;Kleijn et al., 2009). Agricultural practices can threaten arthro-

pods through: (i) habitat loss and fragmentation (Kruess &Tscharntke, 1994; Tscharntke et al., 2002); (ii) fertiliser and

Correspondence: Annette Anderson, UCD School of Agricul-

ture and Food Science, Agriculture and Food Science Centre,

University College Dublin, Belfield, Dublin 4, Ireland. E-mail:

[email protected]

Insect Conservation and Diversity (2012) doi: 10.1111/j.1752-4598.2012.00203.x

� 2012 The AuthorsInsect Conservation and Diversity � 2012 The Royal Entomological Society 1

pesticide use (Kromp & Steinberger, 1992; McLaughlin & Mi-neau, 1995; Plantureux et al., 2005); and (iii) grazing, cutting,

ploughing and reseeding (Plantureux et al., 2005). Heavy graz-ing produces short swards that reduce foraging opportunitiesand habitat for many invertebrates (Morris, 2000), whilst low

stocking rates lead to more complex habitat structure that canfavour invertebrates like spiders (Plantureux et al., 2005).There is now a substantial literature regarding the functional

benefits of maintaining conserved field margins in arable land-scapes to enhance the diversity and abundance of beneficialarthropods (e.g. Marshall & Moonen, 2002; Meek et al., 2002;

Tscharntke et al., 2002;Woodcock et al., 2005b; Haaland et al.,2011).However, thesemargins can also potentially benefit biodi-versity by providing habitats for functionally important speciesin intensive grassland farming systems (Marshall & Moonen,

2002).In intensive grassland farming, conservation field margins

ensure the retentionof bothplant species and structural diversity,

avoid the disturbance of intensive grazing and cutting and thenegative impacts of fertilisers and manures (Lagerlof & Wallin,1993;Meek et al., 2002;Asteraki et al., 2004; Fritch et al., 2011).

Fieldmargins are relatively simple to implement as conventionalmanagement can continue across the rest of the farm, allowingintensive farming, whilst potentially increasing invertebrate bio-diversity. However, in comparison to arable contexts, relatively

few studies have investigated the potential advantages of conser-vation field margins in grassland farming (Cole et al., 2007;Woodcock et al., 2007, 2009; Sheridan et al., 2008;Woodcock&

Pywell, 2010). Despite this, the Irish Agri-Environment Schemeincludes measures to protect biodiversity within grass-basedfarming systems, including the maintenance of an untilled,

unploughed and unsprayed field edge of at least 1.5 m (Anon,2006). However to-date, apart from one study suggesting thatcarabid beetle populations in field margins did not benefit

from such measures (Feehan et al., 2005) very little research hasbeen undertaken within Ireland to validate, or improve theireffectiveness.Our study investigated the response of a broad range of

arthropod groups (Hemiptera, Coleoptera, parasitoid Hyme-noptera, Diptera and Araneae) across five trophic levels (detriti-vores, herbivores, predators, parasitoids and hyperparasitoids)

in a replicated conservation field margin experiment establishedon intensively managed dairy paddocks. Two questions wereaddressed: (i) Do conservation field margins in an intensively

managed dairy increase the abundance and ⁄or taxon richness ofthe five arthropod trophic groups, and (ii) Does the communitystructure of each of these trophic groups differ between thefenced conservationmargin treatments and the grazed field?

Methods

Experimental design

The study was undertaken on a commercially managed dairyfarm at the Teagasc Research Centre, Johnstown Castle, Co.Wexford in S.E. Ireland (52�17¢N, 6�30¢W). All traditional field

hedgerows and their associated herbaceous margins had been

removed from this site during a process of farm intensificationduring the 1970s and 1980s. Electric fencing was used to sub-

divide the site into intensively grazed paddocks dominated byperennial ryegrass (L. perenne).In February 2002, stratified randomised factorial split-plot

field margin experiment was established by fencing off three rep-licate 90 m long strips of 1.5 m, 2.5 and 3.5 mwidth at the edgesof existing paddocks (total = 9 strips). Each strip was subse-

quently divided into three 30 m longplots to accommodate threedifferent fenced margin treatments (a–c) in a fully randomisedblock design. Seed mixtures have been successful in improving

botanical diversity in intensively managed grasslands (Jefferson,2005) and the treatments used in this studywere as follows:

1 Fenced only (FO); the existing grass sward with thestrip simply fenced off from the adjacent paddock.

2 Rotavated and Fenced (ROT); the existing vegetation

removed with a glyphosate-based herbicide and thestrip rotavated and left to revegetate naturally afterbeing fenced off.

3 Rotavated, Reseeded and Fenced (RS); the existing

vegetation removed with glyphosate-based herbicideand the strip rotavated and reseeded with a grass andwildflower seed mixture before being fenced off.

4 Grazed control (Grazed); established adjacent to eachof the 9 replicate blocks. These unfenced marginsremained part of the associated paddock, and contin-

ued to be grazed (2.5 livestock units ha)1), fertilised(200–375 kg N ha)1; 0–50 kg P ha)1; 0–75 kg K ha)1

annually) and cut for silage.

Prior to the start of, and during the experiment, the paddockswere grazed by a Friesian dairy herd on a standard 21-day rota-

tion. All fenced treatments (a–c) were protected from cattlegrazing and nutrient inputs. Vegetation height was measuredusing a Filips Folding Plate Meter (http://www.jenquip.co.nz/

pasturem.htm) and botanical composition and abundance of allplots were determined in May 2004 within 1 m quadrats usingthe Braun-Blanquet scale. To investigate how different plantspecies established across the width of plots, four, eight and

twelve 1 mquadrats were taken from the 1.5, 2.5 and 3.5 mwideplots respectively. Further details on the experimental designand botanical composition of the field margins are provided by

Sheridan et al. (2008). A detailed botanical species list can befound in Table S1.

Arthropod sampling

Arthropods were sampled in both June and August of 2004

and 2005, respectively, using a Vortis suction sampler (BurkardManufacturing Co Ltd, Rickmansworth, UK) (Arnold, 1994).Suction sampling is an effective method for sampling agricul-

tural grassland invertebrates (Brook et al., 2008). An aggregatesample consisting of ten randomly selected spots (each of 10 sduration) was collected from each margin plot, including the

grazed field. The total area sampled per plot was 0.19 m2 oneach sampling occasion. Coleoptera, Hemiptera and Araneaewere identified to species (see Anderson et al., 2008a andHelden

2 Annette Anderson et al.

� 2012 The AuthorsInsect Conservation and Diversity � 2012 The Royal Entomological Society, Insect Conservation and Diversity

et al., 2008a,b for identification literature), parasitoidHymenop-tera to genus (see Anderson et al., 2008b for identification litera-

ture) andDiptera to family (Unwin, 1981).

Trophic levels

All arthropods were assigned to one of the following trophic

levels: detritiviores (including saprophages, mycetophages anddung feeders), herbivores, predators, parasitoids or hyperparasi-toids (Table S2) using published literature (for examples seeAnderson et al., 2008a,b; Helden et al., 2008a,b). Given the di-

chotomoybetween feeding ecology inmost larval and adultDip-tera, feeding by the adult was considered supplementary to thelarva (Oldroyd, 1964) and therefore theDiptera were assigned to

trophic levels according to their larval feeding stage (Table S2).

Data analysis

Trophic group abundance and taxon richness The effects

of margin treatments on the abundance and taxon richness ofthe trophic groups were investigated with generalised linearmixed models (GLMMs) to account for repeated measures atstrip level. A maximal model with a Poisson error structure,

including plot width, sampling year (2004 and 2005), samplingmonth (June and August) and treatment (FO, ROT, RS,Grazed) as fixed effects, and strip as a random effect, was fitted.

The abundance models included an additional observational-level random effect to account for overdispersion and a randomplot effect for repeated measures. The richness models included

a plot random effect for repeated measures only. In some casesthe variance of these random effects were estimated to equal zeroand were therefore not included in the final models. Likelihood

ratio tests (v2) were used to determine significant model parame-ters. Covariates for plant height (cm) and plant species richnesswere also included in the modelling framework to account forpotential suction sampling bias (Brook et al., 2008) and to

directly assess effects on arthropod community that may be dueto plant community attributes.

Community structure of the trophic groups collected fromfield margin treatments Non-metric Multidimensional Scaling

(NMDS), recommended for count data (Minchin, 1987), andPermutational Multivariate Analysis of Variance (PERMANOVA)were used to assess differences in community structure between

the fenced margin treatments and the grazed field. Bothapproaches were based on Bray-Curtis dissimilarity matrix cal-culated after data were square root transformed andWisconson

double standardised (Legendre &Gallagher, 2001). Two dimen-sions were considered adequate for interpretation of NMDSbased on non-metric R2 fit values. PERMANOVA was run over 999permutations. Rare taxa, represented by less than 5 individuals

were excluded from the analysis. In these analyses, all samplescollected from individual margin plots during 2004 and 2005were pooled to create a single sample providing the most com-

prehensive description of arthropod community structure withineach plot. In addition, analysis was also conducted for commu-nities at each sampling date. For clarity, taxa names are left off

the ordination, but a full list of taxa and associations can befound in Table S2.All statisticalanalyseswereconducted inthestatisticalenviron-

ment R version 2.12.1 (R Development Core Team, 2010) using

the lme4 package (Bates&Maechler, 2010) forGLMMsand theveganpackage (Oksanen et al., 2010) formultivariateanalyses.

Results

A total of 23 272 arthropod individuals representing 322 taxawere collected over the 4 sampling dates from a total sampledarea of 27.36 m2. These included: 5466 hymenopteran

parasitoids representing 117 hymenopteran genera, 1016 spi-ders from 29 species, 2725 beetles from 100 species, 1663 hemi-pteran bugs from 48 species, and 12 403 dipteran flies from28 families.

Table 1. Results from likelihood ratio tests, for generalised linear mixed models (lmer) investigating the abundance of all arthropods and

trophic groups in field margin treatments over two seasons and two sampling years.

d.f.

Total Detritivores Herbivores Predators Parasitoids

Hyperparasi-

toids

v2 P-value v2 P-value v2 P-value v2 P-value v2 P-value v2 P-value

Treatment 3 2.12 0.550 2.56 0.464 5.96 0.113 1.16 0.761 6.45 0.092 287.87 <0.001

Season 1 20.13 <0.001 70.32 <0.001 5.74 0.017 59.43 <0.001 84.86 <0.001 30.60 <0.001

Yr 1 81.81 <0.001 3.92 0.048 10.63 0.001 181.80 <0.001 1.13 0.289 2.86 0.090

Vegetation height 1 1.12 0.289 1.51 0.220 10.71 0.001 3.12 0.078 10.29 0.001 11.26 <0.001

Vegetation richness 1 0.95 0.330 0.54 0.464 0.28 0.597 0.05 0.831 5.43 0.019 4.66 0.031

Treat*season 3 1.68 0.641 2.56 0.464 22.14 <0.001 10.48 0.015 8.26 0.041 23.67 <0.001

Treat*yr 3 11.60 0.009 5.25 0.154 8.52 0.036 39.96 <0.001 25.33 <0.001 14.25 0.002

Season*yr 1 0.44 0.507 72.28 <0.001 7.30 0.007 0.30 0.587 15.17 <0.001 25.33 <0.001

Treat*season*yr 3 21.10 <0.001 5.41 0.144 4.66 0.199 13.98 0.003 4.99 0.173 4.02 0.259

Treatment (treat), season, year (yr) and width were included as fixed effects and plot as a random effect. For the likelihood ratio tests,

width was designated the null model and other parameters treat, season and yr and interactions (*) were systematically added to subse-

quent models. Significant (P < 0.05) results are in bold.

Communities of five arthropod trophic groups 3


The response of arthropod abundance to conservation field

margin treatments

Significant interactions between treatment effects and yearand ⁄or season were often found (Table 1; Fig. 1). With the

exception of August 2004, the total abundance of arthropods,predators, parasitoids and hyperparasitoids were generallygreater in the fenced field margin treatments than the grazed

paddocks. There was little differentiation between individualfenced margin treatments (FO, ROT and RS) (Table 1; Fig. 1).In samples collected in August 2004, all groups except detriti-

vores (but especially herbivore, parasitoid and hyperparasitoidgroups) were significantly more abundant in grazed paddocks,comparedwith all fenced treatments (Fig. 1).

The effect of treatments on detritivore abundance varied withseason, with no difference between grazed paddocks or fencedmargins in June of either year. In August 2005, detritivore abun-dance was greater in all fenced treatments, but this effect was

inconsistent, as in August 2004, only the RS fenced treatmenthad a greater abundance of detritivores than the grazed pad-docks (Fig. 1). A similar pattern of inconsistent treatment effects

was seen in the abundance of parasitoid and hyperparasitoidtaxa inAugust of each year studied (Fig. 1).

Herbivore abundance also showed seasonal differences in the

grazed paddock in 2004 (Table 1; Fig 1). This was mirrored byvariation in parasitoid and hyperparasitoid numbers in thegrazed paddock (Fig. 1). More herbivore individuals were col-lected from the grazed paddock, compared with all fenced treat-

ments in August of both years, but the reverse was observed inJune when abundance was greatest in the ROT and RS com-pared with FO and grazed paddocks (Fig. 1). Despite the fluctu-

ations ofmost groups, by the final sampling date inAugust 2005the abundance of all groups except herbivores was greater in allfenced treatments, rather than grazed paddocks (Fig. 1). Plant

height had an additional positive significant effect on herbivores,parasitoids and hyperparasitoid abundances (Table 1). Vegeta-tion richness also had a significant positive effect on parasitoids

and hyperparasitoids (Table 1).

The response of arthropod taxon richness to conservation

field margin treatments

With the exception of herbivores, the dynamics in taxon rich-

ness were more uniform than group abundance; with signifi-cantly greater diversity in August, compared with June samples

Fig. 1. Mean abundance of five arthropod trophic groups collected from fenced field margin treatments and grazed paddocks showing

interactions between sampling year and season. Single point and error bars on left of each panel indicate overall mean and standard error

for each response.



(Fig. 2). The taxon richness of total arthropods, parasitoids andhyperparasitoids was greater in the fenced margin treatments

than grazed paddocks, with the exception of August 2004

(Table 2; Fig. 2). In August 2004, however, more total, predatorand hyperparasitoid taxa were found in the grazed paddock

(Fig. 2). Detritivore taxon richness was similar between grazed

Fig. 2. Mean taxon richness of five arthropod trophic groups collected from fenced field margin treatments and grazed paddocks showing

interactions between sampling year and season. Single point and error bars on left of each panel indicate overall mean and standard error

for each response.

Table 2. Results from the likelihood ratio tests, for generalised linear mixed models (lmer) investigating the richness of all arthropods and

trophic groups in field margin treatments over two seasons and two sampling years.

d.f.

Total Detritivores Herbivores Predators Parasitoids

Hyperparasi-

toids

v2 P-value v2 P-value v2 P-value v2 P-value v2 P-value v2 P-value

Treatment 3 21.81 <0.001 0.12 0.989 5.70 0.127 1.58 0.663 20.74 <0.001 8.42 0.038

Season 1 159.96 <0.001 60.36 <0.001 1.08 0.299 69.63 <0.001 99.69 <0.001 14.24 <0.001

Yr 1 17.70 <0.001 4.41 0.036 1.61 0.204 61.29 <0.001 2.09 0.148 3.81 0.051

Vegetation height 1 5.44 0.020 0.37 0.544 0.23 0.630 11.36 <0.001 3.79 0.052 2.27 0.132

Vegetation richness 1 3.69 0.055 0.43 0.512 1.80 0.180 0.31 0.577 3.61 0.057 2.69 0.101

Treat*season 3 9.06 0.029 1.05 0.790 2.53 0.469 7.58 0.056 5.18 0.159 11.63 0.009

Treat*yr 3 21.59 <0.001 3.63 0.305 1.01 0.798 6.65 0.084 11.60 0.009 4.25 0.240

Season*yr 1 0.64 0.423 0.04 0.841 4.32 0.038 29.13 0.000 22.38 <0.001 3.68 0.055

Treat*season*yr 3 3.09 0.378 0.75 0.757 0.21 0.975 1.14 0.767 3.86 0.277 2.07 0.558

Treatment (treat), season, year (yr) and width were included as fixed effects and plot as a random effect. For the likelihood ratio tests,

width was designated the null model and other parameters treat, season and yr and interactions (*) were systematically added to subse-

quent models. Significant (P < 0.05) results are in bold.



and ROT plots (Fig 2). Herbivore taxon richness differed fromother trophic groups with no evidence of consistent treatment,

season or year effects (Fig. 2). Vegetation height appeared to bean important positive covariate for total arthropod and predatorrichness only (Table 2). Vegetation richness had no significant

effects on the taxon richness of any of the trophic groups(Table 2).

Differences in trophic group community structure

The fenced field margins and grazed plots had two distinct

arthropod communities for all trophic groups (Table 3; Figs 3and 4). In all cases, grazed and fenced field margins wereseparated along the first axis. Separate seasonal ordinations

showed that this community distinction was consistent on all

sampling dates (Fig. 3). Details of trophic group seasonal effectsare given in Fig. S1.

The dung feeding, Megasternum concinnum (Marsham), andmycetophagous beetlesAtomaria atricapilla Stephens,Ptenidiumnitidum (Heer) and P. pusillum (Gyllenhal) were more closely

associated with grazed plots than other decomposers such as themycetophagous Acrotrichis atomaria (De Geer), & Sepedophilusnigripennis (Stephens) (Table S2).

The aphids Rhopalosiphum Koch sp. and MetopolophiumMordvilko sp. and leafhoppersMacrosteles sp. andChloropidaeflies, were more closely associated with grazed paddocks, whilst

other aphid species including Sipha glyceriae (Kaltenbach) andthe plant hoppers Muellarianella fairmarei (Perris) and Parali-burnia clypealis (J. Sahlberg) were usually found within thefenced treatments (Table S2).

Predatory species also showed different affinities with thegrazed paddock and fenced margins. The spiders Erigone atra(Blackwall) and Oedothorax fuscus (Blackwall) and the beetles,

Amischa analis (Gravenhorst),A. decipiens (Sharp) and Tachyp-orus pusillus (Gravenhorst) all had strong associations with thegrazed paddocks (Table S2). Whereas, the spider Lepthyphantes

ericaeus (Blackwall) and the predatory beetles Stenus fulvicornisStephens and S. ossium Stephens were connectedwith the fencedfieldmargins (Table S2).The parasitoids,ChasmodonHaliday, andPolynemaHaliday,

were strongly associated with the grazed paddock. In contrast,GonatocerusNees, InostemmaHaliday,MesopolobusWestwood

Table 3. Summary of results from Permutational Multivariate

Analysis of Variance (permanova).

d.f. F model R2 P-value

Total 3 4.91 0.32 0.001

Decomposers 3 3.13 0.23 0.001

Herbivores 3 9.54 0.47 0.001

Predators 3 14.67 0.58 0.001

Parasitoids 3 5.98 0.36 0.001

Hyperparasitoids 3 22.62 0.68 0.001

Fig. 3. NMDS ordination for all arthro-

pods collected from fenced field margins and

grazed plots over four sampling dates. The

final stress 16.52 was obtained from 20 runs.

Using a non-metric R2, it was determined

that the axes accounted for a combined 97%

of the variance.



andPanstenon oxylus (Walker) were parasitoids generally found

with the fenced fieldmargins (Table S2).

Discussion

Fieldmargins, rich in herbs and grasses, provide potential refuge

and food resources for flower-visiting, herbivorous non-pestinsects and predatory arthropods (Lagerlof & Wallin, 1993).Field margins generally have greater invertebrate abundance

and taxon richness than adjacent agriculturally productive fields(Denys & Tscharntke, 2002; Asteraki et al., 2004; Woodcocket al., 2005a,b; Cole et al., 2007; Moreby, 2007; Ramsay et al.,2007). This increased invertebrate diversity may in turn lead to

an associated increase in ecosystem services such as pollinationand pest control (Tscharntke et al., 2005; Haaland et al., 2011)and more food sources for birds, particularly chicks early in

sampling season (Holland et al., 2006).It is well known that agricultural arthropod populations show

seasonal variation (e.g. Purvis & Curry, 1981). In both years of

our study, the abundance and taxon richness of the trophicgroups were generally greater in August rather than June,probably due to arthropod populations increasing over thegrowing season (Purvis & Curry, 1981). Our results show that,

although there were seasonal fluctuations, recently establishedconservation field margins in an intensively managed grasslandincrease the abundance and richness of most arthropod trophic

groups. This benefit wasmost pronounced on the final sampling

occasion, when overall abundance and richness was at a maxi-

mum within the timescale of the study. This may reflect succes-sional responses (Purvis & Curry, 1980) to increased plantspecies richness and increased vegetation structural complexity

in the studied field margins (Sheridan et al., 2008; Fritch et al.,2011). Similar increases in invertebrate abundance and diversitywith increased vegetation structure have been shown in other

studies (e.g. Woodcock et al., 2009;Woodcock & Pywell, 2010).In our current study, vegetation richness had no effects on therichness of any trophic group investigated. However, Siemann

(1998) and Siemann et al. (1998) showed that increasing plantspecies richness led to increased total, herbivore, predator andparasite, but not detritivore arthropod richness. They suggestedthat the diversity of herbivoresmay also be influenced by, and in

turn influence, the diversity of parasites and predators.However,a single plant species can support a high diversity of herbivores(Siemann, 1998; Siemann et al., 1998). The age of field margins

has also been shown to impact on parasitism rates of the rapepollen beetle in oilseed rape crops (Thies & Tscharntke, 1999).Increased parasitism rates were observed in oilseed rape crops

surrounded by well establishedmargins (6 years old) when com-pared to those surrounded by newly established margins (1 yearold) (Thies & Tscharntke, 1999). Noordijk et al. (2010) foundthat diversity and abundance of herbivores and detritivores

increased, whilst predator abundance decreased, with increasingage of fieldmargins.Perhaps unexpectedly, with the exception of detritivores

abundances were greater in the grazed paddocks than fenced

(c) (d)

(a) (b)

Fig. 4. NMDS ordination for (a) detriti-

vores, (b) herbivores, (c) predators, and (d)

parasitoids collected from fenced margins

and grazed plots. The final stress for detri-

tivores, herbivores, predators and parasi-

toids was 22.34 (28 runs), 20.54 (5 runs),

21.63 (23 runs) and 22.12 (24 runs) respec-

tively. As determined by a non-metric R2,

the axes accounted for a combined 92% of

the variance for detritivores and 95% of

the variance for herbivores, predators and

parasitoids.



field margins in August 2004. This may reflect the relativelyrecent establishment of the field margins, a management effect

(e.g. fertiliser input) or weather conditions such as the relativelywarm dry conditions in the month prior to sampling in August2004 (Fig. S2). In each trophic group one or two taxa accounted

for a third or more of total trophic group abundance in August2004 (Table S3). For example, the aphidRhopalosiphum sp. andthe primarily plant mining Chloropidae accounted for more

than a third of the total herbivore abundance. The staphylinidbeetle, Amischa analis, represented almost half the predatoryabundance.Whilst the parasitoidsTrimorus (parasitoid of Cara-

bid beetle eggs (Masner, 1995)) and Polynema (parasitoid of cic-adellid eggs (Huber, 1997)) represented more than one-third ofparasitoid abundance. Only hyperparasitoids of aphids (Fergus-son, 1980, 1986; Boucek & Rasplus, 1991), were collected in

August 2004, perhaps reflecting the high abundance ofRhopalo-siphum sp. in the grazed paddock (Table S2). Koricheva et al.(2000) presented similar results with leafhopper, carabid beetle

and spider abundance negatively related to plant species rich-ness, but they observed increased aphid numbers with increasednumbers of functional groups. Such observations are consistent

with the resource concentration hypothesis where low plant spe-cies richnessmay support a higher abundance of specialist herbi-vores (Root, 1973).Only herbivores and detritivores responded differently to indi-

vidual fenced margin treatments. The treatments with greaterplant species richness (ROT and RS) (Sheridan et al., 2008;Fritch et al., 2011; Table S1) generally had higher herbivore and

detritivore abundances and richness than FO. This is likely dueto greater host plant diversity and structure providing more her-bivore food sources (Siemann, 1998; Siemann et al., 1998; Mor-

ris, 2000; Woodcock et al., 2009; Woodcock & Pywell, 2010).However, differences between individual fenced treatments weregenerally less obvious than differences between all fenced treat-

ments and the grazed field. This observation follows thoseof Purvis and Curry (1981), who found a more or less immedi-ate response in the abundance and diversity of foliage arthro-pods (especially detritivorous groups) to increased structural

complexity and likely modified microclimatic conditions ingrassland swards. It differs however, from results of Woodcockand Pywell (2010) who found a negative correlation between

plant height and richness of pitfall collected detritivores from thebotanically more diverse chalk grasslands. In other studies,reduced plant species richness, due to increased fertiliser inputs,

resulted in either increased taxon richness (Haddad et al., 2000)or no response in detritivore populations (Siemann, 1998).Keeping in mind that we sampled mobile adults, but based

our trophic level associations on the larvae that may not origi-

nate in the plots, two distinct communities associated with eitherthe fenced field margins or the grazed plots were observed.Withthe presence of grazing cattle and therefore increased levels of

dung it is unsurprising that detritivores like the coprophagousbeetle,M. concinnum, were associated with the grazed paddock.Maelfait et al. (1988) found that coprophagous beetle species

were absent from field edges and boundaries and tended tohibernate in the pasture near droppings. The most abundantaphid collected in our study, Rhopalosiphum sp., was also

strongly associated with the grazed paddocks, likely reflecting

the higher nutrient status of host grasses within the grazed sward(Dixon, 1987; Honek, 1991). Similarly, Moreby (2007) found

more aphid individuals in the centre of a rotational set-aside fieldthan the field edge. The parasitoids, Chasmodon and Polynemalikely reflect the presence of their potential hosts, leaf-mining

Chloropidae (Shaw & Huddleston, 1991) and the cicadellidMacrosteles (Huber, 1997) respectively in grazed paddocks. Themore open habitat associated with the grazed paddocksmay suit

the predatory beetle and spider species associated with this habi-tat. The spiders, O. fuscus and E. atra, build webs in openground (Locket & Millidge, 1953) and were found to be more

abundant in grazed pastures and field centres when compared tofield edges or boundaries (Maelfait et al., 1988; Kromp & Stein-berger, 1992).Perhaps unsurprisingly, the fenced conservation margins had

more herbivorous taxa associated with them reflecting theincreased diversity of host plants (Sheridan et al., 2008; Haalandet al., 2011) and structural complexity of available host plants

(Woodcock et al., 2009;Woodcock&Pywell, 2010). By the finalsampling date in August 2005 there was a more distinct speciesrich herbivore community in the ROT and RS plots (Fig. S1),

reflecting the higher species richness of grasses and forbs in thosefenced treatments (Sheridan et al., 2008; Fritch et al., 2011).Woodcock and Pywell (2010) also found a positive correlationbetween herbivore species richness and forb and grass species

richness. Our data show that this in turn supports amore diversepredator and parasitoid community with more taxa associatedwith the fenced conservation margins. Overall, preventing graz-

ing by fencing field margins, regardless of their width, provideda structurally more complex environment, giving more shelterand likely food sources for the arthropod community.

This study has shown that simply fencing field margins leadsto an almost immediate increase in diversity and abundance ofarthropod trophic groups in intensively managed graslands As

the botanical diversity of the sown margin treatments establish,further benefits to the diversity and functioning of the arthropodcommunity would be expected (Haaland et al., 2011). Thefenced field margins and grazed field of our study had two dis-

tinct communities of all trophic groups. Without fencing, manyof the taxa strongly associated with this margin habitat wouldotherwise not be found in such an intensively managed grazing

system. The increased richness of functionally important groupssuch as detritivores, predators and parasitoids highlights thefunctional as well as conservational importance of these field

margins. Little is currently known of the conservational value offield margins for parasitoids (Haaland et al., 2011) that have aparticular functional significance in all ecosystems and are oftenneglected in conservation strategies (Shaw & Hochberg, 2001).

Thus fencing field margins, provides a relatively simple methodfor conserving diversity of functionally important groups thateven the most intensive grassland farmers might readily under-

take atminimal cost to their production system.

Acknowledgements

This project was funded by the EPA through the ERTDI

(2000–2006) programme as part of the National Develop-



ment Plan. We thank Drs John Finn and Rogier Schulteand the Teagasc Environment Research Centre, Johnstown

Castle, for access to the experiment; Drs Gavin Broad,Andrew Polaszek and John Noyes (Natural HistoryMuseum, London), and Hannes Bauer (Natural History

Museum, Bern) for assistance with verifying and determiningparasitoid identifications; and Dr Jim O’Connor for accessto references and the collection at the National Museum of

Ireland (Natural History).

Supporting information

Additional Supporting Information may be found in the onlineversion of this article under the DOI reference: doi: 10.1111/

j.1752-4598.2012.00203.x:Figure S1. NMDS ordination for (a) detritivores; (b)

herbivores; (c) predators; and (d) parasitoids collected from

fencedmargins and grazed fields over four sampling occasions.Figure S2. (a)Daily rainfall for JohnstownCastle for the sam-

pling period (June–August) in 2004 and 2005. (b) Daily maxi-

mum temperatures for Johnstown Castle for the samplingperiod (June–August) in 2004 and 2005.Table S1. List of plant species associated with fenced

field margin treatments and grazed controls (Sheridan et al.,

2008).Table S2. Trophic groupings of arthropods, used in NMDS

analysis with mean abundances from grazed fields and fenced

margins.Table S3. The mean abundance of the most abundant taxa

collected from the grazed field inAugust 2004.

Please note: Neither the Editors nor Wiley-Blackwell areresponsible for the content or functionality of any supportingmaterials supplied by the authors. Any queries (other thanmiss-

ing material) should be directed to the corresponding author forthe article.

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Accepted 23 February 2012

Editor: Alan Stewart

Associate editor: Peter Dennis



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