the spatio-temporal distribution of weed seed predation differs between conservation agriculture and...

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Agriculture, Ecosystems and Environment 188 (2014) 40–47 Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment j ourna l h omepage: www.elsevier.com/locate/agee The spatio-temporal distribution of weed seed predation differs between conservation agriculture and conventional tillage Aude Trichard a,b , Benoit Ricci a , Chantal Ducourtieux a,c , Sandrine Petit a,a INRA, UMR 1347 Agroécologie, BP 86510, F-21000 Dijon, France b AgroParisTech, F-75005 Paris, France c AgroSup Dijon, F-21000 Dijon, France a r t i c l e i n f o Article history: Received 13 October 2013 Received in revised form 27 January 2014 Accepted 30 January 2014 Available online 12 March 2014 Keywords: Trophic guild No-tillage Cover crop Agroecology Biological regulation MAPCOMP Viola arvensis Capsella bursa-pastoris a b s t r a c t One potentially important ecosystem service in agricultural fields is the regulation of weeds by carabid beetles, but the effect of agricultural management on the level of regulation has so far been poorly documented. In this study, we monitored weed seed predation rates of Viola arvensis and Capsella bursa- pastoris and carabids from March to September using a grid sampling in two adjacent winter-wheat fields, one in conventional tillage (T) and the other converted to direct drilling with cover-crop for five years (DD). At the field level, weed seed predation was positively correlated to the activity of granivores in the tilled field and was marginally higher in DD than in T during wheat growth. After harvest, granivores and predation rates declined sharply in the cover crop of the DD system whereas they increased in the bare tilled field. This result suggests that the dense cover crop set up after harvest was not suitable for the local pool of autumn-breeding granivores. Spatial aggregations of carabid and predation variables were detected using the MAPCOMP software; these spatial patterns differed between the two management systems and were temporally unstable but consistent over large intervals of spatial resolutions. There were significant spatial associations between trophic guilds and predation rates, i.e. predation rates were positively or negatively associated either to granivores or to omnivores, depending on the time and on the management system. These results highlight the spatial and temporal heterogeneity in the level of interactions taking place during a crop cycle, a complexity which explains the important variability in the delivery of ecosystem services such as weed seed predation. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Identifying management practices enhancing the provision of ecosystem services in farmland has become a critical issue in agri- culture (Firbank et al., 2012). One potentially important ecosystem service in agricultural fields is the regulation of weeds by seed pre- dation (Ichihara et al., 2011; Westerman et al., 2003). Seed-eating carabids in particular have been shown to consume substantial numbers of seeds in the field (Honek et al., 2007; Westerman et al., 2003) and their abundance has been related to seed predation level (Menalled et al., 2007; O’Rourke et al., 2006) and to the rate of depletion of the weed seed bank (Bohan et al., 2011). Conservation agriculture, a system that strongly reduces soil tillage operations, has been shown to positively affect carabid Corresponding author. Tel.: +33 3 80 69 30 32; fax: +33 3 80 69 32 62. E-mail addresses: [email protected] (A. Trichard), [email protected] (B. Ricci), [email protected] (C. Ducourtieux), [email protected] (S. Petit). richness and abundance (House and Stinner, 1983) notably as tillage-induced mortality can be important for some carabid species (Shearin et al., 2007). Reduction in soil tillage also leads to an accu- mulation of weed seeds near the surface (Cardina et al., 2002; Yenish et al., 1992), thus providing available resources for potential seed predators. In addition to reduced tillage operations, conserva- tion agriculture often promotes the set-up of cover crops during the intercropping period (Trichard et al., 2013a) and this vegeta- tion cover may provide carabids with favourable habitat conditions (Shearin et al., 2008; Ward et al., 2011) and protection from hyper- predators (Harrison and Gallandt, 2012). Vegetation cover has also been shown to increase weed seed predation by invertebrates (Meiss et al., 2010). Although the effects of conservation agricul- ture on carabids are well documented, its impact on weed seed predation levels appears less consistent in the literature (Gallandt et al., 2005; Sanguankeo and Leon, 2011), possibly because weed seed predation levels often strongly vary over time (Saska et al., 2008; Westerman et al., 2003). Indeed, weed seed predation results from the activity of various omnivorous and granivorous species which may not have similar activity periods over the season. The http://dx.doi.org/10.1016/j.agee.2014.01.031 0167-8809/© 2014 Elsevier B.V. All rights reserved.

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Page 1: The spatio-temporal distribution of weed seed predation differs between conservation agriculture and conventional tillage

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Agriculture, Ecosystems and Environment 188 (2014) 40–47

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment

j ourna l h omepage: www.elsev ier .com/ locate /agee

he spatio-temporal distribution of weed seed predation differsetween conservation agriculture and conventional tillage

ude Tricharda,b, Benoit Riccia, Chantal Ducourtieuxa,c, Sandrine Petit a,∗

INRA, UMR 1347 Agroécologie, BP 86510, F-21000 Dijon, FranceAgroParisTech, F-75005 Paris, FranceAgroSup Dijon, F-21000 Dijon, France

r t i c l e i n f o

rticle history:eceived 13 October 2013eceived in revised form 27 January 2014ccepted 30 January 2014vailable online 12 March 2014

eywords:rophic guildo-tillageover cropgroecologyiological regulationAPCOMP

a b s t r a c t

One potentially important ecosystem service in agricultural fields is the regulation of weeds by carabidbeetles, but the effect of agricultural management on the level of regulation has so far been poorlydocumented. In this study, we monitored weed seed predation rates of Viola arvensis and Capsella bursa-pastoris and carabids from March to September using a grid sampling in two adjacent winter-wheat fields,one in conventional tillage (T) and the other converted to direct drilling with cover-crop for five years(DD). At the field level, weed seed predation was positively correlated to the activity of granivores in thetilled field and was marginally higher in DD than in T during wheat growth. After harvest, granivores andpredation rates declined sharply in the cover crop of the DD system whereas they increased in the baretilled field. This result suggests that the dense cover crop set up after harvest was not suitable for thelocal pool of autumn-breeding granivores. Spatial aggregations of carabid and predation variables weredetected using the MAPCOMP software; these spatial patterns differed between the two managementsystems and were temporally unstable but consistent over large intervals of spatial resolutions. There

iola arvensisapsella bursa-pastoris

were significant spatial associations between trophic guilds and predation rates, i.e. predation rates werepositively or negatively associated either to granivores or to omnivores, depending on the time and onthe management system. These results highlight the spatial and temporal heterogeneity in the level ofinteractions taking place during a crop cycle, a complexity which explains the important variability inthe delivery of ecosystem services such as weed seed predation.

. Introduction

Identifying management practices enhancing the provision ofcosystem services in farmland has become a critical issue in agri-ulture (Firbank et al., 2012). One potentially important ecosystemervice in agricultural fields is the regulation of weeds by seed pre-ation (Ichihara et al., 2011; Westerman et al., 2003). Seed-eatingarabids in particular have been shown to consume substantialumbers of seeds in the field (Honek et al., 2007; Westerman et al.,003) and their abundance has been related to seed predation levelMenalled et al., 2007; O’Rourke et al., 2006) and to the rate of

epletion of the weed seed bank (Bohan et al., 2011).

Conservation agriculture, a system that strongly reduces soilillage operations, has been shown to positively affect carabid

∗ Corresponding author. Tel.: +33 3 80 69 30 32; fax: +33 3 80 69 32 62.E-mail addresses: [email protected] (A. Trichard),

[email protected] (B. Ricci), [email protected] (C. Ducourtieux),[email protected] (S. Petit).

ttp://dx.doi.org/10.1016/j.agee.2014.01.031167-8809/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

richness and abundance (House and Stinner, 1983) notably astillage-induced mortality can be important for some carabid species(Shearin et al., 2007). Reduction in soil tillage also leads to an accu-mulation of weed seeds near the surface (Cardina et al., 2002;Yenish et al., 1992), thus providing available resources for potentialseed predators. In addition to reduced tillage operations, conserva-tion agriculture often promotes the set-up of cover crops duringthe intercropping period (Trichard et al., 2013a) and this vegeta-tion cover may provide carabids with favourable habitat conditions(Shearin et al., 2008; Ward et al., 2011) and protection from hyper-predators (Harrison and Gallandt, 2012). Vegetation cover has alsobeen shown to increase weed seed predation by invertebrates(Meiss et al., 2010). Although the effects of conservation agricul-ture on carabids are well documented, its impact on weed seedpredation levels appears less consistent in the literature (Gallandtet al., 2005; Sanguankeo and Leon, 2011), possibly because weed

seed predation levels often strongly vary over time (Saska et al.,2008; Westerman et al., 2003). Indeed, weed seed predation resultsfrom the activity of various omnivorous and granivorous specieswhich may not have similar activity periods over the season. The
Page 2: The spatio-temporal distribution of weed seed predation differs between conservation agriculture and conventional tillage

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A. Trichard et al. / Agriculture, Ecosy

ffect of richness and abundance in each trophic group on predationevels, as well the role of potential positive or negative interac-ions between and among guilds, are not fully understood (Gainesnd Gratton, 2010; Trichard et al., 2013b). One way to deepen ournderstanding of the processes underlying weed seed predationay spur from a thorough spatial analysis of the movement and

istribution of seed predators and predation through time in fieldonditions. Different spatial analysis methods are available to linkcological processes to spatial patterns such as the spatial analysisy distance indices (SADIE, Perry, 1998; Perry and Dixon, 2002)nd map comparison method (MAPCOMP, Lavigne et al., 2010).hese methods enable to identify significant aggregation patternsf species spatial measures and significant associations betweenwo spatial distributions. For example, they were used to analysepatial pattern and association between arthropods (Thomas et al.,001), to evidence spatial association between predators and preysWinder et al., 2001) or to analyse the temporal evolution of thepatial pattern of insect pests within fields (Ricci et al., 2011).

In this paper, we studied weed seed predation and seed-eatingarabids, measured over a seven months study, in two adjacentropping systems: direct drilling with cover crop (DD) and TillageT). We first conducted a global analysis of the two adjacent systemssing linear models to test the hypothesis that seed predation andarabids differ between the DD and the T systems and that theseifferences vary over time. Second, we performed spatial analysesf the distribution of weed seed predation and seed eating cara-ids over time within each system. We hypothesized that (i) thepatial distributions of predation rates and carabids are clustered;ii) the distributions of carabids and predation rates vary over timeut are spatially associated; (iii) there are associations/segregationsetween carabid trophic guilds that vary over time and (iv) aggre-ation patterns and spatial associations differed between the tworopping systems.

. Material and methods

This study was performed in 2011 in two adjacent calcisol wheatelds that were managed by two different farmers, 50 km Northest of the city of Dijon in North-eastern France (47◦36′12′′N,

◦35′32′′E). The first 5.72 ha field was conducted with traditionalillage operations (T: Tillage) and the second 18.78 ha field hadeen conducted in direct drilling with cover-crop for five years (DD:irect drilling with cover crop).This technique associates no tillagend no superficial soil management with a cover crop (mix species)uring the intercrop period. These differences in soil manage-ent over the years resulted into two clearly different soil profiles

Curmi, pers. com.). The T field had a strong contrast between itsrst two horizons, with a first level from 0 to 15 cm of very finetructures due to tillage, then from 15 to 20 cm angular and compactlusters with large cracks, and below 20 cm, the C horizon exhibitedlays decarbonation. Conversely, the DD field had many stones onhe surface, better water infiltration and good distribution of roots,nd from 0 to 20 cm rough elements of various sizes, less angular,ner in surface, and stronger macroporosity.

The protocol was set up in two contiguous zones of 100 × 100 m,ne within each field, which received no insecticide or fungicide.he margin strip adjacent to the fields consisted of a grassy path anderbaceous borders. The DD and T systems shared the same land-cape of semi-natural habitats and, consequently, a common poolf carabid beetles. Measures were performed during eight consecu-ive sessions, five during the wheat growing season (March 21, April

8, May 16, June 15 and July 11) and three after harvest (August 30,eptember 16 and September 23). After harvest, the T field wasilled and stubble ploughed while the DD field was sown with aegetation mixture of six species (flax, sunflower, vetch, phacelia,

and Environment 188 (2014) 40–47 41

faba bean and runnage pea) resulting in a cover crop that was suc-cessively 0.10 m high on August 30, 1.20 m on September 16 and1.50 m high on September 23.

2.1. Estimation of carabid activity and weed seed predation rates

Within each one of the two zones, 33 measure plots were pos-itioned five or ten m apart (Fig. 1). Each plot was 1-m side squarewith two pitfall traps and two vertebrate exclosure cages placedeach one at a given corner. Pitfall traps were made of plastic con-tainers of 10 cm depth and 8 cm diameter and filled with 150 ml of amixture of anti-freeze and salted water and protected from the rainby plastic roofs above the traps on 10 cm legs. Those plastic con-tainers were placed inside plastic tube settled in the soil to allowthe replacement of containers without disturbing the surroundingsoil. Carabids collected in the two traps of each plot were pooledand identified at species level and assigned to a trophic guild i.e.carnivores, omnivores or granivores (Brooks et al., 2012). Activity-density and richness were computed for each plot per trophic guild.Seed predation by invertebrates was quantified by exposing seedcards protected by vertebrate exclosure cages (18 × 11 × 9, 1 cmwire mesh) using a standard protocol developed by Westermanet al. (2003). Two bundles of 50 seeds of two species commonlyoccurring within the study area (Viola arvensis (Murr.) and Capsellabursa-pastoris (L. Medicus) were glued onto 6 per 14 cm cardsof brown sand paper (grain size 10). The two weed species dif-fered in size: V. arvensis, 0.9–1.0 × 1.4–1.9 mm and C. bursa-pastoris,0.3–0.5 × 0.6–1.0 mm, covering the potential range of seed prefer-ences of carabids of different body sizes (Honek et al., 2007). Seedpredation rate per plot was calculated as the average percentageof seeds lost on the two cards. At each one of the eight sessions ofmeasure, seed cards and pitfall traps were simultaneously placedand then left in place seven days.

2.2. Spatial and statistical analysis

Linear mixed effects models (LMMs) were applied to analysepredation rates of each weed species in relation to session, soiltillage management (T or DD) and the different carabid counts(activity-density and species richness of granivores, omnivoresand carnivores). When a qualitative factor had a significant effecton predation rates, multiple comparisons tests were performedto identify the different groups of factor levels (Pinheiro andBates, 2000). The plot number was included as random factor dueto experimentation i.e. temporal and spatial pseudo-replications.Models were fitted by restricted maximum likelihood (REML) andthe suitability of assumptions was assessed by checking for normal-ity and randomness of residuals. Mixed models were performedwith the lme package and multiple comparisons with the mult-comp package (Hothorn et al., 2008) under the R 2.14.1 software(R Development Core Team, 2010). A comparison of predationrates and carabid counts between the T and DD systems for eachsession was performed using Mann–Witney tests, with XLStat Pro2012.1.02 (Copyright Addinsoft 1995–2012).

The MAPCOMP method, a density map comparison approachdeveloped by Lavigne et al. (2010) was used to explore the spatialdistribution of predation rates and carabid counts. In this method,the interpolation of a spatial distribution includes a bandwidthparameter h which can be used to explore heterogeneity at variousspatial scales. The range of h values is constrained by the distancesbetween sampling points and by the size of the sampled area.

First, we used the MAPCOMP method to assess the intra-field

spatial distribution of each variable Xs

i(V. arvensis predation rate,

C. bursa-pastoris predation rates, granivores activity-density, grani-vores richness, omnivores activity-density, omnivores richness,carnivores activity-density and carnivores richness) at each session

Page 3: The spatio-temporal distribution of weed seed predation differs between conservation agriculture and conventional tillage

42 A. Trichard et al. / Agriculture, Ecosystems and Environment 188 (2014) 40–47

rimen

smtTtwmThHfhe

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Fig. 1. Expe

within each system. MAPCOMP produces a kernel-interpolatedap of Xs

iand compares this interpolated map with the density of

he traps or the cards using the Hellinger distance between maps.he hypothesis H0: “Xs

iis uniform” vs. H1 = “The spatial distribu-

ion of Xsi

is not homogeneous and presents clustering patterns”as then tested using a non-parametric test based on 10,000 per-utations of the values of the variable Xs

ion the sampling points.

he analyses were performed for bandwidth values ranging from = 15 m to h = 40 m with incremental 1 m steps (see Appendix A).eterogeneity was deemed significant when significantly detected

or at least two successive h values, so that the two instances whereeterogeneity was only detected for a single h value were consid-red as uniformly distributed.

Second, within each system, for the variables that were identi-ed as heterogeneous, MAPCOMP was used to assess the temporaltability in the distribution of individual variables from one mea-

ure session to the next (V1 = Xs

iand V2 = Xs+1

i) and to evaluate

patial associations in the distribution of two distinct variablesV1 = Xs

iand V2 = Xs+1

j), here between the distribution of carabid

ounts and predation rates as well as between the distribution of

tal design.

the three carabid trophic guilds. The two interpolated maps of V1and V2 were compared using the Hellinger distance and the hypoth-esis H0: “The spatial intensities of V1 and V2 are independent” vs.H1 = “V1 and V2 are spatially correlated” was tested using a non-parametric test based on 10,000 permutations of the values of V1 onthe sampling points. Spatial associations were deemed significantwhen detected for at least two successive h values.

3. Results

Seed predation rate per session and plot ranged from 0%to 100% and averaged 34.87% for V. arvensis and 52.52% for C.bursa-pastoris. A total of 3919 beetles were caught of which25.1% were omnivores, 43.2% carnivores and 31.7% granivores.They belonged to 55 species, of which six were omnivores (Mainspecies: Calathus fuscipes Goeze; Pterostichus melanarius Illiger; P.

cupreus Linnaeus; P. madidus Fabricius), 25 were carnivores (Mainspecies: Carabus convexus Fabricius; C. coriaceus Linnaeus; Brach-inus crepitans Linnaeus; Ocys harpaloides Audinet-Serville) and 24were granivorous (Main species: Pseudoophonus rufipes De Geer;
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A. Trichard et al. / Agriculture, Ecosystems and Environment 188 (2014) 40–47 43

Table 1Results of mixed models fitted by REML for predation rates.

Factor/variable Levels Viola arvensis Capsella bursa-pastoris

Value ±SE p/group Value ±SE p/group

Management Tillage 16.77 ±3.68 A 42.03 ±3.40 nsDirect Drilling 24.02 ±5.73 B 40.57 ±5.30 ns

Session March 21 11.11 ±7.83 A 33.28 ±7.24 AApril 18 11.27 ±7.33 A 12.12 ±6.77 AMay 16 50.67 ±7.36 B 57.05 ±6.81 DJune 15 67.74 ±7.39 C 71.56 ±6.83 CJuly 11 8.96 ±7.58 A 44.28 ±7.01 AAugust 30 50.60 ±7.27 B 84.89 ±6.72 BSeptember 16 16.77 ±3.68 A 42.03 ±3.40 ASeptember 23 19.68 ±7.14 A 42.17 ±6.60 A

Granivores AD 0.99 ±0.44 * 0.70 ±0.41 nsR 1.44 ±1.39 ns 1.21 ±1.29 ns

Omnivores AD 0.23 ±0.45 ns 0.39 ±0.42 nsR −2.14 ±1.46 ns 0.25 ±1.35 ns

Carnivores AD 0.05 ±0.31 ns −0.10 ±0.29 nsR −0.63 ±1.10 ns 1.04 ±1.02 ns

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ixed terms are soil management, measure session and carabid counts (AD activitean predation rates and associated standard errors. For qualitative factors, letters

actor For quantitative factors, * p < 0.05.

arpalus dimidiatus P. Rossi; Amara consularis Duftschmid; H. affi-is Schrank). All species were caught in both fields except forome only observed in the T field (Nebria brevicollis Fabricius;mara apricaria Paykull; Zabrus tenebrioides Goeze; Amara familiarisuftschmid and Harpalus tardus Panzer) and Amara nitida (Sturm)nly observed in the DD field. The complete list of species is pro-ided in Appendix A.

.1. Weed seed predation and carabid counts

Session had a significant effect on predation rates with higherevels in June, then May and August for the two weed species. Soil

anagement had a significant effect on the predation rate of V.rvensis with higher rates in the DD system (Table 1). The activity-ensity of granivores was the sole carabid count that was relatedo predation, for V. arvensis only (Table 1). The predation rate ofhe two species were generally significantly higher in DD than in

before harvest but became lower in DD than in T after harvestFig. 2a and b and Fig. 3). Granivorous carabids exhibited a tempo-al pattern quite similar to the predation patterns and similar inoth systems before harvest. Conversely, the activity-density andichness of granivorous strongly differed between the two systemsfter harvest. Granivorous counts were very low in the DD sys-em whereas they increased and reached a peak in the T systemFig. 2c and d, Fig. 3). The activity-density and richness of omni-ores were constantly higher in DD than in T except for the lastession (Fig. 2e and f, Fig. 3). The activity-density and richness ofarnivores declined with harvest, then raised in September at ini-ial level, and differed in T and DD but without clear preferences forne system or the other, through more variability was observed inhe T system.

.2. Spatial patterns and associations within each system

Heterogeneous distributions of predation rates and carabidounts were detected in DD and T. Each variable was aggregated ateast once in the experiment but patterns did not appear system-tically in both systems during the same session (Fig. 3). Spatialtructures were detected at several bandwidth values with rangeshat varied for a given variable (Appendix B).

In most cases, there was no spatial association in the distribu-ion of individual variables from one measure session to the nextFig. 3) but spatial patterns that were detected were not neces-arily identical in the T and DD systems. The spatial distribution

sity, R richness) and measure plot is as random term (df = 449). Value ± SE are theate significant differences at p < 0.05 in the effect of different modalities of a single

of predation rates differed between consecutive sessions betweenJuly and August corresponding to harvest time as well as betweenMarch and April, April and May, corresponding to a high increasein mean field values. The spatial distribution of granivores was notaffected by harvest in DD but statistical changes were observed inthe distribution of omnivores in the T system (Fig. 3). Clear shifts inthe spatial distribution of granivores and omnivores were detectedin the DD system between March and April.

There were five instances where a clustered distribution of apredation rate coincided with a clustered distribution of a carabidtrophic guild (Table 2). Three of those occurred in the DD sys-tem, of which two were spatially associated. The distributions ofC. bursa-pastoris predation rate and activity-density of omnivoreswere significantly similar in DD in March whereas in September, thedistribution of predation rates was significantly dissimilar to thedistribution of granivores. Two instances occurred in the T systembut no spatial association could be detected.

There were eight instances where two trophic guilds exhibiteda clustered distribution during the same measure session, four ineach system (Table 2). The distributions of granivores and omni-vores were similar in DD and T, respectively, in April and May. Inthe T system, the distribution of granivores and carnivores were sig-nificantly dissimilar in May whereas the distribution of omnivoresand carnivores were similar in September.

4. Discussion

4.1. Effect of soil management

Our results suggest that weed seed predation as well as activity-density of granivores in the DD system were only marginally higherduring the wheat growing season and significantly lower after har-vest, in comparison to the adjacent conventional T field. Althoughthe comparison of pitfall data collected in two habitats where themobility of carabids might have been differentially affected by habi-tat structure requires caution (Thomas and Marshall, 1999), ourresult is not in line with the markedly higher activity-density ofseed-eating carabids observed by pitfall trapping and weed seedpredation rates observed in no-tilled soybean during late summer(Menalled et al., 2007). Yet, other studies using pitfall trapping sug-

gest a variability in the response of granivores and predation ratesaccording for example to the type of cover crop used in conser-vation agriculture and highlight the fact that higher densities ofgranivores does not necessarily translate into higher weed seed
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44 A. Trichard et al. / Agriculture, Ecosystems and Environment 188 (2014) 40–47

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ig. 2. Predation rates of V. arvensis and C. bursa-pastoris and activity density anditney) between DD (direct drilling, solid line) and T (Tillage, dotted line) are prese

< 0.0001).

redation rates (Gallandt et al., 2005). In our case, during the wheatrowing season, the dominant granivorous species H. dimidiatusnd P. rufipes occurred in the two systems, although catches of P.ufipes were slightly higher in the DD system. At the same time, theichness and activity-density of omnivores, mostly P. melanariusnd P. cupreus, were consistently higher in the DD system com-ared to the T system. After harvest a predation peak was observed

n the two systems and subsequently weed seed predation becameower in the DD system than in the conventional T field. Simul-aneously, the activity-density of granivores decreased sharply inhe DD and increased markedly in the T system. This shift in the

ess of granivores, omnivores and carnivores over time. Comparisons tests (Mannfor each sampling session (ns = not significant; * p < 0.05; ** p < 0.01; *** p < 0.001; ****

distribution of granivores appears to result from several processes;the spring-breeder H. dimidiatus, which dominated in both systemsbefore harvest, became naturally much less active in the two sys-tems in late summer; the autumn-breeding granivorous species A.consularis and A. apricaria appeared in large numbers but solelyin the T system, probably because the dense cover crop that wasin place the DD system did not suit these typical ‘open habitat’

species and finally, P. rufipes almost entirely disappeared from theDD field but became active in the T system, suggesting that indi-viduals actively moved from the DD to the adjacent T system afterharvest. Simultaneously, the activity-density of omnivores, mostly
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A. Trichard et al. / Agriculture, Ecosystems and Environment 188 (2014) 40–47 45

Fig. 3. Density maps of predation rate of V. arvensis and C. bursa-pastoris and activity-density of granivores and omnivores represented for the bandwidth value h = 30 m inthe Tilled system (T on the left) and the direct drilling system with cover crop (DD on the right). An asterix positioned after the letter representing the management system(T or DD) indicates the p value of the spatial clustering (* p < 0.05; ** p < 0.01; *** p < 0.001). Barred parallel lines with a d letter denote a significant dissimilarity and parallellines with a s letter indicate a significant similarity in the distribution of the variable from one session to the next.

Table 2Spatial associations between couples of variables that present clustered distribution in the same system and at the same session.

Variable A Variable B Association I hc

Direct drillingMarch 21 Predation V. arvensis Omnivores AD ns

C. bursa-pastoris Omnivores AD S * 15:16 2R Ns

April 18 Granivores AD Omnivores AD S * 15:26 12July 11 Granivores AD Carnivores AD ns

R Carnivores R nsAugust 30 Omnivores AD Carnivores AD nsSeptember16 Predation V. arvensis Granivores AD D ** 20:40 20

R D * 33:40 8TillageMay 16 Granivores AD Carnivores AD ns

R Carnivores R nsAugust 30 Predation V. arvensis Granivores AD nsSeptember 16 Predation V. arvensis Omnivores AD sSeptember 23 Granivores R Omnivores R S *** 18:40 23

Omnivores AD Carnivores AD S ** 15:27 13

D: distributions are significantly dissimilar and S: distributions are significantly similar (* p < 0.05; ** p < 0.01; *** p < 0.001); ns: no significant spatial association between thetwo variables. I and hc are, respectively, the interval of the bandwidth parameter h and the number of steps over which the association is significant.

Page 7: The spatio-temporal distribution of weed seed predation differs between conservation agriculture and conventional tillage

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6 A. Trichard et al. / Agriculture, Ecosy

. melanarius, increased in both systems. Some caution is of courseecessary in the interpretation of pitfall trapping data as bias coulde induced by differences in the mobility and/or catchability of par-icular species (Baars, 1979; Thomas et al., 1998) and differencesetween the two systems, e.g. the occurrence of crop residues in theD system that could have impeded individual movements or dif-

erences in prey availability levels which can affect the propensityf individuals to move (Firle et al., 1998). Spatial analyses pro-ided us with additional information on processes at play. First,ur results indicate discrepancies in the functional response of thewo trophic guilds in the two systems. The spatial distribution ofmnivores in the T system was significantly different before andfter harvest, i.e. omnivores occupied different portions of the fieldefore and after harvest. Conversely, and despite a drastic drop inheir activity-density, the spatial distribution of granivores in theD system was similar before and after harvest, i.e. they occupied

pace in the same way before and after harvest. Second, the spatialistribution of carabids was clustered at different times in the Tystem and in the DD system. We identified patterns inside fieldshat were not synchronized in both systems, which would suggestifferent dispersal dynamics of predators. Finally, we expected tobserve clustered distributions of carabid trophic guilds that wouldeflect potential spatial segregation that may have been causedy direct predation (Currie et al., 1996) or behavioural avoidancePrasad and Snyder, 2004). Our results indicate that there wasidespread space sharing between trophic guilds in both systems

ver the whole duration of the experiment, hence that intra-guildredation is unlikely to be a major factor shaping the distribution ofarabids in the fields. Carabid distribution is more likely to be driveny species-specific ecological requirements, e.g. vicinity to a fieldoundary or field cores (Holland et al., 2007) or by animal preyvailability, e.g. aphids (Bryan and Wratten, 1984; Winder et al.,005) or the spatial distribution of competitors (Hawes et al., 2013).

.2. Linking carabids to weed seed predation levels

The notion that increasing seed-eating predators abundanceay lead to enhanced weed suppression is widespread yet, while in

ome instances seed-eating carabid abundance has been positivelyelated to seed predation level (Menalled et al., 2007; O’Rourket al., 2006) or depletion of the weed seed bank (Bohan et al.,011), other studies have failed to evidence such links (Davis andaghu, 2010; Gaines and Gratton, 2010; Mauchline et al., 2005;aska et al., 2008). Here, a general statistical analysis highlightshe prime importance of granivores in the delivery of weed seedredation in the T system, as established in other studies (Brookst al., 2012; Trichard et al., 2013b) but a lack of significant effect ofmnivores on seed predation. However, the spatial analysis of thene-scale distribution of carabids and predation rates highlightednly a limited number of spatial associations between predationevels and one or both seed-eaters trophic guilds and suggest thathese links vary according to the session and the management sys-em. We were able to identify significant similarities as well asissimilarities in the distribution of trophic group of seed preda-ors and predation, and hence to spot which guild was consumingr not consuming the seeds at different times during the course ofhe experiment. Interestingly, we failed to detect significant associ-tions during predation peaks, possibly because carabids were busyonsuming weed seeds and as such were not very mobile and thusnlikely to be trapped. It is also possible that in the changing ratioensities of seeds exposed on predation cards and seeds naturallyresent, for example after crop harvest. It is possible that the chance

f carabids to locate the predation cards decreased when naturallyccurring seeds were in excess. There was a great deal of temporalhanges in the distribution of variables in this study, i.e. patternsere often not maintained from one sampling date to the next,

and Environment 188 (2014) 40–47

which would suggest that the sampling interval used here (twoweeks in general) was not sufficient to resolve changes in spatialpatterns through time. As a result, the associations between trophicguilds and predation rates were not stable, a within-field patternalready evidenced in other studies (Pearce and Zalucki, 2006). Thespatial resolution at which aggregations and associations occurredcould be fully explored here as the MAPCOMP method enables totest the whole range of scales at which significant spatial patternscould emerge. Our results indeed indicate that aggregation could bedetected at various scales, probably reflecting the variability in themobility and behaviour among species composing the seed-eatingcarabid community in each system and at each session.

Acknowledgements

We thank Cyrille Auguste for his assistance in the field and PierreCurmi who examined the soil profiles. We are particularly grate-ful to Benoît Lavier and Jean Claude Philisot who lent us part oftheir fields for this experiment. Finally, Claire Lavigne provided uswith very useful insights for the use of the MAPCOMP method. Thiswork was partly funded by the ANR project ADVHERB (ANR-STRA-08-02) and the ANR project PEERLESS (ANR-12-AGRO-0006). AudeTrichard benefited from a PhD studentship funded by the FrenchMinistry of Agriculture.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.agee.2014.01.031.

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