Trophic Structure of the Spider Community of a Mediterranean

Home , Aphantaulax, Aphantaulax trifasciata, Ballus chalybeius, Icius hamatus, Nigma hortensis, Nigma walckenaeri

Basic and Applied Ecology 14 (2013) 413–422

Trophic structure of the spider community of a Mediterranean citrus grove: A stable isotope analysis Laia Mestrea,b,∗, Josep Pinol˜ a,b, José Antonio Barrientosb, Xavier Espadalera,b, Katrin Brewittc, Christiane Wernerd, Christian Platnerc aCREAF, Cerdanyola del Vallès 08193, Spain bDepartament de Biologia Animal, de Biologia Vegetal i d’Ecologia, Univ Autònoma Barcelona, Cerdanyola del Vallès 08193, Spain cExperimental and Systems Ecology, University Bielefeld, Universitätsstr. 25, 33615 Bielefeld, Germany dAgroEcosystem Research, University Bayreuth, Universitätsstr. 30, 95445 Bayreuth, Germany

Received 6 July 2012; accepted 4 May 2013 Available online 2 June 2013

Abstract

Spiders are dominant terrestrial predators that consume a large variety of prey and engage in intraguild predation. Although the feeding habits of certain species are well known, the trophic structure of spider assemblages still needs to be investigated. Stable isotope analysis enables characterisation of trophic relationships between organisms because it tracks the energy flow in food webs and indicates the average number of trophic transfers between a given species and the base of the web, thus being a useful tool to estimate the magnitude of intraguild predation in food webs. Using this technique, we studied the trophic groups of spiders and their links within the arthropod food web of a Mediterranean organic citrus grove. We assessed the trophic positions of the 25 most common spider species relative to other arthropod predators and potential prey in the four seasons of the year, both in the canopy and on the ground. The analyses showed great seasonal variation in the isotopic signatures of some arthropod species, as well as the existence of various trophic groups and a wide range of trophic levels among spiders, even in species belonging to the same family. Differences in ␦15N between spiders and the most abundant prey in the grove usually spanned two trophic levels or more. Our findings provide field evidence of widespread intraguild predation in the food web and caution against using spider families or guilds instead of individual species when studying spider trophic interactions.

Zusammenfassung

Spinnen sind dominante terrestrische Prädatoren, die eine große Vielfalt an Beute konsumieren und oft ‘intraguild predation’ zeigen. Obwohl die Nahrungsbiologie einiger Arten gut erforscht ist, bedarf die trophische Struktur von Spinnen-Gemeinschaften noch intensiver Untersuchung. Die Analyse natürlicher stabiler Isotope ist ein nützliches Verfahren um Nahrungsbeziehungen zwischen Organismen zu charakterisieren, denn sie kann Energieﬂüsse im Nahrungsnetz aufdecken, die Anzahl trophischer Transfers zwischen einer Art und der Basis des Nahrungsnetzes anzeigen und somit ein Maß zur Abschätzung des Ausmaßes der ‘intraguild predation’ liefern. Basierend auf dieser Technologie haben wir die trophischen Gruppen der Spinnen und ihre Verknüpfungen im Arthropoden-Nahrungsnetz auf einer biologisch bewirtschafteten mediterranen Zitrusplantage untersucht. Die trophischen Ebenen der 25 häuﬁgsten Spinnenarten aus den Baumkronen und vom Boden wurden zu vier Jahreszeiten analysiert, jeweils im Vergleich mit anderen Prädatoren und potentiellen Beutetiergruppen. Die Analysen zeigten eine große

∗Corresponding author at: Departament de Biologia Animal, de Biologia Vegetal i d’Ecologia, Univ Autònoma Barcelona, Cerdanyola del Vallès 08193, Spain. Tel.: +34 93 581 3744; fax: +34 93 581 1321. E-mail address: [email protected] (L. Mestre).

1439-1791/$ – see front matter © 2013 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.baae.2013.05.001 414 L. Mestre et al. / Basic and Applied Ecology 14 (2013) 413–422 saisonale Variation der Isotopensignatur einiger Arten sowie die Existenz verschiedener trophischer Gruppen mit einer großen Bandbreite an trophischen Ebenen, sogar zwischen Arten, die zur selben Familie gehören. Die Differenzen der ␦15N Werte zwischen Spinnen und den häuﬁgsten Beutetiergruppen der Obstwiese reichten gewöhnlich über zwei oder mehr trophische Ebenen. Unsere Ergebnisse liefern, unter natürlichen Bedingungen, Belege für eine weite Verbreitung von Intraguildpredation im Nahrungsnetz. Wir mahnen somit zur Vorsicht, Spinnen-Familien oder Gilden als Ersatz für einzelne Arten zu nutzen, um trophische Interaktionen zu untersuchen. © 2013 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved.

Keywords: Agroecosystem; Ants; Food web; Intraguild predation; Trophic niche; ␦15N; ␦13C

Introduction Spiders are dominant generalist predators that consume a great variety of prey, engage in intraguild predation, and Over recent years, the complex network of species inter- reduce herbivore numbers in terrestrial habitats (Hooks, actions in terrestrial arthropod communities has received Pandey, & Johnson 2006; Riechert & Lawrence 1997). But increasing attention. Manipulative experiments have shown despite the determinant role of spiders in food webs, detailed the impact of intraguild interactions on the strength of trophic studies on the trophic ecology of spider communities are cascades (Cardinale, Harvey, Gross, & Ives 2003; Finke & largely lacking. Indeed, identifying arthropods to species is Denno 2005) and have pointed out the relevance of trait- extremely demanding, so almost all studies about trophic mediated effects to the regulation of herbivore populations relationships in arthropods group species into families on the (Schmidt-Entling & Siegenthaler 2009; Werner & Peacor basis that taxonomical relatedness should translate into diet 2003). Ecologists have also begun to disentangle the trophic similarity (but see Sanders & Platner 2007). However, this links that structure arthropod food webs, but data obtained approach is likely to prove inaccurate precisely because of with direct observations or captive feeding studies are limited our lack of knowledge about the biology and feeding habits in scope (Greenstone 1999; Tiunov 2007). of spiders in nature (Cardoso, Pekár, Jocqué, & Coddington The analysis of stable nitrogen and carbon isotope ratios 2011; Greenstone 1999). (expressed as ␦15N and ␦13C, respectively) is a powerful tool We performed a stable isotope analysis to describe the for characterising trophic interactions between organisms trophic structure of the spider community of a Mediterranean (Boecklen, Yarnes, Cook, & James 2011; Vander Zanden organic citrus grove. Because in this system the spider assem- & Rasmussen 2001) and it has been used to study trophic blages of the canopies and the ground are totally different relationships in arthropod food webs both in natural (Collier, (Mestre, Pinol,˜ Barrientos, Cama, & Espadaler 2012) we col- Bury, & Gibbs 2002; Kupfer, Langel, Scheu, Himstedt, & lected arthropods from both layers. The strong seasonality Maraun 2006) and in agricultural settings (McNabb, Halaj, intrinsic to this climate should influence the arthropod com- & Wise 2001; Wise, Moldenhauer, & Halaj 2006). Stable munity and the structure of the whole food web, so we (1) isotopes undergo a fractionation process by which ␦15N compared the trophic signatures of important spider species and ␦13C are enriched in the heavy isotope with increas- and other arthropods at different times of the year. Above ing trophic levels (trophic enrichment). The carbon isotope all, we wanted to (2) assess the trophic positions of the 25 fractionation with each trophic transfer is very small and most common spider species relative to other predators and largely retrieves the isotopic signal of the food source, so potential prey in the grove. Moreover, we aimed to (3) assess it is used to track the energy flow in food webs. By contrast, the extent of intraguild predation in the spider community at the enrichment of ␦15N is considerable and is useful for esti- different seasons. mating the trophic levels of individuals (Gannes, Obrien, & del Rio 1997; Post 2002). Therefore, the ␦15N of a species indicates the average number of trophic transfers between the species and the base of the food web, and the compar- Materials and methods ison of the ␦15N of predators with those of their potential prey gives an integrated measure of intraguild predation in Study site the community (Cabana & Rasmussen 1994; Ponsard & Arditi 2000). This is an especially interesting application The grove is located at La Selva del Camp (Catalonia, NE of stable isotopes because experimental studies show that Spain; 41◦ 13 07 N, 1◦ 835 E), an area with a Mediter- intraguild predation interactions are extremely frequent in ranean climate. There are ca. 300 Clementine trees (Citrus arthropods (Arim & Marquet 2004; Hunter 2009), yet the clementina var. clemenules) grafted on the hybrid rootstock actual prevalence of intraguild predation within arthropod Carrizo citrange (Poncirus trifoliata (L.) Raf. × Citrus sinen- food webs is only beginning to be quantified in the field sis (L.) Osb.); these are watered during dry periods. Grasses (Gagnon, Heimpel, & Brodeur 2011; Sheppard & Harwood and other weeds form a permanent ground cover, which is 2005). mowed a few times every year. L. Mestre et al. / Basic and Applied Ecology 14 (2013) 413–422 415

Sampling methodology and converted to CO2,N2 and H2O. After removing H2O with a water trap, the combustion products were flushed in From February to November 2009 arthropods were col- a helium stream through the dilutor box into the Isotope lected once a month from the canopies of three rows of 23 Ratio Mass Spectrometer (ISOPRIME, Elementar, Hanau, trees each, using beating trays. Ground arthropods were sam- Germany), which was coupled via a variable open-split inter- pled three times (February, June, September) with pairs of face to the elemental analyser. Samples were standardised pitfall traps (7 cm depth and 5.5 cm diameter) on opposite to IAEA-N2 and IAEA-CH-6 (International Atomic Energy sides of all trees. The traps were opened, filled with water Agency, Vienna, Austria) and an IVA protein standard with mixed with detergent and left open for 48 h. Some ground spi- two distinct isotopic signatures (33802155). Repeated mea- der species were collected with corrugated cardboard bands surement precision was 0.15‰ and 0.2‰ for ␦13C and ␦15N, wrapped around the base of the tree trunks. The bands were respectively. Carbon and nitrogen isotope ratios are reported left in place for a few days in September to let spiders use relative to VPDBee and atmospheric air, respectively, and them as shelters. All the arthropods were preserved in 70% expressed as ␦-notation in [‰]: ethanol right after collection. R In the laboratory, arthropods were identified to species, δAX = sample − × R 1 1000 and the most abundant ones were selected for stable iso- s tan dard tope analysis: spiders (15 species from the canopies and 10 where Rsample and Rstandard are the ratios of the heavier iso- from the ground; see Appendix A: Table s.1), ants from the tope to the lighter isotope from the sample and the standard, ground (four species: the herbivorous Messor structor and respectively, and X is the element (C or N) considered. the predaceous Formica rufibarbis, Lasius grandis, and Phei- dole pallidula), and from the canopies (Formica rufibarbis, Lasius grandis), other predators from the canopies (the ear- Statistical analyses wig Forficulapubescens, and the heteropterans Cardiastethus fasciiventris and Pilophorus perplexus), herbivores from the We compared the bivariate (␦15N and ␦13C) isotopic sig- canopies (the aphid Aphis spiraecola, the coccid Ceroplas- nature of spider species with permutational multivariate tes sinensis, and the psocopterans Ectopsocus briggsi and ANOVAs (PERMANOVAs), using the Euclidean distance. Trichopsocus clarus) and detritivores from the ground (the Sample sizes were at least N = 5, usually N = 6 or higher. isopod Armadillidium vulgare and one sample of ∼100 We first run a one-way analysis to test for differences individuals of unidentified epedaphic collembolans from sev- between months for the above-mentioned species of spiders, eral pitfall traps; see Appendix A: Table s.2 for a detailed ants, and other arthropods (Table 1). As some species were account). collected in more than two months, we ran a posteriori pair- When possible, individuals of a given species were taken wise comparisons to test which months had different isotope from different months to test for temporal changes in isotope ratios, if the main analysis was significant. ratios (Table 1). Of the spiders, only Philodromus cespi- Our main aim was to determine the trophic structure tum (Philodromidae) was abundant during several months, of the spider community. We considered four sea- whereas the other species were abundant only on one or two sons: winter (February–April), spring (May–June), summer sampling dates. Due to phenology, individuals of the spiders (August–September), and autumn (October–November). We Clubiona leucaspis (Clubionidae), Icius hamatus (Saltici- tested for differences between the isotope ratios of all spider dae), Philodromus rufus (Philodromidae), and Platnickina species from each season. If these analyses were significant, tincta (Theridiidae) from different months belonged to differ- we conducted a posteriori pairwise comparisons as described ent life stages (see Appendix A: Table s.1). All the individuals above to look for differences between species and classify from the other arthropod species belonged to either imma- them into trophic groups. We defined a trophic group as a ture or adult stages, apart from the primary consumers Aphis group of species sharing a common isotopic signature (␦15N spiraecola and Ceroplastes sinensis. and ␦13C), i.e. no statistically significant differences in their isotopic signatures. We did not conduct statistical analyses of the isotope ratios of the other arthropods because we used Stable isotope analyses them solely as a framework for assessing the relative trophic positions of spiders within the food web (see Appendix A: We loaded about 0.2 mg of dry sample material into tin Table s.3 for the monthly abundances of all species). capsules. For most species, we had to combine individuals We used the ␦15N values of primary consumers as a refer- into one sample, but for large spiders (Dysdera crocata, Olios ence for the trophic positions of spiders and other predators, argelasius), the earwig Forficula pubescens and the isopod because isotopic signatures of primary consumers are less Armadillidium vulgare we only used the anterior part of the variable than those of primary producers (Vander Zanden & body. Rasmussen 2001). To interpret the isotope ratios of arthro- Samples were combusted under O2 excess in an elemen- pods we relied on the study by McCutchan, Lewis, Kendall, tal analyzer (EuroVector EA, Hekatech, Wegberg, Germany) & McGrath (2003), which is the only one that gives average 416 L. Mestre et al. / Basic and Applied Ecology 14 (2013) 413–422

Table 1. ANOVA table of the effect of month on isotope ratios.

Species df Pseudo-F P (perm)

Spiders Clubiona leucaspis 1.14 18.51 0.0002* Dipoena melanogaster 1.13 0.64 0.55 Icius hamatus 1.14 6.37 0.0036* Philodromus cespitum 6.44 5.00 0.0002* Philodromus rufus 1.9 0.17 0.65 Platnickina tincta 1.9 1.44 0.29 Theridion spp. 1.13 0.19 0.80 Ants Formica ruﬁbarbis 2.16 0.51 0.66 Lasius grandis 2.15 0.95 0.42 Messor structor 1.10 1.74 0.22 Pheidole pallidula 1.10 1.61 0.25 Other arthropods Ectopsocus briggsi (psocopteran) 4.17 5.15 0.0029* Trichopsocus clarus (psocopteran) 1.6 0.07 0.88 Cardiastethus fasciiventris (heteropteran) 2.14 16.40 0.0001* Pilophorus perplexus (heteropteran) 1.9 2.12 0.14 Forﬁcula pubescens (earwig) 1.10 2.28 0.15 Armadillidium vulgare (isopod) 1.10 1.61 0.24

* P < 0.01. trophic enrichments for predators of invertebrates: 1.4‰ for P = 0.0001; Fig. 2B). Canopy spiders had very similar iso- ␦15N and 0.5‰ for ␦13C. topic signatures, except for Icius hamatus (Salticidae), which We adjusted the level of signiﬁcance of the a posteriori was isotopically close to ground spiders and was as much as tests by setting a conservative threshold of P < 0.01. Analyses 5.7‰ ␦15N above the primary consumers in the canopies were run with the software PERMANOVA+ for PRIMER v.6 (Fig. 3B). Differences in ␦15N were high in ground spiders, (Anderson, Gorley, & Clarke 2008). Results are shown as the even when they belonged to overlapping trophic groups. For mean ± SE. instance, Pelecopsis spiders (Linyphiidae) had the highest ␦15N values of all predators analysed and were separated by at least 4‰ ␦15N from Pardosa species (Lycosidae), and 5.5‰ Results ␦15N from primary consumers (collembolans; Fig. 3C). There were signiﬁcant differences in the isotope ratios Seasonal differences of spiders in summer (pseudo-F13,109 = 18.69, P = 0.0001; Fig. 2C) despite a large overlap between the trophic groups Isotope ratios changed over time for four predators (the of canopy spiders. Aphantaulax trifasciata (Gnaphosidae), spiders Clubiona leucaspis, Icius hamatus and Philodromus Philodromus cespitum (Philodromidae), and Platnickina cespitum; the heteropteran Cardiastethus fasciiventris) and tincta (Theridiidae) tended to have lower ␦15N values than one primary consumer (the psocopteran Ectopsocus briggsi; the other canopy spiders. Nevertheless, spiders and other Fig. 1 and Table 1). Hence, it was required to compare the predatory arthropods were at least 7.2‰ enriched in ␦15N isotope ratios of spiders for each of the four seasons sepa- compared to psocopterans (Fig. 3D), which were by far rately (see Appendix A: Table s.4 for pairwise comparisons the most abundant primary consumers in that season (see between months). Appendix A: Table s.3). In general, canopy and ground spiders belonged to different trophic groups which dif- ␦13 Trophic groups of spiders fered in their C values except for the theridiid Dipoena melanogaster, whose ratios were indistinguishable from Mar- In winter, all spiders (Clubiona leucaspis, Philodromus those of ground spiders. All ground spider species but ilynia bicolor ␦15 cespitum and Philodromus rufus) belonged to a single (Dictynidae), with very low N values, had similar isotopic signatures (Fig. 2C). In autumn, there were trophic group (pseudo-F2,32 = 0.81, P = 0.88; Fig. 2A), and the difference in ␦15N values between primary consumers three trophic groups of spiders in the canopies, the main ␦15 (psocopterans) and spiders was as high as 6.1‰ (Fig. 3A). In difference between them lying in their N values (pseudo- P spring, the analyses showed distinct trophic groups of spiders F6,48 = 6.17, = 0.0001; Fig. 2D; see Appendix A: Tables 15 s.5-s.7 for pairwise comparisons in every season). The spiders differing considerably in ␦ N values (pseudo-F7,42 = 18.69, L. Mestre et al. / Basic and Applied Ecology 14 (2013) 413–422 417

(A) 10 (B) 12 8 11 8 9 10 10 6 2 9 )

ºº 11 4 8 4 N (º/

15 3 7 6

6 10 5

4 2 P. cespitum 8 E. briggsi 3 C. fasciiventris

-25 -24 -23 -25 -24 -23

13 C (º/ºº)

Fig. 1. Monthly variation of isotope ratios of the spider Philodromus cespitum (A) and the psocopteran Ectopsocus briggsi and the heteropteran Cardiastethus fasciiventris (B) found in the tree canopies. Means ± SE are shown. Numbers indicate months. with the lowest ␦15N values (Neoscona subfusca, Philodro- same family. Spiders and other predators were at least two mus cespitum) were 2.8‰ ␦15N above psocopterans, but their trophic levels above primary consumers in all seasons, pro- ␦13C values were at least 2.0‰ higher than psocopterans viding comprehensive field evidence that intraguild predation (Fig. 3F). is widespread in this food web. There was no common trend in the temporal shifts in isotopic signatures of spiders and other arthropod preda- Isotope ratios of other arthropods in the tors, which were unique to each species. Thus, the seasonal community differences in the isotope ratios of predators were probably driven by fluctuations in prey availability that caused changes In winter, the primary consumers (psocopterans) in the in their diet (Park & Lee 2006), although in the spiders Clu- canopies differed in their isotope ratios (Fig. 3A), but in biona leucaspis and Icius hamatus life stage could also play spring all the primary consumers in the canopies (the aphid a role (Oelbermann & Scheu 2002). Isotopic signatures of Aphis spiraecola, the coccid Ceroplastes sinensis and the pso- Ectopsocus briggsi changed notably between months, but this copteran Ectopsocus briggsi) had similar isotopic contents species always attained the lowest trophic level in the com- (Fig. 3B). The predaceous ants Formica rufibarbis, Lasius munity, consistent with the diet of vegetal material (algae, grandis and Pheidole pallidula showed a high trophic level fungus, etc. scraped from surfaces) of plant-dwelling pso- which remained constant between seasons, whereas the copterans (Resh and Cardé, 2009). The trophic positions of granivorous ant Messor structor was the arthropod with the ants did not undergo strong seasonal changes and largely lowest trophic position in the ground food web (Fig. 3A–E). reflected their feeding habits, as already described in this In spring and in summer, the earwig Forficula pubescens citrus grove by Platner, Pinol,˜ Sanders, & Espadaler (2012). and the heteropteran Pilophorus perplexus had similar ␦15N In winter, there were only three spider species in the values to Formica and Lasius ants but lower ␦13C values. canopies common enough to be analysed, and they belonged to a single trophic group. In spring, there were three abundant spiders in the canopies: the thomisid Xysticus sp. and Discussion the salticid Ballus chalybeius had ␦15N values just above those of the primary consumers, but the salticid Icius hamatus The structure of the arthropod food web varied seasonally was placed higher in the food web and had a similar iso- as indicated by the shifts in isotope ratios of predators and topic signature to that of canopy-foraging ants, which seem primary consumers. Our analyses assigned spider species to to be its preferred prey (Pekár, Coddington, & Blackledge different trophic groups which separated spiders from the 2012). Concerning ground spiders, the polyphagous Dysdera 418 L. Mestre et al. / Basic and Applied Ecology 14 (2013) 413–422

(A) Winter Pe_bucb (B) Spring

Pe_parb c 15 Dy_cro

13 Ic_hamb

Pa_horb

11 Pa_prob

10 Cl_leua a Xyst 9 Ph_rufa Canopy

) Ground a Ba_cha ºº Ph_ces N (º/ 15 (C) Summer (D) Autumn

No_exob 14 c a c Di_mel e Nu_albb 13 Ch_mil Di_melb Cl_leue Zo_sty Ol_arg 12 Ox_linbe There g Eu_epi Ther b Ic_hame 11 Ap_trie Ni_wal a d Ph_ces Pl_tin 10 f Pl_tin Ph_ces Ma_bicg 9 Canopy Ne_sub Ground

-25 -24 -23 -22 -21 -25 -24 -23 -22 -21

13 C (º/ºº)

Fig. 2. Comparison of the isotopic signatures of the 25 spider species sampled in the grove at four different seasons (A–D). Different superscript letters indicate statistically signiﬁcant differences in isotopic signatures between species from each season, delimiting trophic groups. Means ± SE are shown. For clarity, most groups are encircled. Codes: Ap tri (Aphantaulax trifasciata), Ba ch (Ballus chalybeius), Cl leu (Clubiona leucaspis), Ch mil (Cheiracanthium mildei), Dy cro (Dysdera crocata), Di mel (Dipoena melanogaster), Eu epi (Euryopis episinoides), Ic ham (Icius hamatus), Ma bic (Marilynia bicolor), Nu alb (Nurscia albomaculata), No exo (Nomisia exornata), Ne sub (Neoscona subfusca), Ni wal (Nigma walckenaeri), Ol arg (Olios argelasius), Ox lin (Oxyopes lineatus), Pe buc (Pelecopsis bucephala), Pe ces (Philodromus cespitum), Pa hor (Pardosa hortensis), Pe par (Pelecopsis parallela), Pa pro (Pardosa proxima), Ph ruf (Philodromus rufus), Pl tin (Platnickina tincta), Ther (Theridion spp.), Xyst (Xysticus sp.), Zo sty (Zodarion styliferum). L. Mestre et al. / Basic and Applied Ecology 14 (2013) 413–422 419

(A) Winter (canopy) (B) Spring (canopy) Pe_buc (C) Spring (ground) 16 Pe_par 15 Dy_cro

13 Ic_ham Ph_pal La_gra Pa_hor Fo_ruf 12 Pi_per La_gra La_gra Pa_pro Fo_ruf 11 Fo_ruf Fo_pub Coll 10 Cl_leu Ca_fas Ar_vul 9 Xyst Ph_ruf Me_str Ba_cha 8 Ph_ces Ap_spi 7 Ec_bri 4 Ce_sin Ec_bri 6 Tr_cla 5 Spiders Ants 4 Other predators Ec_bri 2 Herbivores 3 Detritivores ) ºº

(D) Summer (canopy) (E) Summer (ground) (F) Autumn (canopy) N (º/ 16 15 15 Pi_per No_exo Nu_alb 14 Ch_mil Ic_ham Di_mel La_gra 13 Ther Ca_fas Cl_leu Di_mel Zo_sty Ph_pal Ol_arg 12 Ox_lin Fo_ruf Fo_ruf Ther 11 Fo_pub Ca_fas Eu_epi Ap_tri Ni_wal 10 Ph_ces Ph_ces Ma_bic Pl_tin Pl_tin 9 Ne_sub 8 Ar_vul Me_str 7 Tr_cla 6 Ec_bri 5

4 Ec_bri 3 32 -26 -25 -24 -23 -22 -21 -26 -25 -24 -23 -22 -21 -26 -25 -24 -23 -22 -21

13 C (º/ºº)

Fig. 3. Isotope ratios of arthropods found in the canopies and on the ground at four different seasons (A–F). Means ± SE are shown. Codes: Ap spi (Aphis spiraecola), Ar vul (Armadillidium vulgare), Ca fas (Cardiastethus fasciiventris), Coll (Collembola), Ce sin (Ceroplastes sinensis), Ec bri (Ectopsocus briggsi; numbers after codes indicate month), Fo pub (Forficula pubescens), Fo ruf (Formica rufibarbis), La gra (Lasius grandis), Me str (Messor structor), Ph pal (Pheidole pallidula), Pi per (Pilophorus perplexus), Tr cla (Trichopsocus clarus). The isotopic signatures of the spider species in Fig. 2 are shown here again to place the spiders in the context of the food web. crocata, which is able to overcome the defensive cuti- linyphiids (Pelecopsis parallela and Pelecopsis bucephala). cle of isopods (Pollard, Jackson, Vanolphen, & Robertson All analysed individuals of the four species were adult 1995), was three trophic levels above Armadillidium vulgare. males. Collembolans are the usual prey of linyphiids and Lycosids (Pardosa proxima and Pardosa hortensis) were iso- Pardosa (Agustí, Shayler, Harwood, Vaughan, Sunderland topically indistinguishable from each other, and so were et al. 2003; Hoekman, Bartrons, & Gratton 2012), as 420 L. Mestre et al. / Basic and Applied Ecology 14 (2013) 413–422 reflected by their isotopic signature in the grove, but Pele- Theridion spp.) also had different isotopic signatures. As copsis had much higher ␦15N values. This was unexpected expected, the ground spider assemblage was isotopically sep- because previous studies report almost identical isotopic arated from the canopy spider assemblage; exceptions to this signatures of linyphiids and Pardosa (McNabb et al. 2001; general result were canopy spiders (Dipoena melanogaster, Wise et al. 2006). Moreover, lycosids are known to prey on Icius hamatus) feeding on ants, a ground-based source. linyphiids, which are much smaller in size (Schmidt-Entling Indeed, these two species become more abundant in the & Siegenthaler 2009), so we could also expect lycosids being trees where ants are present (Mestre, Pinol,˜ Barrientos, & in a higher trophic position than linyphiids. Instead, our Espadaler 2013). Taken together, our results prove that iden- results suggest that linyphiids may be preying on species we tification at the family level cannot be used as a reliable had not considered for the analyses, for instance, fungivorous indicator of trophic position of spiders. Genus-level iden- dipterans, which can have very high ␦15N values (Sanders & tifications could be less misleading, because our analyses Platner 2007). placed the Pardosa, Pelecopsis and Philodromus species in Summer was the season with the greatest diversity of spi- the same trophic group. A similar result was found in a soil ders and the highest difference in ␦15N values between the food web (Scheu & Falca 2000), thus species-level identifi- predators and the primary consumers. This finding suggests cation appears to be indispensable for the study of trophic that intraguild predation was more widespread than in other relationships in arthropod communities. times of the year and that it is a pervasive feature of the spi- The ␦15N values of the whole spider community spanned der community rather than being restricted to certain species. 10.1‰. Even within a given season, there was a wide range of Icius hamatus had lower ␦13C values than ants, which shows trophic levels in spiders: in spring, the ␦15N values between that in summer this spider shifts to prey connected to the species with the highest level (Pelecopsis bucephala) and canopy food web. By contrast, the ant-specialist theridiid those with lowest level (Ballus chalybeius) spanned more Dipoena melanogaster (Umeda, Shinkay, & Miyashita 1996) than 8‰, and about 4‰ in summer (Nomisia exornata vs. was enriched in ␦13C compared to other canopy spiders Marilynia bicolor). These findings reflect the existence of a and had isotope ratios similar to the ant Formica rufibar- high trophic diversity in spiders, probably due to the great bis. The theridiid Euryopis episinoides and the zodariid variety of lifestyles and foraging strategies within this group. Zodarion styliferum, both ant-specialist ground-living spi- In addition, species with known feeding habits (linyphiids, ders (Pekár, Smerda,ˇ Hrusková,ˇ Sedo,ˇ Muster, et al. 2012; Dysdera crocata, Euryopis episinoides, Zodarion styliferum) Porter & Eastmond 1982), exhibited the same isotopic signa- were more than two trophic levels above their usual prey and ture as Dipoena melanogaster and were about three trophic the spider community was 2–5 trophic levels above primary levels above the ant Messor structor. Other ground spiders consumers depending on the season, based on the fraction- also showed similar ␦13C values and higher ␦15N values with ation factors by McCutchan et al. (2003). These patterns respect to ants, so they were possibly preying on them or suggest that intraguild predation plays a key role in structur- on their predators. The food web in the ground comprised ing the arthropod community. Studies of terrestrial arthropod detrital and grazing pathways, represented by the detritivore food webs in soil (Halaj, Peck, & Niwa 2005; Ponsard & Armadillidium vulgare and the herbivore Messor structor, Arditi 2000; Scheu & Falca 2000) and grassland communi- respectively. In summer, ␦13C values of these arthropods ties (Sanders & Platner 2007) have also found predators to be and the spiders were slightly higher than in spring. A possi- more than one trophic level above their potential prey, though ble explanation is that drought-induced stress in herbaceous the range of ␦15N values of spiders they report is much lower, plants induced an increase in their ␦13C values (Farquhar, maybe because they focus almost exclusively on the ground Cernusak, & Barnes 2007) which was carried upwards in layer. the food web. By contrast, the citrus trees were watered dur- Stable isotope data must be interpreted with caution ing summer and thus ␦13C values of canopy arthropods did because fractionation factors—on which estimates of trophic not change between seasons. In autumn, spiders of the low- positions rely—are unknown for most species. Since fraction- est trophic group appeared to be disconnected from the flux ation depends on both consumer species and the organisms of energy coming from psocopterans, and thus carbon could they eat (Vander Zanden & Rasmussen 2001; Vanderklift & have been derived from a source we had not considered. Ponsard 2003), some field studies dealing with few species Acknowledging that seasonality could be the cause of some include laboratory tests to determine fractionation factors. of the reported patterns, the analyses identified several trophic In spiders, however, values have been calculated for only groups in the spider community. This classification based afewPardosa species feeding on collembolans and aphids on isotopic signatures was basically independent of family (Oelbermann & Scheu 2002; Wise et al. 2006). Because classifications because most of the time species from the this procedure becomes impractical when dealing with many same family belonged to different trophic groups. Among species, researchers rely on reviews of published studies that salticids, Icius hamatus reached a higher trophic level than calculate average fractionation values for different groups of Ballus chalybeius, as happened with the gnaphosids Nomisia organisms (McCutchan et al. 2003; Post 2002; Vanderklift exornata and Aphantaulax trifasciata. Theridiids (Dipoena & Ponsard 2003). However, it is increasingly recognised melanogaster, Euryopis episinoides, Platnickina tincta, and that fractionation varies greatly depending on many factors L. Mestre et al. / Basic and Applied Ecology 14 (2013) 413–422 421 such as feeding rate, nutritional status, and trophic position Arim, M., & Marquet, P. A. (2004). Intraguild predation: A (Boecklen et al. 2011; Tiunov 2007). Another limitation is widespread interaction related to species biology. Ecology Let- that stable isotopes can only discriminate between sources ters, 7, 557–564. with different isotopic signatures, which can lead to incor- Boecklen, W. J., Yarnes, C. T., Cook, B. A., & James, A. C. (2011). rect assignations of a source to a consumer. Furthermore, the On the use of stable isotopes in trophic ecology. Annual Review delays between consumption and the incorporation of the iso- of Ecology, Evolution, and Systematics, 42, 411–440. topic signature of the diet are poorly understood (McCutchan Cabana, G., & Rasmussen, J. B. (1994). 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