Sheetweb Spider Research Articles

Ants (Hymenoptera: Formicidae) and Spiders (Araneae) Co-occurring on the Ground of Vineyards from Douro Demarcated Region

This study, held in vineyards from Douro Demarcated Region, aimed to: a) identify the communities and main functional groups of spiders and ants; b) check patterns of co-occurrence between the two communities; and c) evaluate the impact of ground cover and adjacent non-crop habitats in the proximity of vineyards, on the two communities. Samplings were done using pitfall trapping. Twenty species of ants and 44 species of spiders were identified, which included respectively three and nine Iberian endemic species. Aphaenogaster gibbosa (Latreille 1798), Aphaenogaster iberica Emery 1908, Cataglyphis hispanica (Emery 1906), Cataglyphis iberica (Emery 1906), Messor barbarus (L. 1767) and Tapinoma nigerrimum (Nylander 1856) totalized 71.21% of ants. Alopecosa albofasciata (Brullé 1832), Callilepis concolor Simon 1914, Eratigena feminea Simon 1870, Zodarion alacre (Simon 1870) and Zodarion styliferum (Simon 1870) accounted for 38% of spiders. Abundance of both ant-mimicking and ant-eating spiders were positively correlated with Formicinae, while only ant-eating spiders showed positive correlation with Myrmicinae ants. All genera/ species of ant-associated spider were associated with one or more genera/ specie of ants. The abundance of specialist spiders was higher in areas where abundance of ants was also higher. Sheet web weavers spiders were found to be positively correlated with the percentage of ground cover. The present study a) stresses that vineyard agroecosystem support a rich assemblage of ants and spiders evincing that wine production and species conservation is possible and b) the co-occurrence between some species of this two groups is not determined by random patterns.

Abiotic factors and biotic interactions jointly drive spider assemblages in nest-boxes in mixed forests

Although spiders are common inhabitants of tree cavities, factors that drive their community structure in these microhabitats are little known. Here we investigated whether bark type, season, intraguild predation (IGP) among spiders, and presence of vertebrate predators can influence the spider community structure in tree cavities. We examined spider abundance and the taxonomic and functional composition of spiders in nest-boxes within two mixed forest stands in central Slovakia in 2012–2013. In total, 1211 spiders belonging to 31 species were sampled from 60 nest-boxes at two sites over three seasons. Spider abundance peaked in autumn as spiders sought wintering sites. Guilds and taxonomic composition changed seasonally with spring and autumn communities dominated by “Other hunters” (Anyphaenidae, Clubionidae, Philodromidae) while during summer the community was dominated by “Sheet web weavers” (Linyphiidae). The guild and taxonomic turnover may be partly explained by the interaction between spiders' phenology and IGP exerted by winter-active spiders on smaller spiders from autumn until spring. Bark type influenced the guild composition as dominance of “Space web weavers” was higher in trees with rough bark than in trees with smooth bark. The rough bark also reduced the intensity of IGP by Anyphaena accentuata (Sundevall, 1833) on philodromids. The presence of insectivorous birds reduced the abundance of spiders by 67%. The presence of bird predators altered the guild composition as they affected mostly the web spiders. The results show that the biotic interactions and abiotic factors interactively determined the spider community structure in the nest-boxes depending on spiders' functional traits.

Araneofauna (Arachnida: Araneae) de suelo en bosques nativos, exóticos y pajonales del Parque General San Martín, Entre Ríos Argentina

Ground aracnofauna (Araneae: Araneae) in native and exotic Forest and grasslands of San Martin Park, Entre Rios, Argentina. The Exotic plant species invasion in the Argentine Espinal region causes a changein the appearance of the landscape, altering the environment and producing a significant loss of biodiversity. We studied the composition of ground spiders in Parque General San Martin, Entre Rios, during the summerand winter seasons of 2012. Three areas were selected: pajonal (P), native forest (BN) and exotic forest (BE), pitfalls were used for sampling. A total of 1398 spiders (26 families and 120 species/morphospecies) werecollected. The most abundant families in the different areas were Lycosidae (n=574; 41.06%), Tetragnathiidae (n=223; 15.95%) and Linyphiidae (n=137; 9.80%). The guild category “ground hunters” (n=632) “sheet webweavers” (n=262) and “orb webs weavers” (n=244) were highly represented in all sampling areas. Diversity indices showed that BN was more diverse than P and BE. Complementarity values obtained between P andBE and BN were higher than 71%, while BE and BN showed values of 63%. This study constitutes the first record of the spider´s community of Parque General San Martin.

Structural and optical studies on selected web spinning spider silks

This study investigates the structural and optical properties in the cribellate silk of the sheet web spider Stegodyphus sarasinorum Karsch (Eresidae) and the combined dragline, viscid silk of the orb-web spiders Argiope pulchella Thorell (Araneidae) and Nephila pilipes Fabricius (Nephilidae). X-ray diffraction (XRD), Fourier transform infra-red (FTIR), Ultraviolet-visible (UV–Vis) and fluorescence spectroscopic techniques were used to study these three spider silk species. X-ray diffraction data are consistent with the amorphous polymer network which is arising from the interaction of larger side chain amino acid contributions due to the poly-glycine rich sequences known to be present in the proteins of cribellate silk. The same amorphous polymer networks have been determined from the combined dragline and viscid silk of orb-web spiders. From FTIR spectra the results demonstrate that, cribellate silk of Stegodyphus sarasinorum, combined dragline viscid silk of Argiope pulchella and Nephila pilipes spider silks are showing protein peaks in the amide I, II and III regions. Further they proved that the functional groups present in the protein moieties are attributed to α-helical and side chain amino acid contributions. The optical properties of the obtained spider silks such as extinction coefficients, refractive index, real and imaginary dielectric constants and optical conductance were studied extensively from UV–Vis analysis. The important fluorescent amino acid tyrosine is present in the protein folding was investigated by using fluorescence spectroscopy. This research would explore the protein moieties present in the spider silks which were found to be associated with α-helix and side chain amino acid contributions than with β-sheet secondary structure and also the optical relationship between the three different spider silks are investigated. Successful spectroscopic knowledge of the internal protein structure and optical properties of the spider silks could permit industrial production of silk-based fibres with unique properties under benign conditions.

Effects of local and landscape factors on spiders and olive fruit flies

Conservation biological control uses naturally occurring enemies to limit pest populations and to increase agricultural sustainability. The olive is an important perennial crop in Mediterranean countries and there is a high demand for alternative solutions to pesticide applications. As spiders are the most abundant predators in olive trees, they could be effective against the olive fruit fly, Bactrocera oleae (Rossi) (Diptera: Tephritidae), the main pest of this crop. In order to optimise the effectiveness of spiders, it is essential to understand their relationship with environmental factors at both the local and landscape scales. We studied spiders and their potential prey in the canopies of 18 olive orchards in different landscapes of the Monte Pisano (central Italy). The abundance and species richness of spiders as well as the abundance of sheet web spiders were lower in conventional orchards than in organic orchards. The composition of spider communities was affected by the amount of Mediterranean garigue in the surrounding landscape, and the abundance of flies increased with increasing percentage of wood in the landscape. Olive fruit fly densities were negatively correlated with cursorial and sheet web spiders⿿ abundance, suggesting that spiders may be involved in pest suppression. As the response of spiders to local and landscape factors was family and guild specific, tailored management for biological control requires further clarification of the individual and interactive effects of the different spider families and guilds on B. oleae.

Effect of phenology on agonistic competitive interactions between invasive and native sheet-web spiders

The phenologies of introduced relative to native species can greatly influence the degree and symmetry of competition between them. The European spider Linyphia triangularis (Clerck, 1757) (Linyphiidae) reaches very high densities in coastal Maine (USA). Previous studies suggest that L. triangularis negatively affects native linyphiid species, with competition for webs as one mechanism. We documented phenological differences between L. triangularis and three native species that illustrate the potential for the reversal of size-based competitive advantage over the course of the year. To test whether relative size influences interaction outcome, we allowed a resident spider to build a web and then introduced an intruder. We examined whether the outcomes of agonistic interactions over the webs were influenced by the species of the resident (invasive or native), the relative size of the contestants, and the species × size interaction. We found that the importance of relative size differed among species. In interactions between L. triangularis and each of two native species, size played a greater role than resident species on the outcome of interactions, suggesting that competitive advantage reverses over the season based on phenology-related size differences. Linyphia triangularis had a negative impact on the third species regardless of relative size.

Ground Spider Guilds and Functional Diversity in Native Pine Woodlands and Eucalyptus Plantations.

Vegetation structure and floristics have a strong influence on the relative abundance of spider guilds and functional diversity of terrestrial arthropods. Human activities have transformed much of the temperate woodlands. The aim of this study was to test five predictions related to the guild distribution and functional diversity of the ground spider communities of Eucalyptus plantations and native pine woodlands in western Mexico. Spiders were collected every fortnight from September to November from 15 pitfalls positioned in each of the eight sites. We also assessed the cover of grasses, herbs, shrubs, and leaf litter in each site. We found that the abundances of ground hunters and sheet weavers between plantations and pine woodlands were different. Nevertheless, there was not a consistent difference between sites of each of the vegetation types. Most species of ground hunters, sheet web weavers, and many other hunters were associated with litter and the grass cover. Nonetheless, in some cases, species of different families belonging to the same guild responded to different variables. Wolf spiders were related to the grass Aristida stricta Micheaux, 1803, while the species of the other families of ground hunters were associated with leaf litter. One Eucalyptus plantation and one pine woodland had the highest functional diversity of all sites. These sites have a well developed litter and grass cover. Our study suggests that the abundance of litter and a high cover of grasses explain the occurrence of species with different traits, and these habitat components results in a high functional diversity.

Organic versus conventional systems in viticulture: Comparative effects on spiders and carabids in vineyards and adjacent forests

Farming systems and management regimes of vineyards may affect local biodiversity of plants and invertebrates. While most studies have focused on the overall biodiversity of vineyards, there has been little consideration of the response of different ecological guilds to vineyard management, or to how vineyard management affects communities of adjacent semi-natural habitats.We study here two functional guilds of carabids and five of spiders in Langa Astigiana (NW-Italy) with the following aims: (i) to assess the comparative effects of organic and conventional farming systems, along with associated habitat and landscape variables, on species richness and abundance in vineyards; and (ii) to compare the same within forest patches surrounding organic and conventional vineyards.The different guilds exhibited distinct preferences for habitat characteristics (i.e. grass cover), landscape context and farming systems. Generalized Linear Mixed Models showed that spider preferences mostly depended upon habitat variables, while carabid preferences depended on small-scale landscape variables. In general, organic farming increased biodiversity and abundance of arthropod predators, even though different guilds of carabids and spiders responded differently. Brachypterous carabids, ambush spiders, ground-hunter spiders and other hunters preferred organic vineyards, whereas macropterous carabids, specialist spiders (mostly ant-eating spiders) and sheet web weavers selected conventional vineyards. The research we report here shows that preferences for vineyards with different farming systems has been driven not only by farming systems per se (i.e. omission of synthetic pesticides), but also by habitat characteristics and small-scale landscape structure. Arthropod diversity was greater in the forest patches adjacent to organic vineyards than to conventional ones. This suggests that organic systems may sustain a higher diversity of carabids and spiders both in vineyards and in the adjacent forest patches as well. We conclude that although conventional systems may promote the diversity of some guilds, organic systems should take priority.

The effect of thinning on ground spider diversity and microenvironmental factors of a subtropical spruce plantation forest in East Asia

Currently, information about the effect of forest management on biodiversity of subtropical plantation forests in Asia is quite limited. In this study, we compared the spider community structures and guild compositions of subtropical Cryptomeria japonica plantation forests receiving different degree of thinning (0, 25 and 50 %) in central Taiwan. The ground spider diversities and environmental variables were sampled/measured once every 3 months for 1 year before thinning and 2 years after thinning. Results showed that before thinning spider compositions did not differ significantly among three plantation forest types. Two years after thinning, spider species and family compositions of three plantation forest types differed significantly. In all three plantation forest types, the spider composition differed from year to year, indicating existence of temporal variations in spider diversity. Ground hunters (increased 200–600 % in thinned forests), sheet web weavers (increased 50–300 % in thinned forests) and space web weavers (decreased 30–50 % in thinned forests) were the major contributors of the observed spider composition differences among plantation forests receiving different treatments. The stands receiving thinning treatments also had higher illumination, litter decomposition rate, temperature and understory vegetation density. Thinning treatments might have changed the structures of understory vegetation and canopy cover and consequently resulted in abundance and diversity changes of these guilds. Moreover, the heterogeneity in understory vegetation recovery rate and temporal variation of spider composition might further generate spider diversity variations in subtropical forests receiving different degree of thinning.

Competition between introduced and native spiders (Araneae: Linyphiidae)

The European sheet-web spider Linyphia triangularis (Araneae: Linyphiidae) has become established in Maine, where it often reaches very high densities. Two lines of evidence from previous work suggest that L. triangularis affects populations of the native linyphiid spider Frontinella communis. First, F. communis individuals are relatively scarce in both forest and coastal habitat where L. triangularis is common, but more common where L. triangularis is at low density. Second, in field experiments, F. communis species are less likely to settle in experimental plots when L. triangularis is present, and F. communis disappears from study plots when L. triangularis is introduced. Here we test two mechanisms that may underlie these patterns. First, we tested whether L. triangularis invades and usurps the webs of F. communis. When spiders were released onto webs of heterospecifics, L. triangularis was more likely to take over or share webs of F. communis than the reverse. We also observed natural takeovers of F. communis webs. Second, we explored the hypothesis that L. triangularis reduces prey availability for native species. We sampled flying prey in areas with L. triangularis and those where it had been removed, and found no effect of spider presence on measured prey density. We also found no effect of prey supplementation on web tenacity in F. communis, suggesting that F. communis movements are not highly dependent on prey availability. We conclude that web takeover is likely more important than prey reduction in driving negative effects of L. triangularis on F. communis.

Food sources of early colonising arthropods: The importance of allochthonous input

Arthropod predators and detritivores are among the first colonisers on land surfaces undergoing primary succession. In the absence of higher plants and herbivores, they may either have an allochthonous food source (i.e. of geographically distant origin) or local food source that is sustained by e.g. microorganisms. By studying spiders and collembolans on sites along chronosequences on recently emerged nunataks (ice-free land in glacial areas), we analysed whether the food source of early colonisers was of local or distant origin. Also, we measured the potential changes in trophic position with increasing site age. With stable carbon and nitrogen isotope analysis of both spiders and collembolans, we determined that allochthonous food sources are important on the youngest sites. The allochthonous food sources may explain how arthropods can persist in the absence of local primary productivity. Also, some spider species may increase their ability to survive by shifting their trophic position as the community composition changes. The results indicate that a wolf spider species shifted towards a lower trophic level with increasing site age. By contrast, no shifts in trophic position were observed for sheet-web spiders or collembolans.

Spiders (Araneae) from Agricultural fields near foothill of Satpura Mountain ranges of Amravati District, Maharashtra, India.

AbstractThis paper deals with the study of spider distribution in agricultural fields adjoining the Satpura Mountain Ranges of the Amravati district. The total collection of spiders comprises 12 families, 37 genera and 76 species. The Araneidae and Thomisidae families were dominant, followed by Saltisidae from the selected agricultural area. Lycosidae and Tetragnathidae were also found in a decreasing number of species due to their specific habitation. The analysis of guild structure revealed six feeding guilds. Orb web weavers and Ambushers constituted the dominant feeding guild representing 33% and 22% of the total collection respectively. Stalkers and ground runners represent 20% and 17% and foliage hunters and sheet web weavers represent 5% and 3% respectively. A new species of genus Sassacus (Beetle like spider), family Saltisidae was found for the first time in the Vidarbha region. The richness of spider diversity is a sign of the overall biodiversity which is a useful indicator of the species richness of the agro-ecosystem.

Pollen interception by linyphiid spiders in a corn agroecosystem: implications for dietary diversification and risk-assessment

Dietary diversification, including consumption of plant tissues such as pollen, can enhance the fecundity of generalist predators, resulting in improved control of pest prey. Supplemental pollen feeding has been observed in many natural enemies, including sheet-web spiders (Araneae: Linyphiidae), which represent a major component of food webs in agroecosystems. Their horizontal, ground-based webs have the potential to intercept pollen grains during anthesis of crop plants, providing the opportunity for consumption of pollen to occur. In laboratory feeding trials, Frontinella communis and Tennesseellum formicum (Araneae: Linyphiidae) readily fed on pollen grains dusted on their webs, with 82 and 92% of spiders consuming pollen within the 210 min trial. These results revealed a strong potential for dietary supplementation with pollen in ground-based sheet-web spiders, indicating that pollen feeding may be an important component of the feeding biology of linyphiids. To measure pollen and prey interception in simulated linyphiid webs, a 20 m × 20 m grid of miniature sticky traps was established within and downwind of a corn agroecosystem. Traps were exposed for 24 h, all intercepted material was transferred to the laboratory for subsequent identification, and replaced with additional traps for 28 consecutive days in July and August 2008, to encompass periods before, during and after anthesis. Over 150,000 corn pollen grains and 5,000 prey items (dominated by Collembola and Hemiptera) were intercepted at simulated web-sites. Dates of peak anthesis resulted in pollen counts as high as 4,000 grains per web-site in the interior of the cornfield. Spatial Analysis by Distance Indices (SADIE) indicated significant temporal and spatial variability in pollen interception within and outside the corn field, but interestingly there was no significant spatial association between pollen and prey. Furthermore, transgenic Bacillus thuringiensis corn expresses insecticidal proteins in pollen, posing an exposure risk to non-target arthropods. Consumption of corn pollen may be a route to transgenic protein exposure in this important taxon of generalist predators.

Non-crop habitats in the landscape enhance spider diversity in wheat fields of a desert agroecosystem

Spiders (Araneae) are an important group of generalist predators in arable land. In temperate climates, spiders recolonise cropland annually from the surrounding landscape. In arid climates however, irrigated crops and the surrounding landscape matrix offer sharply different habitat conditions and this might negatively affect spider migration into crops. We studied whether the spider fauna in desert crops is influenced by the surrounding landscape in a similar way to that found in temperate climates. Spiders were sampled with pitfall traps in 13 wheat fields ( Triticum aestivum L.) in the Negev Desert (Israel). The fields were situated along a gradient from crop- to non-crop-dominated landscapes (1–72% non-crop habitats). Species richness of spiders in wheat fields increased with the percentage of non-crop habitats in the landscape. In addition, activity-densities of crab spiders (Thomisidae) and cobweb spiders (Theridiidae) were enhanced by high percentages of non-crop habitats in the surrounding landscape. Activity-densities of the dominant sheetweb spiders (Linyphiidae) showed no significant response to landscape composition. As the immigrant spider families employ different foraging strategies than the dominant sheetweb spiders, they functionally enriched the spider fauna in crops and potentially increase the range of prey types consumed by spiders. Thus, non-crop habitats can be expected to increase the potential for biological control by spiders in nearby crops.

Water limitation prevails over energy in European diversity gradients of sheetweb spiders (Araneae: Linyphiidae)

Across large spatial scales, species richness in many taxa is mainly determined by climatic variables. However, some of the mechanisms behind large-scale patterns of species richness and abundance are expected to act on the community level, and on a smaller scale than the resolution of the data commonly used for deriving these patterns. We studied the distribution of sheetweb spiders (Araneae: Linyphiidae) across Europe using both standardised samples from local habitats and large-scale inventories. In the first approach, we analysed species and individual numbers from standardised pitfall traps from 17 locations distributed from northern Sweden to Spain and Greece. We also calculated Simpson indices to correct for different activity densities. Additionally, we analysed whether diversity of sheetweb spiders is sensitive to habitat type. In the second approach, we investigated the known sheetweb spider species richness of European countries and large islands. In both datasets, species richness of sheetweb spiders reached a maximum at about 551N and declined towards the Mediterranean and the Arctic. In addition, species richness decreased from western (maritime) to eastern (continental) regions. Also Simpson diversity showed a hump-shaped relation to latitude, and was the only variable influenced by habitat type. In contrast to species richness and diversity, activity density increased monotonously with latitude. Towards the north, sheetweb spiders are likely limited by energy availability, towards the south, water limitation as well as bottom-up and top-down community interactions may reduce sheetweb spiders. Accordingly, their diversity does not follow the pattern of vertebrates, large insects and aquatic organisms, which depend more strongly on energy availability.

Sex ratio, mating behaviour and Wolbachia infections in a sheetweb spider

The solitary sheetweb spider Pityohyphantes phrygianus has a female-biased primary sex ratio. Earlier studies have indicated that female post-copulatory position may influence the degree of bias, but the adaptive significance of the departure from a Fisherian 1 : 1 sex ratio is not known. In this study we show that there is a significant relationship between abdominal position and offspring sex ratio in undisturbed females. Moreover, female mating behaviour (and thus sex ratio) is correlated with female size, but this relationship may shift from negative to positive depending on the presence of the maternally acquired endosymbiotic bacterium Wolbachia in both sexes. A survey of wild-caught individuals shows that the incidence of Wolbachia infection in natural populations is high. This indicates that Wolbachia may be a significant factor influencing female post-copulatory behaviour and sex ratio in natural populations. We discuss various adaptive reasons for sex-ratio bias and behavioural plasticity in females. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 181–186.

Herbivore release through cascading risk effects

Predators influence prey through consumption, and through trait-mediated effects such as emigration in response to predation risk (risk effects). We studied top-down effects of (sub-) adult wolf spiders (Lycosidae) on arthropods in a meadow. We compared risk effects with the overall top-down effect (including consumption) by gluing the chelicers of wolf spiders to prevent them from killing the prey. In a field experiment, we created three treatments that included either: (i) intact ('predation') wolf spiders; (ii) wolf spiders with glued chelicers ('risk spiders'); or (iii) no (sub-) adult wolf spiders. Young wolf spiders were reduced by their (sub-) adult congeners. Densities of sheetweb spiders (Linyphiidae), a known intraguild prey of wolf spiders, were equally reduced by the presence of risk and predation wolf spiders. Plant- and leafhoppers (Auchenorrhyncha) showed the inverse pattern of higher densities in the presence of both risk and predation wolf spiders. We conclude that (sub-) adult wolf spiders acted as top predators, which reduced densities of intermediate predators and thereby enhanced herbivores. Complementary to earlier studies that found trait-mediated herbivore suppression, our results demonstrate that herbivores can be enhanced through cascading risk effects by top predators.

Sown wildflower areas to enhance spiders in arable fields

Sown wildflower areas (SWA) are cultivated in Switzerland to preserve and enhance farmland biodiversity. Here, the impact of SWA on spider density, diversity and species richness in winter wheat fields was studied. Edge effects and field scale effects of SWA were compared with landscape effects of perennial habitats in general. Adjoining SWA enhanced the densities of crab spiders (Thomisidae), ground spiders (Gnaphosidae), wolf spiders (Lycosidae) and young orb weavers (Araneidae) in wheat fields. In addition, numbers of wolf spiders and ground spiders increased towards field edges. Surprisingly, species richness was unaffected by SWA. Species richness, Simpson diversity, and densities of all spiders, and densities of sheetweb spiders (Linyphiidae) increased with the percentage of perennial habitats in 1.5 km radius. In conclusion, high spider diversity in wheat fields depends on perennial habitats at the landscape scale. To enhance cursorial spider families and orb weavers, SWA should be interspersed between crop fields.

Body size determines the outcome of competition for webs among alien and native sheetweb spiders (Araneae: Linyphiidae)

Abstract 1. Biotic invasions are one of the most important reasons for changes in biodiversity. The alien sheetweb spider Mermessus trilobatus (Araneae: Linyphiidae) has become abundant in large parts of Central Europe within the past three decades. Its invasion might negatively influence native spiders, for instance via competition for webs.2. Laboratory experiments were developed to test if M. trilobatus is competitively superior to native spiders. The alien M. trilobatus and five native sheetweb spiders (Erigone dentipalpis, E. atra, Gnathonarium dentatum, Dicymbium nigrum and Micrargus herbigradus) were compared with respect to their success to take over occupied webs from E. dentipalpis.3. The rate of web takeover or defence was determined by body size, whereby individuals with a wider thorax invaded webs more successfully. After taking body size into account, the frequency of defence or web takeover did not differ between species. In 13% of all confrontations, predation against generally smaller opponents was recorded. Contrary to the predictions, raising the web value with food resources did not raise the effort expended on web defence but reduced predation by the web owner.4. The current study does not indicate that the invasion of the relatively small‐sized M. trilobatus is facilitated by strong competitiveness. Nevertheless, M. trilobatus may displace smaller‐sized immature specimens and thereby threaten native spiders. Still, other reasons are likely to underlie the success of M. trilobatus in Europe, such as rapid reproduction or release from natural enemies.

Dominant insectivorous polyphagous predators in winter wheat: High colonization power, spatial dispersion patterns, and probable importance of the soil surface spiders (Araneae)

Spiders inhabiting the soil surface of a winter wheat field were examined using pitfall traps near Zurich, central Europe. The soil surface spider community was dominated by two species of wolf spiders Pardosa agrestis (Westring) and Pardosa palustris (L.) (Lycosidae) and three species of sheet-web weavers Erigone atra (Blackwall), Erigone dentipalpis (Wider) and Oedothorax apicatus (Blackwall) (Linyphiidae). The identity and average number of spiders collected in the traps over time between field border and center were compared revealing that the number of wolf spiders on the border did not differ significantly from the center. However, the sheet-web weavers expressed a consistent trend of slightly greater numbers in the field's center compared to the border. Computation of the dispersion index (after Morisita) indicates aggregated patterns throughout the field for both Lycosidae and Linyphiidae caught in the pitfall traps. The combined number of spider individuals showed an overall increase as the season progressed. Over 12000 spiders were collected from the 2 ha winter wheat field by means of 45 (7 cm diam.) pitfall traps in less than 3 months, whereby this high number is providing additional evidence for previous work reported in the literature of winter wheat field colonization by large numbers of spiders. The capacity of soil surface spiders to build up these high numbers, potentially consuming field crop insect pests at elevated rates suggests that they may play a significant role as natural control agents. The effectiveness of these spiders as natural enemies may be limited only by their apparently low feeding frequency reflecting a low rate of metabolism.