Contrasting responses of arable spiders to the landscape matrix at different spatial scales

Aim Animal communities can be influenced by the composition of the surrounding landscape through immigration. Depending on habitat preferences, however, the effect of the landscape matrix can be positive or negative and can vary with scale. We tested this idea with arable spiders and tried to infer dispersal distances from relationships between local density and landscape composition at different spatial scales. Location Thirty‐eight landscapes around the cities of Göttingen and Giessen, Germany. Methods Spiders were captured with pitfall traps in one field of winter wheat in each landscape. Landscape composition around the fields was characterized at 11 scales from 95 m to 3 km radius by land‐use mapping and subsequent GIS analysis. Correlation tests were performed between landscape composition and local densities or species richness. Results In both study regions, local species richness was enhanced by non‐crop habitats on a landscape scale. The overall densities of wolf spiders (Lycosidae), long‐jawed spiders (Tetragnathidae), crab spiders (Thomisidae), and dwarf sheet spiders (Hahniidae) increased significantly in landscapes with high percentages of non‐crop habitats. Out of the 40 species tested, 19 responded positively to the percentage of non‐crop habitats in the surrounding landscape, and five responded negatively. Depending on the species, the spatial scales with the highest explanatory power ranged from 95 m to 3 km radius around the study fields, potentially reflecting dispersal distances. Main conclusions Arable spider species showed contrasting responses to the landscape context with respect both to the direction and to the spatial scale of the relationship. The variation in landscape requirements among species ensures high spider densities in a wide range of situations, which contributes to ecosystem resilience. However, species richness of arable spiders depends on heterogeneous landscapes with high percentages of non‐crop habitats.     

Non‐cropped fragments as important spider reservoirs in a Pampean agro‐ecosystem

Agricultural landscapes include patches of cropped and non‐cropped habitats. Non‐cropped fragments are often source habitats for natural pest predators which colonise less suitable agricultural fields. The goals of the present study were: (a) to evaluate the contribution of non‐cropped fragments to agro‐ecosystems as biodiversity reservoirs and ecosystem service providers, by assessing the abundance of spider species and their diversity and (b) to quantify the spatial variation in spider communities across different non‐cropped fragments and crops. We hypothesised that non‐cropped fragments function as spider diversity reservoirs with better conditions for reproduction than crops. We collected spiders from 10 restored fragments having had no disturbance for 20 years and four field edges, along a gradient inside the crop adjacent to each fragment. Overall, we collected 3,591 spiders belonging to 49 species/morphospecies in 14 families. Non‐cropped fragments had a central role in the spider community, as estimated through species–habitat networks. We found differences in the diversity and abundance of spiders between non‐cropped and cropped fragments. However, these differences were only for immature spiders, whose abundance decreased from non‐cropped fragments towards the inside of crops. Our results highlight the importance of non‐cropped fragments in agro‐ecosystems as important source habitat patches, reservoirs of biodiversity and sites where spider reproductive success is possibly higher.     

Spider assemblages in winter oilseed rape affected by landscape and site factors

Spiders are an abundant and diverse group of generalist predators in arable fields. Knowledge on what landscape and site factors affect this group can be valuable for efforts to reduce biodiversity loss in agricultural landscapes and can have implications for natural pest control. We investigated the impact of landscape and site factors on epigeic spiders in 29 winter oilseed rape fields (Brassica napus, OSR) embedded in differently structured landscapes in an agricultural region east of Vienna (Austria). Landscape factors included proportions of non‐crop areas, woody areas and fallows, lengths of road‐side strips and hedges, and landscape diversity at different spatial scales (r=250–2000 m). Site factors included OSR stand density, soil index, soil cultivation intensity, nitrogen fertilisation level, OSR vegetation cover in late autumn, and insecticide applications. Data were analysed using regression, ordination, and variation partitioning. Different characteristics of spider assemblages responded to different landscape factors at different spatial scales. Observed species richness showed the strongest positive reponse to proportions of woody areas at rather small scale (radius 500 m), but the relation remained significant up to the 1250 m radius. Standardised species richness was positively related to non‐crop area at the smallest scale (radius 250 m). Activity density was positively related to length of road‐side strips with maximum effects at large scale (radius 1750 m) and non‐crop area (radius 750 m). Site characteristics (stand density, insecticide applications, and late autumn ground cover) and landscape factors (woody areas and fallows at radius 500 m) were similarly important for explaining species composition. We interpret the scale‐dependency of relations as the result of differences in dispersal power of the studied spider species. These results demonstrate the important, scale dependent influence of natural and semi‐natural habitats on spider assemblages in arable fields.     

Migration patterns and functional groups of spiders in a desert agroecosystem

Abstract 1. Arthropods living in annual crops suffer mortality caused by agricultural practices. Therefore, migration from surrounding habitats is crucial to maintain populations of natural enemies of insect pests in crops. In desert agroecosystems there is a pronounced contrast between managed and unmanaged habitats, where irrigated and fertilised crops are islands of productivity in an arid matrix. This contrast could either enhance or inhibit movement of natural enemies between the landscape components. 2. The importance of the surrounding arid habitats as a source for spiders in crops was examined in the Negev desert of Israel. Spiders were sampled in both arid natural habitat and adjacent wheat fields using pitfall traps and visual searching. In addition, spiders in wheat fields were sampled throughout the winter cropping season using emergence traps at increasing distances from the field edge. Stationary and movable emergence traps were used to distinguish between residents and migrant species. 3. The spider assemblage in the wheat was dominated by three families: Linyphiidae, Theridiidae, and Gnaphosidae. Spider sampling in both natural arid habitat and adjacent wheat fields enabled four functional groups to be recognised that differed in habitat preference, movement patterns, and population dynamics. Thirty‐three per cent of collected individuals were classified as crop residents whereas more than 50% were classified as migrants from the surrounding habitats. These findings suggest that the surrounding habitats influence spider assemblage composition in the fields, in spite of the marked contrast in habitat structure and productivity. 4. Spider assemblages in the wheat fields were dominated by migrant species arriving from the surrounding arid habitats. Migrant spiders inhabited the crop throughout the cropping season. The combined contribution of resident and migrant functional groups may act to prevent insect pest outbreaks in this desert agroecosystem.     

Landscape mosaic of Araucaria forest and forest monocultures influencing understorey spider assemblages in southern Brazil

Abstract This study investigates how abundance, diversity and composition of understorey spiders were influenced by four different forest habitats in a southern Brazilian Araucaria forest. The study area encompasses a landscape mosaic comprised of Araucaria forest, Araucaria plantation, Pinus plantation, and Eucalyptus plantation. Understorey spiders were collected by beating the vegetation inside three patches of each forest habitat. To assess possible predictors of spider assemblage structure, several patch features were analysed: potential prey abundances, estimation of vegetation cover, diversity index of vegetation types, patch ages, patch areas, and geographical distance between patches. To assess the influence of high‐level taxa approaches on spider assemblage patterns, analyses were carried out individually for family, genera and species levels. Additionally, Mantel tests were carried out in underlying similarity matrices between each taxon. Significant differences in spider abundances among forest habitats were found. Pinus plantations showed the highest abundance of spiders and Eucalyptus plantations showed the lowest abundance. Spider abundance was significantly influenced by patch ages, geographical distance and vegetation cover. Expected numbers of families, genera and species did not vary among forest habitats. Spider composition of two Eucalyptus patches differed from the other forest patches, probably due to their low vegetation cover and isolation. Genera composition was the best correlate of species composition, showing that a higher‐level surrogate can be an alternative to the species approach. The understorey spider diversity in this managed area could be maintained when suitable habitat structures are provided, thus ensuring the connectivity between different habitat types. Further studies should focus on individual species responses to the conversion of native forest to monocultures.     

Limited benefits of non-crop vegetation on spiders in Australian vineyards: regional or crop differences?

In crops, invertebrate natural enemies such as spiders have been documented as responding to non-crop vegetation at the local and landscape scales, particularly in northern Europe. Much of this information is based on data from arable or annual crops and it is possible that spider numbers in more persistent perennial systems including vineyards may be less dependent on non-crop vegetation. To test the relationship between spider abundance and non-crop vegetation within the context of Australian vineyards, we sampled spiders in 54 vineyards with adjacent non-crop vegetation, from three different regions. Landscape composition in the area surrounding each of the 54 sites was characterized at 11 spatial scales from 95?m to 3?km radius and spiders were sampled monthly using canopy sticky traps and ground pitfall traps. There were only weak relationships between pasture or woody vegetation and the abundance of spiders in vineyards at all spatial scales. At the local scale, abundance of most spider families tended to be greater in vineyards with adjacent pasture. At the landscape scale there were inconsistent patterns. We discuss possible reasons for these apparent contrasting patterns between perennial and annual crops and European compared to Australian agroecosystems.     

Evidence of a latitudinal gradient in spider diversity in Australian cotton

The most common explanation for species diversity increasing towards the tropics is the corresponding increase in habitats (spatial heterogeneity). Consequently, a monoculture (like cotton in Australia) which is grown along a latitudinal gradient, should have the same degree of species diversity throughout its range. We tested to see if diversity in a dominant cotton community (spiders) changed with latitude, and if the community was structurally identical in different parts of Australia. We sampled seven sites extending over 20° of latitude. At each site we sampled 1–3 fields 3–5 times during the cotton growing season using pitfall traps and beatsheets, recording all the spiders collected to family. We found that spider communities in cotton are diverse, including a large range of foraging guilds, making them suitable for a conservation biological control programme. We also found that spider diversity increased from high to low latitudes, and the communities were different, even though the spiders were in the same monocultural habitat. Spider beatsheet communities around Australia were dominated by different families, and responded differently to seasonal changes, indicating that different pest groups would be targeted at different locations. These results show that diversity can increase from high to low latitudes, even if spatial heterogeneity is held constant, and that other factors external to the cotton crop are influencing spider species composition. Other models which may account for the latitudinal gradient, such as non‐equilibrium regional processes, are discussed.     

Spider communities in urban green patches and their relation to local and landscape traits

Urbanization and urban landscape characteristics greatly alter plant and animal species richness and abundances in negative and positive directions. Spiders are top predators, often considered to be sensitive to habitat alteration. Studies in urban environments frequently focus on ground-dwelling spiders or on spiders in built structures, leaving aside foliage spiders. Effects of habitat, landscape type and structure and local characteristics on spider species composition, richness and relative abundance were evaluated in urban green patches in a temperate city of South America. We also assess whether Salticidae could be an indicator group for the broader spider community in the urban environment. Spiders were sampled with a G-VAC (aspirator) in urban green patches in Córdoba city, Argentina, in urban, suburban and exurban habitats (18 sites; six per habitat) and local and landscape traits were assessed. Overall, the exurban was richer than the urban habitat, however, at the site level Salticidae richness and abundance (but not the total spider assemblage) were significantly lower in urban sites. Species composition moderately differed between urban and exurban sites. Results indicate that on urban green spaces a low impervious surface cover, a coverage of trees, herbaceous vegetation and a vertical structure of vegetation at least up to 1 m in height contribute to higher richness and abundance of spiders, Salticidae being more sensitive than the overall spider community to local effects. In addition, Salticidae richness can predict 74% of the total spider richness recorded and may be used as spider diversity bio-indicators in this climatic region.     

Spider diversity in cereal fields: comparing factors at local, landscape and regional scales

Aim Factors acting at various scales may affect biodiversity, demanding analyses at multiple spatial scales in order to understand how community richness is determined. Here, we adopted a hierarchical approach to test the contribution of region, landscape heterogeneity, local management (organic vs. conventional) and location within field (edge vs. centre) to the species richness and abundance of spiders in cereals. Location Three regions of western and central Germany: Leine Bergland, Soester Boerde, and Lahn‐Dill Bergland. Methods Forty‐two paired organic and conventional winter wheat fields were compared. Field pairs were located in areas ranging from structurally simple to structurally complex landscapes. In May and June 2003, spiders were sampled using pitfall traps. Linear mixed models were used to determine the relationship of spider diversity and abundance with regional spatial factors and landscape heterogeneity within a 500‐m radius, as well as with local management and within‐field location. Results Within‐field location of the traps and landscape heterogeneity were the best predictors of species richness: more species were found in field edges and in heterogeneous landscapes. Region and local management had no effect on species richness. Activity density was higher in field edges and differed among regions. Main conclusions The diversity of farmland spiders was influenced by differences at two of the spatial scales (edge vs. centre, simple vs. complex landscapes), but not at the two others (field management, region), emphasizing the importance of analyses at a variety of spatial scales for an adequate explanation of patterns in biodiversity. Our study suggests that promoting heterogeneity in land use at landscape scales is one of the keys to promoting spider diversity in agroecosystems.     

Comparison of pollinators and natural enemies: a meta‐analysis of landscape and local effects on abundance and richness in crops

To manage agroecosystems for multiple ecosystem services, we need to know whether the management of one service has positive, negative, or no effects on other services. We do not yet have data on the interactions between pollination and pest‐control services. However, we do have data on the distributions of pollinators and natural enemies in agroecosystems. Therefore, we compared these two groups of ecosystem service providers, to see if the management of farms and agricultural landscapes might have similar effects on the abundance and richness of both. In a meta‐analysis, we compared 46 studies that sampled bees, predatory beetles, parasitic wasps, and spiders in fields, orchards, or vineyards of food crops. These studies used the proximity or proportion of non‐crop or natural habitats in the landscapes surrounding these crops (a measure of landscape complexity), or the proximity or diversity of non‐crop plants in the margins of these crops (a measure of local complexity), to explain the abundance or richness of these beneficial arthropods. Compositional complexity at both landscape and local scales had positive effects on both pollinators and natural enemies, but different effects on different taxa. Effects on bees and spiders were significantly positive, but effects on parasitoids and predatory beetles (mostly Carabidae and Staphylinidae) were inconclusive. Landscape complexity had significantly stronger effects on bees than it did on predatory beetles and significantly stronger effects in non‐woody rather than in woody crops. Effects on richness were significantly stronger than effects on abundance, but possibly only for spiders. This abundance‐richness difference might be caused by differences between generalists and specialists, or between arthropods that depend on non‐crop habitats (ecotone species and dispersers) and those that do not (cultural species). We call this the ‘specialist‐generalist’ or ‘cultural difference’ mechanism. If complexity has stronger effects on richness than abundance, it might have stronger effects on the stability than the magnitude of these arthropod‐mediated ecosystem services. We conclude that some pollinators and natural enemies seem to have compatible responses to complexity, and it might be possible to manage agroecosystems for the benefit of both. However, too few studies have compared the two, and so we cannot yet conclude that there are no negative interactions between pollinators and natural enemies, and no trade‐offs between pollination and pest‐control services. Therefore, we suggest a framework for future research to bridge these gaps in our knowledge.     

Effects of agricultural practices and fine-scale landscape factors on spiders and a pest insect in Japanese rice paddy ecosystems

We examined the effects of environmentally friendly (EF) farming and landscape factors on the abundances of major spider guilds and a rice pest, the small brown planthopper (SBPH), Laodelphax striatellus (Fallén), in a Japanese rice paddy ecosystem. The abundances of all spider guilds increased by EF farming, whereas different spider guilds showed contrasting responses to the size of the forest within 200 m of the fields. The abundances of ground spiders and horizontal web weavers increased with increasing forest area, unlike small ground spiders, whose abundance decreased. The abundances of SBPH nymphs and adults decreased by EF farming but responded differently to landscape: nymphs decreased with increasing forest area, but adults did not. Further analysis indicated a potential negative relationship between web-weaving spiders and SBPH nymphs. Our findings suggest that the size of the forest adjacent to fields is an important determinant of spider guild composition and pest abundance.     

Aerial dispersal ability does not drive spider success in a crop landscape

1. Although spiders can colonise ecosystems by air, dispersal capabilities differ among spider species. Web‐building spiders are thought to balloon at higher rates than hunting spiders. Spider success in agricultural systems may also depend on habitat preferences. Few studies have examined the success of aerially dispersing spiders in crop systems, and information about the dispersal capabilities of spiders in putative source habitats is limited. 2. Spiders were monitored in the air and on the foliage of vineyards and adjacent oak woodland in order to compare the aerial spider faunas between these disparate habitats and to determine whether highly dispersive species contributed disproportionately to the spider community in vineyards. 3. The results show that most aerially dispersing spiders in both habitats were web‐building dwarf spiders, Erigone spp. (Linyphiidae), although hunting spiders were also well represented in the air, especially in oak woodland. Most woodland spiders in the air appeared to be residents of oak woodland and probably dispersed only short distances. 4. Conversely, only a subset of the aerial spider fauna established in vineyards in high numbers. Spiders that dominated the aerial fauna were under‐represented on vineyard foliage, whereas several hunting spiders dispersed aerially at low rates but dominated vineyard spider composition. 5. These results suggest that aerial dispersal ability may allow spiders to reach crop systems, but that establishment depends on habitat preferences and/or competitive ability.     

Spider web guilds in cacao agroforestry – comparing tree,plot and landscape‐scale management

Aim Owing to their role as insect predators, web‐building spiders can be important biological control agents within agricultural systems. In complex tropical agroecosystems such as agroforests, management determines plant architecture, vegetation composition and associated ant density, but little is known on how these attributes, together with landscape context, determine spider web density. We hypothesized that all three spatial scales and the presence of Philidris ants significantly contribute to the explanation of spider web density with web types being differently affected. Location In 42 differently managed cacao agroforestry systems in Sulawesi, Indonesia. Methods We surveyed the distribution of five spider‐web types on 420 cacao trees to determine how these relate to habitat variables and a numerically dominant ant species at three different spatial scales, comparing tree, plot and landscape features. We fitted linear mixed‐effects model, selected the best model subset using information‐theoretic criteria and calculated the model‐averaged estimates. We used non‐metric multidimensional scaling (NMDS) to determine and visualize guild level responses to the effects of the tree, plot and landscape‐scale variables. Results The five spider guilds preferred different features of cacao tree architecture. Most frequently recorded webs belonged to the line‐ and orb‐web type. At the tree scale, overall web density was positively related to canopy openness. At the plot scale, a higher number of shade trees was related to a higher web density. At the landscape scale, the altitude determined the distribution patterns of web‐building spiders. Presence of Philidris ants was positively associated with density of orb webs, while no pattern was found for other web types. Main conclusions Results suggest spider web density could be increased by pruning of cacao trees while keeping shade trees at high density in cacao plots. The results emphasize the need to consider scale dependency of crop management and web‐guild‐specific responses that may be related to different functional roles of spiders as a high‐density predator group in agroforestry.     

半自然农田边界与相邻农田步甲和蜘蛛的时空分布

分别于小麦、玉米收获前后,采用陷阱法调查了华北地区典型农业景观中具有不同植被结构的农田边界及其相邻农田中两类重要天敌类群步甲和蜘蛛的多样性.通过比较农田生境及相邻农田边界间两类天敌群落的时空分布格局及其与相邻半自然生境植被群落的相关关系,探讨半自然农田边界对两类天敌类群的保护作用.结果表明: 整个取样季节农田边界处蜘蛛的多度显著高于农田内部;而步甲多样性在农田与边界间无显著性差异,仅呈现不同的群落结构;作物收获后蜘蛛分科数在边界处的增加以及在农田的减少,显示了蜘蛛在农田和边界之间的迁移活动.边界植被结构对蜘蛛和步甲多样性有不同影响:边界较高的草本层盖度和较低的乔木层盖度有利于增加农田中某些步甲优势种的多样性;而较高的草本层盖度有利于增加皿蛛科蜘蛛的多样性.因此,半自然生境的存在可以通过天敌在农田和边界之间的迁移运动促进农田天敌多样性的维持;但不同类型半自然生境植被群落结构可能影响其对不同天敌群落多样性的维持和保护作用.为促进农业景观对天敌的保护作用,提高其害虫控制功能,需要深入了解不同天敌的生境需求及食物需求,精心设计有利于天敌多样性维持的半自然生境.     

The influence of landscape diversity and heterogeneity on spatial dynamics of agrobiont linyphiid spiders: An individual-based model

Spiders are important generalist predators in natural pest control. However, agricultural fields are highly disturbed and ephemeral habitats, which present a number of challenges to the organisms living there; likewise landscape diversity and heterogeneity are also thought to be important factors in determining spider spatial dynamics. To investigate the interactions between these factors, we present an individual-based simulation model, which integrates life history characteristics of a typical agrobiont linyphiid spider with a dynamic spatially explicit landscape representation. The landscape contains several habitat types of varying quality and varies in time and space. Simulations showed that spatial landscape diversity (number of habitat types available for the spiders) is crucial for the persistence of spiders, but that spatial heterogeneity (spatial arrangement of patches) only had little impact on spider abundance. The necessary landscape diversity could either be provided by a diverse crop rotation or by including refuges in the form of less frequently managed habitats in the landscape. The presence of refuges greatly boosted numbers of spiders in the landscape as a whole. The most important characteristics of refuge were sanctuary from pesticides and extra prey availability, whereas tillage frequency mattered less. The simulations indicated that agrobiont linyphiids combination of high dispersal abilities and high reproductive rate enables it to exploit the transient resources of the different habitats in the agricultural landscape.     

Effects of agricultural diversification on the abundance, distribution, and pest control potential of spiders: a review

A review of the literature showed that spider abundance was increased by diversification in 63% of studies. A comparison of diversification modes showed that spider abundance in the crop was increased in 33% of studies by `aggregated diversification' (e.g. intercropping and non-crop strips) and in 80% of studies by `interspersed diversification' (e.g., undersowing, partial weediness, mulching and reduced tillage). It is suggested that spiders tend to remain in diversified patches and that extending the diversification throughout the whole crop (as in interspersed diversification) offers the best prospects for improving pest control. There is little evidence that spiders walk in significant numbers into fields from uncultivated field edges, but diversification at the landscape level serves to foster large multi-species regional populations of spiders which are valuable as a source of aerial immigrants into newly planted crops. There are very few manipulative field studies where the impact of spiders on pests has been measured in diversified crops compared with undiversified controls. It is encouraging, however, that in those few studies an increased spider density resulted in improved pest control. Future work needs are identified.     

Spider assemblages in bird burrows

The arachnofauna of bird burrows, which are excavated in the soil, is incompletely understood. The bird burrows have a potential to serve as refugia for cavity-adopting spider species particularly in anthropogenic habitats formed by sand mining, which otherwise suffer from a limited availability of other types of cavities. We hypothesized that the presence of abundant food resources and safe shelter in bird burrows excavated in the soil allow the existence of specialized assemblage of spiders, similarly to those known from cavities and nests made by other birds and mammals. Here we examined 353 burrows that had been previously occupied by Riparia riparia, Merops apiaster or Passer montanus. The burrows were examined in early May and early September in 24 Czech sand pits and construction sites. The burrows hosted a species-rich assemblage of spiders that, however, occurred at low abundances. The spiders were represented prevalently by species known to have distributions centered in anthropogenic and urban habitats, including species that were never observed outside of heated buildings in the study area and species that only were previously known from mammalian burrows and loess and karstic soil crevices. Spider assemblages strongly differed between bird burrows and cavities and nests made by other birds and mammals. The species composition and overall low abundance of spiders in bird burrows was unexpected and was in strong contrast to previous observations of spider assemblages in cavities and nests made by other birds and mammals. Concluded, we rejected the initial hypothesis and, instead, we have shown that bird burrows are rarely occupied by spiders and if occupied, the spider species that adopt old bird burrows and the nests inside of them predominantly consist of species known to have their distribution centered to anthropogenic and urban habitats.     

Regional and local variation of spider assemblages (Araneae) from coastal grey dunes along the North Sea

Aim This study aims to determine the underlying causes of local and regional patterns of variation in community structure of spiders in coastal grey dunes, and especially whether ecological time constrains the species composition in young and isolated grey dune habitats. Location The study was conducted in coastal dunes from northern France (Boulonnais, Nord‐Pas‐de‐Calais), Belgium (Flemish coastal dunes) and the Netherlands [Amsterdam Water Supply (AWD) dunes and Dunes from the Provincial Water Company North‐Holland]. Methods Spiders were collected with pitfall traps in twenty‐eight grey dune patches in the four areas under investigation. Species composition and environmental parameters (vegetation structure, distance to the sea and the patch‐edge, eolic dynamics and lime richness) were determined. Assemblage composition was related to the regional and local environmental factors with the Primer software package to determine the assemblage‐determining parameters. Differences in species presence were analysed as a function of their habitat preference and distribution range. Results Differences in grey dune spider assemblage structure can mainly be attributed to differences in local sand dynamics and the region. Species from dynamic dunes are mainly present in grey dunes from Belgium and France, while species from non‐dunal xerothermic habitats (chalk grasslands and heathland) occur in both the Boulonnais and the north Holland dune region. These species are absent from geologically young or other xerotherm habitat isolated Flemish coastal dunes. Main conclusion The data show that regional variation in spider assemblage composition results from local landscape characteristics (dynamics in the dune area), the latitude and the connectivity to non‐dunal xerothermic habitats. The strong and moderate geological isolation of dune areas from the Flemish coast and the AWD dunes, respectively, results in the absence of (at least some) species that are primarily bound to heathland and/or chalk grassland. This indicates the importance of ecological time for the assemblage structure. The limited dispersal capacity of the heathland and/or chalk grassland species is probably the main reason for their absence.     

The effects of development,vegetation‐type conversion,and fire on low‐elevation Southern California spider assemblages

California sage scrub (CSS), a native ecosystem type of low‐elevation areas of Southern California, is increasingly threatened by urban development, altered fire regimes, and vegetation‐type conversion to non‐native grasslands. Using pitfall traps, we examined how suburbanization, type conversion, and fire influence ground‐dwelling spider assemblages in eastern Los Angeles County, CA, by surveying spiders in three habitats (CSS, non‐native grasslands, and suburban areas) before and after a fire that occurred in a small portion of our study site. Spider assemblages in the suburban habitat differed from those in CSS and non‐native grassland habitats, but CSS and grassland assemblages did not significantly differ. This suggests that the urban development, but not vegetation‐type conversion to non‐native grasslands, has significant effects on ground‐dwelling spider assemblages. Fire had no observable effect on assemblages. Because ground‐dwelling spiders were not impacted by fire and type conversion, increased fire frequencies, which often result in the establishment of non‐native grasses, may not deleteriously influence this animal group, a differing pattern from other taxonomic groups. However, the rapid urban development occurring in low‐elevation areas of Southern California means that species requiring non‐suburban sites for their survival (15 species, 24.1%) may be threatened and require conservation assessment.     

Spider diversity in a tropical habitat gradient in Chiapas, Mexico

This paper presents an assessment of spider diversity in a complex landscape of southern Mexico. Eighteen different habitats were identified, measured and mapped across this fragmented landscape. Habitat types were characterized by measuring various features, including number of plant forms, tree cover and litter depth. Each month from February to April (dry season) and from June to August 2002 (wet season), spiders were sampled on each habitat by using pitfall traps and direct collection. Correlations between spider diversity and habitat characteristics were carried out to explore the relative contribution of each habitat variable as related to changes in spider composition and richness. In total, 115 spider species were recorded in 18 habitat types, and the mean number and density of species per habitat were 21 (± 3, standard error of mean) and 57 (± 9), respectively. The species recorded represent 41% of the fauna recorded in the Mexican state of Chiapas and 4% of the fauna recorded in Mexico. Relatively pristine habitats (e.g. deciduous forest) contained an important proportion of spider diversity in this fragmented landscape. Epigean spider diversity was significantly correlated with tree cover and with the diversity of plant forms during the rainy season. No correlation was detected between soil spider diversity and the habitat variables measured for any season. The results of this work suggest that in highly fragmented tropical landscapes, some habitat types (e.g. coffee plantations, hedgerows) might play an important role for the persistence of spider populations. The prevalence of relatively stable conditions in some of these habitats can allow spiders species (e.g. Nephila clavipes ) to overcome adverse conditions such as a decrease in humidity and dramatic changes in temperature and wind exposure, allowing them to recolonize when favourable conditions return.