Effects of tree species diversity on a community of weaver spiders in a tropical forest plantation

The effects of producer diversity on predators have received little attention in arboreal plant communities, particularly in the tropics. This is particularly true in the case of tree diversity effects on web‐building spiders, one of the most important groups of invertebrate predators in terrestrial plant communities. We evaluated the effects of tree species diversity on the community of weaver spiders associated with big‐leaf mahogany (Swietenia macrophylla) in 19, 21 × 21‐m plots (64 plants/plot) of a tropical forest plantation which were either mahogany monocultures (12 plots) or polycultures (seven plots) that included mahogany and three other tree species. We conducted two surveys of weaver spiders on mahogany trees to evaluate the effects of tree diversity on spider abundance, species richness, diversity, and species composition associated with mahogany. Our results indicated that tree species mixtures exhibited significantly greater spider abundance, species richness, and diversity, as well as differences in spider species composition relative to monocultures. These results could be due to species polycultures providing a broader range of microhabitat conditions favoring spider species with different habitat requirements, a greater availability of web‐building sites, or due to increased diversity or abundance of prey. Accordingly, these results emphasize the importance of mixed forest plantations for boosting predator abundance and diversity and potentially enhancing herbivore pest suppression. Future work is necessary to determine the specific mechanisms underlying these patterns as well as the top‐down effects of increased spider abundance and species richness on herbivore abundance and damage.     

Phenotypic differentiation within a foundation grass species correlates with species richness in a subalpine community

The effects of species loss on ecosystems depend on the community’s functional diversity (FD). However, how FD responds to environmental changes is poorly understood. This applies particularly to higher trophic levels, which regulate many ecosystem processes and are strongly affected by human-induced environmental changes. We analyzed how functional richness (FRic), evenness (FEve), and divergence (FDiv) of important generalist predators—epigeic spiders—are affected by changes in woody plant species richness, plant phylogenetic diversity, and stand age in highly diverse subtropical forests in China. FEve and FDiv of spiders increased with plant richness and stand age. FRic remained on a constant level despite decreasing spider species richness with increasing plant species richness. Plant phylogenetic diversity had no consistent effect on spider FD. The results contrast with the negative effect of diversity on spider species richness and suggest that functional redundancy among spiders decreased with increasing plant richness through non-random species loss. Moreover, increasing functional dissimilarity within spider assemblages with increasing plant richness indicates that the abundance distribution of predators in functional trait space affects ecological functions independent of predator species richness or the available trait space. While plant diversity is generally hypothesized to positively affect predators, our results only support this hypothesis for FD—and here particularly for trait distributions within the overall functional trait space—and not for patterns in species richness. Understanding the way predator assemblages affect ecosystem functions in such highly diverse, natural ecosystems thus requires explicit consideration of FD and its relationship with species richness.     

Body size and tree species composition determine variation in prey consumption in a forest‐inhabiting generalist predator

Trophic interactions may strongly depend on body size and environmental variation, but this prediction has been seldom tested in nature. Many spiders are generalist predators that use webs to intercept flying prey. The size and mesh of orb webs increases with spider size, allowing a more efficient predation on larger prey. We studied to this extent the orb‐weaving spider Araneus diadematus inhabiting forest fragments differing in edge distance, tree diversity, and tree species. These environmental variables are known to correlate with insect composition, richness, and abundance. We anticipated these forest characteristics to be a principle driver of prey consumption. We additionally hypothesized them to impact spider size at maturity and expect shifts toward larger prey size distributions in larger individuals independently from the environmental context. We quantified spider diet by means of metabarcoding of nearly 1,000 A. diadematus from a total of 53 forest plots. This approach allowed a massive screening of consumption dynamics in nature, though at the cost of identifying the exact prey identity, as well as their abundance and putative intraspecific variation. Our study confirmed A. diadematus as a generalist predator, with more than 300 prey ZOTUs detected in total. At the individual level, we found large spiders to consume fewer different species, but adding larger species to their diet. Tree species composition affected both prey species richness and size in the spider''s diet, although tree diversity per se had no influence on the consumed prey. Edges had an indirect effect on the spider diet as spiders closer to the forest edge were larger and therefore consumed larger prey. We conclude that both intraspecific size variation and tree species composition shape the consumed prey of this generalist predator.     

Management intensity and vegetation complexity affect web-building spiders and their prey

Agricultural management and vegetation complexity affect arthropod diversity and may alter trophic interactions between predators and their prey. Web-building spiders are abundant generalist predators and important natural enemies of pests. We analyzed how management intensity (tillage, cutting of the vegetation, grazing by cattle, and synthetic and organic inputs) and vegetation complexity (plant species richness, vegetation height, coverage, and density) affect rarefied richness and composition of web-building spiders and their prey with respect to prey availability and aphid predation in 12 habitats, ranging from an uncut fallow to a conventionally managed maize field. Spiders and prey from webs were collected manually and the potential prey were quantified using sticky traps. The species richness of web-building spiders and the order richness of prey increased with plant diversity and vegetation coverage. Prey order richness was lower at tilled compared to no-till sites. Hemipterans (primarily aphids) were overrepresented, while dipterans, hymenopterans, and thysanopterans were underrepresented in webs compared to sticky traps. The per spider capture efficiency for aphids was higher at tilled than at no-till sites and decreased with vegetation complexity. After accounting for local densities, 1.8 times more aphids were captured at uncut compared to cut sites. Our results emphasize the functional role of web-building spiders in aphid predation, but suggest negative effects of cutting or harvesting. We conclude that reduced management intensity and increased vegetation complexity help to conserve local invertebrate diversity, and that web-building spiders at sites under low management intensity (e.g., semi-natural habitats) contribute to aphid suppression at the landscape scale.     

Predator diversity and abundance provide little support for the enemies hypothesis in forests of high tree diversity

Predatory arthropods can exert strong top-down control on ecosystem functions. However, despite extensive theory and experimental manipulations of predator diversity, our knowledge about relationships between plant and predator diversity--and thus information on the relevance of experimental findings--for species-rich, natural ecosystems is limited. We studied activity abundance and species richness of epigeic spiders in a highly diverse forest ecosystem in subtropical China across 27 forest stands which formed a gradient in tree diversity of 25-69 species per plot. The enemies hypothesis predicts higher predator abundance and diversity, and concomitantly more effective top-down control of food webs, with increasing plant diversity. However, in our study, activity abundance and observed species richness of spiders decreased with increasing tree species richness. There was only a weak, non-significant relationship with tree richness when spider richness was rarefied, i.e. corrected for different total abundances of spiders. Only foraging guild richness (i.e. the diversity of hunting modes) of spiders was positively related to tree species richness. Plant species richness in the herb layer had no significant effects on spiders. Our results thus provide little support for the enemies hypothesis--derived from studies in less diverse ecosystems--of a positive relationship between predator and plant diversity. Our findings for an important group of generalist predators question whether stronger top-down control of food webs can be expected in the more plant diverse stands of our forest ecosystem. Biotic interactions could play important roles in mediating the observed relationships between spider and plant diversity, but further testing is required for a more detailed mechanistic understanding. Our findings have implications for evaluating the way in which theoretical predictions and experimental findings of functional predator effects apply to species-rich forest ecosystems, in which trophic interactions are often considered to be of crucial importance for the maintenance of high plant diversity.     

Tree Species Richness Strengthens Relationships between Ants and the Functional Composition of Spider Assemblages in a Highly Diverse Forest

In species‐rich ecosystems, such as subtropical and tropical forests, higher trophic level interactions are key mediators of ecosystem functioning. Plant species loss may alter these interactions, but the effects of plant diversity might be modified by intraguild interactions, particularly among predators. We analyzed the relationships between spiders and ants, two dominant predatory arthropod taxa, on tree saplings across a gradient from medium to high woody plant species richness in a subtropical forest in Southeast China. Neither ant nor spider total biomass was significantly related to plant species richness. By contrast, the biomass distribution of web‐building and hunting spiders changed and spider family richness increased in the presence of ants, resulting in more web builder‐dominated assemblages. However, these relationships depended on the plant communities, and were stronger in plots with higher plant species richness. Our results indicate that in addition to potential effects of ants on hunting spiders in particular, ants could indirectly influence intraguild interactions within spider assemblages. The observed shifts in the spider assemblages with increasing ant presence and plant species richness may have functional consequences, as web‐building and hunting spiders have distinct prey spectra. The relationships among ants, spiders, and plant species richness might contribute to explaining the non‐significant relationship between the overall effects of predators and plant diversity previously observed in the same forest plots. Our findings thus give insight into the complexity of biotic interactions in such species‐rich ecosystems.     

Tree diversity promotes generalist herbivore community patterns in a young subtropical forest experiment

Stand diversification is considered a promising management approach to increasing the multifunctionality and ecological stability of forests. However, how tree diversity affects higher trophic levels and their role in regulating forest functioning is not well explored particularly for (sub)tropical regions. We analyzed the effects of tree species richness, community composition, and functional diversity on the abundance, species richness, and beta diversity of important functional groups of herbivores and predators in a large-scale forest biodiversity experiment in south-east China. Tree species richness promoted the abundance, but not the species richness, of the dominant, generalist herbivores (especially, adult leaf chewers), probably through diet mixing effects. In contrast, tree richness did not affect the abundance of more specialized herbivores (larval leaf chewers, sap suckers) or predators (web and hunting spiders), and only increased the species richness of larval chewers. Leaf chemical diversity was unrelated to the arthropod data, and leaf morphological diversity only positively affected oligophagous herbivore and hunting spider abundance. However, richness and abundance of all arthropods showed relationships with community-weighted leaf trait means (CWM). The effects of trait diversity and CWMs probably reflect specific nutritional or habitat requirements. This is supported by the strong effects of tree species composition and CWMs on herbivore and spider beta diversity. Although specialized herbivores are generally assumed to determine herbivore effects in species-rich forests, our study suggests that generalist herbivores can be crucial for trophic interactions. Our results indicate that promoting pest control through stand diversification might require a stronger focus on identifying the best-performing tree species mixtures.     

Effect of an invasive ground cover plant on the abundance and diversity of a forest floor spider assemblage

Ground dwelling spiders are important predators in the detrital food web, which plays important roles in nutrient cycling and energy flow in forest ecosystems. The cursorial spider assemblage in a Beech-Maple forest in southwestern Michigan at sites where and invasive plant, Vinca minor, has invaded was compared to a native site within the same forest and to the forest prior to invasion by the plant. Pitfall traps were used to sample cursorial spiders over the course of a summer. Vinca minor substantially altered the forest floor spider assemblage. The invasive plant reduced the total activity-abundance of spiders by nearly 49% and depressed species diversity and evenness; in contrast, species richness was not affected. We found that V. minor changed the guild and family structure with wolf spiders being common at sites where the plant had invaded. Vinca minor reduced the abundance of vagrant web building and crab spiders. Similarity indices revealed that the spider communities between the two sites were quite dissimilar (Bray-Curtis = 0.506; Jaccard’s = 0.520). Importantly, comparison to a study conducted in the same forest 28 years earlier showed that the cursorial spider assemblage in the forest prior to Vinca invasion was very different than it was after Vinca invaded but was similar to the current native site in species and guild composition. We conclude that invasion by Vinca has caused the striking changes we observed in community organization of this important group of forest floor predators. We suggest that changes in the physical structure of the litter/soil microhabitat with the invasion of V. minor are likely the cause of the substantial impacts of the plant on the spider assemblage.     

Influence of ground cover on spider populations in a table grape vineyard

1. Cover crops and/or resident ground vegetation have been used in California vineyards to increase the number of predators and decrease the number of pestiferous herbivores. The most common resident predators in vineyards are spiders (Araneae). Several observational studies suggest that the addition of cover crops results in an increase in spider density and a decrease in insect pest densities. 2. To test experimentally the effects of cover crops and/or resident ground vegetation (hereafter collectively referred to as ground cover) on spider populations, a 3-year study was undertaken in a commercial vineyard. Large, replicated plots were established with and without ground cover during the growing season. Spider species diversity was analysed on the vines and on the ground cover. 3. On the vines, there was no significant difference in spider species richness or the total number of spiders in plots with and without ground cover. There were differences in the relative abundance of two spiders between treatments, with one species (Trachelas pacificus [Chamberlin & Ivie]) more abundant in plots with ground cover and another (Hololena nedra Chamberlin & Ivie) more common on vines in plots with no ground cover. Annual variation in spider abundance was greater than variation due to ground cover treatment. 4. On the ground cover, the spider species diversity was considerably different from that found on the vines above, suggesting that there is little movement of spiders between the ground cover and the vines. Enhancement of T. pacificus populations on vines with ground covers may be a result of prey species movement between the ground cover and the vines. Spider abundance was sparse on the bare ground. 5. The maintenance of ground cover increased spider species diversity in the vineyard as a whole (vine and ground cover). However, the relatively small changes in spider abundance on the vines indicate there are limitations in the use of ground covers for pest management with respect to generalist predators.     

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.     

Effects of ant competition and bird predation on the spider assemblage of a citrus grove

Characterizing intraguild interactions is key to improving understanding of food webs because they are major forces in the structuring of communities. Spiders are generalist predators with intermediate positions in the food web that establish intraguild interactions with ants and birds, which respectively compete with and prey on them. Research has also found interactions between birds and ants, potentially resulting in non-additive effects of both groups on arthropod assemblages, although studies of their combined impacts with tests for multiple-predator effects are scarce. We thus aimed to discern the relative effect of ants and birds on the spider assemblage of a citrus grove. We used a split-plot design to factorially exclude these groups over 2 years, preventing ants reaching the canopies by placing sticky bands around tree trunks, and birds by enclosing groups of trees in cages. We sampled spiders from the canopies (beating) and the ground (pitfalls) every 3 months, and we identified them to species. We found a strong influence of ants on the canopy spider assemblage, mainly through a negative effect on the families Araneidae and Theridiidae. Since spiders’ weights from ant-excluded and control trees were similar, these results suggest interference competition of ants on spiders rather than competitive exploitation. Bird exclusion did not affect the spider assemblage, contrasting with other studies reporting a marked predatory pressure of birds on spiders; nor were there any non-additive effects of ants and birds. Our findings show that spider assemblages are not uniformly affected by intraguild competitors.     

Environmental and spatial drivers of spider diversity at contrasting microhabitats

The relative importance of environmental and spatial drivers of animal diversity varies across scales, but identifying these scales can be difficult if a sampling design does not match the scale of the target organisms' interaction with their habitat. In this study, we quantify and compare the effects of environmental variation and spatial proximity on ground‐dwelling spider assemblages sampled from three distinct microhabitat types (open grassland, logs, trees) that recur across structurally heterogeneous grassy woodlands. We used model selection and multivariate procedures to compare the effects of different environmental attributes and spatial proximity on spider assemblages at each microhabitat type. We found that species richness and assemblage composition differed among microhabitat types. Bare ground cover had a negative effect on spider richness under trees, but a positive effect on spider richness in open grassland. Turnover in spider assemblages from open grassland was correlated with environmental distance, but not geographic distance. By contrast, turnover in spiders at logs and trees was correlated with geographic distance, but not environmental distance. Our study suggests that spider assemblages from widespread and connected open grassland habitat were more affected by environmental than spatial gradients, whereas spiders at log and tree habitats were more affected by spatial distance among these discrete but recurring microhabitats. Deliberate selection and sampling of small‐scale habitat features can provide robust information about the drivers of arthropod diversity and turnover in landscapes.     

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.     

Landscape heterogeneity at multiple spatial scales enhances spider species richness in an agricultural landscape

Landscape supplementation, which enhances densities of organisms by combination of different landscape elements, is likely common in heterogeneous landscapes, but its prevalence and effects on species richness have been little explored. Using grassland-dwelling spiders in an agricultural landscape, we postulated that richness and abundances of major constituent species are both highest in intermediate mixtures of forests and paddy fields, and that this effect derives from multi-scale landscape heterogeneity. We collected spiders in 35 grasslands in an agricultural landscape in Japan and determined how species richness and abundances of major species related to local and landscape factors across different spatial scales. We used a generalized linear model to fit data, created all possible combinations of variables at 15 spatial scales, and then explored the best models using Akaike's information criterion. Species richness showed a hump-shaped pattern in relation to surrounding forest cover, and the spatial scale determining this relationship was a 300–500-m radius around the study sites. Local variables were of minor importance for species richness. Abundances of major species also exhibited a hump-shaped pattern when plotted against forest cover. Thus, a combination of paddy fields and forests is important for enhancement of grassland spider species richness and abundance, suggesting habitat supplementation. The effective spatial scales determining abundances varied, ranging from 200 to >1000 m, probably representing different dispersal abilities. Landscape compositional heterogeneity at multiple spatial scales may be thus crucial for the maintenance of species diversity.     

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.     

Bromeliad architectural complexity and vertical distribution predict spider abundance and richness

Habitat complexity is a main predictor of the distribution of arthropods on vegetation. However, it remains poorly known whether plant architecture and fine‐scale spatial distribution affect the species richness and composition of associated arthropod guilds. In this study, we extensively sampled bromeliad species with a variety of rosette architectures in a megadiversity region. The aims were to investigate whether (i) possible differences in spider species composition among bromeliad species are related to the distinct architectures of the plants, and (ii) bromeliad architectural complexity (an intrinsic feature) and vertical distribution (an extrinsic feature) are good predictors of spider abundance and richness. Contrary to our expectations, spider species composition did not vary significantly among bromeliad species with different architectures. We found a positive effect of the mean number of leaves on spider abundance and species richness, but it occurred indirectly through spider abundance; factoring out the indirect effect revealed a negative effect of leaf number on species richness. Bromeliad species with wider vertical distributions harboured more spider species. Our results suggest that the dominance of a few spider species and reduced space for orb‐web spiders to attach their webs are the main explanations for lower spider richness on bromeliad species with higher architectural complexity. Our findings highlight the importance of both intrinsic and extrinsic plant features as co‐determinants of predator arthropod diversity.     

On the Relationships between Ground-dwelling Spider (Araneae) Assemblages and Dead Wood in a Northern Sugar Maple Forest

Downed woody material (fallen logs) offers ground-dwelling spiders (Araneae) ideal sites for nesting and foraging, but little is known about what characteristics of dead wood influence spider assemblages. In a maple forest of Forillon National Park, in eastern Québec (Canada), spider assemblages on, adjacent to, and away from fallen logs were compared. We also tested how log type (coniferous vs. deciduous) and decomposition stage influenced spider assemblages. Sampling was done for an intensive four-week period using both litter samples and pitfall traps. A total of 5613 spiders representing 83 species from 16 families was collected. Spiders were affected by the presence of logs, as both species diversity and total number of individuals collected were significantly higher on the log surface compared to the forest floor. Ordination analysis revealed a distinct compositional difference between the spider fauna found on the wood surface compared to the forest floor. Wood type and decomposition stage had few significant effects on spider assemblages, except that less decayed logs supported higher spider diversity than logs in advanced stages of decay. Dead wood is clearly important for generalist predators such as spiders, further supporting the conservation importance of fallen logs in northern forest ecosystems.     

Capture efficiency and trophic adaptations of a specialist and generalist predator: A comparison

Specialist true predators are expected to exhibit higher capture efficiencies for the capture of larger and dangerous prey than generalist predators due to their possession of specialized morphological and behavioral adaptations. We used an araneophagous spider (Lampona murina) and a generalist spider (Drassodes lapidosus) as phylogenetically related model species and investigated their realized and fundamental trophic niches and their efficacy with respect to prey capture and prey handling. The trophic niche of both species confirmed that Lampona had a narrow trophic niche with a predominance of spider prey (including conspecifics), while the niche of Drassodes was wide, without any preference. DNA analysis of the gut contents of Lampona spiders collected in the field revealed that spiders form a significant part of its natural diet. Lampona captured significantly larger prey than itself and the prey captured by Drassodes. As concerns hunting strategy, Lampona grasped the prey with two pairs of legs possessing scopulae, whereas Drassodes immobilized prey with silk. Lampona possess forelegs equipped with scopulae and a thicker cuticle similar to other nonrelated araneophagous spiders. Lampona fed for a longer time and extracted more nutrients than Drassodes. We show that specialized behavioral and morphological adaptations altogether increase the hunting efficiency of specialists when compared to generalists.     

Intraguild interactions among generalist predator functional groups drive impact on herbivore and decomposer prey

Different functional groups of generalist predators may complement each other in controlling prey populations; but intraguild interactions, common among generalist predators, may also reduce the strength of top–down control. In natural communities greater alterations to ecosystem function are expected if a whole functional group declines in abundance or is lost. Therefore studying functional group diversity is important for predicting effects of predator loss. We studied the top–down impact of web‐building spiders, hunting spiders and ants, which are highly abundant generalist predators in most terrestrial ecosystems, on prey from the herbivore and decomposer system of a grassland food web. The density of the three predator groups was manipulated by continuous removal in a three‐factorial designed field experiment, which was carried out for two years. We found no positive effect of increasing predator functional group richness on prey control. However there was evidence for strong composition effects between the functional groups. The presence of ants in predator assemblages reduced the prey suppression through mostly trait‐mediated intraguild interactions, while hunting and web‐building spiders contributed additively to prey suppression and reduced the density of herbivore and decomposer prey by 50–60%. A trophic cascade on plant biomass triggered by web‐builders and hunting spiders was diminished at levels of higher predator group diversity. In conclusion, our experiments showed that intraguild interactions strongly influence the strength of top–down control by generalist predators. Among spiders there was evidence for a positive relation between functional group richness and prey suppression but the overall outcome strongly depended on the occurrence of interference, driven by trait‐mediated indirect interactions.     

Urbanization drives cross‐taxon declines in abundance and diversity at multiple spatial scales

The increasing urbanization process is hypothesized to drastically alter (semi‐)natural environments with a concomitant major decline in species abundance and diversity. Yet, studies on this effect of urbanization, and the spatial scale at which it acts, are at present inconclusive due to the large heterogeneity in taxonomic groups and spatial scales at which this relationship has been investigated among studies. Comprehensive studies analysing this relationship across multiple animal groups and at multiple spatial scales are rare, hampering the assessment of how biodiversity generally responds to urbanization. We studied aquatic (cladocerans), limno‐terrestrial (bdelloid rotifers) and terrestrial (butterflies, ground beetles, ground‐ and web spiders, macro‐moths, orthopterans and snails) invertebrate groups using a hierarchical spatial design, wherein three local‐scale (200 m × 200 m) urbanization levels were repeatedly sampled across three landscape‐scale (3 km × 3 km) urbanization levels. We tested for local and landscape urbanization effects on abundance and species richness of each group, whereby total richness was partitioned into the average richness of local communities and the richness due to variation among local communities. Abundances of the terrestrial active dispersers declined in response to local urbanization, with reductions up to 85% for butterflies, while passive dispersers did not show any clear trend. Species richness also declined with increasing levels of urbanization, but responses were highly heterogeneous among the different groups with respect to the richness component and the spatial scale at which urbanization impacts richness. Depending on the group, species richness declined due to biotic homogenization and/or local species loss. This resulted in an overall decrease in total richness across groups in urban areas. These results provide strong support to the general negative impact of urbanization on abundance and species richness within habitat patches and highlight the importance of considering multiple spatial scales and taxa to assess the impacts of urbanization on biodiversity.