Intraguild interactions between spiders and ants and top-down control in a grassland food web

In most terrestrial ecosystems ants (Formicidae) as eusocial insects and spiders (Araneida) as solitary trappers and hunters are key predators. To study the role of predation by these generalist predators in a dry grassland, we manipulated densities of ants and spiders (natural and low density) in a two-factorial field experiment using fenced plots. The experiment revealed strong intraguild interactions between ants and spiders. Higher densities of ants negatively affected the abundance and biomass of web-building spiders. The density of Linyphiidae was threefold higher in plots without ant colonies. The abundance of Formica cunicularia workers was significantly higher in spider-removal plots. Also, population size of springtails (Collembola) was negatively affected by the presence of wandering spiders. Ants reduced the density of Lepidoptera larvae. In contrast, the abundance of coccids (Ortheziidae) was positively correlated with densities of ants. To gain a better understanding of the position of spiders, ants and other dominant invertebrate groups in the studied food web and important trophic links, we used a stable isotope analysis (¹⁵N and ¹³C). Adult wandering spiders were more enriched in ¹⁵N relative to ¹⁴N than juveniles, indicating a shift to predatory prey groups. Juvenile wandering and web-building spiders showed δ¹⁵N ratios just one trophic level above those of Collembola, and they had similar δ¹³C values, indicating that Collembola are an important prey group for ground living spiders. The effects of spiders demonstrated in the field experiment support this result. We conclude that the food resource of spiders in our study system is largely based on the detrital food web and that their effects on herbivores are weak. The effects of ants are not clear-cut and include predation as well as mutualism with herbivores. Within this diverse predator guild, intraguild interactions are important structuring forces.     

Negative effects of ant foraging on spiders in Douglas-fir canopies

 Spiders and ants are potential competitors and mutual predators. Indirect evidence from previous research has suggested that ant foraging may significantly lower the abundance of arboreal spiders in young Douglas-fir plantations in western Oregon. This study tested the effect of foraging by ants, dominated by Camponotus spp., on spider assemblages in Douglas-fir canopies in a 5-month ant-exclusion experiment. The biomass of potential prey organisms on foliage, dominated by Psocoptera, increased significantly by 1.9- to 2.4-fold following ant exclusion. The removal of ants did not affect the abundance of flying arthropods in the vicinity of tree canopies as indicated by sticky trap catches. The abundance of hunting spiders, the majority being Salticidae, increased significantly by 1.5- to 1.8-fold in trees without ants in the late summer; neither the abundance of web-building spiders nor the average body size of hunting and web-building spiders were significantly affected by ant removal. Spider diversity and community structure did not differ significantly between control and ant-removal trees. The majority of prey captured by ants were Aphidoidea (48.1%) and Psocoptera (12.5%); spiders represented only 1.4% of the ants’ diet. About 40% of observed ants were tending Cinara spp. aphids. Our observations suggest that the lower abundance of hunting spiders in control canopies with ants may be due to interference competition with ants resulting from ant foraging and aphid-tending activities. Direct predation of spiders by ants appeared to be of minor importance in this study system. This study did not provide sufficient evidence for exploitative competition for prey between ants and spiders. Received: 21 February 1996 / Accepted: 14 August 1996     

Prey preferences ofPortia fimbriata,an araneophagic,web-building jumping spider (Araneae: Salticidae) from Queensland

Portia fimbriata from Queensland, a previously studied jumping spider (Salticidae), routinely includes web-building spiders and cursorial salticids in its diet, both of these types of prey being dangerous and unusual prey for a salticid. The present paper is the first detailed study ofP. fimbriata's prey preferences. Three basic types of tests of prey preference were used, providing evidence that (1)P. fimbriata males and females prefer spiders (both web-building spiders in webs and salticids away from webs) to insects; (2)P. fimbriata males and females prefer salticids to web-building spiders; (3)P. fimbriata males and females prefer larger spiders to smaller spiders; (4) there are intersexual differences in the preferences ofP. fimbriata for prey size, females preferring larger prey and males preferring smaller prey; and (5)P. fimbriata's prey preferences are not affected by a prior period without food of 2 weeks. When preferences were tested for by using both living, active prey and dead, motionless lures, the same preferences were expressed, indicating thatP. fimbriata can distinguish among different types of prey independent of the different movement patterns of different prey.     

Interactions between citrus psylla,Trioza erytreae (Hem. Triozidae), and spiders in an unsprayed citrus orchard in the transvaal lowveld

During a two year survey, a total of 3,054 spiders represented by 21 families were sampled in an unsprayed citrus orchard in the Transvaal Lowveld. Numerically the Salticidae was the dominant family (34.4%) followed by the Theridiidae (21.9%), Thomisidae (11.9%), Araneidae (7.9%), Clubionidae (7.0%) and the Tetragnathidae (3.7%). Eighteen species of spiders were observed to prey on citrus psylla,Trioza erytreae (Del Guercio) (Hemiptera: Triozidae), while six species trap nymphs and adults under their retreats and webs. There were significant positive correlations between the weekly psylla populations and the weekly populations of web-building spiders and wandering spiders present one to four weeks later but no significant correlation between the weekly spider populations and the weekly psylla populations present one to five weeks later. This seems to indicate that while spiders are unable to keep citrus psylla populations at acceptable low levels, they may contribute in reducing their numbers.      

Contrasting responses of web-building spiders to deer browsing among habitats and feeding guilds

We examined web-building spider species richness and abundance in forests across a deer density gradient to determine the effects of sika deer browsing on spiders among habitats and feeding guilds. Deer decreased the abundance of web-building spiders in understory vegetation but increased their abundance in the litter layer. Deer seemed to affect web-building spiders in the understory vegetation by reducing the number of sites for webs because vegetation complexity was positively correlated with spider density and negatively correlated with deer density. In contrast, the presence of vegetation just above the litter layer decreased the spider density, and deer exerted a negative effect on this vegetation, possibly resulting in an indirect positive effect on spider density. The vegetation just above the litter layer may be unsuitable as a scaffold for building webs if it is too flexible to serve as a reliable web support, and may even hinder spiders from building webs on litter. Alternatively, the negative effect of this vegetation on spiders in the litter may be as a result of reduced local prey availability under the leaves because of the reduced accessibility of aerial insects. The response to deer browsing on web-building spiders that inhabit the understory vegetation varied with feeding guild. Deer tended to affect web-invading spiders, which inhabit the webs of other spiders and steal prey, more heavily than other web-building spiders, probably because of the accumulated effects of habitat fragmentation through the trophic levels. Thus, the treatment of a particular higher-order taxon as a homogeneous group could result in misleading conclusions about the effects of mammalian herbivores.     

Influence of prey availability and conspecifics on patch quality for a cannibalistic forager: laboratory experiments with the wolf spider Schizocosa

 Because cannibals are potentially both predator and prey, the presence of conspecifics and alternative prey may act together to influence the rate at which cannibals prey upon each other or emigrate from a habitat patch. Wolf spiders (Lycosidae) are cannibalistic-generalist predators that hunt for prey with a sit-and-wait strategy characterized by changes in foraging site. Little information is available on how both prey abundance and the presence of conspecifics influence patch quality for these cursorial, non-web-building spiders. To address this question, laboratory experiments were conducted with spiderlings and older juveniles of the lycosid genus Schizocosa. The presence of insect prey consistently reduced rates of spider emigration when spiders were housed either alone or in groups. Solitary juvenile Schizocosa that had been recently collected from the field exhibited a median giving-up time (GUT) of 10 h in the absence of prey (Collembola); providing Collembola increased the median GUT to 64 h. For solitary spiders, the absence of prey increased by about fourfold the rate of emigration during the first 24 h. In contrast, for spiders in patches with a high density of conspecifics, the absence of prey increased the 24-h emigration rate by only 1.6-fold. For successful cannibals in the no-prey patches, the presence of conspecifics improved patch quality by providing a source of food. Mortality by cannibalism was affected by both prey availability and openness of the patch to net emigration. In patches with no net emigration, the presence of prey reduced rates of cannibalism from 79% to 57%. Spiders in patches open to emigration but not immigration experienced a rate of cannibalism (16%) that was independent of prey availability. The results of these experiments indicate that for a cannibalistic forager such as the wolf spider Schizocosa, (1) the presence of conspecifics can improve average patch quality when prey are absent, and (2) cannibalism has the potential to be a significant mortality factor under natural field conditions because cannibalism persisted in prey patches that were open to emigration. Received: 12 April 1996 / Accepted: 14 August 1996     

Alternative predatory tactics of an araneophagic assassin bug (Stenolemus bituberus)

Predators of dangerous prey risk being injured or killed in counter-attacks and hence may use risk-reducing predatory tactics. Spiders are often dangerous predators to insects, but for a few, including Stenolemus bituberus assassin bugs, web-building spiders are prey. Despite the dangers of counter-attack when hunting spiders, there has been surprisingly little investigation of the predatory tactics used by araneophagic (spider-eating) insects. Here, we compare the pursuit tendency, outcome and predatory tactics of S. bituberus against five species of web-building spider. We found that S. bituberus were most likely to hunt and capture spiders from the genus Achaearanea, a particularly common prey in nature. Capture of Achaearanea sp. was more likely if the prey spider was relatively small, or if S. bituberus was in poor condition. S. bituberus used two distinct predatory tactics, ‘stalking’, in which they slowly approached the prey, and ‘luring’, in which they attracted spiders by manipulating the web to generate vibrations. Tactics were tailored to the prey species, with luring used more often against spiders from the genus Achaearanea, and stalking used more often against Pholcus phalangioides. The choice of hunting tactic used by S. bituberus may reduce the risk posed by the prey spider.     

Side-effects of insecticides on two erigonid spider species

The current rearing technique forErigone atra (Blackwall) andOedothorax apicatus (Blackwall) (Araneae, Erigonidae) was improved. To reduce time spent rearing on live fruit flies the spiders were kept on a culture of the Collembola speciesLepidocyrtus lanuginosus (Gmelin) (Entomobryidae). Side-effects on spiders of two pyrethroid insecticides (fenvalerate and lambda-cyhalothrin) and one carbamate insecticide (pirimicarb) were tested. Sensitivity of adults of both sexes and juveniles to insecticides and their influence on the rate of emergence of spiderlings from cocoons were investigated using topical application, spraying or residual contact. LD₅₀ values for adults ranged from 0.49 to 2.52 ng a.i./spider for lambda-cyhalothrin and from 5.75 to 98.20 ng a.i./spider for fenvalerate. Topical application also resulted in up to a week's delay of web-building. A moving laboratory spraying equipment was used to spray spiders with different insecticide dosages and water volumes. Pyrethroids sprayed onto adults in webs had stronger effects than pyrethroids sprayed onto sitting or walking spiders on the soil surface. Residual contamination caused higher mortality of spiders after contact with lambda-cyhalothrin than fenvalerate. In all tests, males were more susceptible to pyrethroids than females; this difference was related to body weight. Mortality rate was higher forE. atra than forO. apicatus. Both pyrethroids were also toxic to spiderlings. Lambda-cyhalothrin inhibited emergence ofE. atra spiderlings from cocoons. Pirimicarb was harmless to both spider species.     

Differences in venom composition between orb-weaving and wandering Hawaiian Tetragnatha (Araneae)

Spider venoms are complex mixtures of toxins that are used primarily for immobilizing prey. There is evidence of chemical variation in spider venoms among close relatives, yet few studies have analysed their evolution within an ecological and phylogenetic framework. On the Hawaiian archipelago, Tetragnatha, a cosmopolitan orb-weaving genus, has undergone a radiation in which a monophyletic lineage has abandoned web-building and become obligately wandering foragers. This study compares venom composition and details of feeding behaviour between orb-weaving and wandering Hawaiian Tetragnatha. Protein gel electrophoresis patterns indicated that relative to orb-weavers, wandering species had a reduced concentration of low molecular weight (<14kDa) components. Both orb-weaving and wandering Tetragnatha captured flying prey (adult lepidopterans, dipterans), but wandering spiders also captured non-flying prey (insect larvae, spiders). There were no distinct differences between orb-weavers and wanderers in prey capture and immobilization sequences, or in the paralytic effects of bites on prey. However, prey bitten by wanderers took longer to be permanently immobilized than prey bitten by orb-weavers. Contrary to predictions, there was no indication that web-loss in this group was associated with an increase in venom potency.     

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.     

The effect of hunger level on predation dynamics in the spider Nesticodes rufipes: a functional response study

It is well known that a predator has the potential to regulate a prey population only if the predator responds to increases in prey density and inflicts greater mortality rates. Predators may cause such density-dependent mortality depending on the nature of the functional and numerical responses. As spiders are usually faced with a shortage of prey, the killing behavior of the spider Nesticodes rufipes at varying densities of Musca domestica was examined here through laboratory functional response experiments where spiders were deprived of food for 5 (well-fed) or 20 days (hungry). An additional laboratory experiment was also carried out to assess handling time of spiders. The number of prey killed by spiders over 24- and 168-h periods of predator–prey interaction was recorded. Logistic regression analyses revealed the type II functional response for both well-fed and hungry spiders. We found that the lower predation of hungry spiders during the first hours of experimentation was offset later by an increase in predation (explained by estimated handling times), resulting in similarity of functional response curves for well-fed and hungry spiders. It was also observed that the higher number of prey killed by well-fed spiders over a 24-h period of spider–prey interaction probably occurred due to their greater weights than hungry spiders. We concluded that hungry spiders may be more voracious than well-fed spiders only over longer time periods, since hungry spiders may spend more time handling their first prey items than well-fed spiders.     

Utilization of prey from the decomposer system by generalist predators of grassland

We investigated the linkage between the detrital subsystem and generalist predators of meadow ecosystems by manipulating prey availability in two different ways: we increased resource availability for the decomposer subsystem and thereby decomposer prey by adding mulch materials (detritus enhancement), and we added fruitflies (Drosophila melanogaster, Diptera; prey enhancement) to fenced plots. Both supplemented materials significantly differed in their ¹³C/¹²C and ¹⁵N/¹⁴N ratios from those of the natural litter. We measured density responses of detritivorous, herbivorous and predaceous arthropods to the increased resource supply. We used ratios of natural stable isotopes of N and C in arthropod tissues to trace the flux from the added resources to consumers and to relate density responses of consumers to changes in resource supply. Effects of resource enhancement propagated through at least two trophic levels, resulting in higher densities of major decomposer and predator taxa. Effects of detritus enhancement were much stronger than those of prey enhancement. Signatures of δ¹³C proved density responses of Collembola taxa to be related to the added mulch materials. Among generalist predators, densities of juvenile wolf spiders (Lycosidae) responded more to detritus-enhancement than to prey-enhancement treatments. In contrast, the density of the web-building linyphiid and the non-web gnaphosid spiders remained unaffected. Each spider taxon, including those which did not respond numerically, was significantly enriched in ¹³C in detritus-enhancement treatments, suggesting that they gain energy from the decomposer system. Numbers of herbivores—cicadellids and aphids—were similar in each of the treatments, indicating that they were unaffected by changes in predator density. Our results indicate that the lack of a numerical response to resource supplementation is not necessarily due to the absence of a trophic linkage, but may be caused by compensatory changes in mortality factors such as cannibalism and intraguild predation.     

Field predation of twospotted spider mite in a New Zealand strawberry crop

Predation by naturally occurring predatory arthropods was investigated to explain variations in population numbers of twospotted spider mite (Tetranychus urticae Koch) between first and second season strawberry crops. Araneomorph spiders. European harvestman [Phalangium opilio (L.)], Tasmanian lacewing [Micromus tasmaniae (Walker)] and Pacific damsel bug [Nabis kinbergii Reuter] were the only predators found in high numbers. However, spiders and harvestment were more prevalent than lacewings and nabids. Laboratory feedings trials indicated spiders build horizontal webs in the plants and prey predominantly on small flying insects that shelter in the crops. Similar feeding trials cofirmed the palatability of TSSM to spiders and harvestmen. Immunological testing for proteins of TSSM, aphids, Collembola and leafrollers in the intestines of field collected European harvestman, spiders, Tasmanian lacewing and Pacific damsel bug confirmed European harvestman includes TSSM in its diet, but not in large enough quantities to exert a significant regulating pressure on TSSM populations. Lacewings and nabids include TSSM in their diets but only in very small quantities (2% and 1% respectively). Spiders do not take TSSM unless they drop or spin down onto the spider webbing. The immunological testing also showed European harvestman to be a polyphagous and opportunistic feeder. Prey residues were detected more frequently in harvestmen intestines at times of prey abundance which indicated a seasonality to harvestmen diet.      

The impact of altered precipitation on spatial stratification and activity-densities of springtails (Collembola) and spiders (Araneae)

Abstract.  1. A field experiment was conducted to determine how short-term changes in moisture can alter activity-densities of spiders and springtails.
2. In a Kentucky forest 10 unfenced 4-m² plots were divided into two rainfall treatments. A clear roof over five plots excluded rainfall to simulate severe drought conditions ( drought treatment). Water was sprayed on the five uncovered plots at a rate equal to two times the long-term mean in order to establish the high-rainfall treatment. Activity-densities of Collembola and spiders were measured using pitfall traps designed to sample the top, middle, and bottom layers of leaf litter. The experiment ran from 20 July to 23 September 2001.
3. Overall (i.e. litter layers pooled) activity-density (mean number trapped each sampling date) of Collembola was ≈ 60% lower in drought plots than in plots receiving increased precipitation. Surprisingly, overall spider activity-density was ≈ 1.6 times greater in the drought plots.
4. Differences in rainfall affected the spatial stratification of Collembola and spiders in strikingly different ways. Activity-densities of neither group differed between drought and high-rainfall treatments in the bottom litter layer. Collembola activity-density was three times greater in the top and middle litter layers in high-rainfall plots than in drought plots. In contrast, spider activity-density did not differ between treatments in the top layer, but activity-density was decreased by 50% in the middle layer of high-rainfall plots compared with drought plots.
5. Three Collembola families (Sminthuridae, Tomoceridae, and Entomobryidae) accounted for most of the Collembola pattern. The spider response was due to altered activity-density of one family of wandering spider, the Gnaphosidae.     

Species-Specificity of Chemical Signals: Silk Source Affects Discrimination in a Wolf Spider (Araneae: Lycosidae)

Female spiders deposit chemical cues that elicit male courtship behavior with silk. These cues are often assumed to be species-specific although male spiders may court in response to chemical cues of closely-related species. We used behavioral assays to test the extent of species discrimination of female chemical cues by male Schizocosa ocreata, a wolf spider (Lycosidae). Discrimination, expressed as relative courtship intensity of males, varied significantly with phylogenetic distance. Males did not discriminate between female cues of conspecifics and a sibling species, S. rovneri. Courtship response was intermediate for another species within the ocreata clade and not different from control for spiders outside the clade. These findings support the sibling species status of S. ocreata and S. rovneri, and also suggest the composition of female chemical signals is conserved across closely related wolf spider species.     

Experimental assessment of trophic ecology in a generalist spider predator: Implications for biocontrol in Uruguayan crops

Conservative biological control promotes the use of native natural enemies to limit the size and growth of pest populations. Although spiders constitute one of the most important groups of native predators in several crops, their trophic ecology remains largely unknown, especially for several generalist taxa. In laboratory, we assessed the predatory behaviour of a wandering spider (the wolf spider Lycosa thorelli (Keyserling, 1877) against several arthropods varying in size and trophic positions, all found in South American soybean and rice crops. As prey we used the bug Piezodorus guildinii (Westwood, 1837) as well as larvae and adults of the moth Spodoptera frugiperda (Smith, 1797), both being considered important pests in Uruguayan crops. We also used several non-pest arthropods as prey, sarcophagid flies, carabid beetles and wolf spiders. All prey were attacked in more or less high, although not statistically differing, proportions. However, carabids were not consumed, and bugs were consumed in significantly lower proportions than flies. A negative correlation was found between prey size and acceptance rate. Immobilization times were longer against larvae when compared to moths and flies, while predatory sequences were longer for bugs when compared to flies, moths and spiders. In addition, we found a positive effect of prey size on predatory sequence length and complexity. Our results confirm the ability of spiders to attack and feed upon prey with different morphologies, included well-defended arthropods, and their potential use as natural enemies of several pests in South American crops.     

Dynamics of aquatic insect flux affects distribution of riparian web-building spiders

The flux of emerging aquatic insects from streams can provide a significant energy subsidy to riparian web-building spiders. However, despite the high temporality of aquatic insect emergence, the effects of such aquatic insect dynamics on spider distribution are poorly understood. To examine the relationship, the aquatic insect flux from a headwater stream in a northern Japanese deciduous forest was experimentally manipulated by using a greenhouse-type covering, during May to July. Under natural conditions, the aquatic and terrestrial insect abundances dramatically decreased and increased from May through July, respectively. The experimental reduction of aquatic insect flux depressed the density of horizontal orb weavers (Tetragnathidae) in both May and June, but not in July when aquatic insects were scarce, indicating a temporal limitation on spider distribution by aquatic insect flux. In contrast, the densities of both vertical orb weavers (Araneidae) and sheet weavers (Linyphiidae) were unaffected by the manipulation throughout the study period. These various responses, differing among months or spider guilds, may be attributed to the degree of specialization for aquatic prey in the spiders and their mobility in response to aquatic insect flux. The experimental results provided direct evidence that the temporal dynamics of aquatic insect flux, as well as spider characteristics, were primary factors determining the distributional patterns of riparian web-building spiders.     

Contrasting effects of habitat fragmentation,population density,and prey availability on body condition of two orb‐weaving spiders

1. Habitat fragmentation is a major threat to biodiversity because it disrupts movement between habitat patches. In addition, arthropod fitness may be reduced in fragmented habitats, e.g. due to reduced prey availability. 2. We studied the relationship of spider body condition with habitat fragmentation, population density, and prey availability. We expected that prey availability and population density of spiders would be affected by landscape composition and patch isolation. Body condition should be enhanced by high prey availability, but negatively affected by population density due to competition. 3. We sampled spiders on 30 groups of cherry trees that varied independently in the level of isolation from other woody habitats and in the percentage of woody habitat within 500 m radius. As a measure of body condition, we used residuals of the relationship between individual body mass/opisthosoma width and prosoma width of the two most common orb‐weaving spider species, Nuctenea umbratica Clerck and Araniella opisthographa Kulczynski. 4. Body condition of A. opisthographa was positively correlated with the abundance of flies, which increased with the percentage of forest in the landscape. In contrast, body condition of N. umbratica was reduced at high population densities, presumably due to intraspecific competition. In addition, body condition and population density of A. opisthographa was lower at isolated sites. 5. Our study suggests that effects of landscape fragmentation on body condition vary strongly between spider species, depending on the relative role of food limitation and intraspecific competition.     

Importance of habitat structure to the arthropod food-web in Douglas-fir canopies

This study tested the hypothesis that habitat structure dictates the distribution and community composition of arboreal arthropods. A diverse arthropod assemblage of Douglas-fir canopies, which included Araneae, Psocoptera, Collembola and Homoptera, was chosen as a model system. Habitat structural diversity, defined as needle density and branching complexity of Douglas-fir branches, was manipulated in a four-month experiment by needle removal, thinning and tying of branches. Abundance of canopy spiders declined significantly following needle density reduction and branch thinning, branch tying significantly increased spider abundance. Distinct habitat utilization patterns were found among individual spider guilds. Orb weavers (Araneidae) dominated spider assemblages in structurally simple habitats, whereas tied branches were colonized primarily by sheet-web weavers (Linyphiidae) and nocturnal hunting spiders (Anyphaenidae and Clubionidae). Spider species richness and average body size of several spider species increased in structurally more complex habitats. Arboreal spiders appeared to be limited by strong bottom-up effects in the form of habitat quality and, to a lesser degree, prey availability. Habitat manipulations did not affect densities or biomass of flying arthropod colonists in the branch vicinity. Needle removal and branch thinning led to a significant decline in the abundance of Psocoptera and Collembola. Tying of branches resulted in an eight-fold increase in Collembola numbers, organisms most sensitive to habitat alterations. Canopy habitat structure modified vertical dispersal of Collembola from forest litter, which may have significant implications for arboreal consumers. Our results lend strong support to the importance of habitat structural diversity in explaining general patterns of arthropod abundance and diversity on plants.