Habitat selection in a large orb‐weaving spider: vegetational complexity determines site selection and distribution

1. The distribution of the large orb‐weaving spider Argiope trifasciata in old field habitats of North America and the habitat selection process this species used was studied for 2 years. 2. Because web spiders have limited dispersal abilities and an energetically costly prey capture device, they do not have the ability to sample potential foraging sites. Structural complexity of the vegetation to which the web must be attached is relatively easy to assess. The hypothesis that the structural complexity is a primary factor in determining initial web site selection was tested both by relating the natural distribution of the spiders across habitats to vegetational complexity and by manipulating the complexity of the habitats in a series of experiments. 3. Argiope trifasciata was not distributed evenly among three old field vegetation types. Habitat complexity was related to spider density in both years although no measure of insect activity, prey capture, or prey consumption was correlated with spider distribution. 4. Three experimental manipulations were conducted to test the impact of habitat structure on spider establishment: (1) the amount of natural vegetation was reduced, (2) structures were added to a simple habitat, and (3) the complexity of the structures added was varied. In each case, spiders were introduced and establishment of webs was monitored. In all manipulations, spider establishment was related to the complexity of the substrate available. 5. These results are important for understanding the cues that influence foraging site selection and therefore provide insight into the distribution of species with limited dispersal abilities and high site investment requirements.     

Habitat selection and web plasticity by the orb spider Argiope keyserlingi (Argiopidae): Do they compromise foraging success for predator avoidance?

Abstract Orb web spiders face a dilemma: forage in open habitats and risk predation or forage in closed habitats to minimize risk but at reduced foraging profitability. We tested whether Argiope keyserlingi opts for safer habitats at the expense of foraging success by (i) determining habitat selection indices in open and closed habitats; (ii) marking and releasing individual juvenile, subadult and adults over two 4‐week periods to determine if life‐history stage influences habitat selection; and (iii) determining the biotic and abiotic environmental parameters that relate to A. keyserlingi abundance. We found that A. keyserlingi selected closed habitats. Sedge and anthropogenic structures were selected and trees were avoided. Juveniles were never found in open habitats, most likely because of high postdispersal mortality. Subadults and adults may shift from closed to open habitats while juveniles never shifted habitat. Foliage density, plant height, potential prey abundance, and mantid and bird abundance were correlated with A. keyserlingi abundance, with only bird abundance explaining habitat selection. We measured web capture area, spiral distance (distance between spiral threads) and the number of decoration arms (0, 1, 2, 3 or 4) in the field and did laboratory experiments to test the influence of (i) space and vegetation; (ii) prey abundance; and (iii) web damage, on web architecture. Argiope keyserlingi webs exhibited geometric plasticity by having larger prey capture areas and spiral distances in open habitats. Decoration design did not differ between habitats however. Variation in space availability, air temperature, prey abundance and web damage explained the variations in web architecture. Potential prey size and diversity differed between habitats but prey abundance did not. As large prey may be important for spider survivorship, foraging success appears to be compromised by occupying closed habitats.     

Carbohydrates in the webs of Argiope spiders

Glycoproteins are present in the web of the orb-weaving spiders Argiope trifasciata and Argiope aurantia. Periodic acid-Schriff reactive glyco-proteins are confined in large part, to the sticky spiral and sticky spiral-radial junctions. Glycoproteins containing amino sugars appear associated with all fibers, especially the radial fibers. Enzymes may be used to remove glycoproteins selectively from the sticky spiral and stabilimentum.     

Linking properties of an orb‐weaving spider's capture thread glycoprotein adhesive and flagelliform fiber components to prey retention time

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Do stabilimenta in orb webs attract prey or defend spiders?

Orb-weaving spiders are ideal organisms for the study of conflictbetween behavioral investments in foraging and defense becausetheir webs provide physical manifestations of those investments.We examined the impact of including stabilimenta, designs ofbright-white noncapture silk, at the center of orb webs forforaging and defense in Argiope aurantia. Our findings suggestthat stabilimentum building is a defensive behavior, supportingthe "web advertisement" hypothesis that the high visibilityof stabilimenta can prevent birds from flying through webs.Yet, spiders often do not include stabilimenta in their webs,indicating that a serious cost is associated with them. We alsoshow, through comparison of paired webs with and without stabilimenta,that stabilimenta reduce the prey capture success of spidersby almost 30%. This demonstrates the potential impact that defensivebehaviors of spiders can have on their foraging success andsuggests that much of the variation in stabilimenta may be accountedfor by a cost—benefit trade-off made when including stabilimentain webs.     

Augmentation of soil detritus affects the spider community and herbivory in a soybean agroecosystem

If soil detritivores provide a significant prey source for predators in the vegetation, then augmentation of the soil community could affect the grazing food web. Specifically, increases in predator density could enhance any top‐down effects and reduce herbivory. We tested this hypothesis by providing detrital subsidies in the form of composted vegetable matter to 36 m² plots in soybean, Glycine max (L.) Merr. (Fabales: Fabaceae), fields that were managed using either conventional or conservation tillage practices. The foliage‐dwelling spiders, insect predators, and leaf‐chewing insects were censused and the body size of one large spider species, Argiope trifasciata (Forskål) (Araneae: Araneidae), was measured. In addition, the density and size of the plants were assessed and leaf damage was quantified. Any effects of treatments on the palatability of soybean plants to herbivores were determined in two laboratory experiments. Compost increased the density of foliage dwelling spiders and the abdomen size of A. trifasciata. We uncovered no treatment effects on insect predators, herbivorous insects, or plant characteristics except that compost addition reduced leaf damage. In addition, there was a negative correlation across plots between spider abundance and soybean leaf damage and abdomen width of A. trifasciata and weed herbivory levels across plots. These results suggest a connection between the soil community and the foliage food web, but the spiders appear to have exerted a top‐down effect without a shift in herbivore abundance. Further study of the specific seasonality of the herbivores and their behavior in the presence of spiders are needed to uncover the underlying mechanism. Nevertheless, these results provide evidence for complex linkage between the soil and grazing food webs that may be important to biological control.     

Signalling conflict between prey and predator attraction

Predators may utilize signals to exploit the sensory biases of their prey or their predators. The inclusion of conspicuous silk structures called decorations or stabilimenta in the webs of some orb‐web spiders (Araneae: Araneidae, Tetragnathidae, Uloboridae) appears to be an example of a sensory exploitation system. The function of these structures is controversial but they may signal to attract prey and/or deter predators. Here, we test these predictions, using a combination of field manipulations and laboratory experiments. In the field, decorations influenced the foraging success of adult female St. Andrew’s Cross spiders, Argiope keyserlingi: inclusion of decorations increased prey capture rates as the available prey also increased. In contrast, when decorations were removed, prey capture rates were low and unrelated to the amount of available prey. Laboratory choice experiments showed that significantly more flies (Chrysomya varipes; Diptera: Calliphoridae) were attracted to decorated webs. However, decorations also attracted predators (adult and juvenile praying mantids, Archimantis latistylus; Mantodea: Mantidae) to the web. St. Andrew’s Cross spiders apparently resolve the conflicting nature of a prey‐ and predator‐attracting signal by varying their decorating behaviour according to the risk of predation: spiders spun fewer decorations if their webs were located in dense vegetation where predators had greater access, than if the webs were located in sparse vegetation.     

Feeding ecology of the orb-weaving spiderArgiope aurantia [Araneae: Araneidae] in a cotton agroecosystem

Twenty females of the orb-weaving spiderArgiope aurantia Lucas were introduced into a cotton field in east Texas in order to study the feeding ecology of this spider. In the 24 h after the release of these spiders in the cotton field, one had moved over a distance of 53 m. The released spiders spun webs with an average diameter of 33.5 cm with the hub an average of 39 cm above the ground. The diet ofA. aurantia was diverse which characterizes this species as a food generalist. Major food components were aphids (30%), Diptera (26.8%), grasshoppers (17.9%), and Hymenoptera (12.6%). The spiders' prey length ranged from 0.4 to 47 mm (mean =7.7±0.83 mm). Adult females ofA. aurantia have the potential to kill prey of up to ca. 200% of their own size. However, two-thirds of the prey items had a length of <3 mm, while only 25% of the prey items had a length of ≥20 mm.A. aurantia was found to be a predator of the cotton fleahopper (about 1% of the spiders' diet), which is a key pest of cotton.      

Correction to: Testing traditional hypotheses about prey capture efficiency in orb-web spiders

To capture prey, orb-web spiders create complex traps whose efficiency is contingent on a variety of factors that are not yet completely understood, including web size, competition for food, sun exposure, presence of web decorations and web orientation. Here we evaluate such factors in the field and ask which of them are the most influential variables affecting the quantity of prey captured in Argiope argentata webs. Webs were observed during the morning and the number of prey attached to each web was counted. Using the approach of information criteria based on the Akaike information criterion (AIC) values of each candidate model, we averaged the parameters of a global model, finding that the only predictor which 95% confidence interval did not include zero, was exposure to sunlight (whether the web is continuously shaded or continuously exposed to sunlight). All other variables did not explain variation in prey capture. We conclude that only sun exposure has an important effect on orb-web spiders’ prey capture efficiency in A. argentata. We additionally argue that silk decorations have different functions depending on the habitat and the species.

     

Mesh Width Influences Prey Retention in Spider Orb Webs

Orb‐weaving spiders depend upon the sticky capture spirals of webs to retain insects long enough to be captured. However, insects often escape from orb webs before the spiders can attack them. Therefore, the architectures of orb webs likely reflect strong selective pressure to increase retention times of insects. We experimentally increased the mesh width of one side of an orb web while maintaining the original mesh width on the other side as a control, and then tested the effect of this manipulation on the retention times of four different taxa of insects. We found evidence that increased mesh width of Argiope aurantia orb webs resulted in a general reduction in the retention times of insects. However, retention times for different taxa of insects were not predicted by any one specific morphological or flight characteristic. The influence of mesh width on the retention times of insects is very complex, but our results suggest that mesh width can act to selectively favor the capture of certain taxa of insect prey over others. This effect may help to explain both species level differences in web‐building behaviors and variation in the architectures of webs spun by individual spiders on different days.     

Ontogeny of Web-building Behavior in Two Orb-weaving Spiders

Spideis of similar miss but of two different species Araneusdiadematus Cl. and Argiope aurantia L., build webs which havesimilar geometric patterns, but which differ in relative proportionsand thread number. Within species, webs may also differreliably,presumably on a genetic basis. Fine detail of the web undergoeschange throughout the lifetime of the spider, different formale and female; these changes are not simply response to thegrowing weight of the spider. Relatively simple restraints,such as availability of material for thread, result in adaptationsof web-building which may at first appear to be based upon buildingexperience, but which are in fact independent of it.     

Prey analysis of four species of tropical orb-weaving spiders (Araneae: Araneidae) and a comparison with araneids of the temperate zone

Summary The actual prey in the orb webs of four araneid spiders (Nephila clavipes, Eriophora fuliginea, Argiope argentata, and A. savignyi) and the relative abundance of their potential prey (pitfall traps, yellow traps, and sweep-netting) was investigated over 1 year at different locations in Panama. The relative abundance of insects and spiders depends on seasonal fluctuations (Fig. 2) which are reflected by corresponding variations in the effectiveness of the webs. The main prey groups are Nematocera (50%–68%), winged Formicoidea (6%–15%) and Hymenoptera, Coleoptera, and Brachycera (4%–10% each) (Fig. 4-6). The remaining 10%–17% of the prey comes from up to 26 other groups (Table 2). Differences in prey size and prey composition between the spider species are small (Fig. 7). Most prey items are 1–2 mm long: only a few insects exceed 30 mm body length (Figs. 9–12). Relative to the available prey, some groups (e.g. Nematocera, Aphidoidea, Psocoptera) are caught selectively, while other groups (e.g. Heteroptera, Coleoptera, Brachycera, Orthoptera) are underrepresented in the prey spectrum and obviously avoid orb webs (Table 7). The differences in prey composition between araneids of the tropics and of the temperate zone are discussed (Table 8) and compared to those recorded in other studies (Table 9, 10). Most of these report large numbers of big prey items (Odonata, Lepidoptera, wasps/bees). It is pointed out that those studies do not take into account the total available prey in a spider's web but only that part which the spider selects from the web (mainly according to size). The importance of small prey items even for large spiders is explained and an obvious lack of niche partitioning among coexisting araneids is discussed (Table 11).     

Foraging strategies of Eriophora transmarina and Nephila plumipes (Araneae: Araneoidea): Nocturnal and diurnal orb-weaving spiders

Abstract The foraging behaviour, web characteristics and prey availability of two sympatric orb-weaving spiders, Nephila plumipes and Eriophora transmarina (Araneae: Araneoidea), are compared. The spiders are similarly sized but have different temporal foraging patterns. Nephila plumipes spins a relatively permanent web and captures most of its prey during the day. Eriophora transmarina only forages at night, spinning a new web every night and usually dismantling it at dawn. These different foraging activities are most likely to be responsible for the observed differences in the types and rates of prey capture: E. transmarina captured mostly Lepidoptera that were more abundant at night than during the day, while N. plumipes captured mostly Hymenoptera that were more abundant during the day than at night. While nocturnal E. transmarina have less time available for foraging than the diurnal N. plumipes, the former has a substantially higher nocturnal prey capture rate. We argue that the difference between the species in their prey capture rates are likely to be due to differences in the architecture of their webs.     

Effect of abiotic factors on the foraging strategy of the orb‐web spider Argiope keyserlingi (Araneae: Araneidae)

Abstract Environmental conditions such as light level, background contrast and temperature might influence a spider's prey capture success and risk of predation. Thus it may often be advantageous for spiders to adjust web‐building behaviour in response to variation in these environmental conditions. This hypothesis was examined in a study of the construction of webs and web decorations (conspicuous strands of silk at the hub of the web) of the orb‐web spider Argiope keyserlingi. Web decorations are thought to have one or more separate functions. They may attract prey, deter predators or advertise the web to oncoming birds, thus preventing web damage. In this series of experiments, relationships between weather parameters and the construction of webs and web decorations were considered. In complementary laboratory experiments, A. keyserlingi spiders were exposed to two different light levels (700 and 90 lx), background contrasts (black and white) and temperature conditions (20 and 26°C). Of the available weather parameters, only temperature was significantly related to web decorating behaviour but not to web size. In the laboratory, temperature also influenced web‐decorating behaviour, and spiders in dim light (700 lx) constructed larger webs and longer decorations. Background contrast did not significantly alter web size or web decorations. These data suggest that when prey availability is reduced at low temperatures, spiders may use web decorations to attract prey to the web. Similarly, in dim light, spiders may build more and larger decorations to increase the visual signal to approaching prey or to advertise the web to oncoming birds.     

Developmental changes in barrier web structure under different levels of predation risk inNephila clavipes (Araneae: Tetragnathidae)

Individuals of the orb-weaving spider Nephila clavipesbuild complex webs with a region used for prey capture, the orb, and tangle webs opposite either face, the barrier webs. Barrier webs have been hypothesized to serve a variety of functions, including predator defense, and the primary function of the barrier web should be reflected in the relative size of the barrier to the orb under varying conditions of foraging success and predation risk. To investigate the effects of predation pressure and foraging success on barrier web structure, I conducted a comparative study in three disjunct populations that differed in predation risk and foraging success. Although both the orb web and the barrier webs are silk, there was no indication of a foraging-defense trade-off. Barrier web structure did not change during seasonal shifts in orb web size related to changes in preycapture rate, and barrier web silk density and orb radius were positively correlated. The hypothesis that the construction of barrier webs is in part a response to predation pressure was supported. Barrier webs do deflect attacks by some predators, and barrier webs built by small spiders, suffering frequent predation attempts, had a higher silk density than barrier webs built by larger individuals. Additionally, barrier web complexity decreased at a later age in areas with higher predation risk.     

Difference in Web Construction Behavior at Newly Occupied Web Sites Between Two Cyclosa Species

Animals make decisions based on subjective assessments of their environment. To determine their future foraging activities, animals probably assess food availability from past foraging experiences. Thus, foraging also functions as a way for animals to collect information, with the uncertainty of an assessment decreasing as foraging activity increases. This suggests that different needs for a correct assessment may affect the investment made in foraging activities. Orb‐web spiders sometimes relocate their webs and relocation rate differs among species. After web relocation, several spider species have been reported to construct the first webs at newly occupied web sites using less silk than usual, possibly to avoid the risk of an overinvestment at sites where food availability has not been determined. Nevertheless, they may pay a cost, because of inadequate decision‐making, if webs constructed with less silk convey less information and increase the uncertainty of an assessment. We expect that stronger site tenacity necessitates a greater requirement for correct assessment of web site and the degree to which spiders reduce the amount of web silk in the first web after web relocation is smaller in species that use the same site longer. To test this hypothesis, we examined web construction in two orb‐web spiders, Cyclosa octotuberculata and C. argenteoalba. At the same time we found that these two species exhibit different web‐site tenacity, as C. octotuberculata does not relocate its webs as frequently as does C. argenteoalba. After artificially induced web relocation, C. argenteoalba constructed webs that were initially smaller and contained only about 2/3 of the silk in control webs that were constructed at the original site. In contrast, C. octotuberculata did not exhibit such decreases in web size or in the amount of web silk used. This result is consistent with our hypothesis.     

Factors affecting the difference in foraging success in three co-existing Cyclosa Spiders

Species-specific differences in prey-capture success of co-existing web-building spiders are derived from complex factors: various web parameters, web placement, and the spider's response to prey. By examining these, this study revealed prey-capture modes of three species of web-building spiders of the genus Cyclosa living in the same habitat. Cyclosa octotuberculata and C. argenteoalba showed a greater prey capture rate than C. sedeculata , though size compositions of prey were similar in all species. Cyclosa octotuberculata spins thick silk with large adhesive droplets, which may contribute to the higher stopping and retention abilities of the web. Cyclosa argenteoalba constructs webs at open sites where prey is abundant, and has webs of dense mesh size, which may result in the high stopping ability of webs. In C. sedeculata , the web is less effective for stopping and retaining prey, probably owing to the thin silk with a small amount of sticky material, and the response to prey is not rapid. It seems that the former two species achieve a similar level of foraging success by using different sets of foraging traits and the third species has the disadvantage in most aspects of foraging.     

Guiding lights: Foraging responses of juvenile nocturnal orb‐web spiders to the presence of artificial light at night

The reach of artificial light at night (ALAN) is growing rapidly around the globe, including the increasing use of energy‐efficient LED lights. Many studies document the physiological costs of light at night, but far fewer have focused on the potential benefits for nocturnal insectivores and the likely ecological consequences of shifts in predator–prey relationships. We investigated the effects of ALAN on the foraging behaviour and prey capture success in juvenile Australian garden orb‐web spiders (Eriophora biapicata). Laboratory experiments demonstrated that juvenile spiders were attracted to LED lights when choosing foraging sites, but prey availability was a stronger cue for remaining in a foraging site. Field experiments revealed a significant increase in prey capture rates for webs placed near LED lights. This suggests that any physiological costs of light at night may be offset by the foraging benefits, perhaps partially explaining recently observed increases in the size, fecundity and abundance of some orb‐web spider species in urban environments. Our results highlight the potential long‐term consequences of night lighting in urban ecosystems, through the impact of orb‐web spiders on insect populations.     

Competition for a limited space in kleptoparasitic Argyrodes spiders revealed by field experiments

Most kleptoparasitic Argyrodes spiders rely exclusively on host spider webs for obtaining their food. Because their densities occasionally reach high levels within a restricted area, competitive interactions may be important for determining the density of these unique spiders. Here I used two Argyrodes species commonly found on webs of the large orb-web spider Nephila clavata to clarify whether inter- and intraspecific competition influences abundance and within-web distribution by using observational data and field experiment. Removing Argyrodes flavescens from the host webs induced a remarkably high immigration of that species while density on control webs remained almost at the same level, which is evidence for strong intraspecific competition. Larger individuals of A. flavescens were located more frequently at the capture area of the host webs where it is easy to access prey ignored by the host spider, and spiders immigrating into webs from which that species had been removed were smaller in body size, suggesting interference competition for space among conspecific kleptoparasites. Argyrodes bonadea increased in number on webs from which A. flavescens had been removed, and the increase was correlated with the number of A. flavescens removed. This finding is evidence for interspecific competition that is rarely reported in spiders. A multiple regression model including numbers of a conspecific parasite as well as web and body sizes of the host spider could not detect competitive interactions between species, suggesting the importance of experimental approaches. Received: May 22, 2000 / Accepted: December 1, 2000     

Advantages of social foraging in crab spiders: Groups capture more and larger prey despite the absence of a web

Among group‐living spiders, subsocial representatives in the family of crab spiders (Thomisidae) are a special case, as they build protective communal leaf nests instead of extensive communal capture webs. It could thus be inferred that antipredator benefits (e.g., enhanced protection in larger nests) rather than foraging‐related advantages (e.g., capture of more and larger prey) promote sociality in this family. Nonetheless, subsocial crab spiders do share prey, and if this behaviour does not reflect mere food scramble but has a cooperative character, crab spiders may offer insights into the evolution of social foraging applicable to many other cooperative predators that hunt without traps. Here, we performed a comparative laboratory feeding experiment on three of the four subsocial crab spider species—Australomisidia ergandros, Australomisidia socialis and Xysticus bimaculatus—to determine if crab spiders derive advantages from foraging in groups. In particular, we tested artificially composed groups of five sibling spiderlings vs. single siblings in terms of prey capture success and prey size preference. Across species, groups had higher prey capture success (measured in terms of capture rates and capture latency) and were more likely to attack large, sharable prey—dynamics leading to reduced food competition among group members in favour of living and foraging in groups. Within groups, we further compared prey extraction efficiency among the three applied social foraging tactics: producing, scrounging and feeding alone. In A. ergandros, individuals were exceptionally efficient when using the non‐cooperative scrounger tactic, which entails feeding on the prey provided by others. Thus, our multispecies comparison confirms foraging advantages in maintaining a cooperative lifestyle for crab spiders, but also demonstrates the relevance of research into exploitation of cooperative foraging in this family.