Contributions of detrital subsidies to aboveground spiders during secondary succession, revealed by radiocarbon and stable isotope signatures

Prey subsidies originating from detritus add nutrients and energy to arboreal communities. Measurement of this subsidy is required in the understanding of how food web dynamics respond to changes in surrounding environments. Shrub spiders are one of the key predators involved in food web coupling. We evaluate the effects of potential changes in prey availabilities during secondary succession on the contribution of subsidy from detrital food webs to shrub spiders and how different spider feeding guilds used the subsidy of prey from detrital food webs. We measured the relative importance of the subsidy for the spider feeding guilds, using the ratios of stable isotopes of C (δ¹³C), and N (δ¹⁵N) and C isotope discrimination (Δ¹⁴C). Diet age was calculated from Δ¹⁴C values, because old diet ages of spiders indicate that the spiders consume prey from detrital food sources. Dominant aerial prey (Diptera) had a distinctively old diet age compared with arboreal prey, which indicates that aerial prey were subsidized from detrital food webs. Sit-and-wait spiders tended to have an older diet age than active hunting spiders, which indicates that sit-and-wait spiders depended more on subsidies. Diet age varied only slightly for spiders in stands of different ages, indicating that rates at which spiders use grazing and detrital prey are probably determined more by foraging strategies and not by stand age. A dominance of sit-and-wait predators will lead to higher detrital subsidy inputs in shrub habitats. This study highlights the effect of shrub spider community structure (feeding guild composition) on the volume of the subsidy received from the detrital food web.     

Predator hunting mode influences patterns of prey use from grazing and epigeic food webs

Multichannel omnivory by generalist predators, especially the use of both grazing and epigeic prey, has the potential to increase predator abundance and decrease herbivore populations. However, predator use of the epigeic web (soil surface detritus/microbe/algae consumers) varies considerably for reasons that are poorly understood. We therefore used a stable isotope approach to determine whether prey availability and predator hunting style (active hunting vs. passive web-building) impacted the degree of multichannel omnivory by the two most abundant predators on an intertidal salt marsh, both spiders. We found that carbon isotopic values of herbivores remained constant during the growing season, while values for epigeic feeders became dramatically more enriched such that values for the two webs converged in August. Carbon isotopic values for both spider species remained midway between the two webs as values for epigeic feeders shifted, indicating substantial use of prey from both food webs by both spider species. As the season progressed, prey abundance in the grazing food web increased while prey abundance in the epigeic web remained constant or declined. In response, prey consumption by the web-building spider shifted toward the grazing web to a much greater extent than did consumption by the hunting spider, possibly because passive web-capture is more responsive to changes in prey availability. Although both generalist predator species engaged in multichannel omnivory, hunting mode influenced the extent to which these predators used prey from the grazing and epigeic food webs, and could thereby influence the strength of trophic cascades in both food 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.     

Insects from the grazing food web favoured the evolutionary habitat shift to bright environments in araneoid spiders

The Araneoidea comprises a diverse group of web-building spiders, and part of this diversity is believed attributable to habitat expansion to bright environments. We clarified the fitness-related advantages of living in such environments by examining prey availability and the growth rates of 10 species in three families inhabiting grassland (bright) and forest understory (dim) habitats. Spiders in the grassland habitat captured more prey, derived mainly from the grazing food web, than those in the forest-floor environment, and this difference was manifested in their growth rate. Independent contrasts indicated that increased utilization of insects from the grazing food web led to an evolutionary increase in adult body size. These results suggest that the shift to bright environments enabled araneoid spiders to evolve diverse life-history traits, including rapid growth and large size, which were not possible in dim environments.     

Experimental evidence that aboveground predators are sustained by underground detritivores

Detrital infusion into grazing food web is considered to be important in terrestrial communities, but there is hardly any experimental evidence showing that generalist predators aboveground are sustained by belowground detritivores. We established two types of experimental plots in the forest floor, one with sheets on the ground to prevent the emergence of belowground arthropods and the other without sheet, to test the hypotheses that 1) reduced input of detrital arthropods decreases the abundance and species richness of web spiders (major generalist predators in terrestrial ecosystems) and 2) lower number of spiders increases the abundance of herbivorous arthropods. We found that spiders were less abundant in plots where the emergence of detrital arthropods was reduced, while the abundance of herbivores did not significantly increase in these plots. These results provide empirical evidence that organisms moving from underground to aboveground may be important for the maintenance of aboveground predators, although the cascading effect of predator abundance on the grazing food chain was not detected in the present study.     

Are three‐dimensional spider webs defensive adaptations?

Spider webs result from complex behaviours that have evolved under many selective pressures. Webs have been primarily considered to be foraging adaptations, neglecting the potential role of predation risk in the evolution of web architecture. The ecological success of spiders has been attributed to key innovations in how spiders use silk to capture prey, especially the invention of chemically adhesive aerial two‐dimensional orb webs. However, araneoid sheet web weavers transformed the orb architecture into three‐dimensional webs and are the dominant group of aerial web‐building spiders world‐wide, both in numbers and described species diversity. We argue that mud‐dauber wasps are major predators of orbicularian spiders, and exert a directional selective pressure to construct three‐dimensional webs such that three‐dimensional webs are partly defensive innovations. Furthermore, patterns of diversification suggest that escape from wasp predators may have facilitated diversification of three‐dimensional web‐building spiders.     

Predator perception of detritus and eggsac decorations spun by orb-web spiders Cyclosa octotuberculata: Do they function to camouflage the sniders?

Camouflage is one of the most widespread and powerful strategies that animals use to make detection/recognition more difficult. Many orb-web spiders of the genus Cyclosa add prey remains, plant debris, moults, and/or eggsacs to their webs called web decorations. Web decorations resembling spider body colour pattern have been considered to camouflage the spider from predators. While this camouflage is obvious from a human's perspective, it has rarely been investigated from a predator's perspective. In this study, we tested the visibility of web decorations by calculating chromatic and achromatic contrasts of detritus and eggsac decorations built by Cyclosa octotuberculata, against four different backgrounds viewed by both bird (e.g., blue tits) and hymenopteran (e.g. Wasps) predators. We showed that both juvenile and adult spiders on webs with detritus or egg-sac deco-rations were undetectable by both hymenopteran and bird predators over short and long distances. Our results thus suggest that decorating webs with detritus or eggsacs by C. Octotuberculata may camouflage the spiders from both hymenopteran and bird predators in their common habitats.     

Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

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The selective prey of linyphiid-like spiders and of their space webs

Summary 1769 prey animals were collected from the space webs of linyphiid-like spiders, i.e. actual prey, and were compared with more than 110,000 animals from nearby pitfall traps and colored traps (yellow), i.e. potential prey, by means of the Ivlev Index. The catch found in the webs proved to be very selective: certain groups were found in unexpectedly great numbers (especially phytophages insects) while others had nearly always managed to avoid the web (especially predators and pollinating insects). The spider had conducted a further selection in that it consumed only certain animals. The parameters which decide the frequency of capture and of consumption are as follows: flying ability, sense of direction, body type, size, weight and abundance.     

Diel and seasonal patterns of prey available to epigeal predators: Evidence for food limitation in a linyphiid spider community

Linyphiid spiders, as part of the community of natural enemies that frequent agroecosystems, can exert significant pressure on prey populations. Many aspects of linyphiid feeding ecology remain understudied, including temporal and seasonal patterns of prey utilization. To quantify the diversity, quantity, and spatial pattern of availability over diel and seasonal gradients, we monitored prey in an alfalfa crop, using web-site specific sticky traps. Although there were no differences in total number of prey between the day and night, significantly more Collembola were intercepted nocturnally rather than diurnally. Conversely, Diptera and Aphididae were significantly more abundant diurnally. Overall there were more prey at web-sites than in the general crop environment, indicating that spiders were selectively constructing webs where prey was relatively more abundant. However, spiders still may have experienced a high degree of prey limitation because the abundance of Collembola, an important detrital prey, was low over much of the season and below levels calculated as being required to sustain growth and reproductive output in linyphiid spiders. In conclusion, our data suggests that linyphiid spiders, in order to avoid food limitation, may benefit by foraging throughout the diel cycle, consuming herbivorous prey in addition to their core diet of detrital prey. This link between above and below ground food webs indicates the importance of understanding the trophic interactions between epigean spiders and their prey if they are to be integrated into future biological control programs.     

Invasive plants have different effects on trophic structure of green and brown food webs in terrestrial ecosystems: a meta‐analysis

Although invasive plants are a major source of terrestrial ecosystem degradation worldwide, it remains unclear which trophic levels above the base of the food web are most vulnerable to plant invasions. We performed a meta‐analysis of 38 independent studies from 32 papers to examine how invasive plants alter major groupings of primary and secondary consumers in three globally distributed ecosystems: wetlands, woodlands and grasslands. Within each ecosystem we examined if green (grazing) food webs are more sensitive to plant invasions compared to brown (detrital) food webs. Invasive plants have strong negative effects on primary consumers (detritivores, bacterivores, fungivores, and/or herbivores) in woodlands and wetlands, which become less abundant in both green and brown food webs in woodlands and green webs in wetlands. Plant invasions increased abundances of secondary consumers (predators and/or parasitoids) only in woodland brown food webs and green webs in wetlands. Effects of invasive plants on grazing and detrital food webs clearly differed between ecosystems. Overall, invasive plants had the most pronounced effects on the trophic structure of wetlands and woodlands, but caused no detectable changes to grassland trophic structure.     

Predator and detritivore niche width helps to explain biocomplexity of experimental detritus-based food webs in four aquatic and terrestrial ecosystems

In the study of food webs, the existence and explanation of recurring patterns, such as the scale invariance of linkage density, predator–prey ratios and mean chain length, constitute long-standing issues. Our study focused on litter-associated food webs and explored the influence of detritivore and predator niche width (as δ¹³C range) on web topological structure. To compare patterns within and between aquatic and terrestrial ecosystems and take account of intra-habitat variability, we constructed 42 macroinvertebrate patch-scale webs in four different habitats (lake, lagoon, beech forest and cornfield), using an experimental approach with litterbags. The results suggest that although web differences exist between ecosystems, patterns are more similar within than between aquatic and terrestrial web types. In accordance with optimal foraging theory, we found that the niche width of predators and prey increased with the number of predators and prey taxa as a proportion of total taxa in the community. The tendency was more marked in terrestrial ecosystems and can be explained by a lower per capita food level than in aquatic ecosystems, particularly evident for predators. In accordance with these results, the number of links increased with the number of species but with a significantly sharper regression slope for terrestrial ecosystems. As a consequence, linkage density, which was found to be directly correlated to niche width, increased with the total number of species in terrestrial webs, whereas it did not change significantly in aquatic ones, where connectance scaled negatively with the total number of species. In both types of ecosystem, web robustness to rare species removal increased with connectance and the niche width of predators. In conclusion, although limited to litter-associated macroinvertebrate assemblages, this study highlights structural differences and similarities between aquatic and terrestrial detrital webs, providing field evidence of the central role of niche width in determining the structure of detritus-based food webs and posing foraging optimisation constraints as a general mechanistic explanation of food web complexity differences within and between ecosystem types.     

Testing the Functions of Detritus Stabilimenta in Webs of Cyclosa fililineata and Cyclosa morretes (Araneae: Araneidae): Do They Attract Prey or Reduce the Risk of Predation?

Spiders of the genus Cyclosa often add prey remains and other debris to their orb‐webs. The function of silk decorations is generally associated with defense against predators or with the attraction of prey, but few studies have focused on stabilimenta containing detritus. In this study, we used artificial webs with and without the detritus stabilimenta of two species of Cyclosa to investigate whether these structures increase the number of insects intercepted. Artificial models of spiders and stabilimenta were used to compare the frequency of attacks against different shapes. We also conducted choice experiments in laboratory to determine whether detritus columns attracted Drosophila melanogaster (Diptera: Drosophilidae) and Trigona angustula (Hymenoptera: Apidae, Meliponinae) to the webs. The frequency of interception in artificial webs with a stabilimentum was similar to that of webs without such structure. The taxonomic composition and biomass of insects were also similar in both types of artificial webs. The choice experiments showed no significant tendency in attraction to webs with a stabilimentum. However, models of spiders were attacked at a higher frequency than those simulating detritus columns and silk decorations. These findings argue against the prey attraction hypothesis and suggest that the addition of stabilimenta to webs of Cyclosa could reduce the intensity of predation, possibly by disrupting the image of the spider's outline.     

Functionally independent components of prey capture are architecturally constrained in spider orb webs

Evolutionary conflict in trait performance under different ecological contexts is common, but may also arise from functional coupling between traits operating within the same context. Orb webs first intercept and then retain insects long enough to be attacked by spiders. Improving either function increases prey capture and they are largely determined by different aspects of web architecture. We manipulated the mesh width of orbs to investigate its effect, along with web size, on prey capture by spiders and found that they functioned independently. Probability of prey capture increased with web size but was not affected by mesh width. Conversely, spiders on narrow-meshed webs were almost three times more likely to capture energetically profitable large insects, which demand greater prey retention. Yet, the two functions are still constrained during web spinning because increasing mesh width maximizes web size and hence interception, while retention is improved by decreasing mesh width because more silk adheres to insects. The architectural coupling between prey interception and retention has probably played a key role in both the macroevolution of orb web shape and the expression of plasticity in the spinning behaviours of spiders.     

Linking the green and brown worlds through nonconsumptive predator effects

Predators can indirectly affect lower trophic levels by either consuming their prey (consumptive effect, CE) or by changing the physiology or behavior of their prey (nonconsumptive effect, NCE). Cascading effects of predators on primary producers are common, and can be propagated by CEs, NCEs, or a combination of both mechanisms. Predator impacts in detrital food webs (the ‘brown world’) have received considerably less attention than their effects on systems with primary producers at the base (the ‘green world’), and only recently have we begun to appreciate the importance of above‐ground predators indirectly impacting below‐ground processes. Numerous studies reveal the total impact (CEs and NCEs) of predators in brown food webs, but our understanding of the role of isolated NCEs is limited. Many habitats and major taxa have not been studied, and patterns are difficult to distinguish due to frequent reporting of mixed effects. Predators play an important role as connectors between brown and green worlds when they feed from both food webs (multichannel feeding). We are only beginning to understand how NCEs influence detrital food webs, and it is unknown whether multichannel fear is an essential component of predator–prey ecology that regulates ecosystem function. Synthesis Predators have been shown to impact ecosystems through both consumptive and nonconsumptive effects on their prey Historically, herbivory‐based ‘green’ systems have been the venue for documenting these predator effects, while detritus‐based ‘brown’ systems received considerably less attention. However, similar mechanisms exist in green and brown worlds, suggesting strong parallels. We review and synthesize predator effects in detrital systems, highlighting important shortcomings in current understanding. Furthermore, we build upon the idea of multichannel feeding (i.e. consumption of prey from both green and brown food webs) to propose the existence of ‘multichannel fear’. We provide a framework for documenting multichannel fear to facilitate continued exploration of how predators link seemingly disparate systems.     

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.     

Trophic structure of the spider community of a Mediterranean citrus grove: A stable isotope analysis

Spiders are dominant terrestrial predators that consume a large variety of prey and engage in intraguild predation. Although the feeding habits of certain species are well known, the trophic structure of spider assemblages still needs to be investigated. Stable isotope analysis enables characterisation of trophic relationships between organisms because it tracks the energy flow in food webs and indicates the average number of trophic transfers between a given species and the base of the web, thus being a useful tool to estimate the magnitude of intraguild predation in food webs. Using this technique, we studied the trophic groups of spiders and their links within the arthropod food web of a Mediterranean organic citrus grove. We assessed the trophic positions of the 25 most common spider species relative to other arthropod predators and potential prey in the four seasons of the year, both in the canopy and on the ground. The analyses showed great seasonal variation in the isotopic signatures of some arthropod species, as well as the existence of various trophic groups and a wide range of trophic levels among spiders, even in species belonging to the same family. Differences in δ¹⁵N between spiders and the most abundant prey in the grove usually spanned two trophic levels or more. Our findings provide field evidence of widespread intraguild predation in the food web and caution against using spider families or guilds instead of individual species when studying spider trophic interactions.     

The ecological and evolutionary interdependence between web architecture and web silk spun by orb web weaving spiders

Spider orb webs are dynamic, energy absorbing nets whose ability to intercept prey is dependent on both the mechnical properties of web design and the material properties of web silks. Variation in web designs reflects variation in spider web spinning behaviours and variation in web silks reflects variation in spider metabolic processes. Therefore, natural selection may affect web function (or prey capture) through two independent and alternative pathways. In this paper, I examine the ways in which architectural and material properties, singly and in concert, influence the ability of webs to absorb insect impact energy. These findings are evaluated in the context of the evolution of diverse aerial webs. Orb webs range along a continuum from high to low energy absorbing. No single feature of web architecture characterizes the amount of energy webs can absorb, but suites of characters indicate web function. In general, webs that intercept heavy and fast flying prey (high energy absorbing webs) are large, built by large spiders, suspended under high tension and characterized by a ratio of radii to spiral turns per web greater than one. In contrast, webs that intercept light and slow flying prey (low energy absorbing webs) are suspended under low tension, are small and are characterized by radial to spiral turn ratios that are less than one. The data suggest that for spiders building high energy absorbing webs, the orb architecture contributes much to web energy absorption. In contrast, for spiders that build low energy absorbing webs, orb architecture contributes little to enhance web energy absorption. Small or slow flying insects can be intercepted by web silks regardless of web design. Although there exists variation in the material properties of silk collected from high and low energy absorbing webs, only the diameter of web fibres varies predictably with silk energy absorption. Web fibre diameter and hence the amount of energy absorbed by web silks is an isometric function of spider size. The significance of these results lies in the apparent absence of selective advantage of orb architecture to low energy absorbing webs and the evolutionary trend to small spiders that build them. Where high energy absorption is not an exacting feature of web design, web architecture should not be tightly constrained to the orb. Assuming the primitive araneoid web design is the orb web, I propose that the evolution of alternative web building behaviours is a consequence of the general, phyletic trend to small size among araneoids. Araneoids that build webs of other than orb designs are able to use new habitats and resources not available to their ancestors.     

Foraging Benefits in a Colour Polymorphic Neotropical Orb Web Spider

Conspicuous body colouration in sedentary predators such as orb web spiders seems paradoxical as potential prey can see and avoid the webs. Several studies have demonstrated that rather than deterring prey, the colours act as sensory traps for flower‐seeking insects. In chromatically polymorphic species, the existence of more than one colour morph may lead to differing levels of prey attraction. To explore these issues, we studied a neotropical orb web spider, Verrucosa arenata, which shows colour polymorphism with predominantly white or yellow abdomen colours. We asked whether a particular morph is dominant in the population, and whether a particular morph is associated with enhanced foraging success and body condition. Here we showed that although yellow morphs attracted more prey, white morphs were in better body condition. We showed that model prey such as honeybees are able to discriminate between the morphs. We discuss these findings in relation to the functional significance of bright body colouration and colour polymorphism in spiders.     

Conspicuous colouration attracts prey to a stationary predator

Abstract 1. Conspicuous body colouration is counter‐intuitive in stationary predators because sit‐and‐wait tactics frequently rely on concealed traps to capture prey. Consequently, bright colours and contrasting patterns should be rare in predators using traps as they may alert potential prey. Yet, some orb‐weaving spiders are brightly coloured and contrastingly patterned. How can conspicuousness of trap‐building sit‐and‐wait predators be favoured by natural selection? 2. Observations of spiny spiders Gasteracantha fornicata in north‐eastern Australia showed that the size of spiders relative to their orb webs correlated positively with relative prey numbers already captured in their webs. A possible explanation is that the relatively larger appearance of the yellow–black striped dorsal surface of this spider attracts more visually oriented prey items. Prey attracted to webs may get trapped, thereby increasing the spiders' foraging success. 3. To test this hypothesis for the function of conspicuous body colouration, a field experiment was conducted that documented the prey capture rates of spiny spiders after manipulating or sham‐manipulating their appearance. 4. As predicted, spiders that were dyed black on their striped dorsal surface caught relatively fewer prey items than did control spiders. Thus, conspicuous dorsal body colouration may be adaptive in spiny spiders because it increases foraging success and, presumably, survival rates and reproductive outputs. Overall, these data support the colour‐as‐prey‐attractant hypothesis in a stationary, trap‐building predator.