The influence of food supply on foraging behaviour in a desert spider
TL;DR: It is suggested that digestive constraints prevented supplemented spiders from fully utilizing the available prey, and by reducing foraging activities on the surface, spiders in a prey-rich habitat can reduce the risk of predation.
Abstract: We tested the alternative hypotheses that foraging effort will increase (energy maximizer model) or decrease (due to increased costs or risks) when food supply increased, using a Namib desert burrowing spider, Seothyra henscheli (Eresidae), which feeds mainly on ants. The web of S. henscheli has a simple geometrical configuration, comprising a horizontal mat on the sand surface, with a variable number of lobes lined with sticky silk. The sticky silk is renewed daily after being covered by wind-blown sand. In a field experiment, we supplemented the spiders' natural prey with one ant on each day that spiders had active webs and determined the response to an increase in prey. We compared the foraging activity and web geometry of prey-supplemented spiders to non-supplemented controls. We compared the same parameters in fooddeprived and supplemented spiders in captivity. The results support the "costs of foraging" hypothesis. Supplemented spiders reduced their foraging activity and web dimensions. They moulted at least once and grew rapidly, more than doubling their mass in 6 weeks. By contrast, food-deprived spiders increased foraging effort by enlarging the diameter of the capture web. We suggest that digestive constraints prevented supplemented spiders from fully utilizing the available prey. By reducing foraging activities on the surface, spiders in a prey-rich habitat can reduce the risk of predation. However, early maturation resulting from a higher growth rate provides no advantage to S. henscheli owing to the fact that the timing of mating and dispersal are fixed by climatic factors (wind and temperature). Instead, large female body size will increase fitness by increasing the investiment in young during the period of extended maternal care.
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TL;DR: This review first briefly treats the different types of spider cannibalism and then focuses in more depth on evidence relating cannibalism to population dynamics and food web interactions to address the following questions.
Abstract: Cannibalism among generalist predators has implications for the dynamics of terrestrial food webs. Spiders are common, ubiquitous arthropod generalist predators in most natural and managed terrestrial ecosystems. Thus, the relationship of spider cannibalism to food limitation, competition, and population regulation has direct bearing on basic ecological theory and applications such as biological control. This review first briefly treats the different types of spider cannibalism and then focuses in more depth on evidence relating cannibalism to population dynamics and food web interactions to address the following questions: Is cannibalism in spiders a foraging strategy that helps to overcome the effects of a limited supply of calories and/or nutrients? Does cannibalism in spiders reduce competition for prey? Is cannibalism a significant density-dependent factor in spider population dynamics? Does cannibalism dampen spider-initiated trophic cascades?
321 citations
TL;DR: The presented estimates of the global annual prey kill and the relative contribution of spider predation in different biomes improve the general understanding of spider ecology and provide a first assessment of theglobal impact of this very important predator group.
Abstract: Spiders have been suspected to be one of the most important groups of natural enemies of insects worldwide. To document the impact of the global spider community as insect predators, we present estimates of the biomass of annually killed insect prey. Our estimates assessed with two different methods suggest that the annual prey kill of the global spider community is in the range of 400–800 million metric tons (fresh weight), with insects and collembolans composing >90% of the captured prey. This equals approximately 1‰ of the global terrestrial net primary production. Spiders associated with forests and grasslands account for >95% of the annual prey kill of the global spider community, whereas spiders in other habitats are rather insignificant contributors over a full year. The spider communities associated with annual crops contribute less than 2% to the global annual prey kill. This, however, can be partly explained by the fact that annual crop fields are “disturbed habitats” with a low buildup of spider biomass and that agrobiont spiders often only kill prey over short time periods in a year. Our estimates are supported by the published results of exclusion experiments, showing that the number of herbivorous/detritivorous insects and collembolans increased significantly after spider removal from experimental plots. The presented estimates of the global annual prey kill and the relative contribution of spider predation in different biomes improve the general understanding of spider ecology and provide a first assessment of the global impact of this very important predator group.
214 citations
Cites background or methods from "The influence of food supply on for..."
...Due to adverse environmental conditions, prey availability in deserts is very low and these biomes are often populated by spiders in very low densities (Shook 1978; Polis 1991; Lubin and Henschel 1996; Henschel 1997)....
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...In the case of desert spiders, a daily food ingestion rate of 0.01–0.04 mg per milligram spider body mass was used (see Lubin and Henschel 1996; Henschel 1997)....
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TL;DR: Evidence is provided that the behaviour of trap‐building predators is not stereotypic or fixed as was once commonly accepted, rather it can vary greatly, depending on the individual's internal state and its interactions with external environmental factors.
Abstract: Foraging theory was first developed to predict the behaviour of widely-foraging animals that actively search for prey. Although the behaviour of sit-and-wait predators often follows predictions derived from foraging theory, the similarity between these two distinct groups of predators is not always obvious. In this review, we compare foraging activities of trap-building predators (mainly pit-building antlions and web-building spiders), a specific group of sit-and-wait predators that construct traps as a foraging device, with those of widely-foraging predators. We refer to modifications of the trap characteristics as analogous to changes in foraging intensity. Our review illustrates that the responses of trap-building and widely-foraging predators to different internal and external factors, such as hunger level, conspecific density and predation threat are quite similar, calling for additional studies of foraging theory using trap-building predators. In each chapter of this review, we summarize the response of trap-building predators to a different factor, while contrasting it with the equivalent response characterizing widely-foraging predators. We provide here evidence that the behaviour of trap-building predators is not stereotypic or fixed as was once commonly accepted, rather it can vary greatly, depending on the individual's internal state and its interactions with external environmental factors.
143 citations
Cites background from "The influence of food supply on for..."
...Indeed, hungry spiders and antlions were often documented to increase the investment in trap size (Riechert, 1981; Sherman, 1994; Lubin & Henschel, 1996; Herberstein et al., 2000b; Arnett & Gotelli, 2001; Lomascolo & Farji-Brener, 2001)....
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TL;DR: Experienced web-building spiders constructed more asymmetric webs than conspecifics deprived of any prior building experience over a period of several months, revealing that experience can contribute to intraspecific as well as to individual variations in web design.
Abstract: A typical feature of vertical orb-webs is the ‘top/bottom’ asymmetry, where the lower web region is larger than the upper web region. This asymmetry may improve prey capture success, because, sitting in the hub of the web, a spider can reach prey entangled below the hub faster than prey entangled in the area above the hub. While web asymmetry is known to vary intraspecifically, we tested if this variation also exists at the individual level and whether it is the result of experience, using two orb-web spider species, Argiope keyserlingi and Larinioides sclopetarius. The results reveal that experienced web-building spiders constructed more asymmetric webs than conspecifics deprived of any prior building experience over a period of several months. Experienced individuals invested more silk material into the web region below the hub, which covered a larger area. Moreover, web asymmetry was also influenced by previous prey capture experiences, as spiders increased the lower region of the web if it intercepted the most prey over a period of 6 days. Consequently, spiders may be able to use long-term web-building experience as well as short-term prey capture experience to build better traps. In contrast to previous views of spiders, experience can contribute to intraspecific as well as to individual variations in web design.
99 citations
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TL;DR: Modelling the effect of changed food density on optimal foraging effort when a forager faces a trade—off between energy (nutrient) gain and risk of mortality shows that the optimal effort or amount of risk may increase or decrease with increasing food density.
Abstract: Simple theoretical models are used to explore the effect of changed food density on optimal foraging effort when a forager faces a trade—off between energy (nutrient) gain and risk of mortality. Models of simple iteroparous and semelparous life histories show that the optimal effort or amount of risk may increase or decrease with increasing food density. Increases are more likely for iteroparous than for semelparous life histories, and are more likely when the increase in food availability is of short duration. The possibility of starvation makes it more likely that foraging effort will decrease with increasing food density. Organisms whose foraging goal is to minimize mortality are especially unlikely to increase effort in response to increased food availability. If there is a cost associated with the amount of food ingested, effort is more likely to decrease with increasing food. Some limitations of previous theoretical treatments of the foraging effort question are discussed. Previous experimental resu...
222 citations
TL;DR: Nightly variations in web dimensions suggest that hungry spiders invest more effort into foraging, while sated spiders re-allocate energy from continued foraging to egg production, and lend support to a seldom-tested tenet of optimal foraging theory: the direct relationship between foraging success and enhanced reproductive fitness.
180 citations
"The influence of food supply on for..." refers background in this paper
...Features of web design such as web size and the density or length of capture elements may vary with the spider's perception of prey availability in the environment and are thus useful as measures of foraging effort (Ward and Lubin 1992; Rhisiart and Vollrath 1994; Sherman 1994)....
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...Some field studies indicate that spiders invest less in construction, maintenance or defence of webs in prey-rich habitats than in poor ones (Riechert 1981; Pasquet 1984; Higgins and Buskirk 1992; Pasquet et al. 1994; Sherman 1994)....
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TL;DR: Spiders are regarded with keen interest as model organisms in behavioral ecology because of their small size, short lifespan, and the strong influence of genetic control on their behavior.
Abstract: Abundant and ecologically important in many ecosystems as predators of insects (including agricultural pests), spiders have recently gained attention as a result of a number of significant studies. In addition, because of their small size, short lifespan, and the strong influence of genetic control on their behavior, spiders are regarded with keen interest as model organisms in behavioral ecology.
177 citations
TL;DR: The potential for energy-based territoriality to limit population numbers is demonstrated here for a population of the desert spider Agelenopsis aperta (Gertsch) and its consequences to their patterns of speciation, community structure, and predatory role are discussed.
Abstract: The potential for energy-based territoriality to limit population numbers is demonstrated here for a population of the desert spider Agelenopsis aperta (Gertsch). Enclosure experiments run on groups of spiders subjected to different feeding histories and representing two different populations demonstrate that territory size is a fixed characteristic of specific populations which is not influenced by temporal changes in prey abundance or in the numbers of competitors present. Data are presented which further show that territory size is adjusted over evolutionary time to lows in prey availability (times of greatest stringency). The inflexible nature of territorial behavior resulted in the saturation of one habitat offering a limited number of adequate web sites. Removal experiments showed that approximately 35% of the population occupying this habitat in 1978 existed as floaters. Floaters suffered marked daily losses in fitness relative to the gains exhibited by territory holders (measured as changes in bod...
155 citations
"The influence of food supply on for..." refers background in this paper
...Some field studies indicate that spiders invest less in construction, maintenance or defence of webs in prey-rich habitats than in poor ones (Riechert 1981; Pasquet 1984; Higgins and Buskirk 1992; Pasquet et al. 1994; Sherman 1994)....
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