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: The different results for the two sexes are consistent with known differences in their life histories, indicating that a knowledge of an animal's life history will often be necessary to understand the influence of predation risk.
Abstract: The influence of predation risk on patch choice was measured by examining the spatial distribution of 10 guppies (Poecilia reticulata) between two feeders, at one of which there was a risk of predation. The distribution was assumed to be ideal free. Nine unique situations were examined using all possible combinations of three risk levels and three diet levels, for each sex of guppy separately. Both sex and diet level influenced the effect of predation risk on patch choice. For the females the effect of risk was highest at the intermediate diet level. However, the males exhibited the opposite response: the effect of risk of predation was lowest at the intermediate diet level. A simple equation was then used to predict how much extra food (representing the energetic equivalent of risk) must be added to the risky patch for the guppies to become indifferent to the risk differences between the two types of patches. This manipulation caused a similar number of guppies to use both the risky and safe feeders, reducing or offsetting the influence of risk of predation. However, the male guppies were less influenced by this manipulation than were the females. The different results for the two sexes are consistent with known differences in their life histories, indicating that a knowledge of an animal's life history will often be necessary to
393 citations
TL;DR: Comparison of weights and body dimensions of individuals from field populations with those of known nutritional status showed that lack of food is a problem for field populations of both species, suggesting that starved individuals have the capacity to double their body weight through ingestion of large amounts of food when available.
Abstract: Effects of starvation were investigated in two species of spiders to gain insight on how these predators deal with an unpredictable food supply. Comparison of weights and body dimensions of individuals from field populations with those of known nutritional status showed that lack of food is a problem for field populations of both species. Adult survival times under starvation conditions averaged 208 days for the wolf spider, Lycosa lenta, and 276 days for the cribellate web-builder, Filistata hibernalis. Potential adult life spans for fed individuals were estimated to be 305 days for L. lenta and several years for F. hibernalis. Both spiders have metabolic rates significantly lower than those of other poikilotherms of similar size, and when starving, reduce their metabolic rates by 30% to 40% without any apparent decrease in their normal capabilities. The difference in survival times under starva- tion conditions of the two species was found to be inversely related to metabolic rate. This suggests that the relatively low metabolic rates characteristic of spiders as well as their capacity to reduce these rates when starving are adaptive in survival where prey are scarce. That starved individuals have the capacity to double their body weight through ingestion of large amounts of food when available may be an adaptation to their predation.
298 citations
TL;DR: Results suggest that variation in the proportion of time spent foraging can provide an alternative mechanism for traditional functional response forms of predators.
Abstract: Functional responses are derived for several models in which a predator is able to vary the proportion of time that it spends foraging in an adaptive manner. Results suggest that: (1) the functional response of such predators is likely to be very plastic; (2) the traditional type 1, 2, 3 classification is not sufficient to describe the functional responses of such predators; and (3) variation in the proportion of time spent foraging can provide an alternative mechanism for traditional functional response forms.
284 citations
TL;DR: A general discussion of the designs and functions of the silk structures spiders use to capture prey has not been attempted since the book of Witt et al (319).
Abstract: A general discussion of the designs and functions of the silk structures spiders use to capture prey has not been attempted since the book of Witt et al (319). Knowledge has increased so greatly in the intervening years that complete coverage is now impossible in a review article. I have cited primarily more recent and general publications. Related aspects of web biology not emphasized here are reviewed in papers on particular taxonomic groups (260), general feeding ecology (233), choice of websites (131, 232, 234), interand intra-specific competition (and the lack of it) (273, 275, 314), ecophysiology (192), web removal (20), sexual behavior (139, 238), vibration transmission in webs (175), neurobiology (3), attack behavior (237, 279), communication (320), energetics of web-building (227), timing of web construction (229), cues used for orientation during web construction (80, 308), structure and composition of web lines (224, 290), physical properties of silk (109, 322), and silk glands (138). Due to the numbers of papers on orb webs, orbs are emphasized over non-orbs. The imbalance in research is strong (the ratio in the reference section is about 2: 1 orb: non-orb, despite the fact that the ratio in numbers of species must be closer to 1: 2 (H. W. Levi, personal communication). At least partially repairing this imbalance is a pressing need.
269 citations