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 foraging response to increased competition by two species of gerbils is experimentally tested over a range of manipulated population densities in field enclosures located in the Negev Desert of Israel and support the cost-benefit model when the additional energy cost of foraging is important.
Abstract: We studied, both theoretically and empirically, the effect of intra- and in- terspecific competition on the foraging effort of individuals We considered two models, one for a time-minimizer satisfying an energy requirement, the other for an animal max- imizing fitness as a function of multiple inputs subject to a time constraint The goal of satisfying an energy constraint predicts that foraging effort should increase with increased competition The goal of maximizing fitness subject to a time constraint on multiple inputs may also predict that foraging effort should increase with increased competition because of the missed opportunity cost that results when different inputs are complementary However, if the fitness-maximizer with multiple inputs incurs an energy cost of foraging (in addition to missed opportunity costs), then it should often reduce foraging effort in response to an increase in competition We experimentally tested the foraging response to increased competition by two species of gerbils, Gerbillus allenbyi and G pyramidum, over a range of manipulated population densities in field enclosures located in the Negev Desert of Israel Our results support the cost-benefit model when the additional energy cost of foraging is important Per capita activity (as measured by spoor) declined as a function of intraspecific density for each species and as a function of interspecific density for G allenbyi We detected no interspecific effect, however, on G pyramidum
141 citations
TL;DR: The species may be its own most important predator, and the food taken appears to differ at different times of year; this is more probably due to change in abundance of prey than to changes in preference by the spider.
Abstract: 85 % of the prey ofLycosa lugubris (Walckenaer) consists of Diptera, Hemiptera and Araneae. The food taken appears to differ at different times of year; this is more probably due to changes in abundance of prey than to changes in preference by the spider. L. lugubris is not an active predator. It does not run down its prey but remains motionless and captures what comes within reach. It probably feeds infrequently in the field and it carries its prey for between one and two hours. An attempt was made to assess possible predators, and the species may be its own most important predator.
130 citations
TL;DR: Both species responded to additional prey by significantly increasing the number of eggs produced per female, indicating that food was a limited resource for these species.
Abstract: A field experiment was performed to determine if food is a limited resource for adult females of two species of orb-weaving spiders, Mecynogea lemniscata and Metepeira labyrinthea. Spiders built webs after being added to open experimental units located in a mixed deciduous-pine forest in Maryland, USA. Each unit was a frame supporting dead branches of the type used by both species for anchoring webs. Spiders on half the units were exposed to natural prey densities only, while each spider on the other units was given laboratory-reared flies in order to increase prey availability above natural levels. Supplemental feeding continued for 2.5 months. At the end of the experiment all egg sacs were removed from the units.Providing additional prey did not increase the survival rate on the units (net effect of mortality, emigration and immigration). However, both species responded to additional prey by significantly increasing the number of eggs produced per female, indicating that food was a limited resource for these species. Median egg production per female increased from 34 to 62 for Mecynogea lemniscata and from 65 to 145 for Metepeira labyrinthea. Egg weight was not affected.Feeding rates and nearest neighbor distances were determined for spiders in non-experimental populations, which permitted evaluation of the experiment's naturalness. The effects of food supply upon the reproductive rates of the two species are discussed in relation to their numerical response and population dynamics.
129 citations
01 Jan 1987
TL;DR: The life history of any animal is greatly influenced by three factors: the need to grow, the efficiency of growth and the timing of maturation, so it would expect natural selection to have sharpened the animal’s process of making optimal decisions regarding foraging and growth.
Abstract: The life history of any animal is greatly influenced by three factors (Stearns 1976): the need to grow and reach maturity, the need to feed in order to accumulate the energy necessary for growth, and finally, the need to reproduce. Reproduction is often seasonal, and often the reproductive success depends greatly on body size and physical fitness (Thornhill and Alcock 1983). Hence, the effectiveness of foraging, the efficiency of growth and the timing of maturation all directly affect reproductive fitness. Accordingly, we would expect natural selection to have sharpened the animal’s process of making optimal decisions regarding foraging and growth.
125 citations
TL;DR: It is suggested that web—site selection in this species is less precise than expected because of the risk of dying during a move, and movement to larger shrubs resulted in improved spider body condition and, ultimately, greater reproductive success.
Abstract: We investigated web relocation and habitat selection in Latrodectus revivensis (Theridiidae), a widow spider that constructs webs on scattered shrubs in the Negev desert. We used repeated observations of individual spiders' habitat preferences, movements to new web sites, growth, fecundity, and survival under natural and manipulated habitat conditions to examine relationships between movement, habitat, and components of fitness. Our main objectives were to determine the spatial and temporal patterns of movement, and the causes, costs, and fitness consequences of shifting habitat. Repeated censuses of individually marked females showed that web relocation in L. revivensis is associated with spider growth. Spiders tended to move larger shrubs after one or two molts, and the size of web components and web—site features scaled to body size. Moves occurred mainly in Spring (March—May), and most moves (74%) were by juveniles. The main cost of web relocation was increased mortality: there was a 40% chance of death during a move. A comparison of occupied shrubs with those available in the habitat indicated strong site selection involving seasonally varying and age—dependent preferences for particular shrub species, and a general preference for taller shrubs. Prey availability at shrubs (as measured by pitfall traps, and by prey remains in nests) varied spatially and was affected by shrub species and size. However these effects were small compared to habitat—wide temporal variation in prey availability, suggesting that web—site relocations would not result in substantially higher prey capture. Nonetheless, movement to larger shrubs resulted in improved spider body condition and, ultimately, greater reproductive success. The advantages of web—site relocation in this species may relate to architectural features of shrubs that provide support for the web, and minimize exposure by thermal extremes and to predators. However, experimental manipulation of exposure by trimming shrubs did not decrease spider fitness. We suggest that web—site selection in this species is less precise than expected because of the risk of dying during a move.
120 citations