Alternative Foraging Modes of Orb Web Weaving Spiders

Abstract

Forest-understory, orb weaving spiders display at least two, alternative, foraging modes. Large spiders build one large web per feeding period. Although their webs intercept more prey than webs built by small spiders, the biomass of captured prey relative to spider biomass is low. In contrast, small spiders build three to five webs per feeding period. The ratio of prey biomass intake relative to spider biomass, however, is high. Small spiders may be able to tolerate high rates of web loss because of their potentially high rate of prey intake. However, for large spiders, the high rates of web loss may not be tolerable because of the low rate of prey biomass intake. This work shows that orb weaving spiders forage differently and that different patterns of web use have resulted in at least two very different, but equally effective, means of prey capture. ORB SPINNING SPIDERS SPAN body weights over four orders of magnitude. Nevertheless, almost all studies on the foraging ecology of orb weavers has focused on large and medium sized spiders (Kajak 1965; Robinson & Robinson 1972, 1974; Uetz, et al. 1978; Kajak 1978; Eberhard 1979; Wise 1979, 1981; Olive 1980, 1981; Wise & Barata 1983; Spiller 1984). Small orb spinners, including the immature stadia of large web spinners, are more abundant than large spiders and form an important group of terrestrial predators in tropical forests. For web weaving spiders, web silk and the web spinning process are the highest costs associated with foraging (Peakall 1969). Therefore, web loss without prey capture may result in an important energetic loss for the spider. If prey are abundant, however, high rates of web loss may not significantly affect spider foraging effectiveness. Although large and small prey are captured in strong webs, only minute prey can be captured in the webs spun by small spiders. Nevertheless, in a variety of habitats, the most abundant insects are less than or equal to 4 mm in body length (Olive 1980; Greenstone 1984; Nentwig 1983; Shelly 1983, 1984; Wise & Barata 1983; Spiller 1984). In this paper I compare two aspects of spider foraging modes, rate of prey capture, and rate of web breakdown, among five species of tropical forest orb weavers whose size varies over three orders of magnitude. This work predicts that for small spiders prey are abundant and small spiders tolerate high rates of web breakdown. For large spiders, however, resources are less abundant and high rates of web loss are not tolerable. Prey capture at spider orb webs depends on web size, where the web is suspended, and the ability of the web to intercept prey (Kajak 1965, Colebourn 1974, Enders 1977, Brown 1981, Olive 1982, Craig 1987b). The probability of insect encounter at webs and the ability of webs to intercept prey are greatest just after the web is spun. As the web breaks down during the spider's foraging period, however, its effectiveness decreases. Web breakdown results from a variety of both biotic and abiotic factors. The most important biotic events affecting web breakdown and function are prey interceptions. At interception, the kinetic energy an insect transfers to a web is a function of the insect's mass and velocity (E = mv2/2). Most investigators have assumed that small insects are weak fliers (for example see Shelly 1983). Goldsmith et al. (1980), however, have shown that Anartepritchardi (Diptera: Cecydomiidae) is capable of rapid flight velocities up to 3.8 ms-'. Because the kinetic energy of flying insects increases as a function of velocity squared, insect flight speed has a much greater effect on insect kinetic energy than does insect size. Therefore, even an insect of low mass may possess a high kinetic energy when flying fast. This effect is most obvious when insect kinetic energy is calculated relative to insect size or the potential insect area that the web may intercept. Craig (1987b) calculated that the kinetic energy for the small flies (family Cecydomiidae) caught in webs spun by Epilineutes globosus and Leucauge globosa, to be close to the breaking energy of the webs' viscid silk. Hence, when flying fast, even fragile insects are capable of damaging and breaking through