Self-made shelters protect spiders from predation

Abstract

Many animals modify their environments, apparently to reduce predation risk, but the success of such endeavors, and their impact on the density and distribution of populations, are rarely rigorously demonstrated. We staged a manipulative experiment to assess the effectiveness of self-made shelters by web spiders as protection from natural enemies. Scincid lizards were included or excluded from 21 replicated 200-m(2) plots, and spiders therein were classified as exposed or sheltered, depending on whether they were uncovered in their web or hidden in cocoons, leaves/debris, or burrows. We found that exposed spiders were greatly affected by the presence of predatory scincid lizards, whereas sheltered spiders were not. More specifically, lizards, which forage close to the ground, reduced the abundance of exposed spiders by two-thirds but had no effect on the abundance of sheltered spiders. Sheltered spiders were able to avoid predation and share space with lizards, suggesting that shelter construction is a mechanism for reducing predation risk and has important population consequences.

Fig. 1.

Examples of (A) exposed and (B) sheltered spiders from Hinchinbrook Island. (A) Deinopis subrufa (Ai), Argiope aethera (Aii), and Tetragnatha bituberculata (Aiii) are exposed to predatory skinks when at rest in their webs. (B) Cyrtophora hirta (Bi), Phonognatha graffei (Bii), and Theridion (Biii) spp. rest in self-made retreats constructed from silk, leaves and debris, respectively.

Fig. 2.

Height above ground of all lizards observed in lizard access treatments on all sampling occasions. Lizards spend most of their time at ground level, occasionally climbing vegetation to forage at heights within 40 cm from the ground.

Fig. 3.

Total numbers of exposed and sheltered web spiders counted in each study plot in four categories of height above ground. Exposed spiders do not construct any form of shelter in their webs. Sheltered spiders use silk, leaves, and/or debris to construct retreats in which they remain when not active. Ground-dwelling predatory lizards significantly reduced the abundance of exposed spiders at and within 20 cm of ground level. Sheltered spiders did not occur at ground level but were common in the 1- to 20-cm height class, and predatory lizards did not affect their abundance.