Signaling by decorating webs: luring prey or deterring predators?

Many organisms convey false signals to mislead their prey or predators. Some orb-weaving spiders build conspicuous structures on webs called decorations. Web decorations and spider colorations are both suggested to be important signals involved in interactions between spiders and other organisms. There are several hypotheses about the functions of signaling by decorations, among which prey attraction had received much support, but empirical evidence regarding predator defense is controversial. In this study, we conducted field experiments to investigate the effects of spider decoration and coloration on insect interception rates of webs built by Argiope aemula and to evaluate whether presence of decorations may decrease predation risk of spiders. Decorated webs with spiders present had the highest prey interception rate, followed by undecorated webs with spiders, and then undecorated webs without spiders. Such results indicated that decorations of Argiope spiders functioned as visual lures, and so did spiders' bright body colorations. In the field, almost all wasp attack events occurred on medium-sized spiders rather than on large ones. Moreover, medium-sized Arg. aemula on decorated webs received far more attacks than those on undecorated webs. Results of this study thus show that the signals conveyed by decorations can visually lure prey but at the cost of an increased predation risk.