Low Frequency of Stabilimenta in Orb Webs of Argiope appensa (Araneae: Araneidae) from Guam: An Indirect Effect of an Introduced Avian Predator

Abstract

The occurrence and patterns of stabilimenta in 1195 webs of the orb weaver Argiope appensa (Walckenaer) from four islands in the Mariana Archipelago were recorded. Webs on the largest and southernmost island, Guam, had significantly fewer stabilimenta (16.4%, n = 359) than webs of conspecifics from the neighboring islands of Rota (56.9%, n = 211), Tinian (41.9%, n = 315), and Saipan (50.3%, n = 310), as well as from webs of Argiope spp. worldwide (47.8%, n = 5662). The differences in stabilimentum frequency in webs between sites in the Marianas were not correlated with vegetation, light levels, the distribution of individuals within web size classes, or the presence of certain stabilimentum patterns. Factors operating at scales smaller than the sample sites may influence stabilimentum frequency, because several locations showed nonrandom clumping of decorated webs. One theory for the function of stabilimenta is that they reduce damage to the web by warning flying birds of the web's location. Because Guam's native birds have been eliminated recently by the introduction of the brown tree snake, Boiga irregularis (Merrem), the low frequency of decorated webs of A. appensa on Guam might, therefore, be either a behavioral response to the avian extinctions or a result of reduced selection for stabilimenta in the absence of birds.