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

摘要

The occurrence and patterns of stabilimenta in 1195 webs of theorb weaver Argiope appensa (Walckenaer) from four islands in the MarianaArchipelago were recorded. Webs on the largest and southernmost island,Guam, had significantly fewer stabilimenta (16.4%, n = 359) than webs ofconspecifics 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 Argiopespp. worldwide (47.8%, n = 5662). The differences in stabilimentum frequencyin webs between sites in the Marianas were not correlated with vegetation, lightlevels, the distribution of individuals within web size classes, or the presence ofcertain stabilimentum patterns. Factors operating at scales smaller than thesample sites may influence stabilimentum frequency, because several locationsshowed nonrandom clumping of decorated webs. One theory for the functionof stabilimenta is that they reduce damage to the web by warning flying birds ofthe web's location. Because Guam's native birds have been eliminated recentlyby the introduction of the brown tree snake, Boiga irregularis (Merrem), the lowfrequency of decorated webs of A. appensa on Guam might, therefore, be eithera behavioral response to the avian extinctions or a result of reduced selectionfor stabilimenta in the absence of birds.