Orb-weavers spiders (Arachnida-Araneae) community in the Reserva Florestal do Morro Grande region, Cotia, São Paulo State, Brazil

Abstract

An inventory of the orb-weaver spiders guild was carried out at the Reserva Florestal do Morro Grande and nearby regions (Cotia and Ibiúna-SP). Sixteen areas were sampled, eight belonging to the Reserve, and eight to forest fragments. Four areas of the Reserve presented a mature vegetation, while the remaining four, as well as the forest fragment areas, presented secondary vegetation. Sampling was carried out in December 2002 and March 2003, and the method employed was the nocturnal manual searching. Twenty four samples were obtained from each area, totalling 384 samples. A total of 12,683 spiders were collected, 3,148 adults (24.8%), and 9,535 immatures (75,2%). Adult spiders were sorted to 121 species, belonging to seven families. This is the largest richness ever recorded for an Atlantic Forest area. Both sampling periods had 67 species in common and 27 exclusive species. The most species rich family was Araneidae (76 spp), followed by Tetragnathidae (26 spp), Theridiosomatidae (nine spp), Uloboridae (six spp), Anapidae (two spp), Mysmenidae (one sp) and, Symphytognathidae (one sp). The distribution of the species among the families agrees with their richness in Brazil and worldwide, except Theridiosomatidae witch seems to be overepresented. Their relative abundance showed a similar pattern. The dominant species was Micrathena nigrichelis, with 320 specimens, followed by Micrathena guanabara (306 individuals), Mangora sp.2 (258 individuals) and, Wagneriana janeiro (171 individuals). The community presented a low dominance (H'=3.11) and a high evenness (E=0.835), as expected in a tropical environment. Seven non-parametric richness estimators were used, and their estimates ranged from 130.9 (Bootstrap) to 155.9 (Jackknife 2) species. Nevertheless, none of them reached the asymptote, since the number of rare species (singletons, doubletons, uniques and duplicates) remained stable. The distribution of the species among the eight areas of the Reserva do Morro Grande was verified by a DCA, and the results showed a clear distinction between mature and secondary forests. This analysis also showed species related to each kind of environment, and the results were compared to those of other inventories. The large number of species and individuals of orb-weaver spiders collected stresses the importance of the studied forests and especially the Reserva do Morro Grande as a conservation unit.