Diversity of arboreal spiders in primary and disturbed tropical forests

This Study investigates how arboreal spicier communities in SE-Asian primary lowland rain forests charge after anthropogenic disturbance. Two types of secondary forests were distinguished: 1) forests adjacent to each other, which finally merged into primary forest and 2) forests that were isolated by at least 10 km from the primary forest. Three forests of different age were investigated from each type and compared with undisturbed primary forest. All disturbed forests had been used some years for agriculture and were then left between 5 and 50 years to regenerate naturally. Spiders from at least seven trees per forest type were collected using insecticidal knockdown fogging and sorted to species or morphospecies level. Spiders represented between 5-10% of all canopy arthropods. A similar number of spiders were collected per square meter from all trees. However, communities in the primary forest differed greatly in their alpha- and beta-diversity and in community structure from those in the disturbed forest types. Diversity was high in the regenerating forests connected to the primary forest and approximated the conditions of the primary forest during the Course of forest succession. In contrast, the isolated forests were of low diversity and communities showed little change during forest regeneration. These results indicate the importance of a species-source from which disturbed forests can be recolonized. However, even under optimal conditions this process needed decades before spider communities became similar to those of the primary forest. With no species-source available, spider diversity changed little during 50 years of forest regeneration. In the isolated forest we observed a drastic turnover from forest species towards species characteristic of open vegetation and Shrubs. Our results give an indication of how large a loss in diversity can be expected in isolated forest fragments.