Diversity and Trophic Ecology of Arboreal Spider (Araneae) Assemblages along a Transformation Gradient from Lowland Rainforest to Oil Palm Plantations in Jambi Province, Sumatra, Indonesia

Abstract

In chapter 2, I investigated the canopy spider abundance, biomass, richness and community composition across a land-use gradient from tropical lowland rainforest via jungle rubber to monocultures of rubber and oil palm. Overall, I demonstrated that canopy spider communities in oil palm and rubber plantations are less abundant, contain lower biomass, and are less diverse compared to rainforest and jungle rubber systems. I also provided evidence that community composition is similar between less intensive land-use systems, i.e. rainforest and jungle rubber, and differed from more intensive land-use systems, i.e. oil palm and rubber plantations, which also differed from one other. Investigating environmental variables that shape spider communities, I demonstrated that at family level aboveground biomass, number of trees per hectare and canopy openness were major factors determining spider community composition. I also demonstrated that at species level, the most important factors were plant richness and the number of trees per hectare. My results provided detailed information on the importance of rainforests for the conservation of canopy spider communities as only a subset of the community can tolerate the more harsh environmental conditions and disturbances in monoculture plantations. Moreover, the fact that in rainforest and jungle rubber the diversity and community composition are similar suggests that the majority of spiders might tolerate a moderate degree of disturbances. Finally, disturbed agroecosystems, such as oil palm and rubber plantations, may also contribute to the total diversity in a geographic area (γ-diversity) suggesting that landscape scale perspectives are needed for the conservation of spider diversity. In chapter 3, I focused on changes in the trophic structure of canopy spider communities with the transformation of tropical rainforests into rubber and oil palm plantations by measuring bulk tissue 15 N/ 14 N and 13 C/ 12 C ratios. Additionally, I studied in more detail the response of the families Salticidae, Clubionidae, Theridiidae, and Thomisidae, which contributed most to the abundance, biomass, and diversity of the spider communities in the studied land-use systems. Overall, we found that rainforest spider communities respond differently to land-use transformation. Generally, the range in trophic niches and use of basal resources (∆ C values) did not differ between land-use systems, but maximum ∆ 15 N values suggested that some families have narrower trophic niches in plantations. Moreover, functional diversity was similar across the four land-use systems, with the exception of isotopic divergence, which was higher in rainforest and jungle rubber than in monoculture plantations.Further, there was no difference in the trophic positions between cursorial and sheet-web building spiders, but they occupied virtually the same average position. However, maximum ∆ 15 N values in Salticidae, Clubionidae and Thomisidae were higher in plantations than rainforest and jungle rubber indicating that intra-guild predation may be more widespread in free hunting than web-building spiders but only in plantation systems. Finally, we found evidence that transformation of rainforest into monocultures plantations is associated with a shift towards more herbivore prey in canopy spiders. Overall, our study provides first insight into trophic changes in canopy spider communities associated with the transformation of tropical rainforest into plantations.