Food web structure and functional composition
Land-use changes altered the trophic structure of canopy food webs. Biomass-weighted average leaf-normalized Δ15N values of arthropods were significantly higher in rubber plantations (3.3 ‰) than in rainforest, jungle rubber and oil palm (ca. 2 ‰; F3,58 = 4.30, p = 0.0083; Fig. 3). While the maximum of Δ15N values were similar among land-use systems, minimum values were significantly lower in rubber and oil palm plantations than in rainforest (F3,58 = 8.04, p = 0.0001; Fig. 4). Similarly, biomass-weighted average, maximum and minimum Δ13C values of canopy arthropods differed significantly among land-use systems (F3,58 = 37.48, p < 0.001, F3,58 = 26.03, p < 0.0001 and F3,58 = 56.76, p < 0.0001, respectively; Fig. 4), with significantly lower values in plantations, in particular in rubber, compared to rainforest and jungle rubber. Arthropods from rainforest and jungle rubber were strongly enriched in13C compared to leaf material, with maximum Δ13C values of 8.8 ± 1.6 ‰ and 4.7 ± 0.8 ‰, respectively. Minimum Δ13C values were still above 3 ‰ in arthropods from rainforest and jungle rubber, whereas minimum Δ13C values were lower in oil palm and especially rubber (1.4 ± 1.2 ‰ and -0.7 ± 1.0 ‰, respectively).
Δ15N and Δ13C values also differed between canopy arthropod taxa depending on land-use system (Fig. 4, Supplementary Fig. S6; significant taxon ⨯ land-use interaction; F48,919 = 5.58, p < 0.0001 and F48, 869 = 3.37, p < 0.0001 for Δ15N and Δ13C, respectively). Taxa with the highest Δ15N values across land-use systems were Diptera and Elateridae, followed by typical predators/parasitoids, such as Staphylinidae, Araneae and parasitoid wasps. Psocoptera, Collembola and Blattodea featured very low Δ15N values, with minimum δ15N values being in some cases more than 6 ‰ below those of canopy leaves. Δ15N values of most taxa, in particular of those at higher trophic levels, such as Diptera, Staphylinidae and Araneae, were remarkably constant across land-use systems. Δ13C values were generally lower in arthropod taxa from rubber and oil palm plantations than in those from rainforest and jungle rubber.
Trophic structure of canopy arthropod communities, i.e. biomass distribution within Δ15N classes as an indication of trophic level, differed significantly between land-use systems, but the differences varied between landscapes (significant Δ15N class ⨯ landscape ⨯ land-use interaction; F3, 1051 = 3.89, p < 0.0088; Fig. 4). Biomass distribution was close to normal among Δ15N classes in rainforest, but shifted slightly towards lower trophic positions in Bukit Duabelas and towards higher trophic positions in Harapan. By contrast, rubber plantations featured high biomasses at high Δ15N classes, and in case of Bukit Duabelas also at very low Δ15N classes. Jungle rubber in Bukit Duabelas was similar to rainforest in the same region, but biomasses peaked at lower trophic levels compared to rainforest, whereas jungle rubber in Harapan was similar to rubber plantations, with higher biomasses at high trophic levels. Biomasses in oil palm plantations peaked at intermediate and low trophic levels.