Abundance and biomass
Average abundance of canopy arthropods differed significantly between land-use systems (F3, 27 = 28.35, p < 0.0001), being only slightly lower in jungle rubber (221 ± 66 ind/m²) compared to rainforest (267 ± 118 ind/m²), but decreased by ~ 70 % in rubber and oil palm plantations (83 ± 31 and 80 ± 28 ind/m², respectively). Average canopy arthropod biomass was also substantially lower in plantations, with a more severe decline in the Bukit Duabelas (up to 87 %) than in the Harapan landscape (ca. 70 %; significant landscape ⨯ land-use interaction; F3,56 = 3.47, p = 0.0221); this was due to considerably higher arthropod biomass, mainly of large herbivorous/algae-microbivorous orthopterans and cockroaches/termites, in rainforest and to a lesser extent in jungle rubber in Bukit Duabelas (737 ± 335 and 381 ± 131 mg/m², respectively) than in Harapan (respective values of 252 ± 114 and 314 ± 225 mg/m²). By contrast, the biomass in rubber and oil palm plantations was generally low in both landscapes (ca. 90-100 mg/m²).
Similar to total arthropod abundance, differences in the abundance of individual taxonomic groups also depended on land use (significant taxon ⨯ land use interaction; F48,975 = 8.58, p < 0.0001; Supplementary Fig. S3), with the biomass of individual taxa in addition varying with season (significant taxon ⨯ land-use ⨯ season interaction; F51,902 = 1.48, p = 0.0182; Supplementary Fig. S4). In decreasing order, the most abundant groups were Formicidae, Collembola and Diptera across all land-use systems except for oil palm, where Formicidae, Psocoptera and Diptera were most abundant (Supplementary Table S2). Taxa with the highest biomass, by contrast, comprised Orthoptera, Araneae and Blattodea in rainforest, jungle rubber and oil palm, whereas in rubber Araneae and Diptera featured the highest biomasses. Oil palm was dominated by Orthoptera and to a lesser extent by Araneae, but also featured high proportions of herbivorous Lepidoptera and Curculionidae (Fig. 2). The abundance and biomass of most arthropod groups decreased strongly from rainforest towards plantation systems; mean abundance of Collembola in oil palm plantations, for instance, was only one sixth of that in rainforest (5 ± 5 ind/m² in oil palm compared to 32 ± 19 ind/m² in rainforest and even 53 ± 31 ind/m² in jungle rubber). As an exception, compared to rainforest abundance and biomass of Lepidoptera and Curculionidae were only significantly lower in rubber plantations but not in oil palm plantations (Supplementary Table S2, Fig. 2). For details on variations in abundance and biomass of canopy arthropods with season and landscape, see Supplementary Chapter S1, Supplementary Figs. S4 and S5, and Supplementary Table S3.