Sampling and analyses of canopy arthropods and leaves
Arthropods were collected by canopy fogging, i.e. the application of an insecticide to tree and palm canopies, in three target canopies per 50 m ⨯ 50 m plot (Fig. 1). Canopy fogging was conducted twice: Once in dry season 2013 and once in the following rainy season 2013/14. Target canopies were randomly located in the plot, and consisted of interlocking canopies of ca 5 trees in rainforest and jungle rubber, two trees in rubber monocultures and one palm in oil palm plantations. Fallen trees and canopy gaps were avoided. A mixture of 50 mL DECIS 25 EC® (Bayer Crop Science, deltamethrine 25g/L) and four litres petroleum white oil was applied to each target canopy, taking approx. 20 min per fogging event. Underneath each target canopy, 16 square 1 m ⨯ 1 m collection funnels were hung from suspended ropes, each funnel was fitted with a 250 mL plastic bottle containing 100 mL 96% EtOH. Two hours after the application of the insecticide, stunned or dead arthropods were collected, cleaned from debris, the EtOH was exchanged and the samples stored at -20°C until further analysis.
To calibrate stable isotope measurements of the canopy arthropod community to the potential basal resource, five randomly selected leaves per plot were collected at head height near the plot centre. In rainforest, the leaves were from five different tree species; in jungle rubber, two of the leaves were from two different rubber trees, and three leaves from three different tree species in the plot or from as many tree species as present. In rubber, the leaves were from the five nearest rubber trees closest to the plot centre. In oil palm, we collected ca. 40 cm² leaf material from the five different oil palm trees closest to the plot centre.
Collected canopy arthropods were sorted to 12 orders: Acarina, Araneae, Blattodea, Coleoptera, Collembola, Diptera, Hemiptera, Hymenoptera, Lepidoptera, Orthoptera, Psocoptera and Thysanoptera. As large flying taxa such as Apoidea and Vespoidea in part actively evaded the insecticide fog at the time of application (J. Drescher, pers. obs.), the order Hymenoptera in this study is represented by Formicidae (ants) and Braconidae (a family of parasitoid wasps), both of which were highly abundant in the samples (Nazarreta et al. 2020; Azhar et al. 2022). Additionally, four abundant beetle families with contrasting feeding strategies were separately analysed from the rest of the order Coleoptera (henceforth termed ‘other Coleoptera’), i.e. Chrysomelidae, Curculionidae, Elateridae and Staphylinidae (for details see Kasmiatunet al. 2022). Average body length and width were measured for 15 (or if <15 in the sample as many as present) randomly selected individuals from each order/family at plot-level in the dry and rainy season. Biomasses (fresh weight) per square metre were calculated using taxon-specific regressions (Supplementary Table S1).
Stable isotope ratios of carbon and nitrogen (13C/12C and15N/14N) and C-to-N ratios were measured for pooled leaves from each plot and for 15 (or if <15 in the sample as many as present) randomly selected and pooled individuals per plot for each order/family both in the dry and rainy season, resulting in 1073 measured samples of canopy arthropods and 32 measured samples of canopy leaves. Samples were measured with a coupled system of an elemental analyser and a mass spectrometer (Reineking et al. 1993). Isotopic signatures were expressed using the δ notation, with δX (‰) = (Rsample - Rstandard)/Rstandard x 1000; X represents the target isotope, and Rsample and Rstandard represent13C/12C and15N/14N ratios of samples and standard, respectively. PD belemnite (PDB) and atmospheric nitrogen served as the primary standard for δ13C and δ15N, respectively. Acetanilide (C8H9NO, Merck, Darmstadt) was used for internal calibration.