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.