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\begin{document}

\title{Rainforest conversion to plantations fundamentally alters energy fluxes
and functions in canopy arthropod food webs}



\author[1]{Melanie Pollierer}%
\author[1]{Jochen Drescher}%
\author[1]{Anton Potapov}%
\author[2]{Kasmiatun Kasmiatun}%
\author[1]{Amanda Mawan}%
\author[2]{Mega Mutiari}%
\author[2]{Rizky Nazarreta}%
\author[2]{Purnama Hidayat}%
\author[2]{Damayanti Buchori}%
\author[1]{Stefan Scheu}%
\affil[1]{Georg-August-Universitat Gottingen Fakultat fur Biologie und Psychologie}%
\affil[2]{Bogor Agricultural University Faculty of Agriculture}%


\vspace{-1em}



  
  \date{May 22, 2024}


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\selectlanguage{english}
\begin{abstract}
Tropical rainforests around the world are rapidly converted into cash
crop agricultural systems. The associated massive losses of plant and
animal species lead to changes in arthropod food webs and the energy
fluxes therein. These changes are poorly understood, in particular in
the extremely biodiverse canopies of tropical ecosystems. Using canopy
fogging followed by stable isotope and energy flux analyses, we show
that land-use conversion from rainforest to rubber and oil palm
plantations not only causes a drastic reduction in energy fluxes of up
to 75\%, but also shifts fluxes among trophic groups. While rainforest
featured high levels of both herbivory and algae-microbivory, and a
balanced ratio of herbivory to predation, relative fluxes were shifted
towards predation in rubber and towards herbivory in oil palm
plantations, indicating profound shifts in ecosystem functioning. Our
results highlight that the ongoing loss of animal biodiversity and
biomass in tropical canopies degrades animal-driven functions and
restructures canopy food webs.%
\end{abstract}%



\sloppy


\textbf{Introduction}

Conversion of tropical rainforest into cash crops is escalating due to
the pressure to increase food and biofuel production (Laurance \emph{et
al.} 2014). Indonesia has experienced particularly high rates of
deforestation and forest degradation within the last decades
(Hansen\emph{et al.} 2013; Margono \emph{et al.} 2014). Conversion of
rainforest into rubber and oil palm plantations, which are among the
major cash-crops in Indonesia (Juniyanti \emph{et al.} 2021), is not
only accompanied by vast deforestation and above- and belowground carbon
losses, but also by substantial inputs of fertilizers, herbicides and
pesticides (Guillaume \emph{et al.} 2018; Darras \emph{et al.} 2019).
Together, this has resulted in decreases in floral and faunal abundance
and diversity in multiple taxonomic groups (Fitzherbert \emph{et
al.}2008; Gibson \emph{et al.} 2011; Chaudhary \emph{et al.} 2016).
However, functional consequences of biodiversity losses remain little
understood. Biodiversity across trophic levels is of pivotal importance
for maintaining ecosystem multifunctionality (Hunt \& Wall 2002;
Bardgett \& van der Putten 2014; Tilman \emph{et al.} 2014). Decreases
in abundance and diversity of invertebrates may critically impact energy
fluxes in food webs and ultimately lead to losses of ecosystem functions
(Cardinale \emph{et al.} 2006; Barnes \emph{et al.} 2014, 2016;
Clough\emph{et al.} 2016; Drescher \emph{et al.} 2016). However, until
today we do not know how energy fluxes in tropical canopies, which are
one of the most biodiverse habitats on Earth, are affected by land-use
change (Erwin 1983; Ellwood \& Foster 2004; Nakamura \emph{et al.}
2017).

Studies on canopy arthropod groups such as ants, beetles, spiders and
parasitoid wasps suggest substantial decline in species diversity,
density and biomass if rainforest is converted into rubber and oil palm
plantations (Nazarreta \emph{et al.} 2020; Azhar \emph{et al.} 2022;
Kasmiatun \emph{et al.} 2022; Ramos \emph{et al.} 2022). Evaluating
associated changes in energy fluxes may help to understand functional
consequences of this decline (Barnes \emph{et al.} 2014; Zhou \emph{et
al.} 2022). Ecosystem functioning crucially depends on fluxes of energy
and matter, which are closely linked to food web dynamics via trophic
interactions (Reiss \emph{et al.} 2009). Manning \emph{et al.} (2018)
argue that ecosystem functioning should ideally be defined via process
rates involving fluxes of energy and matter between trophic levels and
the environment. This calls for a holistic food web approach, focussing
not only on individual trophic groups, but on energy fluxes through
multiple trophic levels and entire communities (Soliveres \emph{et
al.}2016; Barnes \emph{et al.} 2018; Wan \emph{et al.} 2022). Energy
fluxes through food web nodes, characterizing the energy consumption of
different trophic groups, can be used to infer ecosystem functions such
as predation, herbivory and detritivory, that are closely related to
ecosystem services, such as pest control, decomposition and carbon
sequestration (Barnes \emph{et al.} 2018; Gauzens \emph{et al.} 2019).

Application of the energy flux approach to invertebrate communities in
tropical canopies is challenging, because the trophic structure of
arthropod communities in this habitat is poorly explored
(Nakamura\emph{et al.} 2017). Given the central role of food web
topology for the correct estimation of energy fluxes (Jochum \emph{et
al.} 2021), it is crucial to obtain precise information on trophic
positions and trophic interactions of multiple trophic groups in the
food web (O'Neill 1969; Seibold \emph{et al.} 2018). Thus, in-situ
assessment of trophic positions of taxa and their potential shifts among
tropical land-use systems is needed, but rarely done (Zhou \emph{et al.}
2022). To the best of our knowledge, the trophic structure of arthropod
communities in tropical canopies and land-use associated changes in
energy fluxes therein have not been investigated in a comprehensive way
until today.

Here, using canopy fogging, we collected arthropods in rainforest, and
in three major transformation systems in Southeast Asia, i.e. jungle
rubber (rubber agroforest system), rubber and oil palm plantations,
replicated in two landscapes in Jambi Province, Sumatra. Sampling was
conducted once in the dry and once in the rainy season. For all major
arthropod taxa (12 orders and 5 families, representing consistent
trophic groups) on each site, we recorded abundance and biomass, and
used stable isotope ratios (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C values) to estimate site-specific trophic
positions from primary consumers to top predators and to assess the use
of basal resources such as plants and epiphytic microorganisms. Stable
isotope analysis allowed us to reconstruct realistic trophic positions
of major taxa for calculating energy fluxes and to assess ecological
functions such as herbivory, algae-microbivory and predation. We
compared fluxes and functions among rainforest transformation systems
and hypothesized that (1) due to decline in resource diversity, and
density and biomass of canopy arthropods under anthropogenic
disturbance, land use simplifies the trophic structure and decreases
trophic redundancy; and (2) land use results in a strong reduction of
the total energy flux in canopy food webs and leads to changes in
proportions of key arthropod-driven trophic functions, such as
herbivory, predation and microbivory.

\textbf{Materials and Methods}

\emph{Study region and experimental design}

Canopy arthropods were collected from research plots (n =32) of the
EFForTS (\textbf{E} cological and Socioeconomic~\textbf{F} unctions of
Tropical Lowland Rain\textbf{for} est~\textbf{T}
ransformation~\textbf{S} ystems) project in the lowlands of Jambi
Province, Sumatra, Indonesia (Supplementary Fig. S1; Drescher et al.
2016), a region dominated by smallholder cash crop agriculture (Clough
et al. 2016). Eight 50 m ? 50 m plots each were established in three of
the most dominant agricultural land-use systems in Jambi Province, i.e.
monocultures of rubber (\emph{Hevea brasiliensis} ), oil palm
(\emph{Elaeis guineensis} ), and jungle rubber (rubber agroforest system
in gradually degrading rainforest remnants; Rembold et al. 2017), as
well as in lowland rainforest as control. The 32 research plots were
located within and around two lowland rainforest reserves, i.e. the
Bukit Duabelas National Park and the Harapan Rainforest, which are
covered by degraded primary rainforest (Margono et al. 2014). The plots
inside and near Bukit Duabelas National Park are referred to as
belonging to the ``Bukit Duabelas landscape'', while the research plots
inside and near Harapan rainforest are affiliated to the ``Harapan
landscape'' (Drescher et al. 2016).

\emph{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\selectlanguage{ngerman}® (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 \selectlanguage{english}? 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 -20degC 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
Kasmiatun\emph{et al.} 2022). Average body length and width were
measured for 15 (or if \textless{}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
(\textsuperscript{13}C/\textsuperscript{12}C
and\textsuperscript{15}N/\textsuperscript{14}N) and C-to-N ratios were
measured for pooled leaves from each plot and for 15 (or if
\textless{}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 \selectlanguage{greek}δ
\selectlanguage{english}notation, with \selectlanguage{greek}δ\selectlanguage{english}X (%0) = (R\textsubscript{sample} -
R\textsubscript{standard})/R\textsubscript{standard} x 1000; X
represents the target isotope, and R\textsubscript{sample} and
R\textsubscript{standard}
represent\textsuperscript{13}C/\textsuperscript{12}C
and\textsuperscript{15}N/\textsuperscript{14}N ratios of samples and
standard, respectively. PD belemnite (PDB) and atmospheric nitrogen
served as the primary standard for \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N, respectively. Acetanilide
(C\textsubscript{8}H\textsubscript{9}NO, Merck, Darmstadt) was used for
internal calibration.

\emph{Statistical analyses}

All statistical analyses were conducted with R version 4.1.0
(2021-05-18) ''Camp Pontanezen''. To account for different isotopic
baselines among different land-use types, bulk isotope values
(\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N) of canopy arthropod
orders were normalized to trees or oil palms representing primary
producers by subtracting the isotopic signatures of leaves from the
respective plots. These leaf-calibrated isotope data are denoted as
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C and \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N. To calculate mean,
minimum and maximum of \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C and \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N
for each land-use system in the two landscapes, \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C
and \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values of canopy arthropod orders were
weighted by biomass and scaled between 0 and 1 for each community in a
plot (Cucherousset \& Vill\selectlanguage{ngerman}éger 2015). To estimate biomass distribution
among trophic levels, we assigned `\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N classes' from
the highest to the lowest \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values, each with a
span of 3 %0 (equivalent to approximately one trophic level) and summed
the biomass of taxa in each class for each plot. To test for differences
in abundance, biomass and isotopic composition between canopy arthropod
orders, land-use systems, landscapes and seasons, and for differences in
biomass distribution among trophic levels and land-use systems,
landscapes and seasons, we constructed linear mixed effects models in R
using the packages `lme4' (Bates et al. 2015) and `lmerTest' (Kuznetsova
et al. 2017). `Plot' was included in the models as random effect.
Non-significant effects were eliminated from full models using the
`step' function, but without reducing random effects. To test for
differences among mean, minimum and maximum of \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C
and \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N, we used the R packages `nlme'
(Pinheiro\emph{et al.} 2021) and `mass' (Ripley \emph{et al.} 2019) to
construct linear models with landscape, land-use and season as fixed
factors.

For the reconstruction of the trophic structure of the studied taxa and
the calculation of energy fluxes among them see Box 1. We additionally
ran a sensitivity analysis to test if assumptions used for energy flux
calculations will influence the conclusions of our study (Supplementary
Text S1 and Fig. S2). To inspect differences between ecological
functions depending on land-use, region and season, we again constructed
linear mixed effects models with plot as random term (see above).

\textbf{Results}

\emph{Abundance and biomass}

Average abundance of canopy arthropods differed significantly between
land-use systems (F\textsubscript{3, 27} = 28.35, p \textless{} 0.0001),
being only slightly lower in jungle rubber (221 \selectlanguage{ngerman}± 66 ind/m²) compared to
rainforest (267 ± 118 ind/m²), but decreased by \textasciitilde{} 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 \selectlanguage{english}? land-use interaction; F\textsubscript{3,56} = 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
\selectlanguage{english}? land use interaction; F\textsubscript{48,975} = 8.58, p \textless{}
0.0001; Supplementary Fig. S3), with the biomass of individual taxa in
addition varying with season (significant taxon \selectlanguage{english}? land-use \selectlanguage{english}? season
interaction; F\textsubscript{51,902} = 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.

\emph{Food web structure and functional composition}

Land-use changes altered the trophic structure of canopy food webs.
Biomass-weighted average leaf-normalized \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values
of arthropods were significantly higher in rubber plantations (3.3 %0)
than in rainforest, jungle rubber and oil palm (ca. 2 %0;
F\textsubscript{3,58} = 4.30, p = 0.0083; Fig. 3). While the maximum of
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values were similar among land-use systems,
minimum values were significantly lower in rubber and oil palm
plantations than in rainforest (F\textsubscript{3,58} = 8.04, p =
0.0001; Fig. 4). Similarly, biomass-weighted average, maximum and
minimum \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C values of canopy arthropods differed
significantly among land-use systems (F\textsubscript{3,58} = 37.48, p
\textless{} 0.001, F\textsubscript{3,58} = 26.03, p \textless{} 0.0001
and F\textsubscript{3,58} = 56.76, p \textless{} 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
in\textsuperscript{13}C compared to leaf material, with maximum
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C values of 8.8 \selectlanguage{ngerman}± 1.6 \selectlanguage{english}%0 and 4.7 +- 0.8 \selectlanguage{english}%0,
respectively. Minimum \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C values were still above 3 %0
in arthropods from rainforest and jungle rubber, whereas minimum
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C values were lower in oil palm and especially
rubber (1.4 \selectlanguage{ngerman}± 1.2 \selectlanguage{english}%0 and -0.7 +- 1.0 \selectlanguage{english}%0, respectively).

\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N and \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C values also differed
between canopy arthropod taxa depending on land-use system (Fig. 4,
Supplementary Fig. S6; significant taxon ? land-use interaction;
F\textsubscript{48,919} = 5.58, p \textless{} 0.0001 and
F\textsubscript{48, 869} = 3.37, p \textless{} 0.0001 for
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N and \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C, respectively). Taxa
with the highest \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N 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 \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N
values, with minimum \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values being in some cases
more than 6 %0 below those of canopy leaves. \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N
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. \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C 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 \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N classes as an indication of
trophic level, differed significantly between land-use systems, but the
differences varied between landscapes (significant
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N class ? landscape ? land-use interaction;
F\textsubscript{3, 1051} = 3.89, p \textless{} 0.0088; Fig. 4). Biomass
distribution was close to normal among \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N 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
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N classes, and in case of Bukit Duabelas also at
very low \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N 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.

\emph{Energy fluxes and ecological functions as influenced by land-use
change}

Land-use change influenced energy flux through canopy arthropod
communities (for individual land-use systems see Supplementary Fig. S7).
Total energy fluxes in tree canopies differed significantly between
land-use systems (F\textsubscript{3,59} = 9.22, p \textless{} 0.0001).
They were highest in rainforest (1160 \selectlanguage{ngerman}± 1088 kJ
ha\textsuperscript{-1}year\textsuperscript{-1}) and jungle rubber (747 ±
586 kJ ha\textsuperscript{-1} year\textsuperscript{-1}), but
significantly lower in rubber and oil palm plantations (293 ± 247 and
342 ± 382 kJ ha\textsuperscript{-1} year\textsuperscript{-1},
respectively). Importantly, energy fluxes within different trophic
groups (algae-microbivores, herbivores and predators), as indicators of
different ecological functions, also differed significantly between
land-use systems (significant trophic group \selectlanguage{english}? land-use interaction;
F\textsubscript{6,116} = 6.14, p \textless{} 0.0001; Fig. 5). In
general, algae-microbivory and herbivory were the dominant feeding
strategies in rainforest, jungle rubber and oil palm plantations.
Algae-microbivory, indicated by low \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N signatures,
was highest in rainforest (643 \selectlanguage{ngerman}± 904 kJ
ha\textsuperscript{-1}year\textsuperscript{-1}), even exceeding
herbivory (306 ± 268 kJ ha\textsuperscript{-1}
year\textsuperscript{-1}). In jungle rubber, herbivory and
algae-microbivory were of equal importance, whereas in oil palm
plantations herbivory dominated. Interestingly, predation was dominant
in rubber plantations, with similarly high fluxes as in jungle rubber,
but considerably lower fluxes to herbivory and algae-microbivory. The
ratio between herbivory and predation differed between the land-use
systems (F\textsubscript{3,48} = 7.61, p = 0.0003); it was significantly
greater in oil palm (10.9 ± 20.2) than in rainforest, jungle rubber and
rubber (3.5 ± 6.3, 3.6 ± 6.3 and 1.9 ± 5.3, respectively). For
variations in energy fluxes with landscape and season see Supplementary
Chapter S1 and Supplementary Fig. S8.

\textbf{Discussion}

Here, for the first time we quantitatively explored energy fluxes of
canopy arthropod communities of tropical rainforest transformation
systems. Conversion of rainforest into plantations drastically altered
abundance, biomass and trophic structure of canopy arthropods, resulting
not only in substantially decreased energy fluxes, but also in shifts in
the relative importance of ecological functions such as
algae-microbivory, herbivory and predation. The observed shifts may have
far-reaching consequences for ecosystem functioning and underline the
importance to counteract the rapidly accelerating land-use changes in
tropical regions.

Total abundance and biomass of canopy arthropods strongly decreased from
rainforest and jungle rubber towards monoculture plantation systems of
rubber and oil palm, as has been shown previously for ants, spiders and
parasitoid wasps (Nazarreta \emph{et al.} 2020; Azhar \emph{et al.}2022;
Ramos \emph{et al.} 2022). The decrease of combined average arthropod
abundance across 12 major taxa by ca. 70 \% from rainforest to
monoculture plantations of rubber and oil palm was even more severe than
reported for the abundance of soil invertebrates/arthropods at the same
study sites of ca. 60 \% (Potapov \emph{et al.} 2019a), suggesting more
pronounced effects of land-use changes on aboveground compared to
belowground invertebrates, as has been shown for temperate ecosystems
(Le Provost \emph{et al.} 2021). Notably, differences in biomass of
canopy arthropods varied between the two landscapes studied, with
generally higher abundance and biomass in rainforests of Bukit Duabelas
than in Harapan. This was mainly due to high biomasses of large
herbivorous/algae-microbivorous Orthoptera and Blattodea in the former
and resulted in a more drastic decrease in biomass of up to 87 \% from
rainforests to plantations in the Bukit Duabelas than the Harapan
landscape (ca. 70 \%). The regional differences are presumably due to
differences in fertility and landscape heterogeneity, but in particular
the longer history of rainforest transformation in the Harapan than the
Bukit Duabelas landscape (Allen \emph{et al.} 2015; Harrison \&
Swinfield 2015; Guillaume \emph{et al.} 2018; Sibhatu \emph{et
al.}2022). By contrast, rubber and oil palm plantations featured
similarly low abundance and biomass of canopy arthropods in both
regions, indicating that regional differences are levelled out by strong
disturbance such as the conversion of rainforest into plantation
systems. This highlights the importance of protecting intact tropical
ecosystems at landscape scale.

Taxonomic groups responded differentially to land-use changes. The
abundance and biomass of most arthropod groups decreased strongly from
rainforest towards plantation systems; loss of Collembola abundance was
most pronounced, with a loss of \textgreater{} 85 \% from rainforest and
jungle rubber to oil palm plantations. Abundance of Formicidae was also
reduced by ca. 75 \% from rainforest to plantations and biomass of
Blattodea even decreased by \textgreater{} 90 \%. The strong reduction
in abundance and biomass of these taxa may be related to reduced habitat
complexity, aboveground plant biomass and diversity, and available food
sources (Novotny \emph{et al.} 2006; Kotowska \emph{et al.} 2015;
Drescher \emph{et al.} 2016; Zemp \emph{et al.} 2019). On the other
hand, abundance and biomass of some herbivorous taxa, such as
Lepidoptera and Curculionidae, were not reduced in oil palm plantations.
This is likely due to specialized herbivore species that thrive in oil
palm plantations, such as red palm weevils, bagworms and nettle
caterpillars, and to the introduced curculionid
pollinator\emph{Elaeidobius kamerunicus} , which is the dominating
curculionid species in oil palm plantations in our study region
(Greathead 1983; Kasmiatun \emph{et al.} 2022).

Land-use changes also altered the trophic structure of canopy arthropod
communities. Biomass-weighted mean of trophic positions as indicated by
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values were significantly higher in rubber
plantations, indicating a higher proportion of predators. Low minimum
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values, with \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values being
in some cases more than 6 %0 below those of canopy leaves, suggest that
some canopy arthropod taxa, such as Psocoptera, Collembola and
Blattodea, predominantly feed on algae or lichens which utilize
atmospheric nitrogen and typically are depleted in
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N compared to vascular plants (Chahartaghi \emph{et
al.} 2005; Erdmann \emph{et al.} 2007; Maraun \emph{et al.} 2011).
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values of omnivores such as Hemiptera and
Formicidae, but also of higher trophic level taxa such as Diptera,
Staphylinidae and Araneae, were remarkably constant across land-use
systems. This suggests that, despite high diversity within taxonomic
groups (Basset 2001; Nazarreta \emph{et al.} 2020; Ramos \emph{et
al.}2022), overall trophic niches and therefore ecological functions of
these supra-specific taxa bear phylogenetic signal, which was also
postulated for belowground systems (Potapov \emph{et al.} 2019b). In
contrast to the relatively low \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C values in
plantation systems, in rainforest and jungle rubber canopy arthropods
were on average 5-6 %0 enriched in \textsuperscript{13}C compared to
canopy leaves as potential basal resource; \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C maxima
in rainforest were even close to 9 %0 above those of leaves. Part of
the\textsuperscript{13}C enrichment may be attributed to variations
in\textsuperscript{13}C signatures between leaves high and low in the
canopy, with the latter used for calibration in this study (van der
Merwe \& Medina 1991; Hyodo \emph{et al.} 2010). An additional cause may
be selective utilization of only specific plant/leaf components, such as
water-soluble amino acids and carbohydrates that are more easily
accessible to digestion and are typically enriched
in\textsuperscript{13}C compared to bulk tissue of leaves. Tropical
leaves feature high amounts of structural and hard-to-digest compounds,
such as lignin and cuticular waxes, that are more depleted
in\textsuperscript{13}C than other less condensed carbon compounds
(Pollierer \emph{et al.} 2009). Given the diversity of herbivore feeding
strategies, including leaf chewing and mining, fruit eating as well as
xylem/phloem sucking (Novotny \emph{et al.} 2010), selective utilization
of specific components is likely.

Despite relatively constant trophic levels of taxonomic groups, the
biomass distribution among trophic levels, as indicated by
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N classes, differed significantly between land-use
systems, but this also varied between landscapes. The shift from a
close-to-normal biomass distribution in rainforests, in particular in
Bukit Duabelas, to more uneven distributions in oil palm and rubber,
suggests that, in line with our first hypothesis, land-use
intensification decreases trophic redundancy and causes a more scattered
distribution of trophic niches, possibly mitigated by reduced resource
diversity (Krause \emph{et al.} 2020). Low trophic redundancy may
increase the vulnerability to the loss of trophic functions and to
extinction cascades in intensively managed land-use systems
(Sanders\emph{et al.} 2018).

In line with our second hypothesis, energy fluxes were remarkably
reduced by rainforest conversion into plantations. Total energy fluxes
were reduced by up to 75 \% in rubber and oil palm plantations compared
to rainforest, and reduced by \textgreater{} 30 \% compared to jungle
rubber, mirroring the reductions in biomass. As energy fluxes can be
used to infer ecosystem multifunctionality and stability (Barnes\emph{et
al.} 2018; Manning \emph{et al.} 2018; Potapov 2022), the strong
reduction in energy fluxes in plantation systems points to the loss of
ecosystem functions and potentially to the deterioration of ecosystem
stability. Significantly lower total energy fluxes in plantations
compared to rainforests contrast findings from soil food webs at our
study sites, where the presence of earthworms as large detritivores in
plantations counterbalanced declined energy fluxes in other trophic
groups (Potapov \emph{et al.} 2019a). However, energy fluxes to canopy
arthropods were not only just reduced, but shifted among different
trophic groups, such as herbivores, algae-microbivores and predators,
representing different ecosystem functions. Importantly, shifts in
energy fluxes and respective functions were similar, but not congruent
with shifts in biomass among trophic groups. This highlights that energy
flux approaches go beyond biomass-based approaches by also considering
metabolic rates and assimilation efficiencies of consumers, as well as
trophic structure and preferred prey, thereby more realistically
reflecting community functioning.

In rainforests, algae-microbivory was the most important feeding
strategy, followed by herbivory, while both of these feeding types were
equally important in jungle rubber. Such high energy fluxes to
algae-microbivores have not been demonstrated before for canopy
arthropods and suggest that tropical canopy food webs more heavily rely
on resources other than higher plants, with algae and microorganisms
playing an even greater role than tissue of higher plants such as
leaves. In contrast to vascular plant epiphytes, microbiota in tropical
forest canopies are not well-studied (Nakamura \emph{et al.} 2017).
Microbial communities differ strongly with vertical stratification in
tropical forests and are responsible for decomposition of suspended
detrital substrates (Gora \emph{et al.} 2019). Fungi, algae and lichens
can significantly contribute to the diet of leaf litter and
bark-inhabiting Oribatida and Collembola (Chahartaghi \emph{et al.}2005;
Erdmann \emph{et al.} 2007; Susanti \emph{et al.} 2019). Potentially,
due to the low resource quality and high lignin and wax contents of
leaves of tropical trees, these resources are of particular relevance
for arthropods in the canopy of tropical rainforests. This calls for
further studies to disentangle microbial contributions from fungi,
bacteria and algae to the diet of canopy arthropods and how they are
influenced by land-use change.

While rainforest and jungle rubber were characterized by pronounced
energy fluxes to algae-microbivores and herbivores, predation was the
main feeding type in rubber. Food webs with pronounced energy fluxes to
lower trophic levels, i.e. bottom heavy food webs, as in rainforest and
jungle rubber, are assumed to be more stable than those with higher
fluxes to high trophic levels (Rip \& McCann 2011; Barnes \emph{et
al.}2018). The strong reduction in energy fluxes to lower trophic levels
and the concomitant shift to fluxes at higher trophic levels in rubber
plantations are likely related to the conversion from evergreen to
deciduous forest, causing temporal gaps in resource availability for
herbivores and, due to faster replacement of leaves, lower
algae-microbial colonization. As more energy is required to support high
trophic levels, there is potentially an overall faster transfer and
higher loss of energy along food chains in rubber plantations. This
faster transfer may contribute to higher carbon and nutrient losses in
rubber plantations (De Ruiter \emph{et al.} 1993; Barnes \emph{et
al.}2018; Guillaume \emph{et al.} 2018). On the other hand, a
significantly higher ratio of herbivory compared to predation, as in oil
palm plantations, may indicate reduced pest control and concomitantly
higher losses of oil palm tissue to herbivore pests, necessitating
higher use of pesticides (Corley \& Tinker 2015).

Here, for the first time we documented the dramatic impacts of tropical
land-use change on abundance, biomass, trophic structure and functioning
of canopy arthropod communities. We showed that the conversion of
rainforest into rubber and oil palm plantations not only strongly
reduces energy fluxes, but also alters ecological functions, shifting
from stable systems with high energy fluxes at lower trophic levels and
high trophic redundancy in rainforest to more contrasting trophic niches
in plantation systems. While oil palm plantations were characterized by
a high ratio of herbivory to predation suggesting reduced pest control,
predation was dominant in rubber plantations, providing the risk of
higher energy and nutrient losses. Together, our findings indicate that
the conversion of rainforest into plantation systems compromises
ecosystem multifunctionality and stability, and suggest high sensitivity
of rainforests to even low levels of disturbance such as selective
logging. A combined food web approach, not only considering changes in
abundance and biomass, but also in trophic structure and energy fluxes
is promising to comprehensively trace changes in ecological functions
due to land-use change and may provide a reliable basis to foster
informed management decisions.

\textbf{Acknowledgements}

This study was funded by the Deutsche Forschungsgemeinschaft (DFG),
project number 192626868--SFB 990 in the framework of the collaborative
German-Indonesian research project CRC990. We thank the following
persons and organizations for granting access to and use of their
properties: village leaders, local plot owners, PT Humusindo, PT REKI,
PT Perkebunan Nusantara VI, and Bukit Duabelas National Park. This study
was conducted using samples/organisms collected based on Collection
Permit No. S.710/KKH-2/2013 (PHKA, 23. Dec. 2013) and exported based on
permit No. 773/KKH/SDG/KSA.2/2017 (PHKA, 04. Aug. 2017). MMP was funded
by the DFG Priority Program 1374 ''BiodiversityExploratories'' (SCHE
376/ 38-2).

\textbf{Competing interests}

There are no competing interests to declare.

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\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Fig. 1} : Canopy fogging in rubber monoculture (Photo credit:
Jochen Drescher)\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Fig. 2} : Total biomass (fresh weight, mg/m\selectlanguage{ngerman}²), represented by
the size of the pie charts, and relative biomass proportions (\%) of
canopy arthropod taxa in different land-use systems (rainforest, jungle
rubber, rubber and oil palm plantations). Taxa with relative biomass
contributions \textless{} 6 \% were grouped as ``other''.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Fig. 3} : \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values (normalized to tree
leaves) of canopy-living algae-microbivores (Psocoptera, Blattodea,
Collembola), herbivores and omnivores (Orthoptera, Lepidoptera, Acarina,
Chrysomelidae, Formicidae, Hemiptera, other Coleoptera, Curculionidae,
Thysanoptera) and predators (parasitoid wasps/Braconidae -- Para/Brac,
Araneae, Staphylinidae, Elateridae, Diptera) in different land-use
systems (LU: rainforest -- F, jungle rubber -- J, rubber -- R and oil
palm -- O). Taxa are ordered by increasing mean \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N
values.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Fig. 4} : Biomass and trophic position of canopy arthropod taxa
in different land-use systems (Rainforest, Jungle rubber, Rubber and Oil
palm) in the landscapes Bukit Duabelas (left column) and Harapan (right
column). The dot plots show mean (\selectlanguage{ngerman}± SD) \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C and
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values of arthropod taxa (n = 4 plots * 2 seasons
= 8 per taxon per landscape), with the size of dots representing their
mean biomass. The light blue diamond and horizontal line, and the red
diamond and vertical line represent the biomass-weighted mean and range
of arthropod \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C and \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values,
respectively. The bar plots show mean (\selectlanguage{ngerman}± SEM) biomass (mg/m²) of
arthropods grouped into \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N classes as an indication
of trophic level (turquoise - algae-microbivores, green -- herbivores,
red -- predators, striped -- omnivores); note the different scales for
biomass.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Fig. 5} : Canopy energy flux (mean \selectlanguage{ngerman}± SE) within different
feeding interaction types (algae-microbivory, herbivory, carnivory) and
total energy flux in four land-use systems (rainforest, jungle rubber,
rubber and oil palm) pooled for landscape and season.

\textbf{Box 1:} \emph{Trophic structure and calculation of energy
fluxes}

For reconstructing the trophic structure of the studied taxa and
calculating energy fluxes among them, we generated predator-prey
adjacency matrices for each plot in both the dry and rainy season based
on (1) bulk stable isotope composition, (2) optimum predator-prey
mass-ratios (PPMR) and (3) biomass-based preferences (Potapov 2022).
Bulk isotope composition was used to calculate `optimum' prey or food
resource for each animal group by taking in account a trophic enrichment
of 2.3 \selectlanguage{english}%0 for \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N and of 1 %0 for
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C between prey and predator (Tiunov 2007). Taxa
with \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values below those of plant leaves were
assumed to mainly feed on algae/microbes that have lower
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values than plants (Potapov \emph{et al} . 2019).
PPMR was used as a characteristic that reflects size-based predation
(i.e., small predator feeds on small prey and large predator can also
feed on larger prey) and optimum foraging strategy (i.e., balancing the
energetic profit and handling efforts; Brose \emph{et al.} 2008),
commonly used in food-web ecology (Brose \emph{et al.} 2019). The
optimum PPMR was set to 100, implying that typical prey has 100 times
less mass than the predator (Brose \emph{et al} . 2008). Since this
value is derived from laboratory experiments and modelling, we allowed
for a very broad range for ``optimum'' prey (PPMR width), i.e. body mass
range of the optimum prey was set to be triple the body mass range of
the predator, representing a large niche. Parasitoids (parasitoid
wasps/Braconidae and Diptera) and ants were `allowed' to feed on larger
prey and the range for potential prey was set two times wider than for
other groups due to parasitic lifestyle/pack hunting (Potapov 2022).
Biomass-based preferences were set up assuming that prey preference
scaled with available prey biomass (Gauzens \emph{et al.} 2019). The
three optimum prey (adjacency) matrices above were multiplied to obtain
a final food web matrix for each plot in each season representing
feeding preferences among food web nodes (taxa). The food web matrices
were subsequently used to calculate energy fluxes per square metre at
plot-level using the R package `Fluxweb' (Gauzens \emph{et al} . 2019).
When applying the `fluxing' function, biomass preferences were set to
`false' as they were already accounted for in the food web matrices.
Biomass losses were set to `true', as metabolic losses of taxa were
defined per unit of biomass. Per capita metabolic rates in W based on
metabolic theory scaling (Brown 2004) were calculated assuming a
constant temperature of 25.2 \selectlanguage{ngerman}°C and using general coefficients for
invertebrates (Jochum \emph{et al.} 2021). The efficiency level was set
to `predator', i.e. the efficiencies with which the predator/consumer
assimilates consumed prey were used. Temperature-corrected assimilation
efficiencies of food for predators (0.915) and herbivores (0.573) were
calculated using parameters from Lang et al. (2017) and the mean annual
temperature measured by meteo-stations across all studied plots, i.e.
25.2 °C (Drescher et al. 2016). We assumed assimilation efficiencies of
algae-microbivores to be similar to herbivores. To infer ecological
functions, the fluxes to herbivores were summed up as herbivory, the
fluxes to algae-microbivores were summed up as algae-microbivory and the
fluxes to predators were summed up as predation.

\selectlanguage{english}
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