Introduction
Conversion of tropical rainforest into cash crops is escalating due to
the pressure to increase food and biofuel production (Laurance et
al. 2014). Indonesia has experienced particularly high rates of
deforestation and forest degradation within the last decades (Hansenet al. 2013; Margono et al. 2014). Conversion of
rainforest into rubber and oil palm plantations, which are among the
major cash-crops in Indonesia (Juniyanti 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 et al. 2018; Darras et al. 2019).
Together, this has resulted in decreases in floral and faunal abundance
and diversity in multiple taxonomic groups (Fitzherbert et al.2008; Gibson et al. 2011; Chaudhary 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 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 et al. 2006; Barnes et al. 2014, 2016; Cloughet al. 2016; Drescher 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 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 et al. 2020; Azhar et al. 2022;
Kasmiatun et al. 2022; Ramos et al. 2022). Evaluating
associated changes in energy fluxes may help to understand functional
consequences of this decline (Barnes et al. 2014; Zhou 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 et al. 2009). Manning 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 et al.2016; Barnes et al. 2018; Wan 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 et al. 2018; Gauzens 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 (Nakamuraet al. 2017). Given the central role of food web topology for the
correct estimation of energy fluxes (Jochum 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 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 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 (δ15N and
δ13C 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.