Rainforest conversion to rubber and oil palm reduces abundance, biomass and diversity of canopy spiders

Trophic Change and Community Decline in Acrobat Ants After Rainforest Conversion to Cash Crops

The conversion of tropical rainforests to agriculture causes population declines and biodiversity loss across taxa. This impacts ants (Formicidae), a crucial tropical group for ecosystem functioning. While biodiversity loss among ants is well documented, the responses of individual ant taxa and their adjustments in trophic strategies to land-use change are little studied. Here, we investigated a collection of > 12,000 acrobat ants (Crematogaster) from 14 species collected by canopy fogging in four land-use systems in Jambi Province, Sumatra, including (1) lowland rainforest, (2) jungle rubber (low impact rubber agroforestry), and monocultures of (3) rubber and (4) oil palm. Abundance-weighted trophic positions as indicated by stable isotopes of carbon and nitrogen were significantly different between land uses: Lower average, maximum, minimum and range of δ13C values in monocultures than in rainforest and jungle rubber indicate a shift of Crematogaster towards more plant-based resources, but may be driven substantially by the "canopy effect" of differential carbon assimilation of leaves between lower and higher canopy. Similar Δ15N average, maximum and minimum among the land uses, but a significantly reduced range of Δ15N in monocultures, suggest lowered trophic diversity and increased stochasticity of trophic niches in monocultures. In contrast, community decline in Crematogaster was very pronounced, with density, richness and rarity in monocultures of rubber and oil palm at a fraction of that in rainforest and jungle rubber. Crematogaster communities in oil palm formed a subset of those in rubber, which were a subset of those in jungle rubber, which were a subset of those in rainforest. A notable exception was Crematogaster ferarii, which was exclusively found in oil palm. In conclusion, tropical land-use change induces slight shifts in the trophic structure of Crematogaster communities, but massive declines in its density and diversity. This likely affects Crematogaster-driven ecosystem functions in agricultural systems in Southeast Asia.

Are agricultural commodity production systems at risk from local biodiversity loss?

Compelling evidence for feedbacks between commodity crop production systems and local ecosystems has led to predictions that biodiversity loss could threaten food security. However, for this to happen agricultural production systems must both impact and depend on the same components of biodiversity. Here, we review the evidence for and against the simultaneous impacts and dependencies of eight important commodity crops on biodiversity. We evaluate the risk that pollination, pest control or biodiversity-mediated soil health maintenance services are at risk from local biodiversity loss. We find that for key species groups such as ants, bees and birds, the production of commodities including coffee, cocoa and soya bean is indeed likely to be at risk from local biodiversity loss. However, we also identify several combinations of commodity, ecosystem service and component of biodiversity that are unlikely to lead to reinforcing feedbacks and lose-lose outcomes for biodiversity and agriculture. Furthermore, there are significant gaps in the evidence both for and against a mutualism between biodiversity and agricultural commodity production, highlighting the need for more evaluation of the importance of specific biodiversity groups to agricultural systems globally.

Rainforest transformation reallocates energy from green to brown food webs

Terrestrial animal biodiversity is increasingly being lost because of land-use change1,2. However, functional and energetic consequences aboveground and belowground and across trophic levels in megadiverse tropical ecosystems remain largely unknown. To fill this gap, we assessed changes in energy fluxes across 'green' aboveground (canopy arthropods and birds) and 'brown' belowground (soil arthropods and earthworms) animal food webs in tropical rainforests and plantations in Sumatra, Indonesia. Our results showed that most of the energy in rainforests is channelled to the belowground animal food web. Oil palm and rubber plantations had similar or, in the case of rubber agroforest, higher total animal energy fluxes compared to rainforest but the key energetic nodes were distinctly different: in rainforest more than 90% of the total animal energy flux was channelled by arthropods in soil and canopy, whereas in plantations more than 50% of the energy was allocated to annelids (earthworms). Land-use change led to a consistent decline in multitrophic energy flux aboveground, whereas belowground food webs responded with reduced energy flux to higher trophic levels, down to -90%, and with shifts from slow (fungal) to fast (bacterial) energy channels and from faeces production towards consumption of soil organic matter. This coincides with previously reported soil carbon stock depletion3. Here we show that well-documented animal biodiversity declines with tropical land-use change4-6 are associated with vast energetic and functional restructuring in food webs across aboveground and belowground ecosystem compartments.

Tree species identity, canopy structure and prey availability differentially affect canopy spider diversity and trophic composition

Forest canopies maintain a high proportion of arthropod diversity. The drivers that structure these communities, however, are poorly understood. Therefore, integrative research connecting tree species identity and environmental stand properties with taxonomic and functional community composition of canopy arthropods is required. In this study, we investigated how the taxonomic, functional and trophic composition of arboreal spider communities is affected by tree species composition and associated differences in canopy structure and prey availability in temperate forests. We sampled canopy spiders as well as their potential prey using insecticidal fogging in monospecific and mixed stands of native European beech, native Norway spruce and non-native Douglas fir. Trophic metrics were obtained from stable isotope analysis and structural canopy properties were assessed with mobile laser scanning. Monospecific native spruce stands promoted local canopy spider abundance and diversity, but native beech and beech-conifer mixtures had the highest diversity at landscape scale. Spider community composition differed between monospecific stands, with broadleaf-conifer mixtures mitigating these differences. Irrespective of tree species identity, spider abundance, taxonomic diversity, functional richness and isotopic richness increased in structurally heterogeneous canopies with high prey abundances, but functional evenness and trophic divergence decreased. Our study shows that canopy spiders are differentially affected by tree species identity, canopy structure and prey availability. Broadleaf-conifer mixtures mitigated negative effects of (non-native) conifers, but positive mixture effects were only evident at the landscape scale. Structurally heterogeneous canopies promoted the dominance of only specific trait clusters. This indicates that intermediate heterogeneity might result in high stability of ecological communities.

Response of arboreal Collembola communities to the conversion of lowland rainforest into rubber and oil palm plantations

Background

In the last decades, Southeast Asia has experienced massive conversion of rainforest into rubber and oil palm monoculture plantations. The effects of this land-use change on canopy arthropods are still largely unknown. Arboreal Collembola are among the most abundant canopy arthropods in tropical forests, potentially forming a major component of the canopy food web by contributing to the decomposition of arboreal litter and being an important prey for canopy arthropod predators. We investigated abundance, richness, and community composition of, as well as the influence of a series of environmental factors on, canopy Collembola communities in four land-use systems in Jambi Province, Sumatra, Indonesia: (1) lowland rainforest, (2) jungle rubber (rubber agroforest), and monoculture plantations of (3) rubber and (4) oil palm.

Results

Using canopy fogging in 32 research plots in both the dry and rainy seasons in 2013, we collected 77,104 specimens belonging to 68 (morpho) species. Generally, Collembola communities were dominated by few species including two species of the genus Salina (Paronellidae; 34% of total individuals) and two species of Lepidocyrtinae (Entomobryidae; 20%). The abundance of Collembola in lowland rainforest (53.4 ± 30.7 ind. m−2) was more than five times higher than in rubber plantations, and more than ten times higher than in oil palm plantations; abundances in jungle rubber were intermediate. Collembola species richness was highest in rainforest (18.06 ± 3.60 species) and jungle rubber (16.88 ± 2.33 species), more than twice that in rubber or oil palm. Collembola community composition was similar in rainforest and jungle rubber, but different from monoculture plantations which had similar Collembola community composition to each other. The environmental factors governing community composition differed between the land-use systems and varied between seasons.

Conclusions

Overall, this is the first in-depth report on the structure of arboreal Collembola communities in lowland rainforest and agricultural replacement systems in Southeast Asia. The results highlight the potentially major consequences of land-use change for the functioning of arboreal arthropod food webs.