The "residents" of the rainforest are quietly changing

Produced by: Science Popularization China

Author: Li Yin (School of Life Sciences, Sun Yat-sen University)

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life" to interpret life phenomena and reveal biological mysteries from a unique perspective. Let us delve into the world of life and explore infinite possibilities.

Tropical rainforests are common forest ecosystems distributed near the equator, with a hot and rainy climate and complex and diverse terrain.

The tropical rainforest preserves the most precious biological resources on Earth. It is the area with the richest animal species on Earth and also has a wide variety of plant resources, which provide food sources and habitats for various animals.

Tropical rainforest

(Photo source: veer photo gallery)

Due to the high temperature and humidity, the organic matter in the tropical rain forest decomposes quickly and the material cycle is intense. It is the world's largest biological gene bank and a huge active reservoir for carbon biological cycle transformation and storage.

However, the phenomenon of large-scale reclamation of tropical rainforests is becoming increasingly serious around the world. In addition to causing soil degradation, soil erosion, and biodiversity decline, the changes in the aboveground and underground food webs caused by changes in the tropical rainforest environment, as well as the functional and energy changes of the entire trophic level, are still largely unknown.

Tropical rainforest food web

The distribution of biomass and energy flows in terrestrial ecosystems is dominated by “green” (aboveground) and “brown” (belowground) food web compartments, which together shape ecosystem function and stability.

Leaves in the rainforest canopy absorb sunlight through photosynthesis and then convert it into sugars, and the plants continue to accumulate energy in the process.

In the "green" aboveground environment, herbivorous insects feed on plant tissues, and carnivorous insects, spiders and other canopy arthropods and birds feed on herbivorous insects. In the "brown" underground environment, earthworms and soil arthropods feed on fallen leaves, fungi, etc. The feces of these animals are eventually decomposed by microorganisms and enter the soil.

Energy flows throughout the forest energy system through the food web composed of food chains between plants and animals.

In addition to common economic plants such as rubber plants, cocoa, and cinchona, there are many unique plant and animal species in the tropical rainforest, many of which have medicinal value or other economic value. Tropical rainforests have been reclaimed on a large scale. Due to changes in land development and utilization, the rainforest environment will change, and the animal communities in the rainforest will also change accordingly. The species, number, and distribution of rainforest animals are no longer consistent with the original rainforest ecosystem, and the food web has also quietly changed.

An international research team consisting of the University of Göttingen in Germany, Bogor Agricultural University in Indonesia, the German Center for Integrative Biodiversity Research and the University of Leipzig published their latest research results in the journal Nature in February 2024.

Putting many clues together, the research team conducted a comprehensive analysis of small organisms in the soil and beetles and birds in the tree canopy, comparing energy transfer processes in animal communities in Sumatran rainforests, rubber plantations, and oil palm plantations in Indonesia.

From the ground to the upper canopy, a survey of tropical rainforest arthropods was conducted using 14 different methods:

(1) Trapping and netting; (2) Gall sampling in the volume space of a vertical cylinder; (3) Sticky traps; (4) Aerial composite flight interception traps; (5) Plant and dead branch beating; (6) Manual collection of ants and termites; (7) Ground flight interception traps; (8) Collection and extraction of leaf litter fauna; (9) Collection and extraction of ground soil and floating soil fauna; (10) Wood feeding; (11) Trap method; (12) Ground traps; (13) Canopy spray method; (14) Light trapping.

(Image source: Reference [1])

The results of the study found that changes in land use have led to significant changes in the energy flow of above- and below-ground animal food webs in tropical rainforests.

From rainforest to plantation, the food web changes dramatically

In the Sumatran rainforest region, after the rainforest was cleared for plantations, much of the energy was sent to the animal food web below ground.

The scientists found that oil palm and rubber monocultures had similar or higher total animal energy flows compared to rainforests, but key energy nodes were significantly different: In rainforests, more than 90% of animal energy flows, including both "green" and "brown" food webs, went to arthropods in the soil and canopy; whereas in plantations, more than 50% of energy went to soil annelids - earthworms.

The increase in energy flow associated with earthworms reflects the decrease in energy flow associated with soil arthropods. (The black lines in the figure represent energy flow)

(Image source: Reference [5])

Compared with the rainforest fauna, the food web in the plantation canopy was less rich and less complex, while the food web in the soil changed significantly, with the species-diverse arthropod community being replaced by a relatively homogeneous invasive species, earthworms.

Human agricultural activities are known to affect arthropod population sizes and distributions through pesticide use, fertilization, habitat fragmentation, light pollution, and higher temperatures caused by climate change or associated increases in atmospheric carbon dioxide levels.

As a result of these changes, plantations often have fewer predatory insects, while plant-feeding insects, such as caterpillars and beetles, are relatively abundant.

Plantations often have fewer predatory insects and more plant-feeding insects due to changes in the food web and energy.

(Image source: Reference [5])

However, little is known about the diversity of arthropod species in tropical soils, and thus, the level of biodiversity in tropical rainforest soils may be much higher than has been estimated.

This study did not take into account the energy flow of other vertebrate groups in the rainforest canopy besides birds (such as bats and amphibians). Many vertebrates feed on invertebrates such as arthropods, so the actual energy flow accounted for by arthropods in the rainforest may be greater than the data counted in the study.

Overall, it is clear that the rainforest food web is primarily composed of invertebrates, and most of these are concentrated in the soil layer, which is brown.

The outlook for biodiversity is not optimistic

Earthworms have the potential to alter the soil environment through direct or indirect trophic interactions (e.g., litter removal and microbial feeding), thereby negatively affecting arthropods that depend on soil and litter for survival.

However, the decline in arthropods may also be the result of reduced leaf litter input following changes in the rainforest environment, as well as reduced organic carbon and nitrogen in plantation soils.

Thus, tropical land-use change may lead to dramatic energetic and functional reorganization of food webs in above- and below-ground ecosystem compartments, with consequent impacts on animal biodiversity.

These different organisms, from tiny arthropods to birds, from soil to tree canopies, are connected through food webs and energy flows. Previous studies have found that changes in tropical land use can lead to a 18% to 70% decrease in species richness of arthropods, birds, and other groups. The results of this latest study show that the phenomenon of species reduction is related to fundamental changes in the distribution of energy in the food web.

As rainforests are increasingly converted to plantations, biodiversity is declining and ecosystems are being altered. The results of this study provide the first glimpse into energy processing in soil and canopy animal communities in a highly biodiverse tropical ecosystem.

Studying the connections between different parts of the rainforest ecosystem is undoubtedly of great significance for the protection of the rainforest ecological environment, especially in biodiversity areas where species are endangered. By implementing monitoring studies, we will better assess the impact of anthropogenic changes on the temporal and spatial distribution of rainforest arthropod biodiversity, otherwise the risk of arthropod extinction in tropical rainforests may be seriously underestimated.

References

[1] Basset Y, et al. PLoS ONE, 2015, 10(12):e0144110.

[2] Grass I, et al. Nat. Commun., 2020, 11:1186.

[3] Krashevska V, et al. Biol. Fertil. Soil., 2015, 51:697–705.

[4] Nazarreta R, et al. Myrmecol. News, 2020, 30:175–186.

[5] Potapov AM, et al. Nature, 2024, 627:116

[6] Potapov AM, et al. J. Anim. Ecol., 2019, 88:1845–1859.

[7] Thakur M P. Biol. Lett., 2020, 16:20190770.