Size-trophic spectrum in soil food webs across ecto- and arbuscular mycorrhiza-dominated deciduous forests
Despite distinct microbial resource use between arbuscular and ectomycorrhizal-dominated forests, soil food webs exhibit broadly similar total energy fluxes and a consistent size-trophic spectrum where energy channelling from specific resources peaks in distinct animal size classes.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Beneath the forest floor, hidden from view, lies a bustling world of life that drives the health of the entire ecosystem. This underground community is a complex network where energy flows from plants and microbes up through layers of consumers, from microscopic worms to larger insects and earthworms. Scientists have long known that the type of trees growing above ground influences the soil below. Some trees partner with one kind of fungus, while others partner with a different kind, and these partnerships change how nutrients cycle and how quickly dead leaves break down. A natural question arises: do these different underground foundations create entirely different food webs, or do the animals living there find a way to organize themselves in a similar pattern regardless of the trees above? Understanding this helps us grasp how forests function and how they might respond to changes in the environment.
To answer this, researchers traveled to twenty-four paired forest sites across Denmark and Germany, comparing woods dominated by trees that partner with arbuscular mycorrhizal fungi against those dominated by trees partnering with ectomycorrhizal fungi. They did not just count the animals; they measured how much energy each group consumed and traced exactly where that energy came from. By analyzing the chemical signatures of amino acids in the bodies of soil creatures, the team could distinguish whether an animal was eating plant material, bacteria, fungi, or other animals. They combined this dietary data with measurements of how many animals lived in each square meter and how heavy they were, allowing them to calculate the total flow of energy through the soil food web. The goal was to see if the type of tree partnership above ground dictated the flow of energy below, or if the size of the animals themselves was the true architect of the system.
The results revealed a surprising consistency. Despite the known differences in soil chemistry and decomposition rates between the two forest types, the total amount of energy moving through the food web was nearly identical. The proportion of energy coming from plants, fungi, and bacteria also remained steady across both forest types. The researchers had expected to find that one forest type relied more heavily on bacterial energy while the other leaned on fungal energy, but the data showed no such split. Instead, the way energy was distributed depended almost entirely on the body size of the animals doing the eating. The forest type did not change the rules of the game; the size of the player did.
When the scientists looked closely at the different size groups, a clear and predictable pattern emerged, which they call a size-trophic spectrum. The smallest creatures, the nematodes, which are microscopic roundworms, drew most of their energy from bacteria. As the body size increased to the next group, the microarthropods like springtails and mites, the main source of energy shifted to fungi. The largest group of decomposers, the macroarthropods such as millipedes and woodlice, relied primarily on plant material, specifically the coarse dead leaves and roots. Finally, the earthworms, which are the largest invertebrates in this system, returned to a diet dominated by fungi and bacteria, consuming the soil and the organic matter within it. This pattern held true regardless of whether the forest was dominated by one type of tree partnership or the other.
Predation, or the act of one animal eating another, also followed this size-based structure. The smallest nematodes engaged in the highest amount of predation relative to their total energy intake, likely because they live in tiny water-filled spaces where they frequently encounter other small organisms. As the animals got larger, the relative importance of predation dropped, only to stabilize again among the larger macroarthropods, where spiders and centipedes maintained a steady presence as hunters. This suggests that the size of an animal in the soil is a strong predictor of what it eats and how it fits into the energy flow. The researchers found that while the trees above might change the soil's chemical makeup, the soil animals reorganize these different resources into a consistent flow of energy based on their own physical size.
This discovery changes how we view the underground world. It suggests that the body size of soil animals is a reliable indicator of their role in the ecosystem, acting as a universal map for energy flow. Whether in a beech forest or an oak forest, the smallest animals process bacteria, the medium-sized ones process fungi, and the large ones process plant litter. This consistency implies that if we know the size distribution of the animals in a soil sample, we can predict how energy is moving through that system, even without knowing every single species present. It offers a new way to understand forest health, linking the physical structure of the animal community directly to the vital processes of decomposition and nutrient cycling that keep the forest alive.
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