Who is the biggest foodie underground? The soil animal food web tells you

Produced by: Science Popularization China

Author: Chang Liang, Zhang Sha (Northeast Institute of Geography and Agricultural Ecology, Chinese Academy of Sciences) Qiao Yu (Yunxin Instrument Technology (Shanghai) Co., Ltd.)

Producer: China Science Expo

Editor's note: In order to unveil the mystery of scientific work, the China Science Popularization Frontier Science Project launched a series of articles called "Me and My Research", inviting scientists to write articles themselves, share their scientific research experiences, and create a scientific world. Let us follow the explorers at the forefront of science and technology and embark on a journey full of passion, challenges, and surprises.

When we stroll in a lush green forest or step through a golden wheat field, we often overlook that there is a vibrant "underground kingdom" under our feet, where live a group of "foodies" - soil animals.

Although tiny in size, soil animals and their food webs play an important role in the health of ecosystems.

Today, let us transform into Sherlock Holmes and solve this mystery happening in the mysterious underground world - among the soil animals, who eats whom?

Soil animals under the microscope

(Image source: Eidg.WSL Research Institute)

There are three types of underground "foodies", the largest of which is only a few centimeters

Before we solve this underground mystery of "who ate whom", let's take a look at the "parties" involved in this case:

Soil animals are called "eaters" because they help decompose materials and recycle nutrients by ingesting organic matter, microorganisms or other animals in the soil. Although they are small and inconspicuous, they are essential to soil health and the stability of the ecosystem. Based on their size, soil animals can be divided into:

1. Small soil animals: those with a body width of less than 100 μm, including nematodes, protozoa, rotifers, etc.;

2. Medium-sized soil animals: with a body width between 100μm and 2mm, including mites, springtails, protozoa, diplozoa, earthworms, etc.;

3. Large soil animals: those with a body width greater than 2 mm are large soil animals, including earthworms, spiders, beetles, lipopods, etc.;

Soil animals are a key part of the ecosystem's food web. Although they are small, their activities directly affect soil fertility, plant growth, and the stability of the entire ecosystem. Therefore, studying soil animals can help us better understand and protect the soil and its ecosystem.

Common soil animal classification

(Image source: Paul E A., 2007)

How to find these "foodies"?

However, the largest of these "foodies" is only a few centimeters in size. How do scientists find these tiny creatures in the soil and study the soil animal food web?

Staking out, arresting, and dissecting are simple and crude methods, but they are not enough to "solve the case"

If you are quick enough, direct observation in the field is undoubtedly the most intuitive and fastest method. Scientists only need to wait quietly to find the traces of these "suspects" in the soil, and then observe and capture their predation behavior. However, this method is usually only applicable to large soil animals such as spiders and earthworms that are easy to find with the naked eye. For those small or deep animals in the soil, it is somewhat difficult to observe directly.

Spiders hunting different prey

(Photo source: veer photo gallery)

In addition to direct observation in the wild, capturing soil animals and bringing them back to the laboratory for cultivation and observation is also a common research method. Just like we feed our pets at home, scientists also provide them with food and observe their eating habits to determine what these "foodies" like to eat, thereby locking in the "suspects" and inferring their position in the food web.

Although this method is intuitive and effective, the laboratory cannot completely simulate the wild environment. Therefore, it cannot guarantee that the "foodies" have the same eating habits in the laboratory and in the wild. The accuracy of the indoor cultivation experimental observation method needs to be further determined.

Of course, in addition to directly observing what these "foodies" eat and how they eat, the most direct method is to let the soil animals "prove their innocence."

Microscopic intestinal content analysis allows scientists to use their superb "medical skills" to dissect these soil animals and carefully observe the food residues in their stomachs under a microscope to determine what they have eaten.

This method allows scientists to get a precise picture of the eating habits of these small animals, but the intestinal contents can only reflect what the animal has eaten recently, and some easily digestible foods may be overlooked, so this method is prone to "false positives."

Microplastics in earthworm guts

(Image source: Ke Meng et al., 2023)

Who ate whom? Chemical methods to "solve the case"

All of the above methods determine the food web by observing the feeding behavior of soil animals, but they all have certain limitations. Therefore, scientists have come up with the idea of ​​starting with the food that "foodies" eat, and have developed some more efficient and accurate food web research methods.

Digestive enzyme analysis is a method of inferring the diet of soil animals by analyzing the digestive enzymes in their bodies. Animals have different types of digestive enzymes, and each digestive enzyme is responsible for digesting one or several types of food. Therefore, this method is like using a key to open a lock, allowing us to gain a deeper understanding of the eating habits of these soil animals. For example, studies have found that the digestive enzymes of Eisenia fetida are mainly used to decompose plant residues, indicating that it feeds on plants.

Like digestive enzymes, various nutrients have different properties, and we can determine what soil animals have eaten by examining the nutritional content of the substances they have found.

Take fatty acids as an example. Since fatty acids are not completely decomposed after digestion and are basically absorbed as is, they can well reflect the source of food. Based on this characteristic, scientists use fatty acid analysis to analyze the content and types of fatty acids in soil animals, and then by comparing the fatty acid composition of different soil animals, they can determine "who" eats "who", thereby clarifying the material transfer path in the food web.

Neutral fatty acid extraction process of soil animals (springtails)

(Image source: Scheu Group)

Stable isotope technology plays the role of a "detective". It can reveal the food relationship between soil animals by tracking the transfer and changes of stable isotopes in the food chain. This is like tracking clues in a case to find the suspect. Scientists analyze the carbon and nitrogen isotopes in soil animals to determine their food sources and nutritional levels. This method can not only reveal the feeding relationship at a certain moment, but also let us understand the long-term results of material transfer in the food web.

Stable isotope analysis of herbivorous click beetle larvae and spiders

(Image source: Michael Traugott et al., 2013)

When foodies have an ID card: DNA traces the food web

Entering the 21st century, with the rapid development of molecular biology, scientists have developed a new research method - DNA molecule tracking food chain network technology.

This method uses the specificity of DNA to track food relationships between soil animals...

Imagine that each soil animal is like an individual with a unique "ID card", and DNA is the information on this "ID card". By reading this information, scientists can know what these soil animals eat and how they are related to each other.

In the future, with the continuous improvement of species molecular identification technology and the accumulation of databases, DNA molecular tracking of food chain network technology will become the mainstream method for studying soil animal food webs. The study of soil animal food webs not only allows us to have a deeper understanding of the mysteries of the underground world, but also provides us with new ideas and methods for protecting the ecological environment.

We have reason to believe that in the near future, this mysterious underground world will become clearer and more interesting.

Multiplex PCR Detection of Cereal Pests in Temperate Zones

(Image source: Michael Traugott et al., 2013)

The study of soil animal food web not only allows us to have a deeper understanding of the mysteries of the underground world, but also provides us with new ideas and methods to protect the ecological environment. Let us pay attention to this mysterious underground world and contribute to the harmonious coexistence of our home planet!

(Image source: Pexels)

References:

1. Cui Ying. 2012. Study on material flow in China's coastal ecosystems based on stable isotopes and fatty acid composition (PhD dissertation). Shanghai: East China Normal University.

2.Donovan SE, Eggleton P, Bignell D E. Gut content analysis and a new feeding group classification of termites[J]. Ecological Entomology, 2001, 26(4): 356-366.

3.Berg MP, Stoffer M, van den Heuvel H H. Feeding guilds in Collembola based on digestive enzymes[J]. Pedobiologia, 2004, 48(5-6): 589-601.

4. Ma Xiuhui, Wang Zhijian. DNA-based methods and their applications in studying animal diet composition[J]. Sichuan Animal Science, 2012, 31(3): 497-503.