Worms are crucial to soil quality, but intensive capture and heavy metal pollution are putting them at risk, writes Chen Nengchang.
In China, capturing wild earthworms using electric contraptions has become serious business, with social media users claiming to make a good living from it. But the practice is damaging soil ecology.
This year, it was censured in the first of China’s annual national policy guides – the “No.1 central document” – issued at the start of each year. In the section on strengthening the construction of high-quality farmland, a prominent warning appears: the Party will “severely crack down on soil-destructive behaviours such as… electric trapping [of] earthworms”.
The warning reflects the increasing significance that China attaches to earthworms and soil ecology.
The animals, which were also included in the third national soil survey that launched last year, are vital indicators of soil health and quality. They constitute the highest biomass in the soil, and play a crucial role in promoting the cycle of organic matter, and regulating soil microbial communities.
Why are earthworms being harvested so aggressively?
In traditional Chinese medicine, earthworms are known as “earth dragons”.
The Compendium of Materia Medica, a 16th-century encyclopaedia of traditional Chinese medicine, notes that animals “promote blood circulation, degrade blood clots, and unblock the body’s meridians”. Because of this, they’re often included in traditional herbal remedies widely used in China, Japan, and Korea to treat ischemic or thromboembolic diseases. In recent years, the discovery of lumbrokinase – an enzyme that can degrade blood clots – in earthworms has also led the animal to be used in conventional medicine for the treatment of cardiovascular issues.
The demand for earthworms in both types of medicine over the last few years has turned the capture of the worm into a get-rich-quick industry in rural areas of China.
The 2020 edition of the Chinese Pharmacopoeia only recognises the dried bodies of four species of earthworms as suitable for medicinal use: Pheretima aspergillum (E.Perrier), Pheretima vulgaris Chen, Pheretima guillelmi (Michaelsen) and Pheretima pectinifera Michaelsen. Typically, farming earthworms can produce a more productive and stable supply of the creatures. But as these particular species are very active and likely to escape from confined spaces, they cannot be farmed, and despite the huge demand, earthworms used in medicine are still mainly caught in the wild.
In the past, catching earthworms by hand was laborious, yielding at most 5kg a day. But now, commercially available “electric earthworm machines” can generate an electrical charge into wet soil that stimulates the skin of earthworms, through which they breathe, and forces them to emerge. This method can increase yield tenfold.
However, the boom in electric capture has prevented earthworms from reproducing, and has also damaged soil ecosystems, further affecting soil quality. That is why the No.1 central document prohibits it.
But though they are widely used for medicinal purposes, there are health risks associated with consuming wild earthworms. The invertebrates absorb pollutants from the external environment, and many studies have found their bodies to contain relatively high concentrations of heavy metals. Data from the 2014 National Soil Pollution Survey Bulletin showed that 16.1% of total sample points contained heavy metals exceeding the standard, and on arable land, it increased to 19.4%. When worms are used in medicine or as food ingredients, they may leach these heavy metals, which can then accumulate in the human body.
Tiny earthworms, big roles
Catching large quantities of earthworms for medicinal use not only poses health risks to humans, but also negatively impacts the robustness of the soil and broader ecological environment.
Approximately 4,000 species of terrestrial earthworms have been documented worldwide, of which 306 are found in China. To humans, they are probably the most familiar creature in the soil. Realising their importance in the natural world, scholars have been observing them for centuries. The ancient Chinese believed they fed on soil. “An earthworm eats the dry mould above, and drinks the yellow spring below,” wrote the philosopher Mencius in the 1st century BCE.
We now know that earthworms eat humus – the material that forms in soil when plant and animal matter decays. Meanwhile, the line, “earthworms till the mud, and dragonflies skim the water”, in Song dynasty poem “Desolate Path” by Shi Wenxiang, nods to the role they play in farming – a fact borne out by modern science.
Charles Darwin, the father of evolution, observed and experimented on earthworms for 40 years. He published his final book, “The Formation of Vegetable Mould Through the Action of Worms”, in 1881. It marked the first time the world would understand the fundamental role earthworms play in “bioturbation”, or the burrowing of soil – a key driver of biodiversity.
Darwin estimated that on 0.4 hectares of land, earthworms could ‘plough’ at least 9 tonnes of soil a year, thickening the original layer by half in a decade – vastly improving crop yields and enriching the soil they burrow through.
These days, scientists have an even greater understanding of earthworms. The animals contribute important ecosystem services (or benefits to humans). Among other roles, they are involved in the formation and structural development of soil, its water content, nutrient distribution and climate-regulation capacity. They may even have a role to play in remediating pollution.
As such, they are considered “keystone species”, having an outsized effect on their natural environment, and “ecosystem engineers”, creating microhabitats for other organisms. In short, their impact on soil ecology is disproportionate to their biomass.
Earthworms play three roles in the ecosystem: consumer, decomposer, and modulator. They consume plant residue and leaf litter, and excrete minerals and other nutrients. They affect the physical characteristics of soil, such as bulk density, porosity and stability though burrowing, excreting, and other bioturbation activities. They also improve nutrient availability by stabilising the soil organic carbon pool.
By modifying the composition of their internal microbial communities, they indirectly improve the chemical properties of soil. They transform soil structure, creating an environment conducive to soil microbial activity, and regulating the quantity and activity of soil microorganisms.
In so doing, they enrich plant growth. Earthworms benefit plant growth in five ways, according to a book by entomology professor Clive A. Edwards. They:
Control pests and diseases
Stimulate soil microbes living in symbiosis with plants
Produce plant-growth regulators
Improve soil structure
Increase nutrient availability
On farmland, earthworms play a direct role in increasing crop yields, according to a review of studies looking at the relationship between earthworms and three major global crop pastures – of corn, rice, and wheat – and other types of food. The researchers concluded that the presence of earthworms in such ecosystems leads to an average 25% increase in crop yields and a 23% increase in above-ground biomass, or plant organic matter.
In recent years, there has been significant scientific interest in earthworm activity as indicators of climate change impacts.
In soil polluted with heavy metals, earthworms usually activate its bioavailability, promoting the mobility of heavy metals in the soil-plant system, and causing higher levels of such metals in plants. This poses a health risk to their human consumers.
More importantly, earthworms are bioindicators of heavy metal soil pollution; their permeable skin and constant contact with soil through their digestive tract makes them highly sensitive to changes in soil make-up. Studies have shown that heavy metal pollution is responsible for the absence of earthworms in the soil near the Avonmouth smelter in south-west England, as well as the disappearance of certain types of earthworms in contaminated soil in south-eastern France.
The capacity of earthworms to absorb heavy metals has even led to research exploring the possibility of using earthworms to remediate heavy metal-polluted soil.
How earthworms can be protected
Earthworm numbers and overall soil biodiversity have decreased in China because of modern agriculture’s penchant for fertilisers and pesticides. The special “protected” designation given to the earthworm in the No.1 central document indicates that more attention is being paid to the relationship between agricultural development and biodiversity.
There is no doubting the importance of prohibiting the indiscriminate capture of earthworms. Simultaneously, there should be scientific and holistic understanding and study of the relationship between earthworms, human health, soil, and agriculture – and subsequently, engagement in targeted work to protect the worms.
One such way is by conducting surveys of earthworm species and habitats. In China, there is a lack of investigations into soil biota, with very few relevant articles published. The third national soil survey – the results of which are due in 2025 – is expected to identify the distribution of earthworm species in the soil of different regions, but there is also a need to launch studies into earthworm habitats other than cultivated land.
Additionally, the government could consider enacting a “Soil Biota Protection Act”. Around a quarter of all living creatures on Earth exist in soil, and they play an important part in its evolution. But there is no relevant legislation to protect them, and combatting the electric capture of earthworms has been an uphill battle.
Attention should also be turned to the problem of excessive heavy metals in earthworms used in medicine. At present, such levels are seriously over the limit, especially amongst wild-caught earthworms. The prohibition of electro-capture of such worms must be strengthened to safeguard both soil ecosystems and human health and safety. The possibility of farming and regulating the four “earth dragon” earthworm species should be studied, due to their great potential for medicinal use, and such research should investigate how these products can be free from heavy metal pollution.
The development of eco-agriculture is also fundamental. China is currently endeavouring to turn industrial agriculture ecological. It is only by reducing the use of fertilisers and pesticides, and strengthening protections for the ecological chain, that the number of earthworms in arable land can gradually increase. Only then can they be allowed to serve their roles as ecosystem engineers, soil fertilisers and more.
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