Heavy rains have hit many parts of my country, and dead earthworms can be seen everywhere! Will the population decrease?

Since the beginning of summer in 2024, many parts of our country have been continuously hit by heavy rains. Fortunately, emergency rescue measures have been implemented in place to minimize the losses and ensure the safety of people’s lives and property.

Careful friends will find that with the arrival of rainfall, many earthworms often crawl out of the soil. After the rain, the bodies of earthworms that were drowned or exposed to the sun can be seen everywhere, and birds and other animals that feed on earthworms can also have a good meal. So, will the population of earthworms decrease as a result?

Earthworms. Image source: Pixabay

Why do earthworms burrow out of the ground?

It is obvious that darkness is the paradise of earthworms. Are they "seeking their own death" by coming out on their own initiative?

The answer is no. There are three main reasons why earthworms actively drill out of the ground:

1

Finding new resources

Earthworms breathe through their skin, which requires a relatively high humidity environment. If there are too many earthworms in a certain area, or if there are fewer living resources, the area will become unsuitable for them to survive. Some earthworms will come out at night when the ground is relatively safe and humid, looking for a new place to live.

2

Carry out breeding activities

Earthworms are hermaphroditic and cross-fertilized, and they mostly reproduce in spring and autumn. In order to find high-quality gene sources for their offspring, large numbers of earthworms will come out of the ground during the breeding season and engage in "blind date" behavior.

3

Escape from adverse environment

Many pesticides (such as carbendazim) have a strong killing effect on earthworms. Long-term drought will cause the soil to dry and compact, and flooding will cause the habitat to become hypoxic. Such physical or chemical factors will cause the habitat to no longer be habitable, which will also force earthworms to drill out of the ground.

Will earthworms be killed by rain?

Earthworms often live in soil and organic piles rich in organic waste. They are very sensitive to light and will only come out under specific conditions. For example, heavy rainfall can cause the humidity of the environment in which earthworms depend for survival to increase sharply, and even their habitats may be submerged by stagnant water, causing the earthworms' living environment to quickly become unsuitable.

There are at least two strategies that earthworms have for coping with waterlogging stress. The first strategy is "escape." The earthworms that adopt this strategy are mainly those with low tolerance to waterlogging. They will crawl out of the waterlogged environment and look for a relatively dry place.

The second strategy is "endurance". The earthworms that adopt this strategy are mainly species with high tolerance to waterlogging. They have low oxygen consumption and can survive in low oxygen concentrations and hypoxic conditions for a long time, and will not come out after heavy rain. At the same time, related studies have shown that rainfall enhancement has no significant effect on the number and biomass of earthworms.

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How does the earthworm family stay strong?

1

The special "magic" of rebirth after being severed

We have known since childhood that earthworms have the unique ability to regenerate from severed parts. When an earthworm is cut in two, the wound heals and grows into two earthworms.

Some scholars have conducted relevant experiments, using 18 different cutting methods to observe earthworms. The results show that no matter whether the head is cut off or the tail is removed, the earthworm has a chance to survive, and the survival rate is mainly affected by the number of remaining segments. The more segments there are, the higher the survival rate, but it is difficult for the test individuals to recover to a completely healthy and complete state.

Earthworm amputation experiment (i) Removal of the front end of the earthworm (a); (ii) Removal of the front and back ends of the earthworm (b); (iii) Removal of the back end of the earthworm (c). A total of 18 different types of earthworms were amputated and tested to determine the effect of the number of remaining segments on their regeneration rate. Image source: Literature: Xiao et al., 2011

2

Strong reproductive capacity

The reproductive capacity of earthworms should not be underestimated. Their reproduction method is very peculiar. Earthworms are hermaphrodites, but they need cross-fertilization. When two adult earthworms meet, they will move close to each other in opposite directions, put their female holes close to each other's spermatophore holes, exchange sperm with each other, and complete the fertilization process. When the reproductive activity is over, both earthworms will become pregnant and lay earthworm cocoons/eggs.

The reproduction process of earthworms. Image source: Follow the worms, author Christopher Cudworth

Research results show that when conditions are suitable, earthworms can lay about 5.5 eggs in 28 days, and each egg can hatch 1-7 small earthworms. In other words, each earthworm has the potential to reproduce 35 small earthworms in one month, making it a high-yield species.

Conclusion

Earthworms are considered the most important ecosystem engineers in the soil, playing important roles in loosening the soil, decomposing organic waste, and promoting nutrient circulation. British biologist Darwin was also impressed by the powerful energy and ecological role of earthworms, and once commented that "earthworms are the most important species in the world."

The tiny earthworms hide in the dark quietly, usually quietly "pupa", occasionally popping up to see the outside world. They seem weak, but they have a tenacious power that we are far from understanding. As a survivor who is well versed in the "R strategy (winning by quantity)", the earthworms' silent "prosperity" has contributed indelibly to our living environment. Next time you see them walking on the roadside, be careful not to step on them!

References

[1] Liu Xuelan, Yan Peipei, Li Xia, et al. Effects of environmental temperature, bait humidity and bait pH on earthworm growth and reproduction[J]. Anhui Agricultural Sciences, 2024, 52(07): 90-92.

[2] Shen Zhifeng. Response of earthworm communities to simulated nitrogen deposition and rainfall enhancement in Jigong Mountain and its influencing factors[D]. Henan University, 2019.

[3]Chuang SC, Chen J H. Role of diurnal rhythm of oxygen consumption in emergence from soil at night after heavy rain by earthworms[J]. Invertebrate Biology, 2008, 127(1): 80-86.

[4]Christensen OM, Mather J G. Pesticide-induced surface migration by lumbricid earthworms in grassland: life-stage and species differences[J]. Ecotoxicology and Environmental Safety, 2004, 57(1): 89-99.

[5]Edwards CA, Bohlen P J. 1995. Biology and Ecology of Earthworms. Springer, New York, NY, USA.

[6]Ellis SR, Hodson ME, Wege P. Determining the influence of rainfall patterns and carbendazim on the surface activity of the earthworm Lumbricus terrestris[J]. Environmental Toxicology and Chemistry, 2010, 29(8): 1821-1827.

[7]Johnston ASA, Sibly RM, Thorbek P. Forecasting tillage and soil warming effects on earthworm populations[J]. Journal of Applied Ecology, 2018, 55(3): 1498-1509.

[8]Mather JG, Christensen O. Surface migration of earthworms in grassland[J]. 1992.

[9]Xiao N, Ge F, Edwards C A. The regeneration capacity of an earthworm, Eisenia fetida, in relation to the site of amputation along the body[J]. Acta Ecologica Sinica, 2011, 31(4): 197-204.

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Produced by Science Popularization China

Author: Ji Qiaoqiao, PhD in Agronomy

Producer丨China Science Expo

Editor: Dong Nana

Proofread by Xu Lailinlin

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