In 2019, 15,000 children from primary schools across the UK went to their local playground. Instead of kicking a ball around, they dug up worms, looked for birds and counted both.
“The kids were just so enthusiastic about it. It was incredible,” said Blaise Martay, chief researcher of the British Trust for Ornithology (BTO). Martay was concerned about the data quality – she thought children’s scores might vary with their enthusiasm. But the results “showed exactly what we would expect”, she said: that more worms mean greater numbers of blackbirds, robins and thrushes, the birds that rely on earthworms as an important part of their diet. The data were consistent across school groups.
Earthworms are a cornerstone species with potentially enormous effects on aboveground wildlife and ecosystem functioning, but we still know little about them. They live in a hidden landscape beneath our feet, breaking down organic matter in the soil so that it can be used by other soil organisms. As they spin, they create miniature tunnels, allowing air and water to pass through, with some burrowing worms able to dig tunnels up to 2 meters deep. Sometimes called the “poor man’s tropical rainforest” because of its biodiversity, topsoil is the bedrock of human food systems, and it is true 95% of the planet’s food has grown
Charles Darwin was so obsessed with these extraordinary creatures that he wrote his last book about them, based on a lifetime of study and fascination, and told his son William what he hoped his book would reveal was that “worms have much bigger souls than anyone would suppose”. Darwin thought that “it can be doubted whether there are many other animals that have played so important a part in the history of the world as these lowly organized creatures”, and his book was a bestseller.
But the UK’s first national assessment, published in 2023, found that earthworm populations have declined by a third in the past 25 years. “Such declines are likely to have significant effects on soil health, ecosystem structure and function,” researchers wrote in a 2024 “horizon scan” that identifies the greatest threats and opportunities to biodiversity, published in Trends in Ecology and Evolution.
Large-scale wildlife declines have been reported in oceans, freshwater and on land, but what is happening underground is still relatively unknown. “We know that birds that feed on earthworms are declining, so we wondered what was happening to the worms,” says Dr Ailidh Barnes, a research ecologist from BTO who carried out the national assessment.
His paper found earthworm populations in the UK are in long-term decline of up to 2% per year. It is possible that other countries with similar land use patterns have had equally dramatic declines. The largest declines seen in Barnes’ study were in broadleaf woodland ecosystems. “That was the finding we were most surprised about,” Barnes said. This could be because the climate crisis is drying out the soil, or runoff from surrounding agricultural land.
It is possible that the loss of earthworms could already have an impact on wider woodland ecology. On average, there are 37% fewer forest birds in British forests compared to 1970, with declines accelerating over the past five years. “The loss of worms may play a bigger role than we realize,” Barnes said.
Healthy worm populations are crucial to entire ecosystems, not just birds. Earthworms are ecosystem engineers. As they burrow and feed underground, they break down organic material, which is then conveyed on the conveyor belt to smaller organisms. Wormholes create porous structures through which water and air can pass. They play a crucial role in nutrient recycling and soil fertility, meaning they contribute significantly to global food production.
“Earthworms are essential to caring for the soil, which is the basis of all life and which grows our food,” said Barnes.
If trends revealed by her study hold true elsewhere, the loss could affect our ability to feed a growing human population. Worms’ contribution to the world’s grain harvest matches that of Russia, according to a 2023 study, which found they help make 140 million tons of food a year. This would make them the fourth largest global producer if they were a country.
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Found another paper from last year More than half of the world’s species live in the soil. Despite their importance in supporting ecosystems and providing food for humans, invertebrates are terrestrial “severely neglected” in biodiversity assessments. Extensive drainage, pesticide use and the use of inorganic fertilizers are likely to drive them, but data on trends in population abundance are generally only available from studies covering small areas.
Barnes said: “They are essential to everything. When you start talking to people about earthworms, they are interested, but they are underground so they are forgotten.”
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Welcome to the Guardian’s UK invertebrate of the year competition. Each day between April 2nd and April 12th we will profile one of the incredible invertebrates that live in and around the UK. Let us know which invertebrates you think we should include here. And at midnight on Friday 12 April, voting will take place to decide who our favorite invertebrate is – for now – with the winner to be announced on Monday 15 April.
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