In 2019, 15,000 children from primary schools across the UK went out to their local playing field. Instead of kicking a ball around, they dug up worms, looked out for birds, and counted them both.

“The kids were just so enthusiastic about it. It was incredible,” said Blaise Martay, lead researcher from the British Trust for Ornithology (BTO). Martay had worried about the data quality – she thought children’s counting might vary with their enthusiasm. But the results “showed exactly what we’d expect”, she said: that more worms meant a greater number of blackbirds, robins and thrushes, the birds that rely on earthworms as a vital part of their diets. The data was consistent across school groups.

Earthworms are a keystone species with potentially enormous effects on above-ground wildlife and ecosystem functioning, yet we still know little about them. They live in a hidden landscape below our feet, breaking down organic matter into the soil so it can be used by other soil organisms. As they wriggle around, they create miniature tunnels, so air and water can pass through, with some deep-burrowing worms able to dig tunnels up to 2 metres deep. Sometimes called the “poor man’s tropical rainforest” on account of its biodiversity, topsoil is the bedrock for human food systems, and is where 95% of the planet’s food is 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 that what he hoped his book would reveal was that “worms have much bigger souls than anyone would suppose”. Darwin thought that “it may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures”, and his book was a bestseller.

But the UK’s first national assessment, published in 2023, found that earthworm populations had declined by a third over the past 25 years. “Such declines would likely have significant effects on soil health, ecosystem structure and function,” researchers wrote in a 2024 “horizon scan” identifying the biggest threats and possibilities for 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 under the soil is still relatively unknown. “We know birds that feed on earthworms are declining so we were wondering what was happening with the worms,” said Dr Ailidh Barnes, a research ecologist from BTO who conducted the national assessment.

Her paper found earthworm populations in the UK are in long-term decline of up to 2% a year. It is possible that other countries with similar land-use patterns have had equally dramatic declines. The biggest declines seen in Barnes’s study were in broadleaf woodland ecosystems. “That was the finding we were most surprised by,” said Barnes. It could be because the climate crisis is drying out the soil, or runoff from surrounding farmland.

It is possible that the loss of earthworms could already be affecting broader woodland ecology. On average there are 37% fewer woodland birds in British woods compared with 1970, with declines accelerating in the past five years. “The loss of worms could be playing a bigger part than we realise,” said Barnes.

Healthy worm populations are crucial for entire ecosystems, not just birds. Earthworms are ecosystem engineers. As they burrow and feed underground, they break down organic matter, which is then passed along the conveyor belt to smaller organisms. Wormholes create porous structures for water and air to travel through. They play a crucial role in nutrient recycling and soil fertility, which means they significantly contribute to global food production.

“Earthworms are vital at looking after the soil, which is the basis of all life and what 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 140m tonnes of food a year. This would make them the fourth largest global producer if they were a country.

Another paper from last year found more than half of the world’s species live in the soil. Yet despite their importance in supporting ecosystems and providing food for humans, soil invertebrates have been “woefully neglected” in biodiversity assessments. Extensive drainage, pesticide use and the use of inorganic fertilisers are likely to be driving them, but data on trends in population abundance are generally only available from studies covering small areas.

Barnes said: “They are vital for everything. When you start talking to people about earthworms they are interested, but they’re under the ground so they get forgotten about.”

• Welcome to the Guardian’s UK invertebrate of the year competition. Every day between 2 April and 12 April we’ll be profiling one of the incredible invertebrates that live in and around the UK. Let us know which invertebrates you think we should be including here. And at midnight on Friday 12 April, voting will open to decide which is our favourite invertebrate – for now – with the winner to be announced on Monday 15 April.

• Find more age of extinction coverage here, and follow biodiversity reporters Phoebe Weston and Patrick Greenfield on X for all the latest news and features

Lying on a mound of soft sand inside the nursery, Bani looks like a spoilt child being indulged. Two members of the care team massage her hind leg with oil while the third, sitting at her head, funnels sticks of sugar cane gently into her mouth, clucking reassuringly.

It’s the royal treatment – but Bani, a nine-month-old elephant calf, needs all the medical care and nurturing she can get.

Bani was orphaned in mid-December when she and her pregnant mother were crossing a railway track near Jim Corbett national park in Haldwani. A speeding train smashed into them, killing her mother and flinging baby Bani into a ditch, leaving her with serious injuries and fractured bones.

For several weeks, the frightened calf, unable to stand, was treated locally. When the local forest department caring for her saw no progress, they contacted NGO Wildlife SOS, who sent a team of experts to provide critical care. Once she was strong enough, they transported Bani in a custom elephant ambulance to the Mathura hospital – India’s first specialist elephant hospital.

Since arriving, her life has been an intensive schedule of laser treatment, physiotherapy, hydrotherapy, nerve stimulation and ayurvedic massage. One day, she was able to twitch her tail, to the elation of the staff – it indicated her spine would recover. Then, with the help of a padded harness, she was able to stand for a few minutes, which was a “euphoric moment”, says Kartick Satyanarayan of Wildlife SOS.

“Bani may have to live her life with a handicap but with each day, you can see her becoming less scared and more playful. She loves her bananas and is quite a drama queen, [having tantrums] if she doesn’t get them,” says Satyanarayan.

Bani is the hospital’s first wild elephant and her arrival illustrates the growing threat posed by trains as railway lines cut through forested habitat and migration corridors. Satyanarayan says the Indian Railways’ primary consideration is cost when planning routes, not the need to protect elephants as they forage for food and water.

In India, death from train collisions is the second-highest cause of unnatural elephant deaths, after accidental electrocution. Official data shows that more than 200 elephants were killed in train collisions in the past 10 years. “There’s blood on the tracks when railway lines go through forest areas,” Satyanarayan says.

Indian elephants are classed as endangered, with numbers declining: about 40-50,000 remained in the wild globally at the last assessment in 2019. More than half of the species’ total range has disappeared or been highly fragmented by human settlements, roads and farms. The Wildlife SOS elephant conservation and care centre is a sanctuary for rescued elephants, many of which have come from circuses, hotels, wedding businesses or temples.

Shivam Rai, head coordinator at Wildlife SOS, says most of the 36 elephants in their care have experienced violence at the hands of humans. A number are blind. Many have severe physical disabilities.

“Giving them comfort and dignity is our way of saying sorry – sorry we did this to you, sorry we snatched you from the wild and took you away from your family, sorry for taking everything from you,” says Rai.

The increasing number of elephants being killed by trains has led to calls for changes to the way the railways are managed. Last year, Tamil Nadu in south India installed an AI-enabled surveillance system to monitor elephant movement near railway tracks to help prevent accidents. Sensors pick up elephant movement and alert train drivers, station staff and line controllers.

In other areas, the railways are being fitted with similar systems that sense vibrations, detecting the presence of elephants with nearly 100% accuracy. A system installed in north-east India triggers more than 40 alerts a day.

Flyovers covered with foliage are another option to provide a safe passage for wildlife. In West Bengal, a flyover lined with bamboo and banana trees has been built to encourage elephants to use it and cross the track safely.

But rolling out safety measures is a huge challenge. Indian Railways spans 130,000km (81,000 miles) of track and the country has 150 elephant corridors.

Wildlife SOS believes that if an AI early warning system had been in place, Bani’s mother would be alive and Bani would not be disabled.

“The forest is their home and the trains are invading their homes. Urgent installation of AI-enabled accident prevention systems and strict implementation of speed controls will save hundreds of elephants,” says Satyanarayan.

Of the animals that are hit, Bani is one of the lucky ones. Her progress is steady but slow. Her appetite is gradually improving and she is now able to hold herself up for short periods of time. For the staff who work with her, her recovery is tinged with sadness.

“Bani may never be a normal elephant and can never be wild again,” says Satyanarayan. “She will live with a handicap all her life. Our hope is that she recovers enough to live a life of dignity and freedom.”

Find more age of extinction coverage here, and follow biodiversity reporters Phoebe Weston and Patrick Greenfield on X for all the latest news and features