By Matt Walker
Editor, Earth News
Earthworms eat seeds and seedlings, scientists have found.
The discovery they eat live rather than just dead plants will change the way we think about earthworms, which had been thought to benefit plants by recycling soil nutrients.
It may offer a way for gardeners and farmers to encourage more earthworms into their soil, for example.
But it also means invasive earthworms could be reducing populations of plants in once pristine soils.
Confirmation that earthworms feed on living plants is published in the journal Soil Biology and Biochemistry by Dr Nico Eisenhauer of the Georg-August-University Göttingen in Germany.
With colleagues, Dr Eisenhauer made the discovery studying the behaviour of Lumbricus terrestris, an anecic earthworm that inhabits soils around the world.
Originating in Europe, the worm occurs in grasslands, agricultural fields and forests and is invading soils across the US, Canada, Australia and New Zealand.
Previous studies have shown that some earthworms will swallow plant seeds, while others appear to collect seeds, burying them in their burrows.
But it was not clear why, whether the earthworms were actually seeking rotting seeds, or whether they derived any nutritional benefit from the habit.
So Dr Eisenhauer set up a series of experiments to find out, offering earthworms housed in the lab a selection of food, including seeds and seedlings from different plant species.
The results were startling.
The earthworms selectively fed on slow-germinating, nitrogen-rich seeds and seedlings, actively ingesting the plant material, killing the young plant.
Earthworms allowed to feed on live plants gained weight, and their performance would vary depending on what plants they were offered, such as legumes or grasses, suggesting they do derive nutritional benefits from such a diet.
"However, the most convincing evidence is the change in the N15 signature in earthworm tissue," Dr Eisenhauer told the BBC.
Compared to grasses, legumes generally have lower levels of a isotope of nitrogen called N15, due to the way they fix nitrogen from the air.
When the researchers examined the ratio of N15 within the tissue of earthworms offered legume seeds, it decreased significantly, demonstrating that earthworms feed upon, and prefer, legume seeds and seedlings.
"It was somewhat surprising that we could detect the signature of legumes in plant tissue," says Dr Eisenhauer.
Earthworms' taste for seeds and seedlings could alter the way we think about both the worms, and plant communities.
Earthworms play a fundamental role in nutrient cycling, and in most contexts cannot be regarded as pests of plants, says Dr Eisenhauer.
In agricultural or horticultural settings, farmers or gardeners may even be able to use certain seeds to encourage more earthworms into their soil.
But is some parts of the world, they may pose a previously unrecognised problem.
"They are invading several regions previously devoid of earthworms such as northern North America," says Dr Eisenhauer.
Studies have already shown this can cause the extinction of some plant species, and Dr Eisenhauer's research provides one explanation why.
"The finding that earthworms function as seedling predators highlights the necessity to prevent the further anthropogenic spread of exotic earthworms," he says.