Darwin’s Earthworms: A Groundbreaking Piece of Soil Ecology

Posted byKeironDerekBrownPosted inBlog, Earthworms, entoLIVE BlogTags:, , , ,

Charles Darwin might be best known for his work on evolution and his time on the Voyage of the Beagle, but he is also recognised as the father of earthworm science. After 44 years of studying earthworms, he published a best-selling book dedicated to the group, which is still one of the go-to resources for researchers today. Join us on Darwin Day to look back at the history of earthworm science and research and learn about some of the experiments Darwin did with earthworms.

Kerry Calloway is an earthworm recorder, course tutor and secretary for the Earthworm Society of Britain. She is very passionate about all aspects of natural history but particularly engaging people in the world of the humble but super important earthworm.

Q&A with Kerry Calloway

  • How do earthworms drag the leaves into their burrows?
    Earthworms use their mouth. The mouth is covered by a fleshy lobe known as the prostomium, which is used to grab the leaves and move backwards to pull the leaf into the earthworm burrow.
  • Did Darwin investigate when and how earthworms evolved?
    I’ve not seen any reference from Darwin regarding earthworm evolution. He doesn’t talk about this in his earthworm book and I don’t recall them being mentioned (at least in any significant way) in On the Origin of Species. We now know that earthworm-like organisms (belonging to the class Clitellata) were around as early as the Upper Ordovician Period (around 450 million years ago). The oldest known evidence of an annelid is from the early Cambrian period (around 514 million years ago) and was a polychaete worm called Dannychaeta tucolus. Earthworms don’t fossilise well so they are not recorded in fossil records as readily as animals with exoskeletons or bones, with most fossil evidence coming from fossilised burrows and casts.
  • Does Darwin discuss species diversity mush in his earthworm book?
    No, Darwin doesn’t discuss earthworm species diversity in any detail in On The Formation of Vegetable Mould Through the Actions of Worms. We know from Darwin’s other research that he was interested in species diversity, so I personally find this really surprising. I think there were around 13 described species of earthworm in Britain around that time and Darwin was certainly aware of this, but he didn’t seem to collect and identify any earthworm specimens. Since Darwin’s work, other researchers have been to Down House where he conducted many of his experiments, and they have since found that the dominant species appears to be Aporcetodea longa (a large deep-burrowing species that casts on the surface).
  • Is it true that earthworms are not native to the USA?
    Not exactly. There are earthworms native to North America and these are found across much of the USA. This includes species that have since been introduced here, such as Bimastos eiseni, Bimastos rubidus, and Sparganophilus tamesis. However, there are parts of the USA (such as the Great Lakes area) that have not been naturally colonised by earthworms since the last ice age. European and Asian earthworms have been introduced by humans to these areas and are now causing issues as they change the ecosystem from the ground up. You can learn more about this in The Earthworm Invasion of North America blog by Professor Katalin Szlávecz.
  • Do earthworms die during floods?
    Different species have different tolerances to flooding. Earthworms actually require moisture to be able to breathe through their skin so being in water is not a problem for earthworms – and many species are actually wet soil specialists. However, if the water becomes stagnant and deoxygenated then the earthworms can die through suffocation. Earthworm populations are generally robust due to the cocoon stage of the life cycle, as these can survive more extreme environmental conditions. Even if the adults and juveniles are killed off by unsuitable environmental conditions, the next generation will wait out the unsuitable environmental conditions in the cocoon stage.
  • What is the impact of non-native invasive flatworms on our earthworm populations?
    We have a number of non-native invasive flatworms in the UK, none of which were ever mentioned by Darwin and we have no evidence to suggest that they were around in his day. There has been research into the impact of the New Zealand Flatworm (Arthurdendyus triangulatus) and we know that this impacts the larger deep-burrowing species in particular. We know very little about the impact of the Australian Flatworm (Australoplana sanguinea), Obama Flatworm (Obama nungara) and other earthworm-eating non-native invasive flatworms. the Earthworm Society recommend that people record these flatworms when they see them by taking photos and submitting their records to iRecord so that we can build a picture about where they are and how they are spreading. Once a flatworm is established it is pretty much impossible to get rid of, but understanding how they spread may help us fend off future threats such as the New Guinea Flatworm (Platydemus manokwari), which is impacting earthworm populations on mainland Europe. Check out the Earthworm Society’s Flatworms page for more info.
  • What one thing can people do to improve their garden for earthworms?
    There are lots of different species of earthworm and gardens are really good for housing a wide range of species. Having a range of habitats and places for earthworms to live will help support more diversity in your garden. Having a lawn is good for soil-dwelling earthworms and having leaf litter is great for surface-dwelling species. There are also specialists that you’ll find in compost or associated with deadwood, so introducing these microhabitats to your garden will provide what these species need. Earthworms love residing underneath things, so having plant pots provides this for them. It’s also important to ensure that you leave something for them to eat, such as leaving dead leaves for the deep-burrowing and surface-dwelling species. You can learn more about this in the Gardening For Earthworms: Mutual Benefits of Earthworm-friendly Gardening entoLIVE blog by Keiron.

Literature references

Further info

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Published by KeironDerekBrown

A blog about biological recording in the UK from the scheme organiser for the National Earthworm Recording Scheme.

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