Glorious Earthworms

People understand that earthworms are important, we respect and appreciate them; it is hard to hate an earthworm.

It is also the case that we largely live in different worlds, us on the surface and them under it. Occasionally our paths cross, after rain, and particularly if the soil floods, earthworms are encountered slithering over the surface, and when we dig our gardens we unearth them, and sometimes feel rueful at having sliced them with a spade (no they don’t usually recover, and certainly don’t turn into two earthworms – that trick belongs to the completely unrelated flatworms).

Because we spend little time in the company of earthworms when they are going about their natural lives we don’t really know them very well, we tend to think of them as fleshy soil processing tubes, which at one level they are. But of course they are much more than this, there are over 20 species in the UK alone and 200 in Europe and each one is different, they eat different things, live in different places, have different life cycles and when you get to know them they look different as well.

It may surprise you to know that soil biologists believe that earthworms are the most abundant animal biomass in most terrestrial ecosystems, heavier per hectare than grazing mammals or insects (Lavelle and Spain 2001 and Blouin et al. 2013).

When studied, earthworms increased the above ground production of plant material in 75% of experiments, with an amazing average increase in plant biomass of 56%. The importance of earthworms was not missed by Charles Darwin, whose last great work in 1881 was the book ‘The Formation of Vegetable Mould through the Action of Worms’.

In my opinion the prettiest British earthworm is the Red worm (Lumbricus rubellus), a smallish worm with deep, ruddy, pinkish hue that glistens with turquoise iridescence. But my favourite earthworm is its bigger relative the Lob worm (Lumbricus terrestris), not because of appearances, but because it has a unique and important ecological role and a life history that challenges people’s presumption that bugs live fast, breed prolifically and die young.

Although each earthworm species has its own habitat and food preferences, and idiosyncrasies – for instance worm casts are the work of the Grey worm (Aporrectodea caliginosa) – each species falls into one of three broad approaches to life:-

1.  the classic earthworm – munching its way through the upper horizons of the soil, digesting fungi that it encounters (most earthworm nutrition comes from the fungi and associated decay, rather than dead plant matter itself) (the Grey worm is a typical example),
2.  the humic earthworm – living in decaying matter, leaf litter, rotting wood and manure, munching on decaying organic matter,
3.  the deep burrowing earthworm – living is a single, permanent burrow and dragging leaves down into the burrow where fungi grow on them and the earthworm eats the resulting ferment.

There are many species in the first two categories, but the Lob worm (my favourite) is the only British species in the latter category.  Its burrow is up to five meters deep, so it performs a unique role in soil health and structure – providing drainage and incorporating nutrients back into the soil.  Indeed such species appear to directly prevent soil erosion and the relationship is exponential, so more Lob worms = much less soil erosion.

Having just one burrow brings some difficulties; other worms have sexual encounters in their wanderings underground or through humus, but the Lob worm comes out onto the soil surface on humid nights to find a mate.  After their bisexual exchange each Lob worm will lay about five eggs, this is the worm’s annual output of young. The baby Lob worms disperse, establish their own burrows and slowly grow, it usually takes them five years to reach sexual maturity, and they can live for more than 20 years. One might think that our biggest earthworm would lay more eggs than smaller earthworms, but smaller species all lay many more eggs than the Lob worm, which lives longer and has fewer offspring than most species of mammal.

So earthworms are lovable and abundant in natural ecosystems, indeed they are one of the keystone ecosystem engineers, earthworms help make life happen, but as people concerned about maintaining the natural bounty and richness of our environment we must ask ‘how healthy are their populations?’. The answer unfortunately is that we don’t know. There is no quantitative monitoring of UK worm populations, so we can only hypothesise about how they are faring.

It seems probable that in arable fields modern practices have blasted earthworm populations almost out of existence. I did scientific work on soil in arable fields in the 1990s, and it was hard work finding just a couple of earthworms in some fields, they had been more or less annihilated by appalling toxins such as the now banned methyl bromide. The switch from spring sowing to winter sowing also robbed earthworms of winter leaf litter, burying it beyond their reach with deep ploughing.

There is some hope that the darkest days for the earthworm may have passed, not only have some of the most toxic pesticides been banned, increasingly farmers are getting more sophisticated at soil management, and the current trend is for more and more low-till or no-till arable farming. In these systems the ground is not ploughed, leaving plant material on the surface for the earthworms to incorporate, and allowing a more natural soil structure to develop. But unfortunately we can only hope that things are getting better for earthworms; modern agrotoxins are often toxic to earthworms or their fungal food, so with no monitoring we don’t know if earthworm populations are on the up or the down.

On a species by species basis it is little clearer, we do not know which species are increasing and which are declining, although we do know that six species of earthworm are very rarely encountered and may be in conservation difficulty, but the resources are not available to undertake the ecological surveys needed to establish their conservation status.

Buglife was delighted in the spring of 2015 to welcome the Earthworm Society as a new Buglife member organisation, joining many other specialist and conservation charities that support our work conserving the small things that run the planet.

Hopefully together we can cast more light onto the ecology and conservation of these neglected, but respected, little animals.

Find out more about the Earthworm Society here.

The Names and Status of British Worms

* Statuses adapted from ‘Earthworms in England: distribution, abundance and habitats’ (Natural England 2014)