Today we have another photo-and-text story, this time on earthworms (a favorite of Darwin), concocted by Athayde Tonhasca Júnior. His captions are indented, and you can enlarge the photos by clicking on them.
Underground influencers
“Everything is connected” is the sort of vacuous New Age twaddle churned out by the self-help industry. And yet, stuff and nonsense often holds a grain of truth. For example, we would have to look hard to find a connection between earthworms and bees. But such an association exists, and it is of consequence for pollination services.
Earthworms (mainly of the family Lumbricidae, which includes most European species) are immensely important for the functioning of some terrestrial ecosystems. Their tunnels channel air, water and nutrients into deep layers of the soil, and facilitate root penetration. Their work improves soil structure and reduces runoff, thus decreasing the rates of erosion. By eating soil, plant litter and other materials (depending on the species), earthworms break down organic matter, helping decomposers such as bacteria and fungi release nutrients into the soil. Their food intake, 2 to 20 tonnes of organic matter/ha/year, ends up as castings (worm excrement), which are rich in nitrogen, phosphorus, potassium, magnesium and calcium, all minerals essential for plant growth. Thanks to their relentless burrowing, soil mixing and fertilizing, earthworms are important to soil formation, and consequently vital to plants and every organism that depends on them. You can learn a great deal more about these indefatigable diggers from The Earthworm Society of Britain.
The common earthworm (Lumbricus terrestris) © Fir0002/Flagstaffotos, Wikimedia Commons:
The value of earthworms was not lost on Charles Darwin. His 1881 book, The formation of vegetable mould, through the action of worms, published a few months before his death, was a revelation to the general public about the importance of these secretive and poorly known animals. The book was a huge success, selling 6,000 copies in the first year, more than On the Origin of Species when it was first published.
Darwin and his worms in a caricature from Punch, 1882:
Darwin calculated that in 10 years, castings from 0.4 ha (one acre) of soil would form a 5 cm-thick layer of top soil (what he called ‘vegetable mould’). In his book’s closing paragraph, Darwin justified calling earthworms ‘nature’s ploughs’: ‘The plough is one of the most ancient and most valuable of mans (sic) inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms. 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.’
Diagram of the formation of vegetable mould. Darwin, 1838. Proceedings of the Geological Society of London 2: 274-576:
Considering earthworms’ impressive portfolio as nature’s engineers, we may think they are indispensable, or useful, everywhere. But they are not.
About 10,000 years ago, northern North America was overwhelmed by a vast ice sheet. If there were earthworms in the region, they were killed by the glaciation because there were none when the ice receded. So northern North America was earthworm-free until European settlers started to bring in plants and soil, which inevitably introduced worms such as the ‘night crawler’, the local name for the common earthworm Lumbricus terrestris.
American farmers and gardeners benefited from ‘nature’s ploughs’ as much as Europeans did, but it was a matter of time until earthworms made their way to native habitats such as hardwood forests. And in those environments, earthworms were not welcome at all.
The top layer of the forest floor – known as the litter layer – consists of leaves, bark and stems at different stages of decomposition. In North American native forests, the litter layer is broken down slowly, mainly by millipedes and mites. Organic material accumulates as blanket sheets, which are essential habitats for many insects, amphibians, birds, and flowers.
Deep litter mound at the base of a pine tree © Hood, USDA Forest Service.:
When earthworms move in, the litter layer is consumed in two shakes of a duck’s tail. Decomposition accelerates dramatically, so that nutrients that have been slowly accumulating are released quickly; plants cannot absorb them all. With the loss of litter cover and nutrients, the understory fauna and flora become depleted. Dwindling understory plant biomass has secondary consequences; deer will have no option but to munch on young trees, and non-native plants may take advantage of the impoverished conditions to spread out. These problems worsened after the arrival of the Asian jumping worm (Amynthas agrestis), an earthworm native to Japan and Korea.
But the negative impact of earthworms is not restricted to the litter layer. In Canada, the abundance, biomass, and species richness of the insect fauna above ground are lower in forest plots with invasive earthworms than in earthworm-free areas. Insect abundance was reduced by 61% where earthworm biomass was highest (Jochum et al., 2022).
Effects of earthworm-invasion status on herbivore richness (morphospecies), left; biomass (mg/m2), centre; and abundance (log10 individuals/m2), right, in Alberta, Canada © Jochum et al., 2022:
The reasons for these effects are not known. Scarcity of some plants or altered soil conditions in earthworm areas may reduce the abundance and survival of herbivore and soil-dwelling invertebrates, which may affect the food chain. Invasive earthworms can decrease the concentrations of some plant metabolites used against leaf-chewing insects, so changes in plant chemistry may be involved.
Would this hoverfly be affected by the works of earthworms? Probably yes © Forest Wander, Wikimedia Commons:
Even more worryingly, there is strong evidence that earthworm activity increases emissions of greenhouse gases. Dendrobaena octaedra, another earthworm native to Europe, seems to be spreading in Canadian boreal forests, which are important carbon reservoirs. Wherever this earthworm is found, some of the carbon stock in the forest floor is lost in the form of carbon dioxide. So many soil ecologists have rightly voiced their concerns about a ‘global worming’.
A schematic illustration of invasive earthworm effects on ecosystems that were free of earthworms (left figure) © Ferlian et al., 2017:
The shenanigans of Darwin’s ‘nature’s ploughs’ in northern North America are cautionary tales about species taken to where they do not belong. Few could have expected that earthworms, so beneficial to species and habitats in the Old Continent, are detrimental elsewhere. The buff-tailed bumble bee (Bombus terrestris) and the European honey bee (Apis mellifera) are protagonists of similar tales.
The unpredictability of outcomes is a concern. Only a fraction of invasive species are harmful, but those that are can be disastrous.
Kudzu (Pueraria spp.), ‘the vine that ate the South’, was purposely introduced into the United States for erosion control, but became an environmental nightmare. It is spreading at an estimated rate of 610 km2/year © Scott Ehardt, Wikimedia Commons:
JAC Addition: Here I’m posing (in 2008) with Darwin’s “wormstone” at Down House, his home in Kent. As Darwin Online notes, Darwin used this to “measure the rate of sinking of the stone due to the actions of earthworms.” The site adds, “The stone now at Down House was reconstructed by Horace Darwin’s Cambridge Instrument Company in 1929 when Down House became a museum open to the public.”
