By Athayde Tonhasca
“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 pollinators. But such an association seems to exist, and it could be 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. Earthworm 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 like bacteria and fungi release its nutrients. Their food intake (2 to 20 tonnes of organic matter per hectare each 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.
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 calculated that in 10 years, castings from one acre of soil (0.4 ha) 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.
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.
When earthworms move in, decomposition accelerates dramatically. The litter layer is consumed in a few years, 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 were lower in forest plots with invasive earthworms than in earthworm-free areas. Insect abundance was reduced by 61% where earthworm biomass was highest.
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. Direct links between invasive earthworms and pollinating insects have not been established, but it is safe to assume they exist. Pollinators are not likely to be immune to the profound changes in soil structure, flora composition, and arthropod diversity brought by earthworm activity.
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 occurs, 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’.
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 predicted 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 big worry. Only a fraction of invasive species are harmful, but those that are, can be disastrous.