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Worms
Worms
I don’t think there are any prizes as to what these creatures do. I suspect most readers who have got this far know that that the great engineers of soils are earthworms. If we were to ask most people how soil functions, many would say - worms. We saw in the previous period about how they are responsible for much bioturbation, bringing leaves in and turning over soil continuously.
We saw the origins of earthworms about 200 mya, with the appearance of endogeic worms, that live deep down in soils. In this period they took over the functions we know them so well for. It is most likely that the two main other sorts of worms - classified by their behaviour adapted to the new conditions. As the soil got deeper there would be more opportunities for the endogeic worms to explore - on the surface and vertically. The leaf litter would have provided a new readily available food source. Some nearer the surface - called epigeic could live all their lives in leaves. Others - anecic adapted to pulling the leaves into their vertical burrows.So let’s look more deeply.
Earthworm Society of Britain
Somewhat counter-intuitive, worms appear to have come into soil from sides rather than surface, and thus probably from enchytraeids.
Epigeic
Worms today that remain in the litter layer are called epigeic worms, epi meaning "above" and geic from "Gaia" or "Earth". You may know those in compost piles called ‘redworms’ or ‘wrigglies’. These epigeic worms tend to be smaller than others living deeper and their pigmentation darker perhaps giving protection from light above. Epigeics inhabit nearly every significant land mass on Earth and every continent except Antarctica.
Their prolific range comes at the expense of a poor burrowing ability, which limits their niche within a given ecosystem to the outermost layers of soil. Eisenia fetida is a particularly fecund species of epigeic earthworm, with a range that spans nearly every variety of climate on the planet. This remarkable adaptability of epigeic earthworms, enables converting much surface debris to topsoil, a process critical to the continual renewal of countless ecosystems. Their burrows are not stable.
Anecic
Anecic earthworms make vertical, rather than horizontal burrows. They adapted to the new conditions by pulling leaves down, into vertical tunnels This, possibly the most important soil process, is called bioturbation. These burrows are more or less permanent, giving them a remarkable ability to acquire nutrients from a variety of strata within the soil. These are the ones we know can emerge on the surface of the soil in order to drag leaves into the depths of their burrows and feed on them. Many come back to the surface to leave their poo in casts on top of the soil. These casts are famous for being rich in nutrients. You may know the common earthworm – Lumbricus terrestris - is anecic, noted for their darkly coloured head ends and their relatively pale tail ends.
Vermicasting
Vermicasting
Worms would do more for new soil formation than anything, as they would mix organic matter, now covered a layer of mucus making it attractive to mites, which would then poo out the remains to help build deeper soil structures.
The process of turning organic waste into casts is called Vermicasting. Turning organic wastes into casts takes 22–32 days, depending on density of waste and earthworm maturity. Fresh casts undergo 2 weeks of nitrification where ammonium transforms to nitrate, a form that plants can uptake. Earthworms ingest about 75% of their body weight/day; so a 0.2 g worm eats about 0.15 g/day.
We saw that endogeic worms bring with them their mucous, microbes, mycorrhiza and mineralisation, These functions the newly adapted earthworms like the anecic ones would bring with them, only now adding one spectacularly important one - dispersing the fruits and seeds of flowering plants.
Seeds
Seeds
“Anecic earthworms have been shown to collect, concentrate and bury seeds in their burrows. Moreover, recent studies suggest that earthworms function as granivores and seedling herbivores thereby directly impacting plant community assembly…. seeds and seedlings most likely contribute significantly to earthworm nutrition potentially explaining the collection and concentration of seeds by L. terrestris in its middens and burrows" ( Eisenhauer et al 2010)
There is a growing body of research dedicated to understanding earthworm seed preferences. “Generally, earthworms buried seeds quickly irrespective of seed size and species. Secondary seed dispersal by earthworms affected plant community composition depending mainly on seed size but less on plant functional group identity and diversity: small-seeded species were repressed whereas large-seeded were promoted." (Milcu et al 2006),
Other research has found “Earthworm seed ingestion increased exponentially when available seed density was doubled (+305%) and was reduced when encountering seed that had previously been egested by another earthworm (−30%)" (McTavish & Murphy 2019)
Biomass
Biomass
Do you remember in the home pages, we were talking about biomass of worms weigh in around 20-25kg/m3 in normal soil. That is around the weight of a Labrador dog or lynx. There is twice the weight of worms (0.2Gt of C) in the world than all domesticated animals. We now know that this is the equivalent of doing down there in animal terms. It was not till around 100mya that these creatures dominated the world so much. In temperate forests after broadleaf litter was added to coniferous stands, earthworm communities density and diversity increased (Cesarz et al 2007)
Worm weights
Worm weights
Worm weightWeight in different soils
Other creatures’ metabolism https://sites.google.com/site/soilanimals/look/overview
Converter http://www.kylesconverter.com/area-density/pounds-per-acre-to-kilograms-per-square-meter
= 20-25kg /m3 (presuming not many below metre down)
25 worms in cubic metre https://socratic.org/questions/the-population-density-of-worms-in-a-particular-field-is-25-worms-per-cubic-mete
Other inhabitants https://www.sciencenewsforstudents.org/article/soil-and-its-inhabitants-by-the-numbers
Slugs
Slugs
You may have been wondering about slugs, perhaps our least favourite garden creature. Why have we not heard about them before? The clade Stylommatophora, which includes land snails and slugs, appeared about 150 million years ago, in this Cretaceous period. There a several unrelated families in the group. As groups diversified, as result of adaptive radiation, some of those species persist in the sites where their fossils arose. They evolved from snails, losing the hard shell. The species of the genus Helix, from which the garden snail is perhaps the best known, flourished in the Cretaceous period, although further radiations occur later.
A slug poops through its anus, located under the mantle, on the right side of its body, near the genital opening. The genital opening is located on the slug’s mantle. So, unlike other animals, slugs have their anuses close to their heads. Why, you might be wondering, right?! That’s because they’re relatives of snails, and snails have their anuses outside their shells. The poo resembles a long, thin sticky tube that rolls to the ground, making a nitrogen-rich, mineral-laden fertiliser all ready for plant nutrition.
It is not hard to see how snails have come out of the sea and adapted to these new moist conditions in the leaf litter, becoming slugs
Nematodes
Nematodes
"Fossil invertebrate-parasitic nematodes first appeared in the Early Cretaceous, while the earliest fossil vertebrate-parasitic nematodes are from Upper Triassic coprolites. Specific examples of fossil nematode parasites over time are presented, along with views on the origin and evolution of nematodes and their hosts...fossilized resin (amber) and coprolites have been the most useful media for studying the evolution and early hosts of parasitic nematodes. The second oldest ever found is a "130 mya mermithid, Cretacimermis libani from Early Cretaceous Lebanese amber. The single specimen was coiled up in the body cavity of an adult midge (Chironomidae: Diptera)" (Poinar Jnr 2015). Chironomid larvae live in water. It would be interesting to know if any infected other, soil-dwelling, insect larvae.
The soil-transmitted helminths (geohelminths) are a group of intestinal nematode parasites that are transmitted primarily through contaminated soil. They have a direct life cycle which requires no intermediate hosts or vectors, and the parasitic infection occurs through faecal contamination of soil, foodstuffs and water supplies. They cause immense ill-health in many rural areas of the world, and perhaps the most well-known is Ascaris - the roundworm which can grow very large in human intestines.
By the end of this period, the worms we know today had arrived and were well spread out in soils.
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