Darwin’s win-win for Global Worming? (C) R.J. Blakemore 12th Feb., 2017.
Few realize Charles Darwin had a soft spot for the humble earthworm and scientific data to show why. The topic of his swan song book was: Worms & Vegetable Mould (= earthworms & topsoil humus) in which he calculated that all fertile soils have passed many times through their bodies, and should continue to do so. Darwin’s (1881: 158) estimated from Hensen (1877: 360) that there must exist 133,000 living worms in a hectare of land (ca. 13.3 m-2) with 3 g per worm (Darwin mistakes this for 1 g) (= 40 g/m2 or 0.04 kg m-2), or 53,767 in an acre with this latter number of worms he calculated as weighing 356 pounds per acre. Such a modest estimate gives a global earthworms population of around 1.3 x 1015 or 1.3 quadrillion with (0.4 t ha-1 x 9.5 Gha of productive land) = 3.8 Gigatonne biomass, or about ten times all humanity.
As a cross-check of accuracy, Dr Ken Lee (1985: tab. 7) has summary with average of 273 m-2 and 63 g/m2 (= 0.63 t ha-1) for a range of biome habitats (0.63 x 9.5 Gha) = ~6.0 Gt total earthworm biomass. If global non-ice/sand topsoil land were taken as 12.1 Gha (Ref), this gives (0.63 x 12.1 Gha) = 7.6 Gt or just about double the result from Darwin’s figures.
Interestingly, although often thought of as good fishing bait, the earthworm fresh weight of 3.8-7.6 Gt is double or quadruple the “wet weight” of global fish stocks confidently calculated as between 0.89-2.05 Gt (Ref1, Ref2) of which just 0.15 Gt are total annual combined fish catch & aquaculture (Ref3), the highest on record so far.
Since Dr Ken Lee’s (1985: 33) reference text on Earthworm Ecology has their moisture content at 70%, thus 30% dry, a total is of between 1.14 to 2.28 Gt dry biomass.
Richly organic topsoil populations of earthworms are much higher – averaging 500 worms m-2 and up to 400 gm-2 – such that, for the 7 billion of us, each person alive today has support of 7 million earthworms. There are 7,000 currently described earthworm species (see Blakemore, 2015: 542; Ref) now working day & night, rain or shine, relentlessly recycling organic matter for healthy plants that sustain all Life on Earth from the Sun’s energy.
Darwin (1881: 173) also estimated that earthworms annually eject in the order of 15 tons (his actual mean value was 14.09 tons) of surface castings per acre of pasture/commons land (= 33.6 t ha-1 yr-1 x 9.5 Gha of non-ice/desert land = 319 Gt yr-1 globally, cf. FAO)*. Although this too is surely an underestimate as earthworms can process their own body weight of humus per day and often they cast below ground – sometimes as deep as 15 m – as well as on the surface. Nevertheless, fertile topsoil has >24 kg m-2 of recyclable Organic Carbon (= 2,300 Gt C globally)** in living soil organisms or locked up in humic matter (+ glomalin) produced from activities when alive or their bodies when they die.
Organic farming/gardening preserves substantially higher levels of earthworms (as all good farmers & gardeners know), more water and carbon with greater yields & lower costs (financial, environmental, medical). This was clearly shown at Lady Eve Balfour’s pioneering Haughley farm 36 yrs ago and in a 1981 study to mark Darwin’s book’s Centenary (this study eventually published by Blakemore, 2000); see also Blakemore (2016a, 2016b).
Why all this is especially relevant today is twofold: Firstly, topsoils are degrading with 50-70% now lost to erosion and pollution giving us perhaps only 60 years more of harvests. Secondly, and inter-related, is that we are rapidly losing earthworms too. Populations decline under intensive agriculture – as demonstrated at Haughley – and following deforestation, with many species becoming extinct: e.g., recent review of New Zealand’s 200 native earthworms listed 20 or so (ca. 10%) as extinct or likely soon to be.
Regarding Climate Change (and Global ‘Worming’), the only way proven to remove CO2 from the air (so-called Carbon Capture and Storage or CCS) is via plant photosynthesis and, since Darwin’s earthworms reprocess leaf litter, therefore all atmospheric CO2 is recycled via worm intestines in regular 12-yr cycles (as estimated from NASA data)**.
A positive outcome of the 2015 Paris COP21 meeting is a proposal (4 per 1000 Initiative) to increase soil carbon. Yet it seems Darwin, the “Father of Evolution”, had good prescience in 1881 to already offer us this practical, safe and reasonable soil-based solution to critical trilemma of global Species Extinction, Climate Change & Food Supply.
My humble proposal, along the lines of 4 per 1000 Initiative (that refers to extra 0.04% organic carbon in topsoil needed to reduce global greenhouse gasses), is to aim for “4 worms per 1,000 g soil“. This would give us an entirely reasonable earthworm population of ~400 worms per m2 – as was found in Haughley’s permanent pasture – or 4 million worms per hectare thereby ensuring not only extra carbon, but also greater aeration, drainage, soil water and yields. Plus, importantly, greater biodiversity of earthworms and of whatever else depends upon them (which ultimately is most other organisms)…
Thanks CD; we salute you and our under-appreciated earthworm friends with a debt of gratitude on your birthday. The beauty is Darwin again showing with his worms that, as with his evolution, small changes in time may have massive beneficial consequences. Either way, both earthworms and evolution have brought us this far…
*FAO (2015: 103) estimates median soil formation globally now as just 0.15 t ha-1 yr-1.
**NASA has carbon in SOIL (~2,300 Gt) = all in plants (550), air (800) + oceans (1,000) combined! As >60 Gt atmospheric C is reprocessed via soil per yr (800 / 60) = ~12-13 yrs.
Details of data presented are in latest Cosmopolitan Earthworms CD (Blakemore, 2016) and on VermEcology websites: Annelida (http://www.annelida.net/earthworm/) & veop (https://veop.wordpress.com/). Date: 12th Feb., 2017. Email firstname.lastname@example.org.
Submission of this, and similar articles, to BBC, Guardian and Nature got nary a reply…
[Update 4th May, 2017. I stand corrected, Nature did graciously accept my small submission – “Soil: Restore earthworms to rebuild topsoil” – https://www.nature.com/nature/journal/v545/n7652/full/545030b.html. So, for the worms, things are looking up!].