Earthworms

Although often overlooked, ignored or taken for granted, earthworms are nevertheless keystone soil species, mediators and moderators for rebuilding healthy, biodiverse, high carbon and moisture rich topsoil [Darwin 1881; Blakemore 2016c]. We depend on soils for more than 99% of our food and 100% of our timber and natural fibres [Blakemore 2012, Pimentel 2013].  As an integral part of organic production, earthworms are key to agricultural sustainability and global ecosystem stability.  Ancient in origin (probably pre-Cambrian but certainly more than 500 million years old), the 7,000 known species of earthworms are ubiquitous and invariably associated with topsoil humus.  Earthworms are a basis of terrestrial food webs and the ultimate detritivor [Blakemore 2016c], recently reinstated as key players in the International “4 per 1000 Initiative” [4p1000.org, n.d.] to increase soil organic matter to store carbon. In this section, we discuss the abundance and variety of earthworms and their role in soil health and functionality.

Overview

Extrapolating data from Darwin [1881], their population numbers around 1.3 x 1015 or 1.3 quadrillion globally with biomass of 0.4 t/ha x 9.5 Gha of productive land = 3.8 Gt.  This is about ten times the biomass of all humanity, and twice that of both all domesticated stock and total global fish [Blakemore, 2017]. Forming possibly the largest beneficial animal resource on the planet, earthworms are yet apparently severely depleted by cultivation and agrichemical excesses of industrial farming, often being absent from such soils [e.g. Lee 1985] with both their populations and biodiversity in decline [Blakemore 2016a, b, c].

In comparison to intensive agrichemical farming, studies by Blakemore, [2000, 2016a, b] show a diverse array of up to 23 earthworm species per organic farm site (mean 13 spp), implicated in 16-80% increased crop or pasture yield (mean +39%) plus an average of 12% extra soil moisture storage (range 7-91%) compared to conventional neighbour farms. Carbon sequestration is restored at rates two to three times higher in pasture.  Such findings are highly relevant due to looming species extinction and climate change with requirement to meet the needs of a growing population.  Organic farming can thus produce higher yields and sequester more carbon.

Earthworms may number up to 1,000~2,000/m2 (10-20 million/ha, or 4-8 million/ac) in fertile soils with biomass as high as 3-5 t/ha, (1.2-2 t/ac ) so earthworm stocks may outweigh the above ground stock [Lee 1985; Blakemore 2016c, 2017].  Earthworm abundance and diversity increase in a truly sustainable system as they convert all organic ‘wastes’ into humus-rich compost while processing all atmospheric CO2 in 12 yr cycles [Blakemore 2016a].  Their burrows, as long as 9,000 km/ha (2250 mi/ac) [Kretzchmar 1982] and up to 15 m in depth (49.2 ft) [Sims & Gerard, 1999: 27, as cited in Blakemore 2016c] aerate, improve water infiltration and, importantly, provide habitats for many other beneficial organisms and microbes that they help distribute throughout the entire soil profile.  All rainfall is filtered through their burrows and water is stored in worm-worked humus.  Blakemore [2000] found up to 90% extra water in pasture compared to adjacent arable fields, and organic arable soil stored 40% more water than chemically farmed arable soil [Blakemore, Hochkirch 2017].

Wormless soils are economically and ecologically expensive: they need to be plowed regularly, and require extra irrigation plus subsidized artificial chemical nitrogen fertilizers and biocide sprays to fight off plant infections and infestations [Howard 1945; Balfour 1975].  This toxic burden has severe impact upon non-target organisms and any organism fed the crops – including humans – as well as poisoning the soil, air, waterways and oceans.  Such findings are summarized in Lady Eve Balfour’s IFOAM presentation in 1977 [Balfour 1977]. Another compelling reason for earthworm conservation is that it is impossible to “geoengineer” by addressing isolated variables the many benefits and essential irreducible systems services that earthworms freely and relentlessly provide.  In other words, we have no viable alternative to earthworms.

Soil and Earthworm Relationships

We face a complexity of inter-relating ecological problems.  Intensive chemical agriculture is a major GHG contributor (28-50%) and a major source of extraneous CO2 (currently 10-25% and in total historically up to 40%) [Houghton 2010: 338, 348]:

Globally, the conversion of lands to croplands has been responsible for the largest emissions of carbon from land-use change. . . From 1850 to 2000, land use and land-use change released an estimated 108–188 Gt (billion tons) of carbon to the atmosphere, or about 28–40% of total anthropogenic emissions of carbon (274 Gt C from fossil fuels) [Strassmann 2008].

The FAO [Gerber 2013] found that intensive industrial livestock farming (rather than organic husbandry) contributed 14.5% of human-induced GHG emissions.  A newspaper report [Bryce 2013]  comments:

The FAO’s last livestock report, a 2006 assessment titled Livestock’s Long Shadow, found that farms breeding chickens, pigs, and cows for meat and dairy products, produced a disconcerting 18% of global greenhouse gas emissions . . . Around 30% of global biodiversity loss can be attributed to livestock production, such as the spread of pasture land or turning over forests and savannahs.

Although these figures vary due to different formulas for budgeting, it’s clear that agriculture in all its forms, including the practice of forest clearance, is a major contributor to GHG emissions.

The traditional, innovative & scientific methods of non-chemical, organic farming and Permaculture appreciate the importance of earthworm conservation [Howard 1945; Balfour 1975; Mollison 1988].  As a key player in natural processes and crucial issues, Darwin’s “lowly earthworm”, although neglected, warrants re-ascendency to its former position as premier farm livestock [Howard 1945]. For our own health and for that of our planet, we urgently need wholly natural vermi-composting at all scales (from kitchen to continent) in order to replace synthetic fertilizers and to facilitate rapid transition to broad-acre organics that also has earthworm livestock at its core.   Enabling earthworms to restore healthy soils is vital to stabilizing climate. All organic ‘wastes’ and manures should be recycled via vermi-composting and appropriate management employed to enhance populations of field-working worms.

Earthworm Article Summaries

van Groenigen et al. 2014. In a recent meta-analysis, while not considering organic farming or carbon per se, this study confirmed earthworm presence corresponding to crop yield increases of 25%, which is comparable to average ~39% extra organic yield in soils with earthworm proliferations determined by Blakemore [2000, 2016b]. This supports earlier studies by Wollny [1890: Forschungen auf der Gebiet der Agrikultur-Physik, 13, s. 381] that found addition of earthworms to soil led to a marked increase of cereal grain by 35-50% and of straw by 40%.

Solomon 2013. 

Although earthworms are beneficial in gardens and agricultural fields, they are harmful to Michigan’s forests where they are an invasive species. . . . Earthworms are not native to Michigan and the Great Lakes region, at least not since before glaciers covered the region; they were brought here during European settlement in the 1800s or possibly earlier. Plants, wildlife and forests evolved without any of these creatures around. They are now an invasive species that harms forests.

Hardwood forests without earthworms have a thick layer of slowly decomposing leaves, or “duff” that promotes a rich community of wildflowers, tree seedlings and small animals. Earthworms change that environment dramatically by essentially consuming the duff, thereby destroying habitat and reducing fertility. In contrast to their effect in gardens, earthworms cause forest soils to become more compacted. As a result of habitat loss, fertility declines and soil compaction, these forests may be less productive and have poorer new tree regeneration in the long run.

Another view, from oligochaetologist (worm scientist) Rob Blakemore, is as follows:

Regarding popularized concerns about alien Asian invasive worms threatening to destroy American native forests, this may reasonably be regarded as part of a process that is commonly known as Ecological Succession [Odum 2005].

Ironically, the ecological concept of succession started with Thoreau and Cowles on studies of forest succession and on the Lake Michigan dunes.  Large parts of the northeastern North America were glaciated up to about 10,000 years ago completely destroying all land surfaces and forming the Great Lakes. When the ice retreated Nature returned in successive waves and, gradually, the soil, vegetation, and animals communities re-established and species continue to evolve.  

According to Darwin [1881] earthworms are supremely important for natural productivity and for the recent progress of human civilizations. In this context the woodlands of Michigan seem a relatively minor issue compared to species extinction and climate change.  Healthy soils generally harbour earthworms and it appears there had been insufficient time for these slow-moving and flightless organisms to colonize without fast-track via incidental intervention of most-recent human settlers, often as anglers on the Great Lakes.  

When exotic crops and stock were introduced around the world 10,000 years ago, so too were attendent earthworms and these have now spread to “pristine,” albeit transitional, woodlands.  The many benefits earthworms have for agricultural and unmanaged soils may cause some changes in more natural habitats but this is a virtually unavoidable and irreversible force majeure and fact-of-life.

Certainly there will be a new ecological balance in time, possibly at a different level of productivity and biodiversity.  That is the definition of succession.

Héry et al. 2008. Earthworms have been observed to increase methanotrophy (methane metabolic breakdown) in soil covering a landfill; this is most likely “due to the stimulation of bacterial growth or activity than to substantial shifts in the methanotroph community structure” [Hery 2008: 92].  

Earthworm-mediated bioturbation has been linked to an increase in methanotrophy in a landfill biocover soil (AC Singer et al., unpublished), but the mechanism of this trophic interaction remains unclear. The aims of this study were to determine the composition of the active methanotroph community and to investigate the interactions between earthworms and bacteria in this landfill biocover soil where the methane oxidation activity was significantly increased by the earthworms [Hery 2008: 92].

And

We proposed the hypothesis that earthworms could stimulate the growth or the activity of methanotrophs. We showed that the earthworm-mediated increase of methane oxidation in the landfill biocover soil only weakly correlated with a shift in the structure of the active methanotroph population. Future work needs to focus on the relationship between this earthworm effect on enhanced methane oxidation in landfill cover soil and this effect on bacterial activity and growth. The possible contribution of an enriched population of nitrifying bacteria to methane oxidation also requires further investigation [Hery 2008: 101].