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Friday, 10 March 2006

Earthworms: Renewers of Agroecosystems


by Matthew Werner,

UC Santa Cruz Agroecology Program, and

Robert L. Bugg, SAREP

The Hindu pantheon includes a multitude of deities, but the principal trinity are Brahma the creator, Vishnu the sustainer, and Shiva the destroyer. Creation, sustention, and destruction, they interconnect and operate in balance through the cycles of nature. By contrast, in western religions the destructive forces of the universe are generally seen as evil. Emblematic of these destructive forces is the worm, destroyer of all things mortal. Yet in our efforts to create a more sustainable agriculture, we may need more balance in our appraisal of the great destructive forces of nature, and of the worm in particular.

For all its destructive reputation, the worm, paradoxically, has a crucial role to play in building. As noted by Charles Darwin in his 1882 classic, The Formation of Vegetable Mould Through the Action of Earthworms with Observations on Their Habits, earthworms process huge quantities of plant litter and help convert it into rich topsoil, liberating nutrients for renewed plant growth. More recent studies show that earthworms can help reduce soil compaction, improving permeability and aeration. Earthworms do this through burrowing activities, ingestion of soil along with plant debris, and subsequent excretion of casts. Upon drying, these casts form water-stable soil aggregates. These aggregates are clumps of soil particles bound together by organic compounds, and their presence helps improve soil structure, retain nutrients that might otherwise be leached, and reduce the threat of erosion.

Earthworms are only part of the complex of organisms termed "decomposers" in agroecology. Other destroyers include springtails (Collembola), nematodes, bacteria, protozoa, and fungi. Earthworms themselves fall into several subgroups based on their behavioral ecology: epigeic, endogeic, and anecic.

Epigeic earthworms are those that live in the superficial soil layers and feed on undecomposed plant litter. These worms are usually small and produce new generations rapidly.

Endogeic species are those which forage below the soil surface in horizontal, branching burrows. These species ingest large amounts of soil, showing a preference for soil rich in organic matter. Endogeics may have a major impact on the decomposition of dead plant roots, but are not important in the incorporation of surface litter.

Anecic earthworms build permanent, vertical burrows that extend deep into the soil. This type of worm comes to the surface to feed on manure, leaf litter, and other organic matter. Anecics, such as the nightcrawlers, Lumbricus terrestris and Aporrectodea longa, have profound effects on decomposition of organic matter and the formation of soil.

Deep tillage is generally harmful to earthworms. It can kill them outright, disrupt their burrows, lower soil moisture, and reduce the availability of surface litter. Shallow tillage, ridge-tillage, and surface management of crop residues has often led to increases in earthworm activity compared to areas where deep tillage is practiced. Earthworms favor leaf litter that has a low ratio of carbon to nitrogen, and tend to prefer residues of clovers and other legumes over residues of grass. Residues left as surface mulch are very useful in enhancing earthworm densities.

Certain pesticides are particularly harmful to earthworm populations. These include aldicarb, carbaryl, carbofuran, benomyl, and most soil fumigants. Most herbicides are probably not directly damaging to earthworms at the normal rates of use. Some inorganic fertilizers, especially ammonium sulfate, can be harmful to earthworm populations, possibly due to an acidifying affect.

Much of the research on earthworm agroecology has been conducted in areas with humid cool-temperate climates. Relatively little research has occurred in regions with Mediterranean climates. Native California earthworms particularly are still poorly documented and understood. The most complete collection of native earthworms was destroyed in the 1906 San Francisco earthquake and has never been restored.

There is still little research on the roles of earthworms in Californian agroecosystems. Preliminary observations suggest that earthworms in some California agricultural soils tend to have small populations of endogeic species that are active for short periods during the rainy season. More information is needed to take full advantage of the potential benefits of including earthworms in sustainable agriculture practices. What earthworm species/behavioral groups inhabit California ecosystems? Can earthworm species be introduced that are suited to particular agroecosystems? How does irrigation affect earthworm activity patterns? Are earthworms affected by botanical insecticides? What management practices will sustain crop productivity by promoting earthworm activity? These questions are a starting point for observation and experimentation.

Dindal, Daniel. 1990. Soil Biology Guide. Wiley Publishing, 1349 pages.

Edwards, C.A. 1972. Biology of Earthworms. Bookworm Publishing Co., Russelville, AR, 283 pages.

Edwards, C.A., and Lofti, J.R. 1977. Biology of Earthworms. Chapman and Hall, London.

Gaddie, Ronald E., & Donald Douglas. 1975. Earthworms for Ecology and Profit, in three volumes: Vol. 1, Scientific Earthworm Farming; Vol. 2, Earthworms and the Ecology; Vol. 3, Successful Earthworm Marketing. Bookworm Publishing Co., Russelville, AR.

Lee, K. 1985. Earthworms: Their Ecology and Relationships with Soil. Academic Press, NY, 432 pages.



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