Earthworms’ role in the ecosystem

Charles Darwin is well known for his work on natural selection. He published widely on topics ranging from barnacles to geology to plants. He travelled the world and saw many unusual animals. Near the end of his life in 1881, he wrote in The Formation of Vegetable Mould,

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.

Charles Darwin

Darwin, of course, was referring to earthworms.

Earthworms may lack the charm or excitement of more familiar animals, but their contribution to our world is significant. These ‘lowly creatures’ play a vital part within the natural soil ecosystem. They are also valued for their contribution to ecosystem services – ecosystem functions that are of direct benefit to humans through their action on soil processes.

Earthworm benefits to ecosystems

Earthworms are sometimes known as ‘ecosystem engineers’ because they significantly modify the physical, chemical and biological properties of the soil profile. These modifications can influence the habitat and activities of other organisms within the soil ecosystem.

Earthworms influence (and benefit) the soil ecosystem in a number of ways:

• Recycling organic material: Earthworms, along with bacteria and fungi, decompose organic material. Most people know about earthworms and compost, but earthworms do the same in pasture soils, decomposing dung and plant litter and processing 2–20 tonnes of organic matter per hectare each year, and recycling leaf litter under orchards and in other forested areas.
• Increasing nutrient availability: This happens in two ways: by incorporating organic materials into the soil and by unlocking the nutrients held within dead organisms and plant matter. Nutrients like phosphorus and nitrogen become more readily available to plants after digestion by earthworms and being excreted in earthworm casts. Scientists have measured up to five fold increases in nitrogen availability in earthworm casts compared to undigested soil. Earthworms also take nutrients down through the soil profile, bringing them into closer contact with plant roots.
• Improving soil structure: Earthworm burrows alter the physical structure of the soil. They open up small spaces, known as pores, within the soil. When earthworms are introduced to soils devoid of them, their burrowing can lead to increases in water infiltration rates of up to 10 times the original amount. This brings water and soluble nutrients down to plant roots. Burrowing also improves soil aeration (important for both plants and other organisms living in the soil) and enhances plant root penetration.
• Providing food for predators: Earthworms, like all creatures, are part of food webs. Birds are well known predators, but native earthworms are also food for endangered and endemic land snails.

Earthworm benefits to humans

New Zealand scientists have had a unique opportunity as far as earthworm research is concerned. Once land was cleared for production agriculture, native earthworms quickly disappeared. Unless non-native species of the lumbricid family were introduced to the area, earthworms were absent for periods of time. Scientists have been able to investigate the effects of introducing earthworms to pastoral lands and quantify the benefits they provide.

Earthworms provide these ecosystem services to humans:

• Increasing pastoral productivity: Once lumbricid earthworms become established, pastoral productivity increases by 25–30%. This is equivalent to 2.5 stock units per hectare. Earthworms remove the surface thatch material that can block water from entering the soil, as the thatch can cause it (and soluble nutrients) to run off.
• Facilitating and accelerating mine restoration: By increasing soil fertility, recycling waste products and providing food resources for predators, earthworms help to restore functioning ecosystems both above and below the ground.

Trish Fraser, a soil scientist and earthworm expert says, “The next time you see an earthworm struggling on the footpath, perhaps you will be kind to our little underground ally. Indeed, perhaps you will also think about the rest of the large army of earthworms working hard for us below the ground. Maybe then the important role that this underground army plays in our lives will be forgotten no more.”