What Is The Ecological Role Of Earthworms?

Humans and other species derive invaluable benefits and functions from the ecosystem, which are referred to as ecosystem services. Such services can be categorized into the following groups: provision, which includes products such as food, water, and genetic resources; regulation, such as waste management, climate, and natural hazard regulation; cultural services, which includes spiritual, aesthetic, and recreational benefits; and habitat services. However, the provision of ecosystem services is dependent on the fulfillment of the various ecological roles held by organisms, including earthworms. The "drivers and passengers" hypothesis on ecosystem management stresses that several species are ecosystem engineers, which means that they are relatively more important than others. According to this model of ecological function, the drivers have a relatively larger ecological impact compared to passengers, who have a relatively minimal impact. The more drivers there are in an ecosystem, the more stable the ecosystem is. Earthworms are regarded as typical ecosystem engineers due to their impact on various aspects of the ecosystem, including soil structure. Worms can be found in the soils of almost every terrestrial ecological system in the world. In most cases, earthworms are also the most abundant animal biomass in such ecosystems.

Ecological Classification

Earthworms are divided into three categories that contribute in various distinctive ways to ecosystem processes, and by extension, ecosystem services. Epigeic earthworm species produce casts on the soil surface that affect the distribution of macropores and the soil’s roughness. Anecic species can be found in vertical burrows connected to the soil surface that primarily function as the earthworm’s shelter. Endogeic species are known to form horizontal and randomly oriented holes within the mineral soil. The tunnels of endogeic species are temporary, as they are rarely reused. Based on the terms of the Millennium Ecosystem Assessment, earthworms act as a catalyst for supporting services that include soil formation and nutrient cycling. They also influence services such as flood and climate regulation, water purification, remediation, and restoration. Additionally, earthworms provide cultural services by burying archeological artifacts and providing bait for fishing.

Soil Formation

The process of soil formation, which involves the breakdown of primary minerals and the gradual fusion of organic matter, takes a long time and is partly influenced by climatic conditions and the nature of the parent material. Charles Darwin was among the first people to recognize that biota, particularly earthworms, as partially responsible for soil formation through mixing processes and accumulation of earthworm casts. The recognition of the role played by worms is best illustrated in vermiform soils. Such soils consist of at least 50% of the A horizon layer, and more than 25% of the B horizon, which have earthworm structures such as burrows. Species such as Octodrilus earthworms found in the Romanian Carpathians help in the mineral weathering of the soil by affecting clay mineralogy and the formation of illite in the soil, a process that typically takes long periods without earthworms. Earthworms also play an essential role in humus formation. 

Soil Structure

Soil structure is the arrangement of particles of soil and pore spaces in the soil. It is a function of interacting physical forces including water, actions of biota including earthworms and plant roots, the presence of organic matter, and soil tillage. Worms help in making soil both compact and at times, lose. For example, R. omodei, an endogeic earthworm species, decreases soil porosity leading to compaction, whereas Eudrilidae earthworms increase porosity, leading to loosening of the soil. In general, worms have a positive effect on soil structure. 

Water Regulation

Earthworms affect soil water regulation by modifying soil porosity, which occurs through the production of microporosity, mesoporosity, and macroporosity. The specific configuration of pore shapes and sizes may allow the soil to store or transfer water in various ways. Several studies have shown that some species of earthworms increase the rate of water infiltration. In Mediterranean soils, the amount of biomass, particularly earthworms, significantly affects water percolation through the ground. Experts estimate that an increase in percolation due to worms can decrease soil erosion by up to 50%. Endogeic de-compacting species in the tropics increase the porosity of the soil and water infiltration, thereby reducing run-off. 

Nutrient Cycling 

Earthworms are classified as heterotrophic organisms given that they are involved in the degradation of organic matter. Earthworms are known to accelerate the degradation of organic matter by increasing the surface area involved through comminution. Plants can then reuse mineral nutrients released after digestion. Earthworms play a vital role in nitrogen mineralization through the release of metabolic products such as mucus, casts, and urine, which contain urea, uric acid, and dead tissue. Nitrogen mineralization can also be enhanced indirectly through the fragmentation of organic matter and alteration of the soil’s physical properties. The process of nitrogen mineralization, however, depends on the species of earthworm present in the ground. 

Climate Regulation

Earthworms play a crucial role in climate regulation, as they help in carbon sequestration by enabling the incorporation of organic material into the soil and enhancing the formation of stable macroaggregates through consumption, egestion, and burrowing activities. The storage of stable carbon aggregates in the soil helps prevent the release of carbon in the form of greenhouse gases that cause global warming.

Primary Production of Food

Earthworms have evolved alongside plants for thousands of years. Experts have found that the primary production of food through plants improves significantly due to worms. 

Potential for Ecosystem Restoration

Earthworms can be beneficial in the remediation of contaminated soil. When introduced, earthworms can stimulate the microbial population, which consequently accelerates the degradation of organic contaminants. Worms can also decontaminate the land through the metabolism of ingested soil, which therefore leads to the mineralization of the pollutants. In the presence of inorganic contaminants, modification of the soil chemistry by earthworms can help reduce the timescale needed for phytoremediation. The use of worms to restore ecosystems that have been damaged by open cast mining has also been suggested. Opencast mining can have severe environmental impacts that need to be remedied after the extraction of mineral resources is complete. While a majority of mining companies engage in ecological restoration, most focus on above the ground macro-fauna, while ignoring what happens below the ground. The introduction of earthworms in these areas can help accelerate soil restoration, enhance primary production, and facilitate the recovery of a functional ecosystem.