Earthworms as Soil Ecosystem Engineers

In soils that are the basis of 16 of the 24 ecosystem services recognized by the Millennium Ecosystem Assessment, earthworms play a very important role as soon as conditions allow them to get established. They carry out three types of engineering, physical, community and biochemical. With the energy that they obtain from their mutualistic digestion in association with soil microbial communities, they create a large part of the physical structure of the soil. They create pores of very diverse nature, from the large sub vertical tubular galleries of anecics to porous spaces of diverse shapes and sizes, left between casts deposited in the soil by endogeics. They are also responsible for the aggregation of a significant proportion of soil of the upper horizons, by accumulating casts, in particular those of endogeic communities. The aggregation of several hundred tons of soil per ha in the soils where they are active, greatly influences the provision of hydrological services and C storage in these compact structures. Physical engineering is accompanied by community engineering resulting from the mutualistic mode of digestion which involves a significant selective stimulation of the soil microbial community ingested with soil. In addition, the creation of more or less connected porous spaces affects root penetration and populations of smaller invertebrates which find habitats and nutritional resources there. Finally, the biochemical engineering associated with earthworm activities, the least known of all, allows them, through the release of hormones and other signal molecules produced by the microorganisms under their control, to stimulate the growth of plants and defend them against certain parasites and diseases. Among the various mechanisms involved in biochemical engineering, the modification of the expression of plant genes is the most spectacular. A more in-depth knowledge of these three types of engineering will give stronger arguments to those who want to defend their communities affected by the unsustainable methods of certain agriculture practices and to use them optimally in modern agroecological practices.