There are more species living below ground than above. Soils provide the living space for an enormous diversity of organisms, but in turn, are also formed by soil biota. The activity and diversity of soil organisms are largely affected by biotic and abiotic factors (e.g., climate, soil properties), but also by land use and management practices. Soil organisms play important roles in a number of soil processes such as nutrient cycling, mixing of soil material and development of soil structure. Due to the high diversity of soil organisms, many species are thought to be redundant, which means that they can be replaced by other species fulfilling the same function. However, certain species may play an irreplaceable role in soil functions. Soil structure provides a multitude of niches where different species can coexist with reduced competition for food and reduced stress of being caught by predators. This is a fundamental difference to the living space above ground.